¹M. A. Ivanova, ^2,3S. N. Britvin, ⁴A. N. Krot, ⁴K. Nagashima, ⁵N. G. Zinovieva
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.70234]
¹Vernadsky Institute of Geochemistry of the Russian Academy of Sciences, Moscow, Russia
²Saint-Petersburg State University, St. Petersburg, Russia
³Kola Science Center, Russian Academy of Sciences, Apatity, Russia
⁴Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawaii, USA
⁵M. V. Lomonosov Moscow State University, Moscow, Russia
Published by arrangement with John Wiley & Sons

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%, TiO₂ 4–18, Al₂O₃ 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 [KCa₄ (Si₈O₂₀) (OH,F)·8H₂O] + zeolites [natrolite (Na₂Al₂Si₃O₁₀·2H₂O)] and phillipsite-Na [(Na,K,Ca_0.5)_4–7 (Al_4–7Si₁₂₋₉O₃₂)·12H₂O] assemblage previously unreported in meteorites. Oxygen isotopic composition of calcite is similar to that in CR chondrites (Δ¹⁷O ~2‰, δ¹⁸O ~41‰). Phillipsite and natrolite have Δ¹⁷O close to that of calcite; the lack of proper SIMS standards for these minerals does not allow us to constrain their δ¹⁸O 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: Δ¹⁷O = −22 to −25‰ and show resolvable excess of radiogenic ²⁶Mg corresponding to the inferred initial ²⁶Al/²⁷Al ratio [(²⁶Al/²⁷Al)₀] of (4.97 ± 0.11) × 10⁻⁵ (MSWD = 0.57; δ²⁶Mg*₀ = 0.04 ± 0.05‰). Type C CAI from NWA 3118 consists of melilite (Åk_18–56, 0.1–0.3 wt% Na₂O), anorthite, Al,Ti-diopside (in wt%, TiO₂ 3–5, Al₂O₃ 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.