A geochemical study of the winonaites: Evidence for limited partial melting and constraints on the precursor composition

Abstract

The winonaites are primitive achondrites which are associated with the IAB iron meteorites. Textural evidence implies heating to at least the Fe, Ni-FeS cotectic, but previous geochemical studies are ambiguous about the extent of silicate melting in these samples. Oxygen isotope evidence indicates that the precursor material may be related to the carbonaceous chondrites. Here we analysed a suite of winonaites for modal mineralogy and bulk major- and trace-element chemistry in order to assess the extent of thermal processing as well as constrain the precursor composition of the winonaite -IAB parent asteroid. Modal mineralogy and geochemical data are presented for eight winonaites. Textural analysis reveals that, for our sub-set of samples, all except the most primitive winonaite (Northwest Africa 1463) reached the Fe, Ni-FeS cotectic. However, only one (Tierra Blanca) shows geochemical evidence for silicate melting processes. Tierra Blanca is interpreted as a residue of small-degree silicate melting. Our sample of Winona shows geochemical evidence for extensive terrestrial weathering. All other winonaites studied here (Fortuna, Queen Alexander Range 94535, Hammadah al Hamra 193, Pontlyfni and NWA 1463) have chondritic major-element ratios and flat CI-normalised bulk rare-earth element patterns, suggesting that most of the winonaites did not reach the silicate melting temperature. The majority of winonaites were therefore heated to a narrow temperature range of between ~1220 (the Fe, Ni-FeS cotectic temperature) and ~1370 K (the basaltic partial melting temperature). Silicate inclusions in the IAB irons demonstrate partial melting did occur in some parts of the parent body (Ruzicka and Hutson, 2010), thereby implying heterogeneous heat distribution within this asteroid. Together, this indicates that melting was the result of internal heating by short-lived radionuclides. The brecciated nature of the winonaites suggests that the parent body was later disrupted by a catastrophic impact, which allowed the preservation of the largely unmelted winonaites. Despite major-element similarities to both ordinary and enstatite chondrites, trace-element analysis suggests the winonaite parent body had a arbonaceous chondrite-like precursor composition. The parent body of the winonaites was volatile -depleted relative to CI, but enriched compared to the other carbonaceous classes. The closest match are the CM chondrites, however, the specific precursor is not sampled in current meteorite collections

Late metal–silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling

“…Textures in both the IAB irons and winonaites indicate the presence of molten metal, which implies that large portions of the parent body reached at least the cotectic melting point for the Fe, Ni-FeS system (∼1220-1261 K; Kubaschewski, 1982;Kullerud, 1963) and sets a minimum tem- perature boundary. Additionally, the majority of the body reached a maximum temperature of ∼1470 K, based on peak temperatures obtained from both silicate clasts and winonaite meteorites (Benedix et al, 1998(Benedix et al, , 2000 and winonaite trace-element geochemistry (Hunt et al, 2017a). A body in this temperature range cannot fully differentiate, thus allowing for the preservation of the range of compositions observed in IAB irons and the winonaites.…”

A review of the chondrite–achondrite transition, and a metamorphic facies series for equilibrated primitive stony meteorites

“…In contrast, most winonaites appear not to preserve geochemical evidence for melt loss or gain (cf. Hunt et al 2017). Ratios of chalcophile and siderophile elements can be used to show fractionation of partially melted metal–troilite assemblages, in that the residue is enriched in siderophile elements (Mittlefehldt 2014; Dhaliwal et al 2017).…”

Siderophile element characteristics of acapulcoite–lodranites and winonaites: Implications for the early differentiation processes of their parent bodies

“…Hunt et al. () compared bulk chemical compositions of winonaites and several chondrite groups, and summarized that CM chondrite was the most similar in bulk chemical composition. Because there are not enough data of detailed chemical compositions of CM chondrite metals and aqueous alteration possibly changed chemical compositions of CM metals from its original compositions, we could not verify if CM chondrites and winonaites are similar or not from our data.…”

“…However, we are concerned that Hunt et al. () focused on “normal” winonaites and only referred major element compositions of relict chondrule‐bearing winonaites in their comparison.…”