Formation mechanisms of ringwoodite: clues from the Martian meteorite Northwest Africa 8705

Ting Zhang,Xinyu Zou,Nian Wang,Lixin Gu,Mingming Zhang,Sen Hu,Xu Tang,Yangting Lin

doi:10.1186/s40623-021-01494-1

Abstract

Ringwoodite and wadsleyite are the high-pressure polymorphs of olivine, which are common in shocked meteorites. They are the major constituent minerals in the terrestrial mantle. NWA 8705, an olivine-phyric shergottite, was heavily shocked, producing shock-induced melt veins and pockets associated with four occurrences of ringwoodite: (1) the lamellae intergrown with the host olivine adjacent to a shock-induced melt pocket; (2) polycrystalline assemblages preserving the shapes and compositions of the pre-existing olivine within a shock-induced melt vein (60 μm in width); (3) the rod-like grains coexisting with wadsleyite and clinopyroxene within a shock-induced melt vein; (4) the microlite clusters embedded in silicate glass within a very thin shock-induced melt vein (20 μm in width). The first two occurrences of ringwoodite likely formed via solid-state transformation from olivine, supported by their morphological features and homogeneous compositions (Mg# 64–62) similar to the host olivine (Mg# 66–64). The third occurrence of ringwoodite might fractionally crystallize from the shock-induced melt, based on its heterogeneous and more FeO-enriched compositions (Mg# 76–51) than those of the coexisting wadsleyite (Mg# 77–67) and the host olivine (Mg# 66–64) of this meteorite. The coexistence of ringwoodite, wadsleyite, and clinopyroxene suggests a post-shock pressure of 14–16 GPa and a temperature of 1650–1750 °C. The fourth occurrence of ringwoodite with compositional variation (Mg# 72–58) likely crystallized from melt at 16–18 GPa and 1750–1850 °C. The presence of the four occurrences of ringwoodite was probably due to their very fast cooling rates in and/or adjacent to the thin shock-induced melt veins and small pockets. In addition, the higher Fa-contents of the host olivine (Fa35–39) in NWA 8705 than those in ordinary chondrites (Fa16–32) makes the olivine–ringwoodite transformation prolong to a lower pressure.

Highlights

Meteorites commonly experienced various degrees of shock metamorphism, and some of them contain shockinduced melts and numerous high-pressure minerals (e.g., Stöffler and Keil 1991; Binns et al 1969; Gillet et al 2000; Xie et al 2002; Langenhorst and Poirier 2000; Tschauner, 2014; Beck et al 2004; Tomioka and Miyahara 2017; and references therein)
We found four occurrences of ringwoodite in Northwest Africa (NWA) 8705, and conducted a comprehensive petrographic, mineralchemical and crystallographic analyses to clarify their formation mechanisms, constrain the post-shock P–T conditions, and to shed light on the impact history of the NWA 8705
There are four occurrences of ringwoodite: lamellae in the host olivine within or adjacent to shock-induced melt veins and pockets; polycrystalline ringwoodite olivine entrained in and adjacent to the shock-induced melt vein; rod-like ringwoodite coexisting with wadsleyite and pyroxene in the shock-induced melt vein matrix; the ringwoodite microlite clusters in a very thin shock-induced melt vein across the olivine

Summary

Introduction

Meteorites commonly experienced various degrees of shock metamorphism, and some of them contain shockinduced melts and numerous high-pressure minerals (e.g., Stöffler and Keil 1991; Binns et al 1969; Gillet et al 2000; Xie et al 2002; Langenhorst and Poirier 2000; Tschauner, 2014; Beck et al 2004; Tomioka and Miyahara 2017; and references therein). Previous studies of ringwoodite/wadsleyite in meteorites suggested that they were mainly produced via solid-state transformation from olivine and crystallization from shock-induced melts (Miyahara et al 2008; Walton and McCarthy 2017; Chen et al 2004). The first occurrence of ringwoodite crystallized from shock-induced melt (Xie et al 2006a; Fritz and Greshake 2009), whereas the other two occurrences were probably produced via solid-state phase transformation from olivine, including homogeneous intracrystalline nucleation throughout the olivine (Chen et al 1996), heterogeneous intracrystalline nucleation at defect sites (fractures and stacking faults) of the olivine (Kerschhofer et al 1996, 2000; Miyahara et al 2010; Chen et al 2006, 2007), and grain boundary nucleation and growth mechanism (Liu et al 1998; Kerschhofer et al 1998). Some polycrystalline ringwoodite rims surrounding the olivine show a Fe–Mg diffusion-controlled growth process (Walton and McCarthy 2017; Yin et al 2018; Pittarello et al 2015) with Fe preferentially partitioned to ringwoodite, leading to a higher FeO content of ringwoodite compared with the residual olivine

Results

Discussion

Conclusion