Primary Isotopic Composition Research Articles

ISOTOPE COMPOSITION OF Pb AND Nd OF THE LATE NEOPROTEROZOIC–EARLY PALEOZOIC GRANITOIDS IN EASTERN TUVA (Case study of the Kaa-Khem batholith)

We present the results of Nd and Pb isotope studies of late Neoproterozoic–early Paleozoic granitoids of the Kaa-Khem batholith, which formed in different geodynamic settings within the early Caledonian structures of Eastern Tuva (Altai–Sayan sialic area). Based on the obtained isotope data, we assess the role of different source materials in the formation of melts for these granitoids and compare these rocks with coeval granitoids from the Lake Zone of Mongolia. The plagiogranitoid associations of the island arc stage of the Kaa-Khem batholith formation (572–562 Ma) are characterized by mantle Nd isotope values. In contrast, the Pb isotope characteristics of these intrusive associations correspond to an upper-crust source formed predominantly by terrigenous sediments. The granitoid associations of the accretion–collision stage of the Kaa-Khem batholith formation (512–450 Ma) have a less radiogenic primary Pb isotope composition as compared with the island arc granitoids. Evidently, during the magma formation at this stage, the relative role of terrigenous material decreased, while the role of depleted mantle material increased. At the same time, the decrease in ɛNd values in the granitoids as compared with the depleted mantle indicates that, along with the upper-mantle component, the role of the lower-crust component increased. These Nd and Pb isotope characteristics of the accretionary granitoids of the Kaa-Khem batholith indicate a significant contribution of the lower-crust source to their formation. This source is the crystalline basement of the Tuva–Mongolian microcontinent. The Pb isotope characteristics of the accretion–collision granitoids of the Kaa-Khem batholith are shifted to the upper-crust compositions relative to those of the Lake Zone granitoids. The reason is that the latter formed predominantly through the transformation of early Caledonian island arc complexes, which were generated with a higher portion of mantle component.

Molar-Tooth Carbonate: An excellent proxy for reconstructing lithium isotopic composition of Precambrian seawater

Shallow water carbonates are extensively utilized as archives for reconstructing past seawater chemistry and understanding global geochemical cycles. However, their reliability is often compromised by diagenetic alteration, obscuring primary environmental signals. Molar-tooth carbonate (MTC) exhibits exceptional resistance to diagenesis compared to other carbonate minerals, making it a promising candidate for reliably reconstructing ancient seawater chemistry. This study investigates the preservation of primary lithium isotopic composition in Neoproterozoic (Tonian) MTC and its host carbonate from the Dalian area, northeastern China (ca. 980 Ma – 920 Ma). We assess the reliability of MTC in reconstructing paleo-seawater δ7Li evolution and provide a reconstruction of Tonian seawater δ7Li.Geochemical filter tests indicate that both MTC and host carbonate remain largely unaltered by meteoric and burial diagenesis. Carbonate δ7Li values exhibit a consistent decrease with stratigraphic height, with MTC displaying a narrower range of variation (4–12‰) compared to its host carbonate (1–15‰). Numerical modeling of early marine carbonate diagenesis suggests minimal impact on MTC δ7Li values, while host carbonates show varying degrees of diagenetic alteration. Based on the consistent Li isotopic offset observed between ‘fluid-buffered’ diagenetic host carbonate and MTC in our data, we deduce a fractionation factor of 4.8 ± 1.2‰ for seawater-MTC Li isotopic exchange.Reconstructed paleoseawater δ7Li values reveal a progressive decrease from 16.9 ± 1.2‰ to 8.8 ± 1.2‰ during the early Tonian period. This change is likely linked to the assembly of the Rodinia supercontinent and emplacement of large igneous province (LIP). Furthermore, mass balance modeling and Monte Carlo simulations suggest that this decline in seawater δ7Li can be attributed to the transition towards more congruent weathering during the early Tonian, facilitated by the assembly of Rodinia and associated large igneous province events.Overall, our findings underscore the potential of MTC as a reliable proxy for elucidating Precambrian seawater Li isotopic composition and understanding associated paleoenvironmental and paleoclimatic changes.

