Heterogeneous Oxygen Isotopic Composition of a Complex Wark-lovering Rim and the Margin of a Refractory Inclusion from Leoville

Introduction: Wark-Lovering (WL) rims [1] surrounding many refractory inclusions represent marker events in the early evolution of the Solar System in which many inclusions were exposed to changes in pressure [2], temperature [3], and isotopic reservoirs [4-7]. The effects of these events can be complex, not only producing mineralogical variability of WL rims [2], but also leading to mineralogical [8-10] and isotopic [7, 11, 12] changes within inclusion interiors. Extreme oxygen isotopic heterogeneity measured in CAIs has been explained by mixing between distinct oxygen gas reservoirs in the nebula [13]. Some WL rims contain relatively simple mineral layering and/or are isotopically homogeneous [14, 15]. As part of a larger effort to document and understand the modifications observed in some CAIs, an inclusion (L6) with a complex WL rim from Leoville, a member of the reduced CV3 subgroup was studied. Initial study of the textures and mineral chemistry was presented by [16]. Here we present NanoSIMS oxygen isotopic measurements to complement these petrologic observations. Sample: L6 is a pristine compact Type A (CTA) inclusion with clusters of spinel (typically euhedral, ~50 μm across) and fassaite grains (anhedral) enclosed in visually unaltered melilite that exhibits normal compositional zoning (Åk3-30) within the margin (~200 μm) of the inclusion [16]. At the edge of the inclusion is a nearly continuous and often thick (up to 100 μm) layer of spinel intergrown with the melilite interior. Outside of the main spinel layer are additional layers composed of pyroxene ±melilite. Based on petrography and mineral chemistry [16], five generations of pyroxene were identified in the WL rim. In the section studied herein, the innermost is Ti-bearing (Ti-pyx) and appears intergrown with the “main” spinel layer. This Ti-pyx (≤10 μm) becomes less Ti-rich outward and is intergrown with a mixed layer of fine-grained melilite and spinel of varying thickness. Locally the fine-grained melilite layer has a range of grain sizes and can be found in contact with the “main” spinel layer. Coarser melilite tends to enclose less spinel. In places, a second Ti-pyxmelilite layer exists with a distinct Ti-pyx composition from the first [16]. Finally, the entire inclusion and complex rim is surrounded by a uniform layer of diopside (~15 μm), in some places in direct contact with Ti-pyx from an inner layer and sometimes directly overlying the irregular fine-grained mixed melilitespinel layer (Fig. 1).