Ratios Of Rocks Research Articles

Bi-modular properties of sandstone inferred from seismic moment tensors of acoustic emissions

We propose a novel method for studying the λ/μ ratio, the vP/vS ratio and Poisson's ratio of rocks based on the determination of highly accurate moment tensors of acoustic emissions (AEs). The validity of the method is verified on observations of 539 AEs recorded in a sandstone sample during a semi-circular bend test. We show that the sandstone exhibits the so-called bi-modular behaviour indicating that rock parameters significantly differ under compressive and tensile stress regimes. In addition, the zone characterized by highly tensile fractures is auxetic. For increasing shear component in AEs, Poisson's ratio increases and the auxetic effects disappear. The proposed approach for determining the λ/μ, vP/vS, and Poisson's ratios using AEs is fully independent of standard techniques based on measuring deformation of rock samples. The approach might find applications in detailed studies of rock behaviour under various stress conditions, in particular, in assessing the bi-modularity of rocks measured during a single experiment.

Lead isotopes and rare earth elements geochemistry of global phosphate rocks: Insights into depositional conditions and environmental tracing

Phosphate rock bears both geologically and environmentally significant information. Rare earth elements and yttrium (i.e., REY) characteristics have been commonly used for reconstructing the redox conditions of depositional environments and the effects of post-depositional diagenetic alteration on phosphate rock. In addition, phosphate rock is typically enriched in a range of trace elements such as uranium (U) and cadium (Cd) that can be dispersed as contaminants into the environment with phosphate mining and phosphate fertilizer application. Here we report the lead (Pb) isotope compositions combined with Pb and REY concentrations of both global sedimentary and igneous phosphate rocks, aiming to evalute the geological origin of phosphate rocks over time and the potential of using them for environmental tracing. Phopshate rock samples analyzed in this study were sourced from major economic phosphate deposits in the world, including China, Southern Tethys (e.g., Morocco, Tunisia, Israel), the U.S., India, South Africa and Russia. Our results show a wide range of 208Pb/204Pb (35.70 to 60.58), 207Pb/204Pb (15.20 to 18.25), and 206Pb/204Pb (16.369 to 71.806) ratios in the phosphate rocks, with sedimentary phosphate rocks being significantly more radiogenic than igneous rocks. The majority of sedimentray phosphate rocks show a notable isotopic overprinting by non-radiogenic terrestrial Pb, except for those from Israel and Morocco that have the most radiogenic Pb isotope compositions. Correspondingly, phosphate rocks with more radiogenic Pb isotope ratios show relatively pristine seawater REY features, likely suggesting their preservation of the original oxic seawater conditions and/or minimal diagenetic alteration. In contrast, phosphate rocks with less radiogenic Pb isotope compositions show REY indications for more anoxic seawater redox conditions and/or greater diagenetic alteration. We further evaluate the potential utility of Pb isotopes for tracing the associated contamination with phosphate rock mining and fertilizer application in the environment. In most cases, the radiogenic Pb isotope composition of phosphate rocks and corresponding P-fertilizers is distinctive from both natural crustal Pb and major anthropogenic Pb sources (e.g., Pb ore deposits and pesticides), which provides a great advantage for applying Pb isotopes as an environmental tracer for metal(loid) contamination from phosphate sources. The combination of Pb isotope ratios and REY proxies could further constrain the Pb source discrimination. Overall, this study provides new Pb isotopic and REY geochemical data on global phosphate rocks and fertilizers, which lays the groundwork for future regional and local studies on both their geological and environmental implications.

Phosphorous concentration in iron-rich rocks of the Chilpi Group, Bastar Craton, India: implications on late Palaeoproterozoic seawater palaeo-productivity