Subaqueous carbonate speleothems as paleotemperature archives – clumped isotope thermometry and stable isotope compositions of inclusion-hosted water

Clumped isotope measurements of carbonates and stable isotope analyses of water trapped in fluid inclusions are both promising techniques to determine carbonate formation temperatures. Cave-hosted carbonate deposits (speleothems) are excellent targets for such studies, but kinetic fractionations and diagenetic influences frequently deteriorate the temperature data obtained from these methods. However, subaqueous carbonate deposits may provide reliable data, as kinetic fractionations are less significant in underwater environments. In this study, subaqueous speleothems, whose formation temperatures were directly measured in the water, were investigated. Additionally, temperatures calculated from the oxygen isotope fractionations between calcite and fluid inclusion-hosted water were compared with clumped isotope temperatures obtained for subaqueous carbonate formations in cave-hosted lakes. The clumped isotope temperatures fit the measured and calculated fluid inclusion temperatures within the analytical precisions. Carbonate deposits formed at elevated temperatures (~50°C or above) may undergo post-formational calcite-water oxygen isotope exchange, altering the composition of the inclusion-hosted water. In contrast, subaqueous speleothems formed at about 20-25°C appear to preserve the primary isotopic compositions. Our study shows that subaqueous carbonate speleothems are useful targets for clumped isotope and inclusion water analyses, making them valuable paleotemperature archives.

Barium isotope signatures of barite–fluid ion exchange in Equatorial Pacific sediments

The isotope composition of barium (Ba) in the mineral barite (BaSO4) is emerging as a powerful tracer of the pelagic Ba, carbon, and sulfur cycles. However, it is critical to identify and constrain processes that may alter the primary isotope composition of Ba, δ138Ba, in BaSO4, particularly during early diagenesis. To this end, we analyzed the Ba isotope composition of porewaters and co-located BaSO4 in sediments from the Equatorial Pacific and performed a series of laboratory experiments with these same BaSO4 to assess rates of Ba isotope alteration. We find that sedimentary BaSO4 exhibit Ba isotope compositions ≈+0.1‰ and are offset by ≈−0.16‰ relative to ambient porewaters. Experiments using isotope-labeled seawater show extensive and rapid transfer of Ba ions between the solid and fluid phase through coupled BaSO4 dissolution-precipitation, even at chemical equilibrium. Using published values for Ba isotope fractionation during BaSO4 precipitation and dissolution, we calculate that co-located BaSO4 and porewaters should exhibit Ba isotope offset of −0.17‰ at isotopic equilibrium, similar to the offsets observed in Equatorial Pacific sediments. Altogether, the field data, laboratory experiments, and calculations indicate that ion exchange occurs in Equatorial Pacific sediments and that this process also drives the observed Ba isotope offsets between pelagic BaSO4 and porewaters. This finding implies that ion exchange may alter the isotope composition of Ba in sedimentary BaSO4, though the degree of alteration will depend on the proportion of Ba held in the solid phase. More broadly, this study provides an example of how ion-exchange-mediated processes are widespread in marine geochemistry and that these processes likely affect other metals and minerals beyond Ba and BaSO4.

Iron isotopic variations in basalts from oceanic crust due to low-temperature seawater alteration

Iron (Fe) isotopic compositions of seafloor basalts can provide important insights into high-temperature processes in the upper mantle. However, low-temperature seawater alteration (i.e., seafloor weathering) may significantly affect the primary Fe isotopic compositions. In this paper, we report whole-rock Fe isotopes, major and trace elements, and SrNd isotopes, along with in situ elemental maps for slightly altered Site U1434 basalts collected on IODP Expedition 349 from the South China Sea. We use these data to investigate the influence of low-temperature seawater alteration on the Fe isotopic compositions of seafloor basalts. The basalts have δ57Fe values of +0.12‰ to +0.27‰. No correlation between δ57Fe values versus Sr or Nd isotopes that are source-sensitive and no correlation between δ57Fe values and La/Yb ratios that are melting-sensitive indicate the δ57Fe values have not been affected by mantle source heterogeneity and partial melting. The negative correlations of MgO versus CaO and Al2O3 contents suggest limited clinopyroxene and plagioclase fractionation during magmatic evolution. Possible influence of olivine fractionation on the Fe isotopic variations can further be ruled out by a positive correlation between MgO and SiO2 contents and a weak negative correlation between δ57Fe values and SiO2 contents because olivine fractionation can form basalts with increasing SiO2 and δ57Fe with decreasing MgO. The strong correlations between δ57Fe values and MgO, Al2O3, and TiO2 contents indicate the Fe isotopic variations were caused by low-temperature seawater alteration. This is also evident from in situ elemental mapping of representative Site U1434 basalt sample, which shows that low-temperature alteration was characterized by decreases in MgO and SiO2 contents, and increases in Al2O3 and TiO2 contents. We propose a two-stage alteration model to explain the Fe isotopic variations of the Site U1434 basalts. The alteration appeared to occur under oxidizing conditions: Fe2+-rich external fluid likely derived from the progressive alteration inside the oceanic crust was oxidized at the surface of the oceanic crust, which caused the alteration, Fe isotopic fractionation, and increase in δ57Fe values of the seafloor basalts. This study highlights how low-temperature seawater alteration under oxidizing conditions can modify the Fe isotopic compositions of slightly altered oceanic crust, with critical implications for understanding the Fe cycle in the oceans.

