First Detection of Methane within Chromitites of an Archean-Paleoproterozoic Greenstone Belt in Brazil

Yuri De Melo Portella,Federica Zaccarini,Giuseppe Etiope

doi:10.3390/min9050256

Abstract

Abiotic methane is widely documented in seeps, springs and aquifers associated with mafic-ultramafic rocks in Phanerozoic ophiolites, peridotite massifs and intrusions worldwide. Chromitites in ophiolites, in particular, have been interpreted as the rocks potentially generating methane though CO2 hydrogenation. Here, we document, for the first time, the presence of methane within chromitites in South America. We analyzed, through milling and gas extraction, the content of gas occluded in Cedrolina chromitite samples, belonging to the Pilar de Goiás greenstone belt in Brazil. The chromitites display significant gas concentrations up to 0.31 µg CH4/grock and 2800 ppmv of hydrogen, while the host talc schist is devoid of gas. Stable C isotope composition of methane (δ13C from −30 to −39.2‰) and the absence of organic-matter rich metasediments in the region suggest an abiotic origin. Hydrogen and methane concentrations appear related to high-Cr chromite modal content and to the presence of Ni-sulfides/alloys, which are potential catalysts of CO2 hydrogenation at temperatures above 200 °C. Accessory ruthenium-bearing minerals occurring in the chromitites could also act as catalysts, even at lower temperatures. Geothermometry of chlorite found in the chromitites constrains serpentinization at ~250 °C, during lower greenschist facies retrometamorphism. Hydrogen could be autochthonous, and thus formed under similar temperature, which we hypothesize represents the upper limit for abiotic methane generation in the area (250 °C). The Cedrolina chromitites are the first example of CH4 occurrence in ultramafic rocks related to an Archean-Paleoproterozoic greenstone belt. This may imply that serpentinized Cr-rich chromitites could have been sources of methane for the early Earth’s atmosphere.

Highlights

The abiotic synthesis of methane (CH4 ) after serpentinization of ultramafic rocks has long been documented, often associated with considerable amounts of hydrogen (H2 ), both in hydrothermal mid-ocean ridge submarine environments and in continental serpentinization sites [1,2,3]
The Cedrolina chromitites are the first example of CH4 occurrence in ultramafic rocks related to an Archean-Paleoproterozoic greenstone belt
Mineralogical, stratigraphic and stable carbon isotopic data suggest an abiotic origin for CH4, similar to chromitites studied in Greece [22]

Summary

Introduction

The abiotic synthesis of methane (CH4 ) after serpentinization of ultramafic rocks has long been documented, often associated with considerable amounts of hydrogen (H2 ), both in hydrothermal mid-ocean ridge submarine environments and in continental serpentinization sites [1,2,3]. Serpentinization of peridotitic rocks is driven by seawater under relatively high temperatures (>200 ◦ C; e.g., [3,4]). Continental serpentinization is driven by meteoric water at lower temperatures, typically below 150 ◦ C [5,6]. Methane with a dominant abiotic origin has been discovered in Phanerozoic ophiolites, peridotite massifs and intrusions in at Minerals 2019, 9, 256; doi:10.3390/min9050256 www.mdpi.com/journal/minerals. Abiotic methanogenesis has implications on the origin of life [8], its potential occurrence on Mars and other planets [9,10,11], and in igneous reservoir rocks [2,7,12,13,14]

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Conclusion