# ECHOPLEX

Before yesterdayAmerican Journal of Science recent issues

# Metal isotopes as markers of biogeochemical processes and fluxes in the eroding Critical Zone

12 November 2021 at 09:13
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# Quantifying biotic and abiotic Si fluxes in the Critical Zone with Ge/Si ratios along a gradient of erosion rates

12 November 2021 at 09:13

Silicon (Si) is an important nutrient for many plant and algae species, and the ultimate source of Si is silicate mineral weathering reactions. These topics have inspired the application of Si isotope geochemistry to quantifying Si cycling in the Critical Zone, though the interpretations are often equivocal. Because germanium (Ge) geochemistry is similar to that of Si, the Ge/Si ratio is considered a tracer that provides additional constraints on Si cycling. Here, we provide Ge/Si ratios for three sites that span a gradient of erosion rates and thus time that material spends in the weathering zone before being removed. We present Ge/Si ratios in bulk rock, soil and saprolite, clay-size fractions, plant biomass, and river water from the Central Swiss Alps, the southern Californian Sierra Nevada, and the highlands of Sri Lanka. Our data perform two functions. First, they provide insight into the Ge/Si system. In particular, we document the presence of a substantial pool of Ge in plant biomass that is not associated with phytoliths, suggesting that overall plants do not discriminate against Ge relative to Si during uptake. We also quantify the preferential incorporation of Ge into clay minerals. We show that Ge/Si ratios in secondary clays may be a better proxy for weathering intensity (the fraction of denudation achieved chemically) than the Ge/Si ratio of river solutes. Ge/Si ratios in secondary clay minerals also perform as well as or even better than silicon isotopes as weathering intensity proxies. Second, the Ge/Si data are used in conjunction with silicon isotope data to develop a catchment Si mass-balance model. It suggests that the export of secondary, fractionated solids (largely clays and plant material) becomes increasingly important at shorter regolith residence times: $${80}_{-24}^{+15}\%$$ of total solubilized Si in the rapidly eroding Alps site, vs. $${32}_{-20}^{+22}\%$$ in the slowly eroding Sri Lanka site. The results also suggest that plant material is a surprisingly large contributor to Si export from these catchments, likely equivalent to 25 to110 % of dissolved Si export.

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# The role of vegetation in setting strontium stable isotope ratios in the Critical Zone

12 November 2021 at 09:13

At Earth's surface the stable isotope ratio of strontium (88Sr/86Sr) is predominantly set by biological uptake of Sr and its storage in plant litter. This conclusion was reached from a stable isotope mass balance that was independently validated by direct determination of elemental fluxes between the Critical Zone compartments (rock, soil, vegetation, and stream water) of three field sites located in the Swiss Alps, the US Sierra Nevada, and the tropical highlands of Sri Lanka. These sites cover a gradient in erosion rates, which is inversely related to the residence time of solids in the Critical Zone thereby constituting an "erodosequence". For eroding landscapes, previous stable isotope models predicted that isotope ratios are set by the rate at which secondary solids form during the conversion of rock to regolith. Counter to this expectation we found that, after release from primary minerals, Sr is partitioned into one fraction taken up by plants and the remainder into dissolved Sr flux. The formation of secondary weathering products such as clays and oxides plays a subordinate role in determining the Sr budget. A Sr isotope fractionation factor for biological uptake was determined for each of the three ecosystems from the average Sr stable isotope composition in bulk plants and its dissolved counterpart in stream water. This fractionation factors range from ca. –0.3 for the Alps and Sierra Nevada to ~0 for the tropical Sri Lanka site. That these isotope fingerprints caused by biologic uptake are preserved means that more Sr was physically removed in plant litter than recycled. Such Sr removal in plant litter appears to be strongest at the slowly-eroding site, whereas the dissolved Sr export by streams is highest at the site with the fastest erosion rate. There, all Sr taken up by plants is returned from litter back into solution. The site with short residence time of solids is the only one at which parent material and dissolved export differ in their Sr isotope composition. Our study shows that the behavior of Sr in the Critical Zone is in stark contrast to that of metals of which the isotope fractionation is not affected by biological uptake (for example lithium, mostly set by formation of secondary solids) or affected by both secondary solid formation and biological uptake (for example silicon). Strontium stable isotope signatures offer the new opportunity to quantify nutrient cycling in the Critical Zone as a function of environmental and ecological parameters.

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# Interpreting silicon isotopes in the Critical Zone

12 November 2021 at 09:13

Metal and metalloid stable isotope ratios have emerged as potentially powerful proxies for weathering, element cycling and export in the Critical Zone. The simplest possible interpretative framework for these isotope ratios has three parameters: (i) the isotope ratio of the parent minerals undergoing weathering, (ii) the partitioning of the element between solute and the new secondary phases, and (iii) the fractionation factors associated with the formation of new secondary phases. Using the example of silicon, we show how all three of these parameters vary along a gradient of erosion rate and regolith residence time defined by three sites located on granitoid bedrock. These sites run from the kinetically limited Rhone Valley in the Central Swiss Alps to the tectonically inactive and supply-limited Sri Lankan highlands, with the Sierra Nevada mountains as a site of intermediate weathering intensity. At each site, primary mineral specific 30Si/28Si ratios span >0.4. These minerals weather differentially, such that the isotope ratio of silicon solubilised from rock differs at the three sites and is not necessarily equal to bulk bedrock composition. The partitioning of silicon between secondary clay and solute is reflected in the clay mineralogy and chemical composition: more intense weathering produces Si-poor clays. The clay composition thus comprises a first-order mass-balance control on the extent to which any fractionation factor can be expressed. Finally, the Si isotope fractionation factor associated with clay formation varies systematically with clay mineralogy: the formation of Si-deplete clay minerals is associated with larger fractionation factors. The magnitude of the fractionation may be mechanistically linked to relative aluminium availability. These findings provide the framework needed to use Si isotope ratios as a quantitative proxy to explore Si cycling and reconstruct weathering in the present and past.