Preservation and modification of the isotopic composition (18O/16O and 2H/1H) of structurally-bound hydration water of gypsum (CaSO4·2H2O) in aqueous solution

Early investigations on stable isotopes (δ18O and δD) of structurally-bound gypsum (CaSO4·2H2O) hydration water (GHW) suggested that soon after gypsum precipitation, its primary isotopic composition could be altered via gypsum re-crystallization or by isotope exchange by diffusion of water into the intact crystals. If this occurs, the use of stable isotopes of GHW as a paleoclimate proxy is compromised. Here we investigated the long-term (up to 6 years) stability of GHW in contact with aqueous solution by conducting isotopic exchange experiments at different temperatures and using varying gypsum grain size. We placed gypsum with known hydration water isotopic composition in 18O/D-enriched and 18O/D-depleted aqueous solutions and monitored any change in the stable isotopes of GHW with time. At low temperature (25 °C) and when the solution is in chemical equilibrium with the mineral, GHW preserves its primary isotopic composition after 2 years. In contrast, when the gypsum-solution system is out of chemical equilibrium, the δ18OGHW and δDGHW values are altered, first via gypsum re-crystallization from the solution and later by isotope exchange via diffusion. Importantly, δ18OGHW and δDGHW stabilized after 2 years to values that represent only an 8 % change relative to the original GHW and remain constant during the subsequent 4 years. Our results suggest that at low temperature the effect of isotope exchange on GHW is limited. At higher temperature (65 °C), chemical equilibrium is not attained after 2 years for fine-grained synthetic gypsum (<250 μm) and its isotopic values continues to change. In contrast, the δ18OGHW and δDGHW in experiments using fine-grained natural gypsum at 45 °C stabilized after 1 year and values changed less than 3 % with respect to initial conditions. Experiments with coarser-grained natural gypsum (1–2 mm) at 45 °C did not show measurable isotopic changes of the GHW, indicating the lack of gypsum dissolution/re-crystallization or diffusion isotopic exchange. We conclude that microcrystalline gypsum crystals (<250 μm) are more readily affected by diagenesis resulting from changes in the gypsum saturation state of the solution, whereas larger gypsum crystals are more likely to preserve their original isotopic compositions. Our results indicate that under certain conditions, GHW can preserve the isotopic composition of its parent fluid and provide valuable information about paleoclimate and paleo-hydrologic conditions.

Carbon and oxygen stable isotope record of upper Kimmeridgian shallow-marine ramp carbonates (Iberian Basin, NE Spain): the imprint of different burial and tectonic histories

Bulk carbon and oxygen stable isotopes of ancient shallow-marine carbonates can record the effects of multiple palaeoenvironmental factors, but also the imprint of several post-depositional processes, which may alter the original marine isotopic composition. In this study, carbon and oxygen stable isotope analyses were performed on bulk carbonate, bivalve calcitic-shell (Trichites) and calcite vein samples from two stratigraphic sections (Tosos and Fuendetodos, present-day distance 15km), representing proximal inner- and distal mid-ramp environments, respectively, of the uppermost Kimmeridgian ramp facies deposited in the northern Iberian Basin (NE Spain). These successions underwent different diagenetic pathways that altered the primary marine isotopic composition in each section in different ways. Different burial histories, tectonic uplift and a variable exposure to meteoric diagenesis from the end of the Kimmeridgian to the Cenozoic (following Alpine tectonic uplift) are reflected in the different alteration patterns of the carbon and oxygen stable isotope signatures. A significant deviation to lower values in both δ13O and δ18O is recorded in those carbonates mostly exposed to meteoric diagenesis (distal mid-ramp Fuendetodos section), because of post-depositional tectonic uplift (telogenesis). On the other hand, the deposits mainly affected by burial diagenesis (proximal inner-ramp Tosos section) only record low δ18O with respect to expected values for pristine Kimmeridgian marine carbonates. The different burial and tectonic uplift histories of these deposits in each sector, due to their different tectonic evolution in this part of the basin, resulted in a variable degree of diagenetic resetting. However, in spite of the different diagenetic resetting reported of the carbon and oxygen stable isotope signatures in each section, these carbonates show similar cement types in termsof fabrics and cathodoluminescence properties. The diagenetic resetting reported for these carbonates prevents the use of the δ13O and δ18O records for addressing palaeoenvironmental interpretations, but instead highlights useful features regarding the variable diagenetic overprint of the studied shallow-marine carbonate successions concerning their specific post-depositional history.