AbstractThe concentration of the bio-limiting nutrient element, phosphorus (P), in seawater is important for primary marine productivity and the evolution of life on geological time scales. The molar percentage of P/Fe in banded iron formations (BIF) and iron oxide-rich chemical sediments is a good proxy for the first-order approximation of seawater P concentration. Bio-available concentration of phosphorus in Precambrian, especially during the late Palaeoproterozoic Era (2.0–1.8 Ga), is poorly constrained. We evaluated the P/Fe ratios of iron-rich rocks from the late Palaeoproterozoic Chilpi Group, Bastar Craton, Central India. The bulk rock molar percentage of P/Fe ratios of the Chilpi rocks vary between 0.11 and 1.17 (average 0.51 ± 0.3), and the average of EPMA spot analysis P/Fe molar ratio is 0.32 ± 0.4; both have values similar to Archaean BIFs of the world. The observed low molar ratio is not an artefact of contamination from terrestrial sources, diagenetic alterations or high-temperature hydrothermal inputs; it indicates the deposition from phosphorus-lean seawater. The modelled P/Fe molar ratio in the Chilpi Group suggests that the concentration of phosphorus in the shallow marine environment was less than 0.12 μM. The low level of phosphorus concentration in seawater during the late Palaeoproterozoic Era is interpreted to be a consequence of the low primary production during a period of low atmospheric oxygen content, which might have impeded the evolution of eukaryotes.

The process of crustal thickening in the southern Lhasa terrane during India-Eurasia collision: Constraint from Eocene high Sr/Y rocks in the Quxu pluton

The crustal thickening process is the key to understanding global tectonic evolution and climatic patterns. The Sr/Y, (La/Yb)N ratios of granitoids can be used to quantitatively estimate crustal thickness and reconstruct the crustal thickening process. We present the geochemical characteristics, petrogenesis, and geodynamic implications of Eocene dioritic enclaves (monzodiorite), host granitoid rocks (monzonite), and granite dikes of southern Tibet to reveal the crustal thickening process underway during the collision of India and Asia. The Quxu dioritic enclaves were produced by magma mixing and represent mafic end member. The adakitic host rocks were generated by partial melting of lower crust with normal thickness. The high Sr/Y, (La/Yb)N ratios of the Quxu host rocks were inherited from their magma source region. The ∼40% fractional crystallization of amphibole from magma resembling Quxu host rocks formed the Quxu granite dikes. The continuous crustal thickening history determined by the locally weighted regression analysis shows that the crustal thickness of the Lhasa terrane during 50−45 Ma was no more than 36−46 km, and the crust thickened quickly during the syn-collisional period.

Numerical simulation of fracture propagation in deep coal seam reservoirs

AbstractAs an unconventional oil and gas resource, coalbed methane has become a key area of unconventional oil and gas exploration worldwide, and hydraulic fracturing is the critical technology for the efficient stimulation of coalbed methane. However, due to the strong heterogeneity of deep coal seam reservoirs and the development of natural cleat fractures, the fracture propagation path is quite complex and difficult to capture. In this study, we propose a hydraulic fracture propagation simulation method using the ABAQUS Software for natural cleat fracture networks and a fluid–solid fully coupled two‐dimensional finite element model with the global embedded cohesive zone model (CZM). CZM is established to simulate the propagation of hydraulic fractures in the heterogeneous combination of coal rock and mudstone. The accuracy of this model is verified by using Blanton's experiment criterion. Finally, the model is used to simulate the influence of cleat angle, cleat density, length ratio of coal rock and mudstone combination, and construction displacement on the propagation behavior of hydraulic fractures. This study aims to deepen the understanding of the mechanism of hydraulic fracture propagation in deep coal seams and provides guidance for deep coal seam fracturing and hydraulic fracturing design.

Geochronological and geochemical study of the Late Carboniferous volcanic drilling cores from the Piedmont of the Hala'alate Mountain in West Junggar: Implications for stratigraphic division and tectonic evolution

