Petrographical and geochemical investigations of stannern group eucrites

Introduction: The howardite-eucrite-diogenite (HED) clan of meteorites represents a suite of mafic to ultramafic igneous rocks and breccias of them. They are linked together by O-isotope compositions [e.g., 1] and likely originate from the differentiated asteroid, 4 Vesta [2]. Radiogenic chronometers point to magmatic differentiation on the HED parent occurring within <10 Ma of solar system formation [3, 4], with the Ar-Ar record showing a more extensive history of impact heating [5]. As such, these basaltic eucrites display diverse igneous to metamorphic textures engendered by endogenous crustal processes and impacts [2, 6, 7]. They have fine to medium-grained textures with mineralogy dominantly consisting of plagioclase, pigeonite, and augite with minor to accessory phases including spinel, ilmenite, silica, troilite, Ca-phosphates, olivine, and Nipoor metal [2]. Chemically, basaltic eucrites are divided into subgroups which are typically separated based on their Mg# (=100*molar Mg/(Fe+Mg)), Ti abundance, and incompatible trace element (ITE) abundances [2, 8]. Of these groups, the Stannern group (SG) eucrites are enriched in ITEs relative to the more abundant main group (MG) despite yielding similar Mg#s (42-35) [2, 3, 8, 9]. The purpose of this study is to constrain the petrogenesis of basaltic eucrites to better understand crustal evolution on early-formed planetary bodies. To achieve this, we conducted a detailed comparative investigation of the petrology and geochemistry of representative MG and SG samples. Samples and Methods: This study is part of a collaborative project investigating MG and SG eucrites, including GRO 95533 and HOW 88401 (both MG), and LEW 88010, PCA 82501 and PCA 91179 (all SG). Here, our primary focus was on samples GRO 95533, PCA 91179 and HOW 88401. Our typical protocol was analysis of a thin section of each sample using optical microscopy, X-ray mapping using a scanning electron microscope (SEM), and determination of mineral phase composition using electron microprobe analysis (EPMA). Results: General Petrography. Within this section, only eucrites containing potassium feldspar (K-spar) confirmed by EPMA, will be discussed. Our section of GRO 95533 has a discrete internal boundary separating distinct textural domains. The same phases are present in both domains, but they exhibit textural and chemical differences. We will refer to one domain as GRO 95533 N, and the other as GRO 95533 S. Main group GRO 95533 (N & S) and HOW 88401 eucrites display sub-ophitic to ophitic magmatic textures, but also exhibit evidence for subsolidus mineral growth and have areas of fragmental texture. Pyroxene grains exhibit exsolution of high-Ca pyroxene from lowCa pyroxene with lamellae of variable widths, thinning as grains transition from core pigeonite to rim augite. Plagioclase grains range from anhedral to nearly euhedral in GRO 95533 N, whereas they are anhedral to subhedral in HOW 88401 and GRO 95533 S. In all samples Carlsbad and albite twinning are prevalent, but on a few grains no twinning is visible. Patchy zoning of plagioclase was found in GRO 95533 N with interior areas of grains more sodic (blue in Fig 1) and inclusion-free, while some rims are more calcic (green in Fig 1) and inclusion rich.