The Ca and Mg isotope record of the Cryogenian Trezona carbon isotope excursion

The Trezona carbon isotope excursion is recorded on five different continents in platform carbonates deposited prior to the end-Cryogenian Marinoan glaciation (>635 Ma) and represents a change in carbon isotope values of 16–18‰. Based on the spatial and temporal reproducibility, the excursion previously has been interpreted as tracking the carbon isotopic composition of dissolved inorganic carbon in the global ocean before the descent into a snowball Earth. However, in modern restricted shallow marine and freshwater settings, carbon isotope values have a similarly large range, which is mostly independent from open ocean chemistry and instead reflects local processes. In this study, we combine calcium, magnesium, and strontium isotope geochemistry with a numerical model of carbonate diagenesis to disentangle the degree to which the Trezona excursion reflects changes in global seawater chemistry versus local shallow-water platform environments. Our analysis demonstrates that the most extreme carbon isotope values (∼-10‰ versus +10‰) are preserved in former platform aragonite that was neomorphosed to calcite during sediment-buffered conditions and record the primary carbon isotope composition of platform-top surface waters. In contrast, the downturn and recovery of the Trezona excursion are recorded in carbonates that were altered during early fluid-buffered diagenesis and commonly are dolomitized. We also find that the nadir of the Trezona excursion is associated with a fractional increase in siliciclastic sediments, whereas the recovery from the excursion correlates with a relative increase in carbonate. This relationship suggests that the extreme negative isotopic shift in platform aragonite occurred in concert with periods of increased input of siliciclastic sediments, changes in water depth, and possibly nutrients to platform environments. Although the process for generating extremely negative carbon isotope values in Neoproterozoic platform carbonates remains enigmatic, we speculate that these excursions reflect kinetic isotope effects associated with CO2 invasion in platform waters during periods of intense primary productivity.

The shocking state of apatite and merrillite in shergottite Northwest Africa 5298 and extreme nanoscale chlorine isotope variability revealed by atom probe tomography

The elemental and chlorine isotope compositions of calcium-phosphate minerals are key recorders of the volatile inventory of Mars, as well as the planet’s endogenous magmatic and hydrothermal history. Most martian meteorites have clear evidence for exogenous impact-generated deformation and metamorphism, yet the effects of these shock metamorphic processes on chlorine isotopic records contained within calcium phosphates have not been evaluated. Here we test the effects of a single shock metamorphic cycle on chlorine isotope systematics in apatite from the highly shocked, enriched shergottite Northwest Africa (NWA) 5298. Detailed nanostructural (EBSD, Raman and TEM) data reveals a wide range of distributed shock features. These are principally the result of intensive plastic deformation, recrystallization and/or impact melting. These shock features are directly linked with chemical heterogeneities, including crosscutting microscale chlorine-enriched features that are associated with shock melt and iron-rich veins. NanoSIMS chlorine isotope measurements of NWA 5298 apatite reveal a range of δ37Cl values (−3 to 1‰; 2σ uncertainties <0.9‰) that is almost as large as all previous measurements of basaltic shergottites, and the measured δ37Cl values can be readily linked with different nanostructural states of targeted apatite. High spatial resolution atom probe tomography (APT) data reveal that chlorine-enriched and defect-rich nanoscale boundaries have highly negative δ37Cl values (mean of −15 ± 8‰). Our results show that shock metamorphism can have significant effects on chemical and chlorine isotopic records in calcium phosphates, principally as a result of chlorine mobilization during shock melting and recrystallization. Despite this, low-strain apatite domains have been identified by EBSD, and yield a mean δ37Cl value of −0.3 ± 0.6‰ that is taken as the best estimate of the primary chlorine isotopic composition of NWA 5298. The combined nanostructural, microscale-chemical and nanoscale APT isotopic approach gives the ability to better isolate and identify endogenous volatile-element records of magmatic and near-surface processes as well as exogenous, shock-related effects.

Vendian Taseeva Group, Southwestern Margin of the Siberian Platform: Isotope, Geochemical, and Geochronological Data, Age, and Correlation

Abstract —A high-amplitude negative δ13C anomaly (–11‰ V-PDB) has been found in the carbonate horizons of the Chistyakovka Formation of the Taseeva Group in the type sections of the southern Yenisei Ridge. The δ18O values (–2.7 to –5.4‰ V-PDB) and the lithologic features of carbonates indicate that these are primary sedimentary rocks and their isotope parameters reflect the primary isotope composition of the paleobasin waters. The negative δ13C excursion in the Chistyakovka Formation is close in stratigraphic position and amplitude to similar isotope excursion in the Vendian strata of the southern Siberian Platform and to the global middle Ediacaran Shuram–Wonoka anomaly. According to the data on clastic zircons, the age of the Chistyakovka Formation is no older than 580 Ma, which agrees with the earlier estimated age of the Shuram–Wonoka event. As follows from the data obtained, the age of the basement of the Taseeva Group is no older than 600 Ma. This provides a more accurate correlation of the Vendian complex along the southwestern margin of the Siberian Platform. The different stratigraphic scales of the sediments indicate that the marginal trough here began to form at different times (from late Riphean to late Vendian), which explains its segmented structure.