The West Junggar tectono-magmatic processes are critical for reconstructing the amalgamation history of the Central Asian Orogenic Belt (CAOB). However, the lack of consensus regarding the geochronology, petrogenesis, and tectonic setting of the complex igneous rocks poses a challenge to comprehending the Late Paleozoic tectonic evolution of West Junggar. This study focuses on systematic geochronological and geochemical research of well-preserved drill cores from the Piedmont of the Hala'alate Mountain, including rhyolite, felsic tuff, basalt, and andesite. The ages of these rocks in Late Carboniferous (310–301 Ma) are constrained by zircon UPb dating, excluding them from the Jiamuhe Formation of the Early Permian. The rhyolites and felsic tuffs exhibit apparent affinity to the A-type granite series, characterized by high total alkali and FeOT/MgO values. Plagioclase controls the main residual facies and depleted isotopic compositions (εHf(t) = +10.2 to +13.4; εNd(t) = +7.6 to +8.1), indicating that the magma source originated from the partial melting of juvenile continental materials in an extensional background. The basalts and andesites display high LILEs/LREEs, LILEs/HFSEs ratios, along with low HFSEs, which correspond to arc-affinity geochemical compositions. The high zircon εHf(t) (+7.5 to +14.3) and whole-rock εNd(t) (+7.8 to +7.9) values indicate a depleted pyroxene-rich mantle source modified by metasomatic slab-derived fluids. This implies a large-scale upwelling of mantle materials driven by the overlying lithospheric crustal thinning. Combining the temporal variations of crustal thickness recorded by the Sr/Y and (La/Yb)N ratios in igneous rocks, the tectonic processes of West Junggar during the Carboniferous to Early Permian can be divided into three stages. The first stage involves sustained subduction processes from 350 Ma to 320 Ma, followed by the closure of the oceanic crust and collision orogenesis from 320 Ma to 310 Ma. Finally, and a post-collision extension setting occurred after 310 Ma.

The soft rock can promote the improvement of aeolian sandy soil in Mu Us Sandy Land, China

This study focuses on the significance of improving the land degradation of Mu Us Sandy Land to increase cultivated land area and promote ecological green development. The research objects were four kinds of mixed soils, and rhizosphere soils were collected during the crop harvesting period. The volume ratio of soft rock to sand was 0:1 (control check, CK), 1:5 (composite soil one, PS1), 1:2 (composite soil two, PS2), and 1:1 (composite soil three, PS3). The results showed that the large aggregates were primarily mechanically stable aggregates, while the small aggregates were mainly water-stable aggregates. The soft rock promoted the increase of clay and silt content in sandy soil, and the soil texture changed from sand to loam. The contents of organic matter, available phosphorus, and available potassium increased significantly under PS2 and PS3 treatments, but there was no significant difference between them. Total nitrogen had no significant difference among treatments. Actinobaciota, Proteobateria, and Chloroflexi were the dominant bacteria in rhizosphere soil, accounting for about 75% of all microorganisms. At the Genus level, the soft rock contributes to richer species composition. The diversity index, evenness index, and richness index was higher in PS1, and the available phosphorus and available potassium content promoted the increase of diversity. Therefore, when the proportion of soft rock and sand compound soil is between 1: 5 and 1: 2, it can be used as an important basis and technical parameter for Mu Us Sandy Land improvement.

Cd isotopic constraints on the sources of Zn-Sb deposits: A case study of the Jianzhupo Zn-Sb deposit, Guangxi Province, China

Antimony (Sb) is identified as a critical metal in many countries. The source of hydrothermal Sb-bearing deposits is currently debated in two opposing models (magmatic fluids or country rocks). Some Sb-bearing hydrothermal systems host abundant cadmium (Cd). Due to the close association of Cd and Sb in hydrothermal systems and the distinct Cd isotopic signatures between magmatic and sedimentary rocks, Cd isotopes have the potential to trace the metal origin in Sb-bearing deposits. Here, we conducted Cd isotope analyses of sulfide (jamesonite and sphalerite) collected from the Jianzhupo Zn-Sb deposit, SW China. A narrow range of δ114/110Cd relative to NIST SRM 3108 Cd standard was observed in sphalerites (−0.15‰ to +0.18‰; mean = 0.03‰ ± 0.10‰, one standard deviation [1SD]), identical to that of intermediate igneous rocks (−0.20‰ to +0.15‰); in contrast, pure jamesonites show a large range of δ114/110Cd (−0.42‰ to +0.17‰; mean = −0.22‰ ± 0.20‰, 1SD), differing from those of sphalerite. Different Cd isotope signatures between jamesonite and sphalerite are unlikely to have been triggered by sulfide precipitation, vapor-liquid phase separation, diffusion, and different Cd-S bond strengths. Instead, based on a comparison of δ114/110Cd and Zn/Cd ratio of sulfide and potential source rocks, we propose that a mixing of two ore-forming endmembers, derived from igneous and sedimentary rocks, may better explain the sulfide Cd isotopic signatures. This is supported by the well-defined positive correlation between δ114/110Cd and Zn/Cd ratio in sulfides. This study shows a novel application of Cd isotopes for metallogenetic tracing and demonstrates that Sb-bearing hydrothermal systems can incorporate metals from multiple sources.

