Contrasting geochemical signatures on land from the Middle and Late Permian extinction events

The end of the Palaeozoic is marked by two mass‐extinction events during the Middle Permian (Capitanian) and the Late Permian (Changhsingian). Given similarities between the two events in geochemical signatures, such as large magnitude negative δ 13 C anomalies, sedimentological signatures such as claystone breccias, and the approximate contemporaneous emplacement of large igneous provinces, many authors have sought a common causal mechanism. Here, a new high‐resolution continental record of the Capitanian event from Portal Mountain, Antarctica, is compared with previously published Changhsingian records of geochemical signatures of weathering intensity and palaeoclimatic change. Geochemical means of discriminating sedimentary provenance (Ti/Al, U/Th and La/Ce ratios) all indicate a common provenance for the Portal Mountain sediments and associated palaeosols, so changes spanning the Capitanian extinction represent changes in weathering intensity rather than sediment source. Proxies for weathering intensity chemical index of alteration, ∆ W and rare earth element accumulation all decline across the Capitanian extinction event at Portal Mountain, which is in contrast to the increased weathering recorded globally at the Late Permian extinction. Furthermore, palaeoclimatic proxies are consistent with unchanging or cooler climatic conditions throughout the Capitanian event, which contrasts with Changhsingian records that all indicate a significant syn‐extinction and post‐extinction series of greenhouse warming events. Although both the Capitanian and Changhsingian event records indicate significant redox shifts, palaeosol geochemistry of the Changhsingian event indicates more reducing conditions, whereas the new Capitanian record of reduced trace metal abundances (Cr, Cu, Ni and Ce) indicates more oxidizing conditions. Taken together, the differences in weathering intensity, redox and the lack of evidence for significant climatic change in the new record suggest that the Capitanian mass extinction was not triggered by dyke injection of coal‐beds, as in the Changhsingian extinction, and may instead have been triggered directly by the Emeishan large igneous province or by the interaction of Emeishan basalts with platform carbonates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108696/1/sed12117.pd

Timescales of magmatic processes and eruption ages of the Nyiragongo volcanics from 238U-230Th-226Ra-210Pb disequilibria

Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 288 (2009): 149-1157, doi:10.1016/j.epsl.2009.09.017.The silica-undersaturated Nyiragongo volcanics, located in the East African rift, have globally unique chemical compositions and unusually low viscosities, only higher than carbonatite lavas, for terrestrial silicate magmas. We report 238U-230Th-226Ra-210Pb series disequilibria in 13 recent and prehistoric lava samples from Nyiragongo including those from the 2002 flank eruption and a 2003 lava lake sample. (230Th/238U) ranges from 0.90- 0.97 in the recent lavas and from 0.94-1.09 in the prehistoric lavas. To explain the variable 230Th and 238U excesses in these lavas, we hypothesize that different processes with opposite effects in terms of fractionating Th/U in the mantle source are involved. These processes include: 1) low degree partial melting of a phlogopite-bearing mantle source (consistent with low K/Rb) with residual garnet (consistent with high chondrite-normalized Dy/Yb), to produce the observed 230Th excesses; and, 2) carbonate metasomatism for the 238U enrichment, consistent with high Zr/Hf in the Nyiragongo lavas. The Nyiragongo volcanics have higher (230Th/232Th) values than observed in most mantle-derived rocks, especially ocean-island basalts, suggesting that their mantle-source was affected by carbonate metasomatism less than 300 ka ago. Several Nyiragongo samples display significant 226Ra excesses implying rapid magma transport (less than 8 ka) from the mantle-source to the surface. Modeling the observed (226Ra/230Th) versus Zr/Hf correlation in the lavas indicates that the 2002, 2003 and a few pre-historic lavas incorporated 50-60% of a carbonate-metasomatized mantle source while the other prehistoric lavas show 10-22% contribution of this source. This result indicates that the Nyiragongo lavas were derived from a heterogeneous, non-uniformly carbonated mantle source. The 2002 lava shows (210Pb/226Ra) equilibrium, whereas the 2003 lava lake sample shows initial (210Pb/226Ra) < 1. The latter observation suggests that Nyiragongo magmas degas as they rise to the surface over years or decades before eruption. (210Pb/226Ra) equilibrium in the 2002 lava suggests that the 2002 magma may have stagnated for more than a decade before eruption. The high CO2 content, high emission rates, extreme fluidity, along with the inferred short residence time and our inferences of rapid magma transport and high eruptive frequency suggest that the volcanic hazards of Nyiragongo, both from lava flows and gas emissions, are higher than previously estimated.Lava samples were collected as a part of a field work in Nyiragongo, supported by UN-OCHA grants. The field team included Paolo Papale, Alba Santo, Dario Tedesco and Orlando Vaselli with support from the staff of Goma Volcanological Observatory, D. R. Congo. The 2003 lava lake sample was collected by Jacques Durieux. Funding for U-series analyses was covered by NSF-EAR 063824101 and NSF-EAR 083887800 to KWWS. 210Po analyses were funded with EAR0738776 to MR. Sample preparation and dissolution was covered by NSF-EAR 0732679 to ARB. RC acknowledges guest student award from WHOI

