Aqueous alteration of chondrules from the Murchison CM carbonaceous chondrite: Replacement, pore filling, and the genesis of polyhedral serpentine

M R Lee,P Lindgren

doi:10.1111/maps.12644

Abstract

AbstractForsterite and clinoenstatite in type IAB chondrules from the Murchison CM carbonaceous chondrite have been partially serpentinized, and the mechanisms of their alteration reveal crystallographic and microstructural controls on the reaction of silicate minerals with parent body aqueous solutions. Grains of forsterite were altered in two stages. Narrow veinlets of Fe‐rich serpentine formed first and by the filling of sheet pores. Most of these pores were oriented parallel to (010) and (001) and had been produced by earlier fracturing and/or congruent dissolution. In the second stage, the subset of veinlets that were oriented parallel to (001) was widened accompanying the replacement of forsterite by Mg‐Fe serpentine. This reaction proceeded most rapidly parallel to [001], and crystallographic controls on the trajectory of retreating vein walls created fine‐scale serrations. Murchison clinoenstatite grains have a skeletal appearance due to the presence of abundant veinlets and patches of phyllosilicate. Two alteration stages can again be recognized, with initial water–mineral interaction producing tochilinite‐rich veinlets by the filling of (001)‐parallel contraction cracks. Pores then formed by congruent dissolution that was guided principally by orthopyroxene lamellae, and they were subsequently filled by submicrometer‐sized crystals of polyhedral serpentine. This finding that Murchison forsterite and clinoenstatite grains have been altered demonstrates that aqueous processing of magnesium silicate minerals started much earlier in CM parent body history than previously believed. Our results also show that the occurrence of polyhedral serpentine can be used to locate former pore spaces within the parent body.

Highlights

The occurrence within CM carbonaceous chondrite meteorites of minerals including phyllosilicates and carbonates is good evidence for the former presence of liquid water in the interiors of their parent body(ies) (e.g., DuFresne and Anders 1962; McSween 1979; Bunch and Chang 1980; Browning et al 1996)
The focus of this study is on the serpentinization of olivine and clinopyroxene within type IAB chondrules and chondrule fragments, and in these objects, the two minerals have close to end-member chemical compositions (Lovering and Le Maitre 1971; Fuchs et al 1973)
Grains of augite and orthopyroxene occur in type IAB chondrules, but as they are unaltered in the samples studied, these two minerals are not considered further

Summary

Introduction

The occurrence within CM carbonaceous chondrite meteorites of minerals including phyllosilicates and carbonates is good evidence for the former presence of liquid water in the interiors of their parent body(ies) (e.g., DuFresne and Anders 1962; McSween 1979; Bunch and Chang 1980; Browning et al 1996) These secondary minerals formed within a few million years of the earliest solar system solids (Endress et al 1996; Hutcheon et al 1999; Brearley and Hutcheon 2002; De Leuw et al 2009; Fujiya et al 2012; Lee et al 2012) and by the alteration of primary anhydrous constituents such as Fe, Ni metal and Fe-sulfides, silicate minerals, and amorphous materials (e.g., Bunch and Chang 1980; Barber 1981, 1985; Richardson 1981; Tomeoka and Buseck 1985; Zolensky et al 1993; Greenwood et al 1994; Lee and Greenwood 1994; Browning et al 1996; Chizmadia and Brearley 2008; Howard et al 2009, 2011; Velbel et al 2012, 2015). Clinopyroxene, and orthopyroxene were less reactive and so are present in all but the most highly aqueously processed CMs, which are classified as “CM1” (Zolensky et al 1997) or “CM2.0” (Rubin et al 2007).

Methods

Results

Discussion

Conclusion