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# Rock weathering and nutrient cycling along an erodosequence

How flowing water and organisms can shape Earth's surface, the Critical Zone, depends on how fast this layer is turned over by erosion. To quantify the dependence of rock weathering and the cycling of elements through ecosystems on erosion we have used existing and new metrics that quantify the partitioning and cycling of elements between rock, saprolite, soil, plants, and river dissolved and solid loads. We demonstrate their utility at three sites along a global transect of mountain landscapes that differ in erosion rates – an "erodosequence". These sites are the Swiss Central Alps, a rapidly-eroding, post-glacial mountain belt; the Southern Sierra Nevada, USA, eroding at moderate rates; and the slowly-eroding tropical Highlands of Sri Lanka. The backbone of this analysis is an extensive data set of rock, saprolite, soil, water, and plant geochemical and isotopic data. This set of material properties is converted into process rates by using regolith production and weathering rates from cosmogenic nuclides and river loads, and estimates of biomass growth. Combined, these metrics allow us to derive elemental fluxes through regolith and vegetation. The main findings are: 1) the rates of weathering are set locally in regolith, and not by the rate at which entire landscapes erode; 2) the degree of weathering is mainly controlled by regolith residence time. This results in supply-limited weathering in Sri Lanka where weathering runs to completion in the regolith, and kinetically-limited weathering in the Alps and Sierra Nevada where soluble primary minerals persist; 3) these weathering characteristics are reflected in the sites' ecosystem processes, namely in that nutritive elements are intensely recycled in the supply-limited setting, and directly taken up from soil and rock in the kinetically settings; 4) the weathering rates are not controlled by biomass growth; 5) at all sites we find a deficit in river solute export when compared to solute production in regolith, the extent of which differs between elements. Plant uptake followed by litter export might explain this deficit for biologically utilized elements of high solubility, and rare, high-discharge flushing events for colloidal-bound elements of low solubility. Our data and new metrics have begun to serve for calibrating metal isotope systems in the weathering zone, the isotope ratios of which depend on the flux partitioning between the compartments of the Critical Zone. We demonstrate this application in several isotope geochemical companion papers.

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# A Laurentian cratonic reference from the distal Proterozoic basement of Western Newfoundland using tandem in situ and isotope dilution U-pb zircon and titanite geochronology

19 October 2021 at 06:38

The Humber Margin of Newfoundland preserves the most distal exposures of Proterozoic basement in northeastern Laurentia. Age uncertainty has permitted a range of hypotheses for its origin and links to subsequent tectonic events. One hypothesis has proposed large-scale orogen-parallel displacement between basement blocks in western Newfoundland. The apparent absence of Grenville- (~1250–950 Ma sensu lato) or Taconic-aged (~480–450 Ma) magmatism or metamorphism on the Corner Brook Lake Block (CBLB), which are defining features of the Humber Margin, has been reconciled by restoring the CBLB to a pre-Taconic position in Labrador with >400 km of post-Taconic dextral motion along the Humber River Fault. To test this model and better define the basement and Paleozoic rifted margin of North America, we conducted a geochronological study of the CBLB and the basement of the adjacent Humber Margin at Indian Head Range using tandem in situ and isotope dilution U-Pb zircon and titanite geochronology. These basement blocks, separated by the Humber River Fault, consist of ~1500 and ~1250 Ma protoliths, 1140 to 1135 Ma magmatism, 1000 to 970 Ma metamorphism, and ~607 Ma intraplate magmatism. These basement blocks are also overlain by similar late Ediacaran to Cambrian siliciclastic successions with similar detrital zircon age spectra. From this set of geological data, we conclude that the Humber River Fault did not accommodate significant orogen-parallel displacement. New basement ages and a revised compilation of detrital zircon ages from overlying rift-related deposits contribute to a geochronologic cratonic reference datum for western Newfoundland's crystalline basement, whose protolith has a restricted age range from circa 1500 to 950 Ma. New age constraints for metasedimentary rocks are also used to document a 1250 to 1135 Ma succession at Indian Head Range and a ~1000 Ma succession on the CBLB associated with Grenvillian orogenesis. Protracted late Grenvillian tectono-thermal events are inferred from cores and metamorphic overgrowths of ~990 to 920 Ma detrital titanite in late Ediacaran conglomerate overlying CBLB basement.

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# Exploring multiple steady states in Earth's long-term carbon cycle

19 October 2021 at 06:38

The long-term carbon cycle regulates Earth's climate and atmospheric CO2 levels over multimillion-year timescales, but it is not clear that this system has a single steady state for a given input rate of CO2. In this paper we explore the possibility for multiple steady states in the long-term climate system. Using a simple carbon cycle box model, we show that the location of precipitation bands around the tropics and high mid-latitudes, coupled with the response of the terrestrial biosphere to local surface temperature, can result in system bi-stability. Here, maximum CO2 drawdown can occur when either the tropics or high mid-latitudes are at the photosynthetic optimum temperature of around 25°C, and a period of instability can exist between these states. We suggest that this dynamic has influenced climate variations over Phanerozoic time, and that higher steady state surface temperatures may be easier to reach than is commonly demonstrated in simple ‘GEOCARB style’ carbon cycle models.

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# A comparative study of clay mineral authigenesis in terrestrial and martian lakes; an Australian example

19 October 2021 at 06:38

Clay mineral-bearing mudstones are a prominent component of ancient fluvial-lacustrine deposits, 100s of meters thick, documented by the Mars Science Laboratory (MSL) rover, in Gale crater, Mars. Most of the clay minerals documented by MSL are hypothesized to have formed in situ, at or close to the time of deposition ~3.5 Ga ago, by aqueous alteration of basaltic detritus. Here we study the mechanisms, controls, and timescales of clay mineral authigenesis in a series of lakes with a wide range of water chemistries from the Western Volcanic District, Victoria, SE Australia, as an analog to the Gale crater mudstones. X-ray diffraction (XRD) analysis reveals that the sediments of most of the Western Volcanic District lakes studied contain mixtures of kaolinite, illite, mixed-layer illite-smectite (I-S), and dioctahedral smectite clay minerals. Comparisons of this mineral assemblage with regional soils and creek bedload material confirm previous assertions of significant inputs of detrital clay minerals into the lakes. A trioctahedral clay mineral phase is also detected, making up to 39 wt.% of bulk sediments. The abundance of trioctahedral clay minerals correlates with contemporary lake hydrology and proxies for past lake water Mg concentration. This indicates in situ formation of trioctahedral clay minerals by the uptake of Mg and Si from lake waters and pore fluids at rates determined by local physico-chemical conditions. Examination of crater lake sediments, where detrital clay mineral input is minimized, demonstrate that neoformed trioctahedral clay minerals are poorly crystalline trioctahedral smectites. Neoformation of trioctahedral smectites also occurs in lakes where detrital clay minerals are more abundant. However, an additional authigenic transformation process is indicated by the proportions of Mg and Si added to detrital clay minerals as well as evidence for the uptake of K from lake waters. The transformation process probably involved the incorporation of Mg into the octahedral sheets of detrital clay minerals, leading to irreversible uptake of K into interlayer sites (illitization). The distribution of trioctahedral smectites and radiocarbon ages from sediment cores show that clay mineral authigenesis occurred before sediment consolidation, on timescales of years to 100s of years. These results support syndepositional interpretations of analogous Mg-rich clay minerals documented by MSL, and their use as proxies for chemical conditions in ancient Gale lakes. In comparison with the Western Volcanic District lakes, clay mineral-bearing lacustrine mudstones from Gale crater exhibit only modest chemical weathering of basaltic detrital materials and rarely contain carbonate minerals in quantities detectable by XRD. These observations highlight significant differences in weathering regimes and regolith mineralogy on ancient Mars that could be linked to lake catchment geomorphology, climate, atmospheric CO2 content, and the absence of biotic processes on Mars.

