Abiotic formation of condensed carbonaceous matter in the hydrating oceanic crust

Marie Catherine Sforna,Daniele Malferrari,Daniele Brunelli,Valerio Pasini,Bénédicte Ménez,Céline Pisapia

doi:10.1038/s41467-018-07385-6

Abstract

Thermodynamic modeling has recently suggested that condensed carbonaceous matter should be the dominant product of abiotic organic synthesis during serpentinization, although it has not yet been described in natural serpentinites. Here we report evidence for three distinct types of abiotic condensed carbonaceous matter in paragenetic equilibrium with low-temperature mineralogical assemblages hosted by magma-impregnated, mantle-derived, serpentinites of the Ligurian Tethyan ophiolite. The first type coats hydroandraditic garnets in bastitized pyroxenes and bears mainly aliphatic chains. The second type forms small aggregates (~2 µm) associated with the alteration rims of spinel and plagioclase. The third type appears as large aggregates (~100–200 µm), bearing aromatic carbon and short aliphatic chains associated with saponite and hematite assemblage after plagioclase. These assemblages result from successive alteration at decreasing temperature and increasing oxygen fugacity. They affect a hybrid mafic-ultramafic paragenesis commonly occurring in the lower oceanic crust, pointing to ubiquity of the highlighted process during serpentinization.

Highlights

Thermodynamic modeling has recently suggested that condensed carbonaceous matter should be the dominant product of abiotic organic synthesis during serpentinization, it has not yet been described in natural serpentinites
The strict spatial association between the organic and mineralogical phases suggests that the different forms of Carbonaceous Matter (CCM) are in paragenetic equilibrium with secondary mineral phases
A CCM genesis after thermal degradation of pristine biogenic material can be excluded based on Raman spectra that do not display the expected broad bands of a Bastite Hadr-1 C-Fe-Mg

Summary

Introduction

Thermodynamic modeling has recently suggested that condensed carbonaceous matter should be the dominant product of abiotic organic synthesis during serpentinization, it has not yet been described in natural serpentinites. The third type appears as large aggregates (~100–200 μm), bearing aromatic carbon and short aliphatic chains associated with saponite and hematite assemblage after plagioclase. These assemblages result from successive alteration at decreasing temperature and increasing oxygen fugacity. The oceanic sequence is formed by mantle peridotites, once exposed at the seafloor, intruded by sparse gabbroic bodies and a discontinuous basaltic cover This association presents strong similarities with present-day non-volcanic passive margins and ultra-slow spreading ridge settings[20,21]. Mineral paragenesis and nature of the organic matter disseminated in these serpentinites confirm previous studies showing that this unit was not affected by metamorphic overprint during orogenetic exhumation and obduction[23]

Methods

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Conclusion