The layered structure model for winonaite parent asteroid implicated by textural and mineralogical diversity

Xiaojia Zeng,Shijie Li,Yang Li,Shijie Wang,Yingli Shang,Xiongyao Li

doi:10.1186/s40623-019-1015-9

Abstract

The winonaites are primitive achondrites that are associated with IAB iron meteorites. They provide valuable insights into differentiation processes on asteroids in the early Solar System. However, there is still little understanding of the lithological diversity as well as the structure of the winonaite parent asteroid. In this work, we report the petrologic texture and mineralogy of a suite of winonaites (i.e., Northwest Africa (NWA) 725, NWA 6448, NWA 4024, Grove Mountains (GRV) 022890, GRV 021663, and Sahara (SAH) 02029) that exhibit a wide diversity of petrographic textures from primitive chondritic texture to coarse-grained equigranular texture. In particular, we recognized an unusual winonaite (SAH 02029) with a distinctive mineralogy and mineral chemistry (e.g., depleted in troilite, plagioclase contains melt inclusions, high plagioclase An values, and LREE-depleted clinopyroxene). The petrological and mineralogical features of SAH 02029 indicate that this meteorite has undergone silicate partial melting and may represent the residue of ~ 5–10 vol% partial melting. The textural and mineralogical diversity among winonaites suggests that the winonaite-IAB parent asteroid would have formed a four-layered structure during its evolution history: (1) surface layer consisting of precursor chondritic materials; (2) subsurface layer composed of diverse lithologies that experienced limited metamorphism and FeNi–FeS partial melting; (3) deep residues of silicate partial melting; and (4) interior layer consisting of incomplete differentiation metal pools. This conclusion enables us to establish constraints on the evolution history of winonaite-IAB parent body.

Highlights

The primitive achondrites can provide a glimpse into the early differentiation and initial partial melting processes of chondritic planetesimals in the early Solar System (e.g., Prinz et al 1980; McCoy et al 1996; Benedix et al 1998; Goodrich et al 2011)
Petrographic texture The studied six winonaites (i.e., Northwest Africa (NWA) 725, NWA 6448, Grove Mountains (GRV) 022890, GRV 021663, NWA 4024, and SAH 02029) exhibit a wide diversity of petrographic textures ranging from a chondritic texture, to fine- and coarse-grained
For the studied SAH 02029, it is lithologically different from other reported partial melt residues, because this meteorite shows unusual petrologic textures and mineral chemistry (e.g., Ca-rich plagioclase and clinopyroxene depleted in light rare earth element (LREE)) (Figs. 3a–d, 5c, and 6b; Benedix et al 1998, 2000)

Summary

Introduction

The primitive achondrites (e.g., acapulcoites, lodranites, brachinites, and winonaites) can provide a glimpse into the early differentiation and initial partial melting processes of chondritic planetesimals in the early Solar System (e.g., Prinz et al 1980; McCoy et al 1996; Benedix et al 1998; Goodrich et al 2011). The winonaite group is discriminated from all of other primitive achondrite groups based on its mineralogy, mineral composition, and importantly the oxygen isotopic composition (Graham et al 1977; Prinz et al 1980; Kimura and Tsuchiyama 1992; Clayton and Mayeda 1996). The winonaites are characterized as they are highly heterogeneous in grain size, petrologic texture, and modal mineralogy (e.g., Benedix et al 1998; Floss et al 2007; Li et al 2011; Hunt et al 2017). Such diverse lithologies suggest that complex and varied processes have acted on the winonaite parent asteroid (Benedix et al 1998; Floss et al 2007). Investigations on the textural and mineralogical variation of winonaite achondrites are limited and only about a dozen samples have been studied in detail

Objectives

Discussion

Conclusion