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

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.

Testing Trace-Element Distribution and the Zr-Based Thermometry of Accessory Rutile from Chromitite

“…The Cedrolina chromitite body is located within the lower stratigraphic units of the greenstone belt, where metakomatiites and metabasalts (talc-chlorite-schist, amphibolite and amphibole-schists) predominate over inter-bedded metasediments (metacherts, BIF). The chromitite has a tabular geometry of about 230 m × 100 m and 1-2.4 m thickness, being in contact with micaschists to the SE, and amphibole rich BIF to the NW [13,51]. At the scale of hand samples, the chromitite consists of rounded nodules, up to 1 cm in diameter, embedded in a matrix of chlorite and talc, with a mean modal composition of 60% chromite, 25% talc, and 15% Cr-rich chlorite.…”

“…Single crystals of chromite, variable in size from few microns up to 1 mm and, eventually, with hexagonal and octahedral shapes, have been also described [13,51], but are not shown in the pictures. According to these authors, the Cedrolina chromitite suffered intense metasomatism related to hydrothermal fluids emanated by albite-granite intrusions, which generated a complex suite of exotic accessory minerals such as monazite-Ce [52], uraninite, thorianite, and zircon.…”

Petrological factors controlling abiotic methane formation in mafic-ultramafic rocks

“…-Alkaline spring in Ranau where preliminary results showed that its water contains CH4 of potentially abiotic origin -Despite that no chromitites were found, the majority of Ranau peridotites are Lazar et al, 2012, Foustoukos et al, 2015, Portella et al, 2019. Moreover, awaruite is known to result from the breakdown of pentlandite from an alkaline Aluid (Garuti & Zaccarini 1997).…”