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# Multiphase ophiolite formation in the Northern Altyn Tagh Orogen, southeastern Tarim

6 October 2021 at 11:14

Early Paleozoic ophiolitic mélanges in the Altyn Tagh Orogen, southeastern Tarim, yield a large range of formation ages and geochemical affinities. This study focused on the Hongliugou ophiolitic mélange in the North Altyn Tagh subduction-accretion belt and involved mineral chemical, zircon geochronological, and whole-rock elemental and isotopic investigations of the ultramafic and mafic rocks. In the studied lherzolite samples, Cr-spinel, olivine, and pyroxene show mineral chemistry akin to that of abyssal peridotite. Subhedral-anhedral Cr-spinel grains with high Cr# values (100*Cr/[Cr+Al] of 30–40) and negative oxygen fugacity (fO2) values represent original spinel formed during mid-ocean ridge basalt (MORB) melt extraction. Other subhedral-anhedral and euhedral Cr-spinel with low Cr# values of 2 values, indicating interaction of peridotite with MORB magma. New zircon U-Pb dating results record ages of ca. 490 Ma for gabbro and ca. 486 Ma for diabase in the Hongliugou ophiolitic mélange. Whole-rock geochemical compositions suggest that the gabbro and diabase samples are tholeiitic and show MORB affinities or transitional affinities between MORB and island arc tholeiite. These mafic rocks might have been derived from a depleted MORB-source mantle that was variably metasomatized by subduction-induced fluids based on their variable Th/Yb, high La/Nb, low Th/Nb, and depleted isotopes (initial 87Sr/86Sr ratios of 0.703073 to 0.703385 and Nd(t) values of +6.2 to +6.3). New results, when integrated with previous work, clarify that ca. 520 to 510 Ma ophiolites formed in the initial subduction setting and ca. 490 to 480 Ma ophiolites in a back-arc setting. Minor ca. 450 Ma ophiolite probably represents a late phase of ophiolite formation due to the presence of a very minor remnant ocean.

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# Deciphering paleogeography from orogenic architecture: Constructing orogens in a future supercontinent as thought experiment

6 October 2021 at 11:14

Orogens that form at convergent plate boundaries typically consist of accreted rock units that form an incomplete archive of subducted oceanic and continental lithosphere, as well as of deformed lithosphere of the former upper plate. Reading the construction of orogenic architecture forms the key to decipher the pre-orogenic paleogeographic distribution of oceans and continents, as well as bathymetric and topographic features that existed thereon such as igneous plateaus, seamounts, microcontinents, or magmatic arcs. Current classification schemes of orogens divide between settings associated with termination of subduction [continent-continent collision, continent-ocean collision (obduction)] and with ongoing subduction (accretionary orogenesis), alongside intraplate orogens. Perceived diagnostic features for such classifications, particularly of collisional orogenesis, hinge on dynamic interpretations linking downgoing plate paleogeography to upper plate deformation, plate motion changes, or magmatism. Here, we show, however, that Mesozoic-Cenozoic orogens that undergo collision almost all defy these proposed diagnostic features and behave as accretionary orogens instead. To reconstruct paleogeography of subducted and upper plates, we therefore propose an alternative approach to navigating through orogenic architecture: subducted plate units comprise nappes (or mélanges) with Ocean Plate Stratigraphy (OPS) and Continental Plate Stratigraphy (CPS) stripped from their now-subducted or otherwise underthrust lower crustal and mantle lithospheric underpinnings. Upper plate deformation and paleogeography respond to the competition between absolute motions of the upper plate and the subducting slab. Our navigation approach through orogenic architecture aims to avoid a priori dynamic interpretations that link downgoing plate paleogeography to deformation or magmatic responses in the upper plate, to provide an independent basis for geodynamic analysis. From our analysis we identify ‘rules of orogenesis' that link the rules of rigid plate tectonics with the reality of plate deformation. We use these rules for a thought experiment, in which we predict orogenic architecture that will result from subducting the present-day Indian Ocean and colliding the Somali, Madagascar, and Indian margins using a published continental drift scenario for a future supercontinent as basis. We illustrate that our inferred rules (of thumb) generate orogenic architecture that is analogous to elements of modern orogens, unlocking the well-known modern geography as inspiration for developing testable hypotheses that aid interpreting paleogeography from orogens that formed since the birth of plate tectonics.

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# Foreword

6 October 2021 at 11:14
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# Geodynamic environment of the ca. 3800 Ma Outer Arc Group, Isua (Greenland)

The arcuate, 35 km long Isua supracrustal belt (ISB, southern West Greenland) contains the world's largest remnants of Eoarchean volcanic and sedimentary sequences. The ISB is broadly divided into: (i) the northern Inner Arc Group of 3720 to 3690 Ma rocks, and (ii) the southern Outer Arc Group of ca. 3800 Ma rocks which is bounded on its northern side by the highly tectonized ca. 3750 Ma Dividing Sedimentary Unit. The boundary between the two groups is a mylonite formed between 3685 and 3660 Ma. Despite the generally high strain, amphibolite facies metamorphism and layer-parallel dislocations that can thin or altogether excise some units, domains of lower deformation comprising Mafic Volcanic formation in which rarely-preserved pillow structures indicate both their predominantly subaqueous eruption and also their stratigraphic facing. They erupted >3800 Ma, because they were first intruded by subconcordant sheets of fine-grained hypabyssal tonalite dated at 3803±3 Ma (Crowley, 2003) and then by coarser-grained 3795 to 3791 Ma tonalite-granodiorite, which forms a large deformed pluton along the south side of the ISB. This formation is succeeded by the Sedimentary formation whose base consists of discontinuous rare, thin fuchsitic quartzites with 3890 to 3805 Ma detrital zircons. Overlying is a diverse package of dolostones, marls and siliceous rocks. Although they are extensively modified by metamorphism and metasomatism, producing widespread growth of talc or tremolite, relict graded sedimentary layering, chemical and isotopic signatures indicate originally sedimentary protoliths. Detrital zircons in these rocks range in age from ca. 3820 to 3805 Ma. This unit shows an upwards transition from ‘pure' chemical sedimentary rocks with distinct seawater-like trace element signatures into lithologies increasingly contaminated by felsic material that is locally preserved as graded layers, which are interpreted as an increasing volcanogenic input. Succeeding the sedimentary rocks is the Felsic Volcanic formation, an extensive unit of mostly schistose 3807 to 3802 Ma felsic potassic-altered rocks with carbonate-rich interludes and veins. Locally-preserved andesitic units with graded layering, massive vesicular lavas, polymict breccias, resorbed quartz phenocrysts and fiammé, attest to volcanic and volcano-sedimentary protoliths. Whole rock geochemistry and oxygen isotope analyses on these rocks and their zircons indicate predominantly felsic volcanic protoliths that experienced massive alteration in a surficial environment, probably following subaerial eruption. Massive volcanic rocks are commonest in the west of the ISB, suggesting this part of the belt was proximal to a volcanic center. Using these stratigraphic data, we conclude that the ISB volcanic and sedimentary rocks formed in a mobile geodynamic regime resembling plate tectonics, and not within a stagnant lid regime.