In situ sea urchin carbonates analysis by LA-MC-ICP-MS to identify lead isotopic signatures: A case study at Trindade Island (South Atlantic Ocean versus anthropogenic)

Isotope ratios of lead (Pb) are increasingly applied to trace pollution sources in marine settings, using mostly carbonate remains of living organisms. However, this data has never been acquired in sea urchin carbonates. Sea urchins are widely distributed in marine settings and have been recognized for their ability to bioaccumulate heavy metals and their skill to track environmental changes. Thus, this work intends to perform, for the first time in-situ analysis of Pb isotope ratios using LA-ICP-MC-MS in sea urchin spines collected on the beaches from Trindade Island (SE Brazil). Trindade Island is a protected environmental area, almost uninhabited, located in a remote region of the South Atlantic Ocean (1200 km from the mainland). The results obtained in sea urchins spines collected on the beaches from Trindade Island varied from: 1.03 to 1.30 (mean 1.18 ± 0.06) for 206Pb/207Pb; 1.44 to 2.23 (mean 2.10 ± 0.07) for 208Pb/206Pb and 2.22 to 2.87 (mean 2.48 ± 0.15) for 208Pb/207Pb. The biplot of 208Pb/206Pb versus 206Pb/207Pb ratios show greater dispersion of isotopic values in the sea urchin spines than in the geogenic data of the Vitória-Trindade Ridge (VTR; used as reference) and the surface water from the Tropical Atlantic Ocean. While the Pb isotope ratios of rocks and sediments of the VTR are quite similar and present intermediate values, a significant variation of the isotopic signature in the sea urchin spines was observed. The comparison of the sea urchin spines 206Pb/207Pb ratios with data published in the literature from several materials suggests the presence of anthropogenic contamination on Trindade Island caused using fossil fuels. The range of variation of Pb isotopes analyzed in sea urchin spines suggests mixing between the natural source (the rocks and sediments of the Vitória-Trindade Ridge) and surface water masses of the Tropical South Atlantic and anthropogenic sources.

Precursors of Cyclic Loading and Unloading Sandstone Failure Based on “Acoustic-Thermal” Loading–Unloading Response Ratio

Coal mining often causes periodic disruption in the rock mass around the stope. The study of the deformation and failure characteristics of cyclic loading and unloading sandstone is very critical for gaining a thorough understanding of the mechanisms of rock damage, degradation, and failure. This kind of investigation is very helpful in determining the precursors of rock failure and the instability of engineering structures. In this research study, the properties of acoustic emission and infrared radiation of cyclic loading and unloading sandstone are explored using a cyclic loading and unloading sandstone experiment. Based on acoustic emission and infrared radiation, the loading–unloading response ratio of rock is established. It is found that the response variables of sandstone during the loading stage based on acoustic emission (AE) counts and the loading–unloading response ratio based on average infrared radiation temperature (AIRT) both rise suddenly in the last cycle, which may be a precursor of “acoustic-thermal” approaching rock failure. On this basis, the quantitative analysis index of infrared radiation of differential infrared energy change rate (DIECR) is proposed, that is, the change of square of ΔAIRT in unit time, and based on AE counts and DIECR, the loading–unloading response ratio of “acoustic-thermal” is defined. It is found that the “acoustic-thermal” loading–unloading response ratio suddenly increases during the penultimate cycle of loading and unloading. This feature can be taken as the initial precursor of rock failure. Together with the “acoustic-thermal” imminent failure precursor of rock, it constitutes the “initial precursor-imminent failure precursor” combined with the internal fracture and surface infrared radiation temperature field during the cyclic loading and unloading process of rock, realizing the hierarchical monitoring and early warning of cyclic loading and unloading rock failure. The research results lay a theoretical and practical foundation for using infrared radiation to monitor engineering disasters caused by rock fracture and failure in mining engineering.

