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The volatile concentrations of the martian mantle and martian magmas remain important questions due to their role in petrogenesis and planetary habitability. The sulfur and chlorine concentrations, and their spatial distribution, in clinopyroxenes from nakhlites MIL 03346, Nakhla, and NWA 998 were measured to provide insight into these volatiles in the parental melts and source regions of nakhlites, and to constrain the evolution of the nakhlite melts. Sulfur and chlorine in four clinopyroxene crystals from MIL 03346, four from Nakhla, and five from NWA 998 were measured in crystal cores and rims by synchrotron X-ray fluorescence using beamline I18 at the Diamond Light Source. Portions of two crystals from MIL 03346 and one from Nakhla were mapped for S and Cl; a few reconnaissance analyses of Cl and F in MIL 03346 and Nakhla were made by ion microprobe. Clinopyroxene cores in Nakhla and NWA 998 contain ∼10 ppm S, ∼10 ppm Cl and ∼74 ppm F (only Nakhla analyzed), whereas the cores of MIL 03346 contain ∼10 ppm S, ∼5 ppm Cl and ∼53 ppm F. Using the volatile concentrations in the cores combined with previously determined partition coefficients we calculate that these clinopyroxenes crystallized from evolved basaltic melts containing ∼500 ppm S, ∼500–1900 ppm Cl, and 160–420 ppm F. These evolved melts can be used to calculate primitive melts in equilibrium with martian peridotite and the concentrations of S, Cl and F in the mantle source region of the nakhlite melts. Depending upon the extent of melting (5–30%) necessary to produce the primary melts associated with nakhlites, our calculations indicate that the nakhlite source region has a S concentration between 20 (5% melting) to 120 ppm (30% melting), Cl between 16 and 97 ppm, and F between 14 and 48 ppm. These concentrations in the nakhlite magma source region are similar to previous estimates for the martian mantle; our calculated source region concentrations of F and Cl agree best with previous estimates if the martian mantle undergoes 10–20% melting to produce primary magmas that evolve to be parental to nakhlites. However, our maximum estimated sulfur concentration of the source (calculated for 30% melting) is near previous minimum estimates for the martian mantle, suggesting the possibility that the nakhlite source region is depleted in sulfur relative to much of Mars' mantle. Mapping the spatial distribution of volatiles in three clinopyroxene crystals demonstrates that S and Cl concentrations of the evolving melts changed significantly from the core to the rim, particularly those in MIL 03346. Increasing S and Cl concentrations between the core and rim of MIL 03346 crystals are attributed to incorporation of additional volatiles through assimilation, but the Nakhla crystal shows no such evidence. However, concentrations of Cl and S at some outer crystal rims of one MIL 03346 crystal decrease, most probably due to volatile degassing during the final stages of clinopyroxene growth. [ABSTRACT FROM AUTHOR]

The article presents a study which suggested that volcanic sulfur degassing from such activity could have caused repeated short-lived global drops in temperature, stressing the ecosystems long before the bolide impact delivered its final blow at the end of the Cretaceous. Topics include Western Ghats volcanostratigraphy and sample set, in situ determination of sulfur and fluorine in clinopyroxene, and sulfur concentrations calculated for equilibrium melts.

Magmatic systems are dominated by five volatiles, namely H2O, CO2, F, Cl, and S (the igneous quintet). Multiple studies have measured partitioning of four out of these five volatiles (H2O, CO2, F, and Cl) between nominally volatile-free minerals and melts, whereas the partitioning of sulfur is poorly known. To better constrain the behavior of sulfur in igneous systems we measured the partitioning of sulfur between clinopyroxene and silicate melts over a range of pressure, temperature, and melt composition from 0.8 to 1.2 GPa, 1000 to 1240 °C, and 49 to 66 wt% SiO2 (13 measurements). Additionally, we determined the crystal-melt partitioning of sulfur for plagioclase (6 measurements), orthopyroxene (2 measurements), amphibole (2 measurements), and olivine (1 measurement) in some of these same run products. Experiments were performed at high and low oxygen fugacities, where sulfur in the melt is expected to be dominantly present as an S6+ or an S2- species, respectively. When the partition coefficient is calculated as the total sulfur in the crystal divided by the total sulfur in the melt, the partition coefficient varies from 0.017 to 0.075 for clinopyroxene, from 0.036 to 0.229 for plagioclase, and is a maximum of 0.001 for olivine and of 0.003 for orthopyroxene. The variation in the total sulfur partition coefficient positively correlates with cation-oxygen bond lengths in the crystals; the measured partition coefficients increase in the order: olivine < orthopyroxene < clinopyroxene ≤ amphibole and plagioclase. At high oxygen fugacities in hydrous experiments, the clinopyroxene/melt partition coefficients for total sulfur are only approximately one-third of those measured in low oxygen fugacity, anhydrous experiments. However when the partition coefficient is calculated as total sulfur in the crystal divided by S2- in the melt, the clinopyroxene/melt partition coefficients for experiments with melts between ~51 and 66 wt% SiO2 can be described by a single mean value of 0.063 ± 0.010 (1σ standard deviation about the mean). These two observations support the hypothesis that sulfur, as S2-, replaces oxygen in the crystal structure. The results of hydrous experiments at low oxygen fugacity and anhydrous experiments at high oxygen fugacity suggest that oxygen fugacity has a greater effect on sulfur partitioning than water. Although the total sulfur clinopyroxenemelt partition coefficients are affected by the Mg/(Mg+Fe) ratio of the crystal, partition coefficients calculated using S2- in the melt display no clear dependence upon the Mg# of the clinopyroxene. Both the bulk and the S2- partition coefficients appear unaffected by IVAl in the clinopyroxene structure. No effect of anorthite content nor of iron concentration in the crystal was seen in the data for plagioclase-melt partitioning. The data obtained for orthopyroxene and olivine were too few to establish any trends. The partition coefficients of total sulfur and S2- between the crystals studied and silicate melts are typically lower than those of fluorine, higher than those of carbon, and similar to those of chlorine and hydrogen. These sulfur partition coefficients can be combined with analyses of volatiles in nominally volatile-free minerals and previously published partition coefficients of H2O, C, F, and Cl to constrain the concentration of the igneous quintet, the five major volatiles in magmatic systems. [ABSTRACT FROM AUTHOR]

Volcanic gases can have devastating impacts on Earth's environment and life. However, measurement of volatile elements in prehistoric magmas is challenging. We present a new method for determining sulfur contents in magmas using its concentrations in clinopyroxenes and an experimentally determined crystal-melt partition coefficient. Using this method we compared magmatic sulfur contents in the Paraná-Etendeka large igneous province (LIP; South America) and two other similarly sized LIPs, the Central Atlantic magmatic province (CAMP) and the Deccan Traps (India). We found that the CAMP and Deccan Traps, both associated with major extinction events, contained high magmatic sulfur concentrations, up to 1900 ppm. Conversely, the Paraná-Etendeka LIP, lacking significant environmental effects, had substantially lower magmatic sulfur concentrations, less than 800 ppm. [ABSTRACT FROM AUTHOR]