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# Composition and provenance analysis of beach sands in an almost isolated sedimentary system - A field study of the Galapagos Archipelago

The Galápagos Archipelago is the surface expression of an active hotspot or long-lived mantle plume. The Archipelago consists of a group of 13 main islands which are located in the eastern central Pacific Ocean about 1,000 km west of the northern edge of the South American continent, east of the East Pacific Rise and south of the Galápagos spreading center. Because of the large distance to the nearest continental land mass, Galapagos can be seen as an almost isolated sedimentary system. A provenance study conducted on samples collected from seventeen beaches on eleven islands, demonstrates that mineral grains and particles were derived from weathering of predominantly basaltic rocks and were transported within the islands, between the islands or inside the coastal area around the Archipelago. The exclusion of external sources allows advanced studies about erosion processes, transport pathways of particles and the accumulation of autochthonous sediments. The combined usage of optical particle size and shape analysis with RAMAN spectroscopy allows a successful spatial delimitation of host rocks and a reconstruction of transport pathways. The analyzed samples can be subdivided into three groups: 1) Type-A sediments: fine-grained and sampled on beaches of the oldest islands in the eastern part of Galápagos. The composition of volcanic minerals corresponds to the alkaline character of the basaltic source rocks. 2) Type-B: well sorted sediments characterized by medium-grained olivine, pyroxene, plagioclase and even a small amount of quartz grains. The islands of this group are located in the central region of the Archipelago. 3) Type-C samples: olivine and pyroxene are the predominant volcanic minerals. These samples indicate bimodal, coarse-grained size distributions and large proportions of pumice and are found in Floreana in the south and the youngest islands Isabela and Fernandina in the west of Galápagos.

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# Petrogenesis and tectonic implications of TTG granitoids from the Daqingshan Complex of the Khondalite Belt, North China Craton

6 October 2021 at 11:14

Located in the Western Block of the North China Craton, the Khondalite Belt is one of the three Paleoproterozoic tectonic belts that were linked to the final assembly of the craton. At present, a popular model is that the Khondalite Belt was formed by the collision between the Yinshan and Ordos blocks at ~1.95 Ga. However, the initiation of oceanic subduction and its related arc magmatism and accretionary process before the collisional event were poorly constrained. The Daqingshan Complex is located in the middle East part of the Khondalite Belt, and contains highly deformed and metamorphosed rock assemblages, and thus represents a key area to decipher the above issue. In this study, we carried out petrological, geochemical and geochronological analysis on the TTG granitoids of the Daqingshan Complex. Zircon U-Pb results from three typical TTG samples yielded upper intercept ages of 2545 ± 50 Ma, 2484 ± 68 Ma and 2452 ± 32 Ma, indicating that the TTG granitoids were emplaced in the late Neoarchean. Metamorphic zircons from two samples gave 207Pb/206Pb weighted mean ages of 1892 ± 53 Ma and 1906 ± 27 Ma, respectively, recording the timing of a continent-to-continent collisional event. Thirteen TTG granitoid samples are geochemically low-, medium- and high-K calc-alkaline, with metaluminous to peraluminous trends and are enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, La, Ce, Nd, and depleted in high field strength elements (HFSEs) such as Nb and Ta. Chondrite-normalized rare earth element (REE) patterns show fractionation with (La/Yb) N ratios ranging from 8.20 to 27.47, with weak Eu negative anomalies (Eu = 0.50 – 0.98). In addition, TTG granitoids of the Daqingshan Complex belong to I-type granitoids, and their igneous protoliths were intimately related to a subduction-related magmatic arc environment. New results of this study reveal that the initial oceanic lithosphere subduction operated since ~2.55 Ga along the southern margin of the Yinshan Block, and generated the coeval arc-related TTG granitoids. Closure of the ocean led to the final collision between the Yinshan and Ordos blocks and the amalgamation of the Western Block at 1.95 to 1.85 Ga.

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# An Early Paleoproterozoic back-arc system along the southern margin of the Yinshan Block: Evidence from a newly-defined bimodal volcanic sequence in the Daqingshan Complex, Khondalite Belt

As one of the 2.1 to 1.9 Ga orogenic belts that welded the Columbia supercontinent, the Khondalite Belt in the North China Craton is a typical continent-continent collisional orogen that formed through the collision between the Yinshan and Ordos Blocks. Previous studies mostly focused on the collisional event in the Khondalite Belt but paid little attention to how the subduction system operated before the final closure of the ocean. To address this issue, we identified a series of interlayered meta-mafic and felsic rock assemblages in the Daqingshan Complex and implemented geochemical and geochronological analyses. Petrological and geochemical studies revealed that these rocks are bimodal and include plagioclase amphibolite (Group 1) and biotite plagiogneiss (Group 2). Geochemically, Group 1 samples show tholeiitic affinity, whereas Group 2 samples belong to the high-K calc-alkaline series. Geochemical data indicate that the protolith magma of Group 1 was most likely derived from the partial melting of lithospheric mantle with minor crustal contamination, whereas Group 2 rocks represent highly differentiated magma derived from the partial melting of ancient crustal materials. All the samples show depletion of HFSEs and enrichment of LILEs, indicative of a subduction-related magmatic arc environment. Zircon U-Pb dating results show that the protoliths of Group 1 samples yield crystallization ages of ~2.47 Ga and metamorphic ages of 1.95 to 1.85 Ga, whereas the protoliths of Group 2 samples yield crystallization ages of ~2.40 Ga and metamorphic ages of ~1.85 Ga. Our new results and available geochemical, petrological, and isotopic data demonstrate that the bimodal volcanic sequence of the Daqingshan Complex was developed in a 2.47 to 2.40 Ga back-arc system along the southern margin of Yinshan Block. Subsequent collision between the Ordos and Yinshan Blocks resulted in the formation of the Khondalite Belt and final amalgamation of the Western Block between 1.95 and 1.85 Ga.