Correlations between Uniaxial Compressive Strength and Dynamic Elastic Properties for Six Rock Types

The assessment of the behavior of the ground is highly critical for any geotechnical and geological projects. Uniaxial compressive strength (UCS) is an essential mechanical parameter for any rock mass characterization, whereas the elastic properties of rocks help us understand the deformational behavior of the rocks. However, the determination of these parameters is challenging, time-consuming, destructive, and expensive and requires a skilled workforce. In contrast, the dynamic elastic properties require P-wave velocity (VP), S-wave velocity (VS), and density (ρ) for calculation, which are easy, quick, nondestructive, and cost-effective to determine. Therefore, in this paper, to understand the behavior of different rocks, UCS has been correlated with the dynamic elastic properties, including dynamic bulk modulus (KD), dynamic shear modulus (GD), dynamic Young’s modulus (ED), and dynamic Poisson’s ratio (µD) for carbonate, serpentinite, shale, sandstone, volcanite, and ignimbrite rocks incorporated from the databases of eight previous studies. Based on the lithology, different correlation equations have been proposed to predict the UCS from dynamic elastic properties using simple regression (SR) analysis. From SR analysis, it was observed that the coefficient of correlation (R) values depended on the type of rock. The R-values for the overall rocks, including all six rock types, for different correlations, were observed to be inferior to the R-values obtained on the basis of lithology. The principal component analysis (PCA) reduced the dimension of the data sets, and the trendline obtained for each rock type was observed to be dependent on the lithology. Moreover, descriptive statistics were also used to validate the lithological control for six different rock types.

Insight into a rift volcanism with the petrogenesis of ultramafic enclaves and the host basalts: Kula Volcanic Field, Western Anatolia, Turkey

The Quaternary Kula Volcanic Field (KVF), located on an E-W trending Plio-Quaternary horst, consists of at least 80 scoria cones, associated fissural lava flows and maars. Volcanic activity is divided into three main stages as BI (ca. 2-0.9 Ma), BII (ca. 300-50 ka) and BIII (ca. 10-0.7 ka). The volcanic products of BII and BIII contain ultramafic enclaves. They are generally spherical or elliptical varying in size from 1.7), and Nb/U (>31) ratios, reflecting that crustal contamination is insignificant. Petrogenetic modelling shows that Kula enclaves and host rocks are probably originating from 3-5% partial melting of garnet and spinel-peridotite mantle source mixtures. Rb/Sr (0.03-0.10) and slightly high Ba/Rb (9.5-23) ratios for host rocks demonstrate the presence of amphibole and phlogopite in their source, while low Rb/Sr (0.01-0.09) and high Ba/Rb (18-115) reveal that amphibole is present in the source. Estimated T/P conditions of early crystallizations are between 19.5–9.5 kbar, 63–31 km, and 1260–1130 °C for host rocks and 18.3–11.1 kbar, 59–36 km, and 1324–1208 °C for the enclaves, which suggests that the crystallization might have taken place in the lithospheric mantle.

O- and H- Isotope constraints on Neogene high-K calc-alkaline and shoshonitic volcanic rocks and borates in the Kırka (Eskişehir) basin, W Anatolia

ABSTRACT The O and H isotopic ratios of borates, clay minerals, and volcanic rocks from the Kırka basin (south of Eskişehir, W Anatolia) have been analysed in order to better constrain the conditions that led to precipitation of the borates. The δ18Owr values of the volcanic rocks range from + 7.6 to + 15.9‰, but many samples are enriched in 18O by post-eruption hydration, as evidenced by the positive correlation between 18O and H2O (LOI up to 3.1 wt.%) contents. The δ18O values of the borate and clay minerals (smectite) range from 10.0 to 18.8‰, and 19.5 to 31.3‰, respectively. The large variation among the δ18O values of the borate minerals either is likely related to oxygen isotope fractionation during their formation and/or indicates that they formed at different stages of evaporation of the original brines over a range of temperatures. Considering that borax was a primary phase in the basin, decreasing δ18O values from borax to colemanite and ulexite is compatible with decreasing formation temperatures. The δD values of borate and clay minerals do not show significant differences (−64 – −88‰, and −106 – −125‰, respectively). The calculated δ18OH2O and δDH2O values of the fluid in equilibrium with the borate minerals (−11.6–5.93‰ and −85 – −65‰, respectively) and in equilibrium with the smectite (14.4–17.0‰ and −126 – −67‰, respectively) indicates that the basin brines were dominated by geothermal fluids. Overall, the oxygen isotope systematics of the borates and smectite in the Kırka basin are consistent with the smectite and borate minerals forming from brines that resulted from the ponding and evaporation of geothermal fluids that had undergone water-rock interaction with the local felsic volcanic rocks.

