Constraining the fluid history of a CO2 -H2 S reservoir: insights from stable isotopes, REE and fluid inclusion microthermometry

Reservoirs that host CO2‐H2S‐bearing gases provide a key insight into crustal redox reactions such as thermochemical sulfate reduction (TSR). Despite this, there remains a poor understanding of the extent, duration, and the factors limiting this process on a reservoir scale. Here we show how a combination of petrography, fluid inclusion, rare earth element (REE), and carbon (δ13C), oxygen (δ18O), and sulfur (δ34S) stable isotope data can disentangle the fluid history of the world's largest CO2 accumulation, the LaBarge Field in Wyoming, USA. The carbonate‐hosted LaBarge Field was charged with oil around 80 Ma ago, which together with nodular anhydrite represent the reactants for TSR. The nodules exhibit two distinct trends of evolution in δ13C with both δ34S and δ18O that may be coupled to two different processes. The first trend was interpreted to reflect the coupled dissolution of anhydrite and reduction to elemental sulfur and the oxidation of organic compounds and associated precipitation of calcite during TSR. In contrast, the second trend was interpreted to be the result of the hydrothermal CO2 influx after the cessation of TSR. In addition, mass balance calculations were performed to estimate an approximate TSR reaction duration of 80 ka and to identify the availability of organic compounds as the limiting factor of the TSR process. Such an approach provides a tool for the prediction of TSR occurrence elsewhere and advancing our understanding of crustal fluid interactions

Water in Martian Magmas: Clues from Light Lithophile Elements in Shergottite and Nakhlite Pyroxenes

There is abundant geomorphic evidence that Mars once had potentially significant amounts of water on its surface. Bulk martian meteorites are curiously dry, and hydrated minerals found in some of these rocks are also surprisingly low in water content. We look for evidence of pre-eruptive magmatic water by analyzing the abundances of Li, Be, and B, light lithophile elements that have proven useful in tracking water-magma interactions in terrestrial studies because of their solubility differences. We performed secondary ionization mass spectrometer (SIMS) analysis of these incompatible elements in pyroxenes of two nakhlites and two basaltic shergottites, with quite different results. In Nakhla and Lafayette, all three elements behave as incompatible elements, with increasing abundance with magma evolution from pyroxene cores to rims. In Shergotty and Zagami, Be increases, but both B and Li decrease from pyroxene cores to rims. From terrestrial studies, it is known that Be is virtually insoluble in aqueous hydrothermal fluids, whereas B and Li are quite soluble. We suggest, therefore, that the elemental decreases in the shergottite pyroxenes reflect dissolution and loss of B and Li in a hot, aqueous fluid exsolved from the magma. Consistent with our results, recent experimental work proposes that the shergottite parent magmas contained 1.8 wt% water (Dann et al., 2001). We suggest that the pyroxene cores grew at depth (\u3e4 km) where the water would remain dissolved in the magma. Once the magma began to ascend, the volatile component could gradually exsolve, removing the soluble species from the melt in the process. Upon eruption, the volatile component might then be lost through degassing, leaving a B- and Li-depleted magma to crystallize pyroxene rims and plagioclase. This magmatic water might have derived from the martian mantle or resulted from deep crustal contamination. If the water contents proposed for the shergottite magmas, and implied by our results, are typical of basaltic magmas on Mars, this mechanism could provide an efficient method of delivering substantial amounts of water to the martian surface at later times in martian history

High-precision oxygen isotope analysis of picogram samples reveals 2 μm gradients and slow diffusion in zircon

Ion microprobe analysis with a sub-micrometer diameter spot reveals a sharp, 2 μm gradient in oxygen isotope ratio proving that oxygen diffusion in zircon is slow even under prolonged high-grade metamorphism. The data are consistent with an oxygen diffusion coefficient of 10−23.5±1 cm2/s. Furthermore, this gradient is found in a zircon that contains clear textural evidence of recrystallization in nearby regions. This finding shows that through careful textural and chemical analysis, primary information can be extracted from a zircon that has also undergone partial recrystallization. The oxygen isotope ratios found in zircon have been used to infer magmatic and pre-magmatic histories, including the presence of liquid water on the surface of earliest Earth. Recently, these interpretations have been questioned with the assertion that zircon may not retain its primary oxygen isotope signature through metamorphism. The slow diffusion confirmed by these results supports interpretations that assume preservation of magmatic compositions

Geochemical Evidence for Magmatic Water Within Mars from Pyroxenes in the Shergotty Meteorite

Observations of martian surface morphology have been used to argue that an ancient ocean once existed on Mars. It has been thought that significant quantities of such water could have been supplied to the martian surface through volcanic outgassing, but this suggestion is contradicted by the low magmatic water content that is generally inferred from chemical analyses of igneous martian meteorites. Here, however, we report the distributions of trace elements within pyroxenes of the Shergotty meteorite—a basalt body ejected 175 million years ago from Mars—as well as hydrous and anhydrous crystallization experiments that, together, imply that water contents of pre-eruptive magma on Mars could have been up to 1.8%. We found that in the Shergotty meteorite, the inner cores of pyroxene minerals (which formed at depth in the martian crust) are enriched in soluble trace elements when compared to the outer rims (which crystallized on or near to the martian surface). This implies that water was present in pyroxenes at depth but was largely lost as pyroxenes were carried to the surface during magma ascent. We conclude that ascending magmas possibly delivered significant quantities of water to the martian surface in recent times, reconciling geologic and petrologic constraints on the outgassing history of Mars