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# Reconstructing lost plates of the Panthalassa Ocean through paleomagnetic data from circum-Pacific accretionary orogens

The Panthalassa Ocean, which surrounded the late Paleozoic-early Mesozoic Pangea supercontinent, was underlain by multiple tectonic plates that have since been lost to subduction. In this study, we develop an approach to reconstruct plate motions of this subducted lithosphere utilizing paleomagnetic data from accreted Ocean Plate Stratigraphy (OPS). We first establish the boundaries of the Panthalassa domain by using available Indo-Atlantic plate reconstructions and restorations of complex plate boundary deformation at circum-Panthalassa trenches. We reconstruct the Pacific Plate and its conjugates, the Farallon, Phoenix, and Izanagi plates, back to 190 Ma using marine magnetic anomaly records of the modern Pacific. Then, we present new and review published paleomagnetic data from OPS exposed in the accretionary complexes of Cedros Island (Mexico), the Santa Elena Peninsula (Costa Rica), the North Island of New Zealand, and Japan. These data provide paleolatitudinal plate motion components of the Farallon, Phoenix and Izanagi plates, and constrain the trajectories of these plates from their spreading ridges towards the trenches in which they subducted. For 83 to 150 Ma, we use two independent mantle frames to connect the Panthalassa plate system to the Indo-Atlantic plate system and test the feasibility of this approach with the paleomagnetic data. For times prior to 150 Ma, and as far back as Permian time, we reconstruct relative and absolute Panthalassa plate motions such that divergence is maintained between the Izanagi, Farallon and Phoenix plates, convergence is maintained with Pangean continental margins in Japan, Mexico and New Zealand, and paleomagnetic constraints are met. The reconstruction approach developed here enables data-based reconstruction of oceanic plates and plate boundaries in the absence of marine magnetic anomaly data or mantle reference frames, using Ocean Plate Stratigraphy, paleo-magnetism, and constraints on the nature of circum-oceanic plate boundaries. Such an approach is a crucial next step towards quantitative reconstruction of the currently largely unknown tectonic evolution of the Earth's oceanic domains in deep geological time.

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# Chronological and geochemical variations of the late Mesozoic granitoids in the Taihang Mountains and middle-southern Tan-Lu Fault: Implications for lithosphere destruction of the North China Craton

6 October 2021 at 11:14

In the Late Mesozoic, the North China Craton (NCC) underwent significant lithospheric thinning and destruction, especially in the eastern part, but the mechanism and timing related to this process are still contentious. The Taihang Mountains (TH) are located in the western part of the eastern NCC and the Tan-Lu Fault (TLF) is in the eastern part, which are two essential magmatic areas that reveal deep processes of magma origin. We investigated the spatial-temporal distribution of igneous rocks from these two areas to constrain the tectonic setting and magmatic sources. SHRIMP zircon U-Pb ages of the granitoids within the Fangshan pluton in northern TH area range from 136 to 128 Ma. Their Hf(t) values and 18O values show ranges of –27.7 to –18.5 and 6.68 to 7.26 permil, respectively. Hence, we conclude that the rocks were formed by mixing between underplating magma and the melts from the lower crust. The O-Hf isotopic compositions of six granitoid samples from the Yunmengshan complex in northern TH are also reported. In combination with previous studies, we propose that the geochemical characteristics of the magmatic rocks from the TH area had insignificant changes during late Mesozoic time, but the rocks from the TLF area varied greatly. The difference between those two areas may reflect the diverse impact of the Paleo-Pacific subduction process. The high Mg# adakitic rocks (HMA) from TLF area have higher Mg# values than the HMA rocks from TH area. Our conclusion is that the HMA rocks in the TLF area were mainly formed by delaminated lower crust interacting with mantle materials and that the Paleo-Pacific subduction had limited impact on TH magmas. Based on chronology and geochemical characteristics, we recognize three stages: 1) ~166 to 140 Ma, multi-directional compression resulted in crustal shortening and thickening in the NCC, accompanied by regional partial melting of the crust and underplating of mafic magmas, 2) 140 to 125 Ma, the TLF underwent left-lateral strike-slip movement. Subsequent delamination of the lower crust around the fault and the NCC evolved into an extensional tectonic environment, 3) after 125 Ma, a large-scale extension of the NCC occurred likely due to stress relaxation after delamination. The TLF acted as a favorable channel for transporting mantle material and fluids, which implies that the large-scale fault zone was a key factor of the NCC lithosphere destruction.

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# Preface: THIS IS THE SECOND ISSUE DEDICATED TO THE MEMORY OF DISTINGUISHED SCIENTIST ALFRED KRONER WHO SADLY PASSED AWAY ON 22 MAY 2019

6 October 2021 at 11:14
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# A highly dynamic hot hydrothermal system in the subduction environment: Geochemistry and geochronology of jadeitite and associated rocks of the Sierra del Convento melange (eastern Cuba)

A U-Pb zircon date of ~113 Ma revealed that a variety of jadeitites and related omphacitite, chloritite and albite-rich rocks from the subduction-related Sierra del Convento block-in-serpentinite-matrix mélange (eastern Cuba) formed nearly synchronously with MORB metabasite-derived anatectic trondhjemitic liquids at high-temperature and pressure in a hot subduction environment. Field, petrologic and geochemical data indicate hydrothermal/metasomatic processes triggered by juvenile fluids likely evolved from the crystallizing hydrous trondhjemitic melts. These fluids, variably mixed with sediment-derived fluids and channelized along fractures in the supra-slab mantle, precipitated relatively pure jadeitite with geochemical patterns depleted in REE and HFSE and epidote-rich jadeitite with LILE- (notably Ba) enriched compositions with respect to N-MORB. The crystallization of jadeitite veins was accompanied by formation of chloritite blackwalls at the vein-ultramafic rock contact and omphacititic patches at the outer parts of the veins, denoting wall rock-fluid interactions. Further pervasive flow of external fluid within the rock bodies triggered modal and cryptic (geochemical) metasomatic transformation of earlier jadeitite, producing mica-rich jadeitite and albite-epidote (-chlorite) rocks. Altogether these rocks document a discrete episode of massive flow of fluid in the supra-slab mantle roughly coeval with hydrous melting of subducted MORB metabasite.

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# Origin and magmatic evolution of late Neoproterozoic post-accretion high-K calc-alkaline adakitic volcanics in the northern Arabian-Nubian Shield

13 September 2021 at 14:09

In the northernmost segment of the Arabian–Nubian Shield, a post-collisional high-K calc-alkaline volcanic sequence is exposed along Wadi Abu Ma’amel, Eastern Desert of the Nubian Shield. It comprises a series of intermediate to silicic volcanics and associated pyroclastics that include the Imperial Porphyry and calc-alkaline volcanics typical of the Dokhan Volcanics. The Imperial Porphyry occurs as subvolcanic sill-like intrusions forming the young member of the Dokhan Volcanics. The volcanic sequence extruded through synorogenic granite and was intruded by post-collisional granite, which also caused thermal contact metamorphism. The red and purple colors of the Imperial Porphyry reflect hydrothermal alterations, which resulted in the formation of dispersed flakes of hematite, epidote, and piemontite. The entire high-K calc-alkaline volcanic sequence, ranging from andesite through dacite and rhyodacite, exhibits post-collisional geochemical characteristics. Most samples of the Imperial Porphyry and some of the typical Dokhan Volcanics have characteristics of adakitic rocks, including high Sr (694–889 ppm), low Y (10.6–18.8 ppm), high Sr/Y (41.1–83.8), (La/Yb)n (8.6–15.6), and low (Yb)n (5.4–9.0). The mostly calc-alkaline character and other traits of the studied volcanics that were previously interpreted to indicate arc magmatism reflect, instead, remelting of earlier (pre-collisional) arc-related material. The formation of Wadi Abu Ma'amel volcanics resulted from upwelling of hot asthenospheric material during thinning of the previously thickened lithosphere as a consequence of lithospheric delamination. The parental magma was generated by partial melting of mafic lower crust that mixed with upper-crust-derived magma. It evolved mostly through fractionation of clinopyroxene and plagioclase, accompanied by apatite and Fe–Ti oxides in the more-evolved dacitic and rhyodacitic rocks.