Silicon isotopes: from cosmos to benthos

Silicon is the second most abundant element on the Earth and one of the more abundant elements in our Solar System. Variations in the relative abundance of the stable isotopes of Si (Si isotope fractionation) in different natural reservoirs, both terrestrial (surface and deep Earth) as well as extra-terrestrial (e.g. meteorites, lunar samples), are a powerful tracer of present and past processes involving abiotic as well as biotic systems. The versatility of the Si isotope tracer is reflected in its wide-ranging applications from understanding the origin of early Solar System objects, planetary differentiation, Moon formation, mantle melting and magma differentiation on the Earth, ancient sea-water composition, to modern-day weathering, clay formation and biological fractionation on land as well as in the oceans. The application of Si isotopes as tracers of natural processes started over six decades ago and its usage has seen a sudden increase over the last decade due to improvements in mass spectrometry, particularly the advent of multi-collector inductively coupled plasma mass spectrometers, which has made Si isotope measurements safe and relatively easy while simultaneously improving the accuracy and precision of measurements

A geochemical and Nd, Sr and stable Ca isotopic study of carbonatites and associated silicate rocks from the similar to 65 Ma old Ambadongar carbonatite complex and the Phenai Mata igneous complex, Gujarat, India: Implications for crustal contamination, carbonate recycling, hydrothermal alteration and source-mantle mineralogy