A Pb isotope model for the Proterozoic of northern Australia with a focus on the McArthur Basin

The Pb isotopic composition of mineral exploration samples can be used to help determine their metallogenic association and hence to assess prospectivity in geological provinces where there is an established Pb isotope database. We have developed such a database, available online, and models that relate Pb isotopic signatures to known metallogenic events in Proterozoic terrains in northern Australia. Two terrain-specific Pb isotope model curves, the Western Fold Belt curve and the Eastern Fold Belt curve, were developed based on major ore systems in the Mount Isa and McArthur terrains. The Western Fold Belt curve was optimised to give an age of 1640 Ma for the McArthur Formation HYC deposit. This is the well-constrained age of the Barney Creek Formation, host of the mineralisation. To validate the accuracy of the models, we attempted to determine the initial Pb isotope ratios of granitic rocks from across the Proterozoic terrains with ages varying from 1870 to 1550 Ma. The curves give model ages for major ore systems including Mount Isa, George Fisher Zn–Pb, Century and Cannington that are consistent with their stratigraphic locations and accepted metallogenesis. Focusing on the McArthur Basin, a database of 59 deposits and prospects was developed. In addition to the 1640 Ma HYC event, an older event at ca 1700 Ma has been fingerprinted, a younger Upper McArthur event at ca 1570 Ma, an event coeval with the Mount Isa Orogeny at ca 1550–1500 Ma and a late Roper event younger than 1500 Ma. There is also evidence of hydrothermal activity of Cambrian age and recent weathering-related metal concentration.

Prediction of rock burst intensity based on multi-source evidence weight and error-eliminating theory.

Rock burst is the main geological hazard in deep underground engineering. For the prediction of the intensity of rock burst, a model for prediction of rock burst intensity on the basis of multi-source evidence weight and error-eliminating theory was established. Four indexes including the ratio of rock's compressive-tensile strength [Formula: see text], the stress coefficient of rock [Formula: see text], the elastic energy index of rock Wet, and integrality coefficient Kv were chosen as the prediction variables of rock burst; the index weights are calculated by different weighting methods and fused with evidence theory to determine the final weight of each index. According to the theory of error-eliminating, taking "no rock burst" (I in classification standards of rock burst intensity) as the objective and using the error function to process 18 sets of typical rock burst data and the weight of evidence fusion as the normalized index limit loss value, a model for prediction of rock burst intensity was built. It is verified by the actual situation and three other models. Finally, the model has been applied to rock burst prediction of Zhongnanshan tunnel ventilation shaft. The results show that evidence theory fuses multi-source index weights and improves the method of determining index weights. The index value is processed by error-eliminating theory, and the limit value problem of index value normalization is optimized. The predicted results of the proposed model are consistent with the situation of Zhongnanshan tunnel. It improves the objectivity of the rock burst prediction process and provides a research idea for rock burst intensity prediction index.

Light fractions can also be heavy hitters: Comparison of detrital zircon U–Pb and detrital K‐feldspar Pb–Pb as provenance indicators—A case study from Cretaceous strata of southern Utah, USA

AbstractComparison of detrital zircon U–Pb geochronology and detrital K‐feldspar Pb–Pb isotopic data from Upper Cretaceous sandstones of southern Utah reveals important provenance information that is unrecorded by more chemically and physically durable detrital minerals like zircons. To better understand these signals, we also present an updated database of Pb‐isotopic ratios in ores, whole rocks and feldspars, for potential sediment source terranes in North America. Detrital K‐feldspar Pb‐isotopes indicate sediment provenance of the Western Cordillera/Mojave, Yavapai‐Mazatzal and Wyoming terranes, with limited input from recycled Sevier fold‐thrust belt strata. The K‐feldspar data broadly support previous interpretations, largely based on detrital zircon datasets, of a Cretaceous palaeo‐river system transporting sediment from the southwest. However, the light‐fraction data also provide important, complementary and otherwise unrecognized perspectives as compared to detrital zircon datasets. For example, K‐feldspars indicate a stronger apparent influence of Western Cordillera/Mojave and Wyoming terrane sources than previously interpreted. We interpret the K‐feldspar Wyoming terrane signal as the result of south‐directed longshore drift, with preferential loss of zircons due to hydraulic sorting processes. As relatively new detrital K‐feldspar detrital methods continue to be developed, this multi‐proxy approach to provenance could be informative in other sedimentary basins where suitable source terranes can be identified.

