Oxygen and Al‐Mg isotopic constraints on cooling rate and age of partial melting of an Allende Type B CAI, Golfball

Abstract

AbstractCoarse‐grained, igneous Ca‐Al‐rich inclusions (CAIs) in CV chondrites formed through multiple melting events. We conducted in situ O‐isotope analysis and Al‐Mg systematics by secondary ion mass spectrometry of relict and overgrown minerals from a partial melting event in an Allende Type B CAI, Golfball. Golfball has a Type B CAI bulk composition and a unique structure: a fassaite‐rich mantle enclosing a melilite‐rich core. Many of the blocky melilite crystals in the core have irregularly shaped, Al‐rich (Åk5–15) cores enclosed in strongly zoned (Åk30–70) overgrowths. Since the Al‐rich melilite grains could not have formed from a melt of Golfball, they are interpreted as relict grains that survived later melting events. The O‐isotopic compositions of the blocky melilite crystals plot along the carbonaceous chondrite anhydrous mineral line, ranging between Δ17O ~ −14‰ and −5‰. The Al‐rich relict melilite grains and their overgrowths exhibit the same O‐isotopic compositions, while the O‐isotopic compositions are varied spatially among melilites. We found that the O‐isotopic compositions steeply change across several melilite crystals within few tens of micrometers, indicating the O‐isotopic compositions of the melt could not have been homogenized during the partial melting in that scale. According to the time scale of O self‐diffusivity in the melt, the cooling rate of the partial melting event is calculated to be >6 × 104 K h−1. Al‐Mg isotope data for core minerals plot on a straight line on an Al‐Mg evolution diagram. A mineral isochron for Golfball gives initial 26Al/27Al of (4.42 ± 0.20) × 10–5 and initial δ26Mg* of −0.035 ± 0.050‰. The chemical and O‐isotopic compositions of melilite and those initial values imply that its precursor consisted of fluffy Type A and/or fine‐grained CAIs. The partial melting event for Golfball may have occurred in very short order after the precursor formation.