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# Presentation and applications of mixing elements and dissolved isotopes in rivers (MEANDIR), a customizable MATLAB model for Monte Carlo inversion of dissolved river chemistry

13 September 2021 at 14:09

The dissolved chemistry of rivers has been extensively studied to elucidate physical and climatic controls of chemical weathering at local to global spatial scales, as well as the impacts of chemical weathering on climate over short to geologic temporal scales. Within this effort, mixing models with Monte Carlo uncertainty propagation are a common tool for inverting measurements of dissolved river chemistry to distinguish among contributions from end-members with distinct elemental and/or isotopic compositions. However, the methods underlying prior river inversion models have typically been opaque. Here we present Mixing Elements ANd Dissolved Isotopes in Rivers (MEANDIR), a set of MATLAB scripts that enable highly customizable inversion of dissolved river chemistry with Monte Carlo propagation of uncertainty. First, we present an overview of the mathematics underlying MEANDIR. This overview includes, among other topics, derivation of equations for mass balance, implementation of chlorine critical values, construction of cost functions, normalization to the sum of dissolved variables, quantification of river sulfate sourced from pyrite oxidation, resolution of petrogenic organic carbon oxidation, representation of secondary phase formation with isotopic fractionation, and calculation of the impact of weathering on atmospheric carbon dioxide. Second, we apply MEANDIR to five previously published datasets to demonstrate the sensitivity of results to parameter choices. We invert data from two global compilations of river chemistry (Gaillardet and others, 1999; Burke and others, 2018), the major element chemistry and sulfate sulfur isotope ratios of rivers in the Peruvian Amazon (Torres and others, 2016), the major element chemistry of Icelandic rivers (Gíslason and others, 1996), and the major and trace element chemistry of water samples from the Mackenzie River (Horan and others, 2019). MEANDIR and its user guide are freely available online.

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# Paleoaltimetry of the Western Andes in Northern Chile (~18.5-19.5{degrees}S)

13 September 2021 at 14:09

Establishing the timing of surface uplift in the Central Andes is essential for evaluating the geodynamic mechanisms responsible for mountain building and their role in the development of dry conditions along the western coasts of Peru and Chile. Here, we present new stable hydrogen isotopic values from stream waters and hydration water in volcanic glass from northern Chile (18.5–19.5°S) that show that the Western Cordillera was already elevated by the early Miocene. The hydrogen isotopic values of reconstructed surface waters obtained from ancient and modern volcanic glass indicate that the Western Cordillera in northern Chile attained modern elevations by at least 22.8 Ma. When combined with paleoaltimetric records from the Altiplano and northwestern Puna, these results demonstrate that surface uplift of the Andean plateau was a time-transgressive process that varied not just from west to east but also from north and south along the strike of the orogen. Our paleoaltimetry reconstruction also suggests that the Western Cordillera has blocked moisture coming from the east since at least the early Miocene, consistent with previously published evidence of arid-semiarid conditions in the Atacama Desert. However, hyperaridity on the western Andean slope developed later and appears to correspond with the timing of uplift in the Eastern Cordillera and Altiplano. Our results suggest that the growth of the Central Andean rain shadow relied not only on the elevation of the Western Cordillera but also on the widening of the plateau.

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# Silurian-Devonian tectonic evolution of mid-coastal Maine, U.S.A.: Details of polyphase orogenic processes

29 July 2021 at 07:24

Detailed bedrock mapping, structural geology, meta-igneous whole rock geochemistry, and U-Pb geochronology from rocks sampled along a portion of a complexly deformed tectonic boundary between the Ordovician peri-Gondwanan Liberty-Orrington belt and Silurian syn-orogenic strata of the Fredericton trough (a.k.a. the Dog Bay Line) in mid-coastal Maine aid in deciphering the Silurian-Devonian tectonic evolution of the region. The new results provide constraints on several key events. First, initial terrane juxtapositioning occurred along the east-verging Boothbay thrust fault (D1). This tectonism occurred prior to 423 Ma and is associated with the accretion of the Ganderian microcontinent to the Laurentian margin (that is, the Salinic orogeny). Subsequently, intrusion of an ultra-potassic magma, the protolith of the Edgecomb Gneiss, occurred at ca. 413 Ma. Its distinctive whole rock geochemical signature allows for correlation with rocks of similar composition and age along a relatively narrow 140 kilometer long distance on the northwestern margin of the Fredericton trough. This restricted area of ultra-potassic magma generation is attributed to the breakoff of the descending Salinic oceanic slab that triggered decompression melting of a previously metasomatized mantle wedge region beneath the accreted Ganderian microcontinent. Early thrust faults (D1) and the ca. 413 Edgecomb Gneiss igneous protolith were overprinted by an episode of upright folding (D2) and low-pressure amphibolite facies metamorphism associated with the Early to Middle Devonian Acadian orogeny. Zircon overgrowths in the Edgecomb Gneiss dated at ca. 399 Ma grew during this tectonic episode. Comparisons with previous geochronological studies across the region suggest this dominant phase of Acadian deformation and metamorphism was long-lived (ca. 40 m.y.) and associated with the outboard accretion of the Avalonian microcontinent. Dextral shear structures represent the final phase of deformation (D3) superimposed on this terrane boundary and are associated with the Norumbega fault and shear zone system that was active in Middle Devonian-Carboniferous time.

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# Terrestrial biomarker isotope records of late Quaternary climate and source-to-sink sediment transport processes in southwestern Taiwan

Fluvial sediments are important archives of paleoenvironments. However, variations in sediment production and transport processes greatly influence sediment geochemistry and resultant interpretations of ancient conditions. Tectonically-active tropical regions are particularly sensitive to climate feedbacks because these areas are often characterized by high precipitation rates, rapid erosion and short sediment residence times. We analyzed the hydrogen and carbon isotope composition of plant-derived n-alkanes (2Hn-alkane and 13Cn-alkane) in sediment cores along the Gaoping River-submarine canyon system in southwestern Taiwan to examine climatic and geomorphic controls on isotope geochemical signatures of fluvial sedimentary archives. These records span the last ~26 kyr and provide critical insight into the temporal and spatial variations in sedimentary biomarker isotopes within a source-to-sink system. Isotope data are coupled with new results from an iCESM 1.2 Earth System Model of precipitation isotopes during the last glacial-interglacial cycle. Biomarker isotope and modeling results support two important conclusions. First, biomarker isotope values change by ~10 to 15 in 2Hn-alkane and ~1 to 2 13Cn-alkane in offshore SW Taiwan through the late Quaternary deglaciation. These shifts are consistent with iCESM predictions and other records from the South China Sea and are best explained by a shift in isotope hydrology due to regional warming and biologic responses to increased atmospheric pCO2. Second, the 2Hn-alkane of biomarkers preserved in onshore sediments proximal to the mountain range is ~15 to 20 more negative than biomarkers deposited in offshore sites, and the temporal change in carbon isotopes exceeds that observed in the offshore deposits. The onshore core locality is proximal to the orogen and characterized by a mean elevation > 1 km compared to the offshore site, which has a mean catchment elevation of ~500 m. These data show that depositional setting and catchment hypsometry strongly bias the geochemical signature of sediments transported through the river system. The magnitude of isotopic variability generated by catchment geometry and sediment integration greatly exceeds the change associated with warming during deglaciation. This result suggests that catchment integration processes may play a similar or larger role in shaping fluvial geochemical records in tropical mountain systems than climatic factors.