Isotopic composition of lead from dikes and ores of the Vorontsovkoe gold deposit (Northern Urals)

Research subject . The Vorontsovskoe deposit belongs to gold deposits, in which gold is disseminated finely across carbonate-terrigenous rocks (Carlin-type). This study was aimed at identifying the sources of lead in various ore types (in skarn, jasperoid, tuff-sandstone and limestone breccia) and evaluating the role of magmatic rocks (apophises of Auerbakh intrusions and dykes of medium-basic composition) in the mineralization. M aterials and methods . Ore and rock samples for isotopic analysis were collected in the northern quarry of the deposit, as well as were retrieved from exploration holes. The lead isotopic composition was studied in the analytical centre Geoanalytic , Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences, using a mass spectrometer Neptune Plus on the basis of the TL N MC ICP MS method. Results. The lead isotopic composition of ores and igneous rocks indicates mixed mantle-crustal sources. The primary lead isotopic composition of igneous rocks corrected for the age of 400 Ma is characterized by a linear trend with wide variations in 207 Pb/ 204 Pb (15.6824–15.5182) and 206 Pb/ 204 Pb (18.1973–17.7707). The lamprophyre dykes feature the least radiogenic lead located between the evolutionary curves for the mantle and orogen in the Doe–Zartman model. The most radiogenic lead is typical of the granodiorite of the Auerbakh intrusion apophises. The analysis points of diorite porphyrite, gabbro-diorite and diorite dykes are located between the orogen and upper crust curves. The sulphide ores are characterized by lower variations in the primary lead isotopic composition compared to igneous rocks – 207 Pb/ 204 Pb (15.6009–15.5421) and 206 Pb/ 204 Pb (18.0434–17.8404). The most productive ores in tuff-sandstones and limestone breccias are characterized by a significant predominance of the crustal component over the mantle component. Their points in the diagram 207 Pb/ 204 Pb– 206 Pb/ 204 Pb are located between the curves for the orogen and the crustal source with µ 2 ( 238 U/ 204 Pb) = 9.74. Conclusion. Mantle lead in the dykes under study appears to reflect the nature of their parent magmas and may have been partially extracted by melts from the footwall mafic rocks containing pyrite ores. However, crustal lead is likely to have resulted from assimilating host rocks when magmatic melts reached the upper crust level. Crustal lead migrated with the metamorphic fluid formed by the heat action of the Auerbakh intrusion on sulphide-bearing volcano-sedimentary rocks. Mantle lead might have been transported by the fluid flow from a deepseated magmatic chamber.

Preservation of primordial signatures of water in highly-shocked ancient lunar rocks

Spurred by the discovery of water in lunar volcanic glasses about a decade ago, the accessory mineral apatite became the primary target to investigate the abundance and source of lunar water. This is due to its ability to contain significant amounts of OH in its structure, along with the widespread presence of apatite in lunar rocks. There is a general understanding that crustal cumulate rocks of the lunar magnesian (Mg) suite are better candidates for recording the original isotopic compositions of volatile elements in their parental melts compared to eruptive rocks, such as mare basalts. Consequently, water-bearing minerals in Mg-suite rocks are thought to be ideal candidates for discerning the primary hydrogen isotopic composition of water in the lunar interior. Mg-suite rocks and most other Apollo samples that were collected at the lunar surface display variable degrees of shock-deformation. In this study, we have investigated seven Apollo 17 Mg-suite samples that include troctolite, gabbro and norite lithologies, in order to understand if shock processes affected the water abundances and/or H isotopic composition of apatite. The measured water contents in apatite grains range from 31 to 964 ppm, with associated δD values varying between −535 ± 134‰ and +147 ± 194‰ (2σ). Considering the full dataset, there appears to be no correlation between H2O and δD of apatite and the level of shock each apatite grain has experienced. However, the lowest δD was recorded by individual, water-poor (<∼100 ppm H2O) apatite grains that are either directly in contact with an impact melt or in its proximity. Therefore, the low-δD signature of apatite could be a result of interactions with D-poor regolith (solar wind derived H), facilitated by shock-induced nanostructures that could have provided pathways for migration of volatiles. In contrast, in relatively water-rich apatites (>∼100 ppm H2O), regardless of the complexity of the shock-induced nanostructures, there appears to be no evidence of water-loss or alteration in their δD. The weighted average δD value of 24 such water-rich apatites is −192 ± 71‰, and, of all 36 analyzed spots is −209 ± 47‰, indistinguishable from that of other KREEPy lunar lithologies or the Earth's deep mantle. Despite experiencing variable degrees of shock-deformation at a later stage in lunar history, water-rich apatite in some of the earliest-formed lunar crustal material appears to retain the original isotopic signature of H in the Moon.

