Aqueous alteration without initial water: possibility of organic-induced hydration of anhydrous silicates in meteorite parent bodies

Abstract

Early evolution of Solar System small bodies proceeded through interactions of mineral and water. Melting of water ice accreted with mineral particles to the parent body results in the formation of secondary minerals, the so-called aqueous alteration. Formation of phyllosilicates from anhydrous silicates is a typical alteration effect recorded in primitive meteorites. In addition to mineral and water, organic matter could have been also a significant component in meteorite parent bodies. However, the role of organic matter in the alteration of silicates is not well understood. We conducted a heating experiment of anhydrous silicate (olivine) with a mixture of organic compounds which simulated primordial organic matter in the Solar System. Dissolution and precipitation features were confirmed on the olivine surface after heating at 300 °C for 10 days, and proto-phyllosilicates were formed in the precipitation area. Magnesite was also detected as concomitant mineral phase. These minerals could be the evidence of aqueous alteration and carbonation of olivine induced by water generated through decomposition of the organic compounds with hydroxy groups. Our result showed that the in situ formation of hydrated silicates through a mineral–organic interaction without the initial presence of water. It further implies that formation of phyllosilicates on the olivine surface in contact with organic matter can occur in meteorite parent bodies which formed inside the H2O snow line but accreted with organic matter, initially without water. Water formed through decomposition of organic matter could be one candidate for hydrous silicate formation, for example, in ordinary chondrites from S-type asteroids inside the H2O snow line. Although the origin of water in ordinary chondrites is under debate, water generation from organic matter may also explain the D-rich water in ordinary chondrites because primordial organic matter is known to be D rich.