Abstract

The concentrations and isotopic compositions of N and C were obtained for five melanophlogite samples, ideal formula 46SiO2·6(CO2,N2)·2(CH4,N2), from various localities in Italy and California, U.S.A. The melanophlogite crystals enclathrate 132 to 1674 ppm N presumed to be speciated as molecular N2 and with δ15Nair ranging from −6.1 to +5.7‰. The higher δ15 N values overlap those for organic/sedimentary N, the latter largely with values between 0 and +10‰. The samples also contain 1.2 to 2.9 wt% total C, with δ13CVPDB of −42.9 to −8.7‰, obtained from analyses of the bulk C in samples with probable varying proportions of CO2 and CH4 in the melanophlogite cages. Although the lower δ15N values for the melanophlogites (−6.1 and −2.8‰) are near upper mantle values (−5 ± 2‰), the full range in N2 δ15N can be explained by equilibration with NH4+ in clay minerals bearing an organic-influenced N isotope signature, at temperatures of near 100 °C estimated for melanophlogite crystallization. The lower δ13C values (as low as –42.9‰) are suggestive of equilibration with carbonaceous matter (poorly recrystallized organic material) at high cage CH4:CO2, perhaps representing lower oxygen fugacities. The growing number of reports of melanophlogite at terrestrial localities, and its occurrences in organic-rich settings, makes this clathrate mineral an intriguing candidate for preserving records of past surface or near-surface biogeochemical cycling on Earth and perhaps on Mars.

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