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

Kristin E. Lazzeri, Gray Edward Bebout, Charles A. Geiger

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3 Citations (Scopus)

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 δ15N 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 CH4CO2, 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.

Original languageEnglish
Pages (from-to)686-689
Number of pages4
JournalAmerican Mineralogist
Volume102
Issue number3
DOIs
Publication statusPublished - Mar 1 2017
Externally publishedYes

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Keywords

  • biogeochemistry
  • carbon isotopes
  • Mars
  • mass spectrometry
  • Melanophlogite
  • microporous minerals
  • nitrogen isotopes
  • silica clathrate

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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