ALBERT

Description: Variability in the sulfur isotopic composition in sediments can reflect atmospheric, geologic and biological processes. Evidence for ancient fluvio-lacustrine environments at Gale crater on Mars and a lack of efficient crustal recycling mechanisms on the planet suggests a surface environment that was once warm enough to allow the presence of liquid water, at least for discrete periods of time, and implies a greenhouse effect that may have been influenced by sulfur-bearing volcanic gases. Here we report in situ analyses of the sulfur isotopic compositions of SO2 volatilized from ten sediment samples acquired by NASA's Curiosity rover along a 13 km traverse of Gale crater. We find large variations in sulfur isotopic composition that exceed those measured for Martian meteorites and show both depletion and enrichment in 34S. Measured values of Δ34S range from - 47 ± 14‰ to 28 ± 7‰, similar to the range typical of terrestrial environments. Although limited geochronological constraints on the stratigraphy traversed by Curiosity are available, we propose that the observed sulfur isotopic signatures at Gale crater can be explained by equilibrium fractionation between sulfate and sulfide in an impact-driven hydrothermal system and atmospheric processing of sulfur-bearing gases during transient warm periods. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

Description: Clastic sedimentary rocks, deposited on eastern North America in response to the Taconian Orogeny, commonly have Sm/Nd isotope relationships indicating substantial isotope disturbance near or subsequent to the time of sedimentation that may be associated with severe depletion in light rare earth elements (LREE). Affected units [Normanskill Formation (Austin Glen and Pawlet Members), Frankfort Formation and Perry Mountain Formation] are widely separated both geographically (western New York to western Maine) and stratigraphically (Middle Ordovician to Silurian). A model is proposed for the most likely explanation of the observed REE and Sm/Nd isotope relationships involving a two-stage process. In the first stage, REE are redistributed on a mineralogical scale (dissolution/precipitation on a sample scale) often with the involvement of REE-enriched trace phases such as apatite and monazite. This stage typically takes place during diagenesis but may also take place later during metamorphism and/or recent weathering, and results in isotope re-equilibration on a sample scale. The second stage occurs when one or more of these phases is redissolved and REE are transported on large advective scales. Where LREE-enriched phases are involved, this gives rise to LREE depletion in whole rocks. The timing of this second stage cannot be constrained from Sm/Nd isotope data and may take place at any time subsequent to the isotope re-equilibration. Such complex histories of REE redistribution may result in serious errors in estimating Nd model ages but not in estimating the Nd isotope composition at the age of sedimentation. Thus, Sm/Nd ratios even of unmetamorphosed sedimentary rocks have to be carefully evaluated before the calculation of depleted mantle model ages for the provenance.