Zeolite formation via aqueous alteration of calcium‐aluminum‐rich inclusions on the CR and CV parent asteroids

Abstract We describe here zeolites and zeolite‐like phases in Ca,Al‐inclusions (CAIs) from Kaidun microbreccia and Northwest Africa (NWA) 3118 (CV3) chondrite studied using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), secondary ion mass spectrometry (SIMS), Fourier transform infrared (FT‐IR) spectroscopy, and single‐crystal X‐ray diffraction (SCXRD). Large (~4–8 μm in size) fragments of a coarse‐grained igneous type B CAI from Kaidun consist of melilite (Åk 35–75 ), anorthite, magnesian spinel (<1 wt% FeO), and Al,Ti‐diopside (in wt%, TiO 2 4–18, Al 2 O 3 16–22); perovskite, hibonite, and Fe,Ni‐metal are minor. The CAI experienced aqueous alteration that resulted in replacement of melilite by the calcite + apophyllite [KCa 4 (Si 8 O 20 ) (OH,F)·8H 2 O] + zeolites [natrolite (Na 2 Al 2 Si 3 O 10 ·2H 2 O)] and phillipsite‐Na [(Na,K,Ca 0.5 ) 4–7 (Al 4–7 Si 12−9 O 32 )·12H 2 O] assemblage previously unreported in meteorites. Oxygen isotopic composition of calcite is similar to that in CR chondrites (Δ 17 O ~2‰, δ 18 O ~41‰). Phillipsite and natrolite have Δ 17 O close to that of calcite; the lack of proper SIMS standards for these minerals does not allow us to constrain their δ 18 O values. These data and textural observations suggest co‐precipitation of calcite and zeolites. Calcite–zeolite assemblages are commonly observed in terrestrial rocks that experienced aqueous alteration in the presence of low‐temperature alkaline‐ and carbonate‐rich solutions. We suggest that aqueous alteration and formation of calcite–zeolite–apophyllite assemblage in Kaidun CAI occurred in fractures, cracks, and veins from volatile‐rich, alkaline hydrothermal fluids (pH = 8–10) at ~20–150°C. This alteration has little effect on oxygen and aluminum–magnesium isotope systematics of primary minerals in the Kaidun CAI: melilite, spinel, pyroxene, and anorthite have solar‐like oxygen isotope compositions: Δ 17 O = −22 to −25‰ and show resolvable excess of radiogenic 26 Mg corresponding to the inferred initial 26 Al/ 27 Al ratio [( 26 Al/ 27 Al) 0 ] of (4.97 ± 0.11) × 10 −5 (MSWD = 0.57; δ 26 Mg* 0 = 0.04 ± 0.05‰). Type C CAI from NWA 3118 consists of melilite (Åk 18–56 , 0.1–0.3 wt% Na 2 O), anorthite, Al,Ti‐diopside (in wt%, TiO 2 3–5, Al 2 O 3 16–24), and spinel (up to 3.3 wt% FeO). Melilite and anorthite in the peripheral part of the CAI are replaced by nepheline, sodalite, Na‐bearing plagioclase, and hedenbergite. Melilite in the CAI core is replaced by a hydrated silica‐rich amorphous phase, which may have been originally a zeolite that subsequently experienced structural and compositional changes during thermal metamorphism in the CV parent asteroid.