Sr isotopes in metasomatized fault rocks constrain the age of juxtaposition of oceanic and continental lithosphere (Dun Mountain Ophiolite, New Zealand)

The juxtaposition of oceanic and continental lithosphere along major fault zones is a rare but globally significant process that is commonly associated with the closure of an ocean basin. However, it is often difficult to determine the timing in the absence of a high-grade metamorphic sole that can be dated. We show that the timing of mantle and crust juxtaposition along the base of the Dun Mountain Ophiolite in New Zealand can be constrained using Sr isotope data from metasomatic rocks that developed due to the extreme chemical contrast between ultramafic (mantle) and quartzofeldspathic (crustal) lithologies along the basal Livingstone Fault. Fluids that migrated through ultramafic rocks into greenschist facies quartzofeldspathic metasediments resulted in formation of actinolite, diopside, hedenbergite, epidote and garnet, as well as the disappearance of quartz and white mica, and the near-complete removal of Rb, in a rodingite and albite-rich zone up to c.10 m wide. Major and trace element compositions indicate that albite-rich rocks and rodingites have quartzofeldspathic protoliths equivalent to rocks outside of the metasomatic zone. Due to loss of the main Rb-bearing phase (mica), the timing of metasomatism can be broadly constrained by age correction of the 87Sr/86Sr ratios of unaltered quartzofeldspathic rocks to those of the strongly metasomatized equivalents. The calculated age, c. 173 Ma, represents juxtaposition of continental and oceanic lithosphere along the Livingstone Fault. Although obduction of the Dun Mountain Ophiolite may have initiated prior to the dated event, the new age is consistent with published paleogeographic reconstructions, and indicates that a >2000 km long fore-arc ocean basin that existed along the Gondwana margin was closed by the middle Jurassic. Removal of Rb from metasedimentary rocks in contact with ultramafic rocks in ophiolite settings provides an opportunity to constrain the timing of juxtaposition at lower-greenschist facies conditions.

Carbon isotope fractionation by an ancestral rubisco suggests that biological proxies for CO2 through geologic time should be reevaluated

The history of Earth's carbon cycle reflects trends in atmospheric composition convolved with the evolution of photosynthesis. Fortunately, key parts of the carbon cycle have been recorded in the carbon isotope ratios of sedimentary rocks. The dominant model used to interpret this record as a proxy for ancient atmospheric CO2 is based on carbon isotope fractionations of modern photoautotrophs, and longstanding questions remain about how their evolution might have impacted the record. Therefore, we measured both biomass (εp) and enzymatic (εRubisco) carbon isotope fractionations of a cyanobacterial strain (Synechococcus elongatus PCC 7942) solely expressing a putative ancestral Form 1B rubisco dating to ≫1Ga. This strain, nicknamed ANC, grows in ambient pCO2 and displays larger εp values than WT, despite having a much smaller εRubisco (17.23 ± 0.61‰ vs. 25.18 ± 0.31‰, respectively). Surprisingly, ANC εp exceeded ANC εRubisco in all conditions tested, contradicting prevailing models of cyanobacterial carbon isotope fractionation. Such models can be rectified by introducing additional isotopic fractionation associated with powered inorganic carbon uptake mechanisms present in Cyanobacteria, but this amendment hinders the ability to accurately estimate historical pCO2 from geological data. Understanding the evolution of rubisco and the CO2 concentrating mechanism is therefore critical for interpreting the carbon isotope record, and fluctuations in the record may reflect the evolving efficiency of carbon fixing metabolisms in addition to changes in atmospheric CO2.

