Nitrogen and carbon concentrations and isotopic compositions of the silica clathrate melanophlogite

Abstract

The concentrations and isotopic compositions of N and C were obtained for five melanophlogite samples, ideal formula 46SiO 2 ·6(CO 2 ,N 2 )·2(CH 4 ,N 2 ), 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 N 2 and with δ 15 N air ranging from −6.1 to +5.7‰. The higher δ 15 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 δ 13 C VPDB of −42.9 to −8.7‰, obtained from analyses of the bulk C in samples with probable varying proportions of CO 2 and CH 4 in the melanophlogite cages. Although the lower δ 15 N values for the melanophlogites (−6.1 and −2.8‰) are near upper mantle values (−5 ± 2‰), the full range in N 2 δ 15 N can be explained by equilibration with NH 4 + in clay minerals bearing an organic-influenced N isotope signature, at temperatures of near 100 °C estimated for melanophlogite crystallization. The lower δ 13 C values (as low as –42.9‰) are suggestive of equilibration with carbonaceous matter (poorly recrystallized organic material) at high cage CH 4 :CO 2 , 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.