The linkage of nitrogen isotopic composition and depositional environment of black mudstones in the Upper Triassic Yanchang Formation, Ordos Basin, northern China

In this study, we report new analyses of stable nitrogen isotopic compositions (δ15N) of the black mudstones from the Chang-7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin, China, to examine the possible linkage of such isotopic compositions with the depositional environment. These mudstones were deposited mainly under suboxic bottom water column conditions. The core descriptions and trace element concentrations differ between samples obtained from the Chang-73 and Chang-71,2 members, situated at the bottom and top of the Chang-7 Member, respectively, such that the depositional condition of the former implies a more reduced water regime. The bulk rock nitrogen isotope (δ15Nbulk) values of the samples obtained from the Chang-73 Member are higher than those of the Chang-71,2 Member. No significant differences were identified in the organic carbon isotopes (δ13Corg), total organic carbon (TOC), or total nitrogen (TN) values between the two units. The results of δ15Nbulk cross-plotting with the TOC, TN, and carbon to nitrogen atomic ratio (C/N) show an absence of significant modification of the primary nitrogen isotopic compositions from post-depositional processes. Given the depositional history of these units, we suggest that the sedimentary redox environment can have a significant influence on δ15Nbulk. Denitrification, which occurs mainly in suboxic conditions, plays an important role in such an environment, resulting in 15N enrichment during the original deposition. Thus, owing to its sensitivity to sedimentary redox environments, the δ15Nbulk index can be an effective tool for evaluating depositional redox conditions, particularly for further subdivision of suboxic environments.

Evaluation of paired carbon isotopic signals from the Ediacaran Doushantuo phosphorites: Diagenetic or primary?

This study reports high-resolution C-isotopic data of carbonate carbon (δ13Ccarb) and organic carbon (δ13Corg) from the Ediacaran Doushantuo carbonate and phosphorite as well as from the underlying tillite of the Nantuo Formation from the Datang mine and Chuanyuandong sections. For the Nantuo tillite in the Datang section, the striking negative δ13Ccarb values ranging mostly from −10‰ to −16‰ with negative δ18O values down to −27.2‰ and clear covariance between δ13Ccarb and δ18O, suggest that nearly all of the primary C and O-isotopic compositions have been reset by meteoric diagenesis. The near complete modification of the primary isotopic signals may be due to frequent exposure to fresh water conditions during diagenesis and the lithification of porous tillite sediment. The extremely negative δ13Ccarb values down to −16.8‰ suggest the contribution of 12C-enriched dissolved inorganic carbonate which originated from the remineralization of organic carbon during burial diagenesis. These observations suggest that caution is needed when using tillite samples for paleoenvironmental reconstruction.The correlative lower phosphorite deposits from the Datang and Chuanyuandong sections show clear covariance between δ13Ccarb and δ18O, although the Chuanyuandong phosphorites have more negative values of δ13Ccarb and δ18O than the Datang section. Although most of the δ18O values of the lower phosphorites may have been altered by secondary diagenetic processes, the δ13Ccarb values of >−3.5‰ and the δ13Corg values of <−28.0‰ may represent the primary isotopic compositions of contemporaneous phosphate-rich seawater. In contrast, the upper phosphorites, characterized by δ13Ccarb values ranging from −0.9‰ to 1.7‰ with the δ18O values from −0.2‰ to −4.5‰ and the absence of covariance between δ13Ccarb and δ18O, may have recorded the primary isotopic signals of seawater from which the phosphorites were formed. It appears that the chemical composition of phosphate-rich seawater may have changed during the two phosphogenic events of the Ediacaran Doushantuo Formation.