Major, trace element concentrations and Nd, Sr and Ca stable isotopic compositions (delta Ca-44/40 and delta Ca-44/42 w.r.t. NIST SRM915a) of carbonatites and associated igneous silicate rocks from the -65 Ma old Ambadongar carbonatite complex and the surrounding Phenai Mata igneous complex of western India are reported. Samples of fluorspar from Ambadongar and the Bagh Limestone and Sandstone, which are part of the country rocks at Ambadongar, have also been analysed. The Ambadongar carbonatites are primarily calcio- and ferro-carbonatites while the silicate rocks from these two complexes are alkaline and tholeiitic in composition. The delta Ca-44/40 values of the carbonatites (0.58-1.1 parts per thousand, n = 7) and the associated igneous silicate rocks (0.50-0.92 parts per thousand, n = 14) show a broad range. The low K/Ca values of the carbonatites (<0.2) and silicate rocks (<2) along with their young eruption age (similar to 65 Ma) rule out any effect of radiogenic Ca-40 ingrowth due to decay of K-40 on the delta Ca-44/40 values. The lack of correlations between delta Ca-44/40 and Mg# as well as La/Yb-(N) values suggest that the variability in delta Ca-44/40 is not controlled by the degree of partial melting. The delta Ca-44/40 values of the carbonatites (0.58-1.1%0) overlap with that of the upper mantle/Bulk Silicate Earth and is mostly higher than the delta Ca-44/40 value of the Bagh Limestone (0.66 parts per thousand) suggesting that assimilation of these crustal limestones by the magma is unlikely to have caused the variability in delta Ca-44/40 of the carbonatites. In plots of delta Ca-44/40 versus epsilon Nd-(t) and Sr-87/Sr-86((t)), the igneous silicate rocks from the Ambadongar and Phenai Mata complexes plot on a mixing trend between a primitive (plume) mantle source and the continental crustal basement suggesting the role of continental crustal contamination during eruption of the Reunion plume. While simple binary mixing calculations yield unrealistically high amounts of crustal contamination (40%), assimilation and fractional crystallization (AFC) models suggest up to 20% contribution from a heterogeneous basement for these igneous silicate rocks. The role of continental crustal contamination in the genesis of the igneous silicate rocks is further supported by their unradiogenic epsilon Nd-(t), radiogenic Sr-87/Sr-86((t)) and low Ce/Pb values. In contrast, carbonatites plot away from the mixing trend between a primitive mantle (plume) source and continental crust in Ca-Sr-Nd isotopic diagrams suggesting that the Ca isotopic variability of carbonatites is not caused by continental crustal contamination. In contrast, the isotopic composition of the carbonatites can be explained by mixing of the plume end-member with up to 20% of similar to 160 Ma-old recycled carbonates suggesting their derivation from a highly heterogeneous, recycled carbonate-bearing plume mantle source. The composition of one carbonatite sample showing unusually high delta Ca-44/40 and highly radiogenic Sr-87/Sr-86((t)) is explained by hydrothermal alteration which is also invoked for the formation of massive fluorspar deposits with high delta Ca-44/40 (1.44 parts per thousand) at Ambadongar. In a plot of delta Ca-44/40 versus K/Rb, the carbonatites plot towards the phlogopite end-member (delta Ca-44/40 = 1 parts per thousand, K/Rb = 40-450) while the igneous silicate rocks plot towards the amphibole end-member (delta Ca-44/40 = 0.44 parts per thousand, K/Rb >1000). Phlogopite, especially if F-rich, is stable at greater depths in the mantle compared to amphibole. Hence, the correlated delta Ca-44/40 and K/Rb values of the carbonatites and associated igneous silicate rocks suggest the derivation of these carbonatites from a relatively deeper mantle source compared to the silicate rocks, both within the Reunion mantle plume. The origin of the carbonatites from the F-rich phlogopite-bearing mantle is also consistent with the occurrence of large fluorspar deposits within the Ambadongar carbonatite complex. (C) 2019 Elsevier B.V. All rights reserved

Large Ca stable isotopic (delta Ca-44/40) variation in a hand-specimen sized spheroidally weathered diabase due to selective weathering of clinopyroxene and plagioclase

