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

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.

Micro‐FTIR Spectroscopy of Lunar Pyroclastic and Impact Glasses as a New Diagnostic Tool to Discern Them

“…It was carbon‐coated and then investigated using a series of in situ analytical techniques. The glass fragments (they usually have a smooth surface and do not show the texture of mineral fragments) in NWA 7948 were recognized using a high‐resolution (the magnification is ~500 times) Back‐scattered electron (BSE) image collected with a FEI Scios dual‐beam focused ion beam/scanning electron microscope (SEM) at the Institute of Geochemistry, Chinese Academy of Sciences (Zeng et al, ). The operating conditions were 15 kV accelerating voltage, 3.2 nA beam current, and 7–9 mm working distance.…”

Oldest Immiscible Silica‐rich Melt on the Moon Recorded in a ~4.38 Ga Zircon

“…Back-scattered electron (BSE) images were collected by using a FEI Scios dual-beam focused ion beam/scanning electron microscope at the Institute of Geochemistry, Chinese Academy of Sciences. The operating conditions were 15-20 kV accelerating voltage, 1.6-3.2 nA beam current, and 7 mm working distance (Zeng et al, 2019). Cathodoluminescence (CL) image of zircon was collected using a Gatan MonoCL4 CL spectrometer attached to a JEOL JSM-7800F field emission scanning electron microscopy at the Institute of Geochemistry, Chinese Academy of Sciences.…”

Meteorite petrology versus genetics: Toward a unified binominal classification

“…The matrix‐rich chondrites seem overall less depleted in RI than chondrule‐rich chondrites, even in the NC superclan. The few known NCD chondrites seem to be all matrix‐rich; such are the K chondrites and GRV 020043 (38 vol% matrix), which may be an acapulcoite precursor (Li et al., 2018); also, the winonaite NWA 725, which really is a type 5 chondrite, only contains 5 vol% relict chondrules (Zeng et al., 2019) in contradistinction to H5 and H6 chondrites which contain 22 and 11 vol%, respectively (Rubin et al., 2001). Perhaps NC matrix‐rich chondrites accreted earlier than their chondrule‐rich counterparts and were thus more prone to at least partial differentiation because of more abundant aluminum‐26.…”

“…(1995) had proposed <5 cm −2 for ECs). This type 7 includes most acapulcoites and winonaites (Zeng et al., 2019). Some chondrites previously classified as type 7 that are merely extensions of type 6 (with relict chondrules; e.g., Irving et al., 2019) may be lumped back therewith or be given a transitional type 6/7.…”

“…Aside from lodranites, the type 8 thus defined includes, for example, Itqyi (which appears as EH7‐an in the MetBullDB), most brachinites (except the near‐chondritic Brachina which should be type 7; Keil, 2014) and ureilites. Other primitive achondrites showing evidence for significant silicate melt mobilization but not meeting the above threshold, such as transitional acapulcoites/lodranites or “enriched” acapulcoites (Keil & McCoy, 2018) or the Sahara 02029 and Tierra Blanca winonaites (Zeng et al., 2019) may be ranked as type 7/8. Watson 012, Tait et al.’s (2014) prototype for their type 7 redefinition, shows net gain of basaltic melt relative to H6 chondrites and might likewise be ranked as H7/8.…”