Diagenesis of shallowly buried Miocene lacustrine carbonates from the Hoh Xil Basin, northern Tibetan Plateau: Implications for stable-isotope based elevation estimates

The oxygen isotopic composition of lacustrine carbonate deposits has been frequently used to reconstruct paleoaltimetry of orogenic belts. However, the primary oxygen isotopic composition of lacustrine carbonate can be partially or wholly altered by post-depositional diagenetic processes, such as deep burial recrystallization and the interaction with geologically younger meteoric waters. To better understand how ancient lacustrine carbonates are diagenetically modified by meteoric waters, we present detailed mineralogy, petrography, stable carbon and oxygen isotopic compositions, and major and minor element geochemistry of lacustrine carbonates from the Miocene Wudaoliang Group, which underwent only shallow burial (<300 m) in the Hoh Xil Basin, northern Tibetan Plateau. Our observations indicate that the primary mineralogy in the Wudaoliang carbonate muds consists of a mixture of both low-Mg and high-Mg calcite. During or shortly after deposition, the carbonate muds experienced stabilization and associated carbonate cementation in a closed phreatic meteoric diagenetic system. Interaction with 18O-depleted meteoric waters during diagenesis slightly reduced 18Oc values, but overall the Wudaoliang carbonates largely retained primary oxygen isotope compositions. Our data support previous interpretations that δ18Oc values of the Wudaoliang Group lacustrine carbonates are indicative of the primary δ18O composition of Miocene meteoric water and can be used for paleoelevation estimate of the Miocene Hoh Xil Basin, northern Tibetan Plateau. By demonstrating the oxygen isotopic fidelity of the Wudaoliang carbonates, we are able to rule out open-system meteoric diagenesis from younger meteoric waters and subsequent isotopic alteration. This study highlights that integration of scanning-electron-microscope and cathodoluminescence petrography with stable isotopic and mineralogical data enables the determination of the extent of meteoric diagenetic alteration in lacustrine carbonates, particularly whether the diagenetic systems were open or closed with respect to the oxygen isotope.

Reconstructing carbonate alteration histories in orogenic sedimentary basins: Xigaze forearc, southern Tibet

Abstract Carbonate clumped isotope thermometry (T( Δ 47 )) and the oxygen stable isotope record ( δ 18 O ) provide critical constraints on the temperature of carbonate mineral formation and isotopic composition of ancient waters used in reconstructions of past climate, tectonics, and ecological and environmental change. The robust use of these proxies requires that carbonate minerals in the rock record retain primary isotopic compositions through often complex post-depositional thermal histories. New carbonate clumped isotope thermometry and sedimentology data are paired with existing thermochronology and isotope exchange reaction modeling to provide unique constraints on the alteration history of the carbon and oxygen isotopes in Tethyan marine carbonates (Jialazi Fm, Tso Jianding Group) of the Xigaze forearc, southern Tibet. Within the framework of the textural and isotopic data, the effects of early and late burial alteration on Δ 47 and δ 18 O values of marine rocks that were buried to 4 km above sea level are modeled. The alteration products are considered within two well-studied mechanistic frameworks: water-rock recrystallization and solid-state reordering of 13 C 18 O bonds at high temperature. The proposed alteration model is as follows: (1) shallow, early diagenesis with water of similar composition and temperature to formation waters infills primary porespace with microspar; (2) high-temperature, rock-buffered water-rock exchange and partial solid-state reordering at >3–4 km depth; and (3) near-surface, low-temperature water-rock exchange on the exhumation pathway. The Δ 47 heterogeneity within individual samples (0.456 to 0.721) requires significant differential alteration on the retrograde path (high to low temperature, low water-rock ratio) in which primary compositions and compositions inherited during early burial diagenesis are overprinted with modern meteoric values. A low T( Δ 47 ) value is often assumed to record the primary temperature of the depositional environment. However, samples yielding low T( Δ 47 ) values in the Jialazi Fm have undergone extensive high-temperature water-rock alteration, and therefore, require a mechanism for increasing 13 C 18 O bond ordering on the retrograde pathway while preserving many primary biogenic structures and micritic textures. This study highlights the potential significance of late-stage, low-temperature alteration of carbonates derived from orogenic sedimentary basins for studies of terrestrial paleoenvironments and tectonics. This study also provides an example framework for combining observational and analytical data to reconstruct a carbonate alteration history of a buried sedimentary package.