Calcium stable isotopic compositions (delta Ca-44/40(SRM 915a)) as well as selected major and trace element concentrations are reported for micro-drilled samples of a hand-specimen sized spheroidally weathered diabase (synonymously used as dolerite or microgabbro) from southern India. A sample of the similar to 2.37 Ga old unweathered diabase (UW) dike was also analyzed. X-ray micro-CT imaging of a representative portion of the weathered sample shows the presence of two dominant mineral phases, plagioclase and clinopyroxene, with minor proportions of an iron-bearing phase (possibly haematite or ilmenite). Two different generations of crosscutting micro-fractures are identified from the micro-CT image. The older fracture is sealed with secondary mineral deposits. The delta Ca-44/40 values of the weathered samples range from 0.42 parts per thousand to 0.84 parts per thousand with samples showing both higher and lower values than that of the un-weathered diabase dike which shows a delta Ca-44/40 value of 0.65 parts per thousand. The variation in delta Ca-44/40 in the weathered samples is significantly higher than the external reproducibility of our measurements (< 0.1 parts per thousand) based on multiple measurements of Ca isotopic standards SRM 915a, SRM 915b and seawater (NASS-6). Mass balance calculations based on Sr/Ca and delta Ca-44/40 values rule out the contributions of secondary silicate as well as carbonate minerals in causing the variability of delta Ca-44/40 in the weathered samples. The positive correlations between delta Ca-44/40 and Mg/Al, Mg/Na in the weathered samples further negate the possible contribution of secondary carbonates in causing the variability in delta Ca-44/40, as Al and Na are unlikely to be present in carbonates. The samples with lower delta Ca-44/40 show higher CIA (Chemical Index of Alteration) values and Al/Ca than the UW diabase while samples having higher delta Ca-44/40 show lower CIA and Al/Ca than the UW diabase. The weathered sample having the lowest delta Ca-44/40 (close to the delta Ca-44/40 of plagioclase) exhibits the highest value of europium anomaly (Eu/Eu*). These observations are explained by the presence of variable relative proportions of residual clinopyroxene and plagioclase in the weathered diabase, due to selective weathering of these minerals. Since plagioclase and clinopyroxene could not be separated from the diabase, we measured the Ca isotopic compositions of a clinopyroxene (delta Ca-44/40= 1.06 parts per thousand) from the San Carlos mantle peridotite and plagioclase from the Chilka anorthosite complex (delta Ca-44/40= 0.40 parts per thousand) from the Eastern Ghats and used these isotopic values as representative of the compositions of these minerals in the diabase. Based on mixing models, using delta Ca-44/40, Al/Ca and Sr/Ca of the plagioclase and clinopyroxene end-members, we estimate that the delta Ca-44/40 of the UW diabase can be explained by 31 parts per thousand clinopyroxene and 69 parts per thousand plagioclase, which is consistent with the modal proportion of these minerals in an average unweathered diabase. Based on mixing calculations, the delta Ca-44/40, Sr/Ca and Al/Ca variability of the micro-drilled samples of the weathered diabase are explained by the presence of varying proportions of residual clinopyroxene (20-65 parts per thousand) and plagioclase (80-35 parts per thousand) in the weathered rock. This study demonstrates that selective weathering of major rock-forming minerals in nature can result in significant variation in delta Ca-44/40 in weathered rocks and that selective weathering of rock-forming minerals should be considered as an additional mechanism to explain the delta Ca-44/40 variability in rivers

A novel sample loading method and protocol for monitoring sample fractionation for high precision Ca stable isotope ratio measurements using double-spike TIMS

The external reproducibility (2 sigma(SD)) of Ca stable isotope ratio measurements (delta Ca-44/40) using double-spike thermal ionization mass spectrometry (TIMS) shows a large range from <0.1 parts per thousand to 0.5 parts per thousand. We demonstrate that using a Ca-43-Ca-48 double spike, which allows simultaneous measurements of delta Ca-44/40 and delta Ca-44/42, and analyses at moderate signal strengths (Ca-40 ranging from 6 to 10 V), high precision delta Ca-44/40 (external reproducibility better than +/- 0.08 parts per thousand, 2 sigma(SD)) as well as delta Ca-44/42 can be obtained if samples and standards are analyzed under similar fractionation conditions. To monitor the fractionation conditions, the usage of a parameter beta is proposed, which measures the deviation of the Ca-43/Ca-48 ratio of the sample-double spike mixture from that of the pure double spike. A novel, low-cost sample loading technique using a combination of Re and Ta filaments and a Ta2O5 activator is presented which results in a steady signal. We report the delta Ca-44/40 and delta Ca-44/42 values, calculated w.r.t. NIST SRM 915a and reported as deviations in parts per mil (parts per thousand), of NIST standards SRM 915a (0.01, -0. 02) and SRM 915b (0.73, 0.32), NASS 6 (seawater, 1.80, 0.89), USGS silicate rock standards BHVO-2 (0.86, 0.47) and BCR-2 (0.89, 0.48), Geological Survey of Japan carbonate standards JCp-1 (coral, 0.85, 0.45) and JCt-1 (clam shell, 0.83, 0.46) and ECRM 752-1 (limestone, alternative name BCS-CRM 393) (0.83, 0.46) from the Bureau of Analyzed Samples Ltd. UK. The Ca stable isotopic compositions of JCt-1 and ECRM 752-1 are reported for the first time