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# Thermochemistry of melilites I. Towards resolving an inconsistency in nebular condensation calculations

29 July 2021 at 07:24

A thermodynamic model is formulated for (Ca,Na)2(Mg,Fe2+,Al,Fe3+)T1 (Al,Fe3+,Si$${)}_{2}^{\mathbf{T2}}$$O7 melilites. It employs the compositional vertices: åkermanite (Ca2MgSi2O7, 1), gehlenite (Ca2Al2SiO7, 2), iron åkermanite (Ca2Fe2+Si2O7, 3), ferrigehlenite (Ca2$${\hbox{ Fe }}_{2}^{3\hspace{0.17em}+\hspace{0.17em}}$$SiO7, 4), sodium melilite (NaCaAlSi2O7, 5), and the convergent ordering variables: s = $${X}_{{\mathbf{Al}}^{3\hspace{0.17em}+\hspace{0.17em}}}^{\mathbf{T2a}}$$$${X}_{{\mathbf{Al}}^{\mathbf{3+}}}^{\mathbf{T2b}}$$ and t = $${X}_{{\mathbf{Fe}}^{\mathbf{3}\hspace{0.17em}+\hspace{0.17em}}}^{\mathbf{T2a}}$$$${X}_{{\mathbf{Fe}}^{\mathbf{3}\hspace{0.17em}+\hspace{0.17em}}}^{\mathbf{T2b}}$$ to describe the distribution of Al3+, Fe3+ and Si4+ between T2 subsites T2a and T2b. It is calibrated for åkermanite–gehlenite melilites based on the calorimetric data of Charlu and others (1981), the assumption that the synthetic samples of Charlu and others approached "equilibrium" states of Al-Si tetrahedral ordering at 970 K, and analogy with the Al2(MgSi) – 1 substitution in CaMgSi2O6 – CaMg1/2$${\hbox{ Ti }}_{1/2}$$AlSiO6 – CaAl2SiO6 fassaites (for example, Sack and Ghiorso, 2017). In this model gehlenite has a disordered Al-Si distribution on T2 sites above 1443 K (1170 °C), consistent with the crystallographic data on c/a ratios of lattice parameters as a function of annealing temperature (Woodhead and Waldbaum, 1974) and the high-temperature heat capacities inferred from drop calorimetric data (Pankratz and Kelley,1964). However, above this critical temperature a partially ordered Al-Si distribution persists between T2a and T2b sites in åkermanite – gehlenite solid solutions with intermediate X2 (for example, 0.19 $${X}_{2}$$ ai = Xi), particularly in gehlenite-rich compositions, but those of åkermanite component display pronounced temperature dependence in intermediate compositions. Enthalpies of formation of åkermanite and gehlenite from the elements at 298.15 K, $$\Delta {\overline{H}}_{f\hspace{0.17em}298.15}^{\mathrm{o}\hspace{0.17em}\mathbf{AK}}$$ and $$\Delta {\overline{H}}_{f\hspace{0.17em}298.15}^{\mathrm{o}\hspace{0.17em}\mathbf{GEHL}}$$, consistent with the experimental brackets on decarbonation equilibria of Walter (1963), Hoschek (1974), and Shmulovich (1974), the thermodynamic model for åkermanite-gehlenite melilites developed here, the thermodynamic properties of the other phases in these reactions tabulated by Berman (1988), and the revised estimates for $${\overline{C}}_{p}$$ and $${\overline{S}}_{298.15}^{\hbox{ o }}$$ of diopside of Sack and Ghiorso (2017), are roughly 1 and 3 (kJ/gfw) more positive than those estimated by Berman (1988). More positive standard enthalpies of formation of both endmembers, together with a decrease in the vibrational heat capacity of gehlenite and less negative deviations from ideal mixing compared with previous calibrations, all contribute to reducing the stability of melilites in this model. Together these effects will decrease the predicted temperature of condensation of melilite from nebular vapors, bringing calculated temperatures of melilite condensation into closer alignment with those of MgAl2O4 spinel than the 80 to 100 K separating their appearances in previous calculations (for example, Yoneda and Grossman, 1995; Petaev and Wood,1998; Ebel and Grossman,2000). These effects, together with a possible increase in spinel stability due to non-negligible solubility of Al8/3O4 alumina component, may allow equilibrium models to match the observed condensation sequence of spinel before melilite in calcium-aluminum inclusions (CAIs) in carbonaceous chondrites, without the need to invoke kinetic effects.

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# Carbon cycle evolution before and after the Great Oxidation of the atmosphere

5 May 2021 at 13:27

The rock record of organic carbon abundance and its isotopic composition is consistent with the evolution of life more than 3800 million years ago (Ma). Despite this, there are very few insights as to the ecology of this ancient biosphere or to its level of activity. One possible insight, however, comes from the isotopic composition of inorganic and organic carbon in ancient rocks. This isotope record can be used, in principle, to determine the proportion of inorganic carbon entering the oceans that was buried in sediments as organic matter, and thus it helps constrain the activity level of the ancient biosphere. A quantitative analysis of this isotope record, however, requires that we understand how the Earth-surface carbon reservoir has evolved over time, as burial rates of organic matter in marine sediments depend on the input rates of carbon to the oceans. We must also know how organic matter is weathered as a function of atmospheric oxygen concentrations, thus indicating how much of the organic matter in sediments is newly formed or recycled. To explore these issues, a carbon cycle model is developed here that includes an evolving Earth-surface carbon reservoir as well as the oxygen dependency of the organic matter weathering in rocks. The model also allows for the release of CO2 from organic matter during metamorphism and it contains a rock cycle with young and old reservoirs with appropriate transfer fluxes between them. The model shows that before the Great Oxidation Event (GOE) about 2400 Ma, only about 5 percent to 10 percent as much organic matter was buried into marine sediments as compared with today. Such low rates of organic matter burial would be consistent with a subdued marine biosphere. Such a subdued biosphere could possibly be consistent with primary production driven by anoxygenic photosynthesis coupled to an iron cycle. In association with, and in the aftermath of, the GOE, the biosphere likely increased its activity level by an order of magnitude. This large increase would have completely transformed the biology of the Earth and could have resulted from either the evolution and/or expansion of oxygen-producing cyanobacteria or a dramatic increase in the availability of nutrients to fuel oxygenic phototrophs.