Re-Os isotope geochemistry of komatiite-hosted Ni-Cu-PGE deposits in Finland

We have studied the Re-Os isotope systematics of komatiite-related Ni-Cu-PGE sulfide deposits in Finland, including the Archean Ruossakero Ni-(Cu), Vaara Ni-(Cu-PGE), Tainiovaara Ni-(Cu-PGE) deposits and Ni-PGE sulfide mineralization related to a dunitic body at Tulppio, and the Paleoproterozoic Lomalampi PGE-(Ni-Cu) deposit. We analyzed chromite separates from both mineralized and non-mineralized komatiitic rocks, bulk-sulfide separates, and country-rock black schist from one deposit, and for comparison, chromite separates from non-mineralized Archean komatiitic rocks from the Kovero and Kuhmo greenstone belts.We have observed open-system behavior of sulfides in three deposits, with their Re-Os isotope compositions indicating resetting at ca. 1.83–1.87 Ga by metamorphic events related to the Svecofennian orogeny. Most of the chromite separates, especially those with >10 ppb Os, retain their primary isotope compositions with near-chondritic initial 187Os/188Os ratios, indicating that their Os was derived from a mantle source with a chondritic Re-Os isotope evolution. These include the Vaara deposit, which yielded a chromite Re-Os isochron age of 2719 ± 140 Ma and low initial γOs of −1.2 ± 0.02, in spite of the evidence for the presence of mass-independent S isotope fractionation in this deposit, suggesting a low-Os, sulfur-bearing contaminant. In the case of the Lomalampi deposit, chromites yield an isochron with an age of 2058 ± 93 Ma and initial γOs of +0.9 ± 0.02, consistent with earlier isotope data from Central Lapland komatiites. However, one chromite from a mineralized cumulate gave a γOs(2050 Ma) value of +24 ± 18, indicating a minor/moderate degree of crustal contamination involving material from country rock black schists. This is also supported by earlier S isotope data. In the Archean Tainiovaara deposit, chromite also suggests crustal contamination (γOs + 13 ± 1), but country rocks are Os-poor Archean gneisses, leaving the source of radiogenic Os unclear, though it is possible that contamination occurred at a greater depth. Our results also provide evidence that the Re-Os age of the Ruossakero deposit is ca. 2.9 Ga, with the deposit likely representing komatiitic magmatism that is older than the komatiitic magmatism elsewhere in Finland. Re-Os results also confirmed that the mineralized Tulppio Dunite is Archean in age.

Isotopic and geochemical characteristics of Cambrian phosphorites of Karatau Basin (Southern Kazakhstan)

«Вестник Санкт-Петербургского университета. Науки о Земле» — научно-теоретический рецензируемый журнал, обобщающий результаты исследований в области геологии, гидрогеологии и геохимии, петрологии и экологии, ландшафтного планирования и геоморфологии, экономической и социальной географии.

Extreme source heterogeneity and complex contamination patterns along the Cameroon Volcanic Line: New geochemical data from the Bamoun plateau

We investigated mafic and felsic volcanic rocks from the Bamoun plateau, a magmatic province located north of Mount Cameroon, in the continental part of the Cameroon Volcanic Line (CVL). Basalts and dacites were probably emplaced more than 40 Ma ago, while basanites represent very young volcanic eruptions. Among the basalts, some of them have suffered crustal contamination during their uprise through the continental crust, and their primary trace element and isotopic compositions have been slightly modified. The formation of the dacites was also accompanied by some crustal contamination. Non-contaminated rocks show that the oldest magmas are transitional basalts formed by relatively high degrees of partial melting of a moderately enriched mantle source, probably containing pyroxenites. Recent basanites were produced by very low partial melting degrees of an enriched mantle source with HIMU composition, but different from the source of the nearby Mount Cameroon lavas. The mantle beneath the CVL is thus very heterogeneous, and the tendency towards more alkaline mafic-ultramafic compositions in the youngest volcanic manifestations along the CVL seems to be a general feature of all CVL.

Do Hf isotopes in magmatic zircons represent those of their host rocks?

Lu–Hf isotopic system in zircon is a powerful and widely used geochemical tracer in studying petrogenesis of magmatic rocks and crustal evolution, assuming that zircon Hf isotopes can represent initial Hf isotopes of their parental whole rock. However, this assumption may not always be valid. Disequilibrium partial melting of continental crust would preferentially melt out non-zircon minerals with high time-integrated Lu/Hf ratios and generate partial melts with Hf isotope compositions that are more radiogenic than those of its magma source. Dissolution experiments (with hotplate, bomb and sintering procedures) of zircon-bearing samples demonstrate this disequilibrium effect where partial dissolution yielded variable and more radiogenic Hf isotope compositions than fully dissolved samples. A case study from the Neoproterozoic Jiuling batholith in southern China shows that about half of the investigated samples show decoupled Hf isotopes between zircons and the bulk rocks. This decoupling could reflect complex and prolonged magmatic processes, such as crustal assimilation, magma mixing, and disequilibrium melting, which are consistent with the wide temperature spectrum from ∼630 °C to ∼900 °C by Ti-in-zircon thermometer. We suggest that magmatic zircons may only record the Hf isotopic composition of their surrounding melt during crystallization and it is uncertain whether their Hf isotopic compositions can represent the primary Hf isotopic compositions of the bulk magmas. In this regard, using zircon Hf isotopic compositions to trace crustal evolution may be biased since most of these could be originally from disequilibrium partial melts.