Mn‑carbonate deposition in a seafloor hydrothermal system (CLAM field, Iheya Ridge, Okinawa Trough): Insights from mineralogy, geochemistry and isotope studies

A seafloor hydrothermal system located at the Iheya Ridge (Okinawa Trough), named CLAM, deposits Mn‑carbonate chimneys that have no analogue found so far on the seafloor. The chimneys are composed of Mn-calcite and Ca-rhodochrosite. The crystallographic differences between these carbonates appear to control the rare earth elements (REE) partitioning between them that results in enrichment of the Ca-rhodochrosite in middle and heavy REE, and enrichment of the Mn-calcite in light REE. Chemistry of the CLAM hydrothermal fluids suggests: (1) low water/rock ratio of the hydrothermal system; (2) phase separation and dominance of low-chlorinity vapor phase; (3) sub-seafloor formation of Na-rich alteration minerals during fluid/rock reactions; (4) removal of some elements from the seawater to the host rocks during the seawater/rock interaction; (5) high Mn/Ca ratio of the basement rocks is responsible for the high Mn concentration in the hydrothermal fluids. C-O-isotope compositions of the CLAM Mn‑carbonates suggest they precipitated through binary mixing of end-member hydrothermal fluid and seawater accompanied by progressive degassing and cooling of the fluid. Mn-calcite precipitated from almost pure end-member hydrothermal fluid, whereas Ca-rhodochrosite precipitated from seawater-dominated vent fluid. Mg-isotope fractionation during Mn‑carbonate precipitation is assumed to depend on carbonate growth conditions and resulting carbonate mineralogy. S-isotope composition of the CLAM Mn‑carbonates suggests that the Ca-rhodochrosite precipitated in oxic conditions through rapid mixing of hydrothermal fluid and seawater, whereas the Mn-calcite precipitated in reduced conditions (thermochemical or microbial sulfate reduction) through slow mixing of hydrothermal fluid and seawater. Sr-isotope composition of the CLAM hydrothermal fluids is close to that of Okinawa Trough deep seawater. In contrast, Sr-isotopes in the CLAM Mn‑carbonates are more variable, indicating that Sr was derived from seawater, local lavas and sediments. Nd-isotope composition of the Mn‑carbonates indicates that Nd was derived from the local lavas and sediments. Pb in the majority of the CLAM Mn‑carbonates is of sedimentary origin (Pb isotope data), but involvement of anthropogenic Pb in the hydrothermal system is inferred for some Mn-calcite samples. Stability phase diagram modeling coupled with C-O-S-Sr-isotope data suggest that in the CLAM vent fluid the rhodochrosite is stable in a narrow Eh-pH range (6 < pH < 10; Eh > 0) and in a wide range of [Mn] and [Ca] activities, whereas calcite precipitates from a close to the end-member hydrothermal fluid in reduced conditions (Eh < 0).

Evolution of the Pristane/Phytane Ratio with Maturity of Lacustrine Source Rocks: Implications of a Thermal Simulation Experiment

The pristane/phytane ratio (Pr/Ph) of source rocks has been extensively used for the identification of paleosedimentary environments and oil–source rock correlations. However, the influences of maturity on the Pr/Ph ratio are not fully understood, which largely limits the applicability of this parameter under geological conditions. Based on a thermal simulation experiment performed on typical lacustrine source rock samples and systematic research on geological source rock samples, the evolution of the Pr/Ph ratio with increasing maturity was investigated in this study. The results indicate that the Pr/Ph ratios of the two source rock extracts progressively decrease with increasing maturity, and the decrease in this parameter is more significant for shallow lacustrine samples than for deep lacustrine samples. The Pr/Ph ratios of the two types of source rocks are different at the oil generation stage with EasyRo less than 1.02% but are similar at the oil cracking stage with EasyRo larger than 1.02%. The Pr and Ph of source rocks are contributed by both biogenic and thermal origins, which are largely determined by the sedimentary environment and maturity, respectively, and the contributions of the two origins are diverse at different maturity stages. The Pr/Ph ratio is mainly controlled by the sedimentary environment at the diagenetic stage, by both sedimentary and thermal maturity at the oil generation stage, and by thermal maturity at the oil cracking stage. The Pr/Ph ratio of source rocks can be used to identify sedimentary environments and oil–source rock correlations at diagenetic and oil generation stages.