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# 40Ar/39Ar and LA-ICP-MS U-Pb geochronology for the New England portion of the Early Cretaceous New England-Quebec igneous province: Implications for the postrift evolution of the eastern North American Margin

The Early Cretaceous New England-Quebec igneous province is a classic example of postrift magmatism along the eastern North American passive margin. Although a suite of 40Ar/39Ar ages has been available for the Monteregian Hills lobe in the Quebec portion of the New England-Quebec igneous province for many years, only a single high accuracy radiometric age has been published for the Burlington lobe and none for the Taconic lobe in the New England portion of the province. As a result, the timing of and driving mechanisms behind the magmatism have remained unresolved, and a hotspot origin for the entire province persists in the literature. We have dated four dikes and one pluton in the Burlington and Taconic lobes using 40Ar/39Ar and U–Pb geochronology to improve understanding of the age of magmatism in the New England portion of the province. In the Burlington lobe, 40Ar/39Ar plateau ages include a 137.55 ± 1.80 Ma biotite age and a 136.9 ± 4.2 Ma amphibole age for a lamprophyre dike from Charlotte, Vermont, and a 133.6 ± 2.2 Ma biotite age for a lamprophyre dike from Colchester, Vermont. In the Taconic lobe, ages include an 40Ar/39Ar plateau amphibole age of 107.09 ± 1.32 Ma for a lamprophyre dike from Castleton, Vermont, a 122 Ma minimum 40Ar/39Ar biotite age for a lamprophyre dike from Poultney, Vermont, and a 103.13 ± 0.53 Ma LA-ICP-MS U–Pb zircon age from the quartz syenite of the Cuttingsville complex. These results show that magmatism spanned at least 35 Ma, from ~138 to 103 Ma, which is broadly consistent with the span of magmatism suggested by workers in the 1970s and 1980s based on K–Ar and Rb–Sr ages. This extended span of magmatism for the Burlington and Taconic lobes is in contrast to the brief 1 to 2 Ma episode of magmatism at ~124 Ma inferred for the Monteregian Hills lobe. The New England-Quebec igneous province has traditionally been attributed to passage of the Great Meteor hotspot. However, given the close proximity of the Burlington and Taconic lobes, the magmatism in these lobes should span only a few Ma if the product of a hotspot. The age data are also difficult to reconcile with a more complex expression of hotspot magmatism in continental lithosphere related to either plume head magmatism or long-distance migration of plume material. Instead, the extended duration of Early Cretaceous New England-Quebec igneous province magmatism in New England may represent an expression of edge-driven convection, a process known to occur along passive margins and inferred to be operating beneath the eastern North American margin today.

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# Unmixing multiple metamorphic muscovite age populations with powder X-ray diffraction and 40Ar/39Ar analysis

A combination of modal estimates from powder X-ray diffraction (XRD) experiments and argon isotopic data shows that muscovite 40Ar/39Ar total gas age correlates with muscovite composition near the retrograde Bald Mountain shear zone (BMSZ) in Claremont, New Hampshire, and that the shear zone was active at ~245 Ma. Petrologic study demonstrates that chemical disequilibrium is preserved in muscovite grains in these samples. The recognition of this preservation is critical to the interpretation of the 40Ar/39Ar step-heating experiments, which never produce age plateaus and yield spectra with steps that range in age by ~20 Ma. Petrographic, compositional, and crystallographic data all indicate that the age spectra reflect dissolution of metastable Na-rich muscovite and precipitation of stable Na-poor muscovite associated with deformation in the BMSZ.Comparison of whole rock and muscovite concentrate XRD patterns from individual samples demonstrates that the mineral separation process can fractionate these muscovite populations. Therefore, four muscovite concentrates of varying magnetic susceptibility were prepared from a single hand sample, analyzed by XRD, and dated. These four splits define a mixing line that resolves end-member ages of 244.5 ± 4.2 Ma and 302.5 ± 12.5 Ma (1). Although the ages are imprecise, the petrologically supported conclusion that these schists preserve two discrete ages is distinct from an interpretation that the spectra reflect cooling through closure at ~270 Ma, as might be concluded in the absence of petrologic characterization. The XRD results also demonstrate that, even well above anchizone conditions, petrologic information relevant to 40Ar/39Ar dating is observable in subtle variations in the crystallography of muscovite grains.

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# Timing and Nd-Hf isotopic mapping of early Mesozoic granitoids in the Qinling Orogen, central China: Implication for architecture, nature and processes of the orogen

23 April 2021 at 05:07

The Qinling orogen, one of the most important orogens in Asia, belongs to the northeastern part of the Tethyan orogen. The architecture and processes of the Qinling orogen remain controversial. In this study, we present 15 new zircon U–Pb ages, 20 whole-rock geochemical and 46 Sm-Nd isotopic analyses, and 30 zircon Lu–Hf isotopic data for early Mesozoic granitoids in this orogen, combining with data from literature, to delineate the crustal architecture and processes of the orogen. A total of 181 zircon U–Pb ages show three phase (252–230, 230–198, and 190–185 Ma) of granitoids. The first-phase granitoids occur mainly in the westernmost segment of the orogen and formed in a subduction setting during the closure of the Mianlue Ocean (a northern branch of the Paleo-Tethyan Ocean). The second- and third-phase granitoids, distributed in the middle to eastern parts of the Qinling orogen, were generated in late syn-collisional and post-collisional tectonic settings, respectively. Whole-rock Nd and zircon Hf isotopic mapping of these granitoids yield six and seven isotopic provinces, respectively. These provinces display that the southern margin of the North China Block and the northern margin of the South China Block are dominated by ancient deep crust, that is, early Paleoproterozoic (2.3–1.8 Ga) and late Paleoproterozoic (~1.7 Ga) components, respectively. By way of camparison, the North Qinling contains younger Mesoproterozoic [Nd(t) = –10.7 to –0.2; TDM = 1.4–1.0 Ga] basement, evidencing that it is an independent terrane different from the North China Block. The isotopic mapping also reveals a deep-seated NNE–SSW-trending zoned architecture that is approximately perpendicular to the WNW–ENE-trending of the orogen. This provides new evidence for the "Spaghetti Junction model" for the Qinling orogen. The old Nd (2.2–1.0 Ga, mostly 2.0–1.2 Ga) and Hf (2.3–0.8 Ga, mostly 2.0–1.2 Ga) model ages indicate that the continental growth in this orogen occurred mainly during the Paleoproterozoic and Mesoproterozoic, with only minor amounts of juvenile [Nd(t) = ~0, TDM = ~0.1 Ga] continental growth along the Shangdan and Mianlue sutures. These characteristics suggest that the Qinling orogen is dominantly formed by the collision of ancient continental blocks, distinct from some typical accretionary orogens, such as the Central Asian Orogenic Belt with voluminous juvenile crust.

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