Element concentrations and different index analysis of the spheroidally-weathered diabase and the un-weathered rocks sampled near Yeragumballi, southern India

Major, trace element concentrations and Nd, Sr isotope ratios were measured in micro-drilled samples of a 2.37 Ga-old, hand-specimen sized spheroidally weathered diabase from southern India. A sample of the un-weathered diabase dike was also analyzed. X-ray micro-CT imaging of the weathered sample shows three dominant mineral phases which are plagioclase, pyroxene, and a Fe-bearing phase (possibly hematite and/or ilmenite). This imaging documents the pervasive nature of two generations of ribbon-like, cross-cutting fractures. The older fracture is sealed while the more recent fracture is open without any in-filling. The values of the Chemical Index of Alteration (CIA) of the samples show a wide range but are less than 50. Despite being a relatively less weathered rock, we observe that concentrations of major, minor and trace elements vary significantly with the percentage relative standard deviation (%RSD) for the elements ranging from 10.2?41.8. The CIA of the samples do not show any trend with the position of the sample in the hand-specimen. Barring Ca and Li, whose concentrations decrease from the core to the rim of the sample, there is no significant spatial trend in the concentrations of the elements. Concentrations of Na, Al, and Sr increase with increasing CIA values while concentrations of Mg, Fe, and Sc decrease with increasing CIA. The strong positive correlations of Na and Al, as well as Na and Sr indicates preferential weathering of plagioclase in the diabase. Na/Ca increases while Mg/Al, Mg/Na, Mg/Ca, Fe/Al and Sc/Sr decrease with increasing CIA values and the un-weathered rock plots in the middle of these trends. Such variations are explained in terms of differential weathering of plagioclase (in samples with lower CIA than the un-weathered rock, W1-type) and pyroxene (in samples with higher CIA than the un-weathered rock, W2-type) which have varying resistance to weathering. At the hand-specimen scale, the variability in the weathering indices like CIA are controlled by differential weathering of minerals and might not accurately reflect the intensity of weathering. Chondrite-normalized La/Sm and Gd/Lu co-vary with CIA values indicating mobility of the REEs during spheroidal weathering even at the hand-specimen scale. The Eu anomaly also increases with increasing CIA values which is explained by differential weathering of pyroxene and plagioclase. We observe large percentage deviations of the Nb-normalized concentrations of elements from the un-weathered rock in specific samples but no spatial trend is observed. Overall, the variations in element concentrations can be explained by varying fluid mobility of the elements, selective weathering of the minerals in the diabase, and ambient environmental conditions. Considerable Nd and Sr isotopic variability is observed at the hand-specimen scale and is explained in terms of weathering-related fractionation of parent/daughter ratios. This elemental fractionation must have happened long time ago to allow for radiogenic decay of the long-lived isotopes of 87Rb and 147Sm. The spread (%RSD) in the initial Sr and Nd isotope compositions of the weathered samples reach a minimum value around 1.2?1.3 Ga which we interpret as the timing of the peak weathering event which led to fractionation of the parent/daughter ratios. For Nd isotopes, the average epsilon-Nd (1.2 Ga) of the weathered samples coincides with the epsilon-Nd (1.2 Ga) of the un-weathered rock. The timing of the weathering event coincides with the timing of the breakup of the Columbia supercontinent and follows wide-spread alkaline volcanism in the Indian subcontinent. This is the first such attempt to determine the timing of a weathering event in rocks using long-lived radioactive isotope