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

1Noriyuki Kawasaki,2Shoichi Itoh,3Naoya Sakamoto,4Steven B. Simon,5Daiki Yamamoto,1,3Hisayoshi Yurimoto
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13701]
1Department of Natural History Sciences, Hokkaido University, Sapporo, 060-0810 Japan
2Department of Earth and Planetary Sciences, Kyoto University, Kyoto, 606-8502 Japan
3Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo, 001-0021 Japan
4Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, 87131 USA
5Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, 252-5210 Japan
Published by arrangement with John Wiley & Sons

Coarse-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.

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