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Warkite, Ca₂Sc₆Al₆O₂₀, a new mineral in carbonaceous chondrites and a key-stone phase in ultrarefractory inclusions from the solar nebula

Ma, Chi and Krot, Alexander N. and Beckett, John R. and Nagashima, Kazuhide and Tschauner, Oliver and Rossman, George R. and Simon, Steven B. and Bischoff, Addi (2020) Warkite, Ca₂Sc₆Al₆O₂₀, a new mineral in carbonaceous chondrites and a key-stone phase in ultrarefractory inclusions from the solar nebula. Geochimica et Cosmochimica Acta, 277 . pp. 52-86. ISSN 0016-7037. https://resolver.caltech.edu/CaltechAUTHORS:20200403-125805955

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Abstract

Warkite (IMA 2013-129) is a new Sc-rich ultrarefractory mineral in the rhönite group of the sapphirine supergroup. It has a P aenigmatite-type structure with a = 10.367 Å, b = 10.756 Å, c = 8.895 Å, α = 106°, β = 96°, γ = 125°, and Z = 2, and general formula of Ca₂ (Sc,Ti,Al,Mg,Zr)Al₆O₂₀. Warkite occurs as micrometer-sized crystals in eleven ultrarefractory Ca,Al-rich inclusions (UR CAIs) from the CM, CV, CO, and CH chondrites. In the CM, CO, and CV CAIs, warkite in the cores coexists with a Ti-rich oxide, either perovskite or kangite; the cores are generally mantled and rimmed by davisite and/or Sc-diopside. In the CH CAIs and one CO CAI, warkite in the cores coexists with perovskite and grossite; the cores are mantled by grossite ± gehlenite, and rimmed by low-Sc, Al-diopside. Therefore, there are two basic families of warkite-bearing inclusions, those containing Sc-rich clinopyroxene but no grossite and those containing grossite but no Sc-rich clinopyroxene. Scandian clinopyroxene in warkite-bearing CAIs generally formed by the reaction of warkite, which supplied most or all of the Sc, Ti, and Al, and a nebular gas that supplied much of the Ca and O and virtually all of the Mg and Si. The presence of Sc-rich clinopyroxenes may reflect exposure of some warkite-bearing CAIs to a dust-rich environment, which would enhance partial pressures of Si-, Mg-, and Ca-bearing species in the vapor and make it oxidizing relative to a dust-poor gas. Warkite in grossite-bearing inclusions is generally Ti³⁺-enriched relative to those in davisite-bearing inclusions, consistent with their formation in relatively dust-poor, more-reducing environments. Warkite compositions are sensitive to the presence or absence of spinel, melilite, Sc-rich clinopyroxenes, and grossite. Compositional variations of perovskite and warkite grains indicate a connection but, except for late-stage Fe exchange, they did not equilibrate with each other. The presence of at least two trends in Y-Sc among perovskites without corresponding trends in warkite suggests that at least some perovskite formed separately. Warkite-bearing CAIs from CM2s and CO3.0s are uniformly ¹⁶O-rich (Δ¹⁷O ∼ −23‰), whereas those from metamorphosed COs and CVs are isotopically heterogeneous: warkite, kangite, perovskite, melilite, and davisite are ¹⁶O-depleted to various degrees (Δ¹⁷O range from −22 to −2‰) relative to hibonite, spinel, and forsterite, all having ¹⁶O-rich compositions (Δ¹⁷O ∼  − 25 to −20‰). We infer that warkite-bearing CAIs originated in an ¹⁶O-rich nebular gas. Subsequently, CAIs from metamorphosed CVs and COs experienced O-isotope exchange with an ¹⁶O-depleted external reservoir, most likely aqueous fluids on the CV and CO chondrite parent asteroids; however, O-isotope exchange in the solar nebula cannot be excluded.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/j.gca.2020.03.002DOIArticle
ORCID:
AuthorORCID
Ma, Chi0000-0002-1828-7033
Krot, Alexander N.0000-0002-2278-8519
Tschauner, Oliver0000-0003-3364-8906
Rossman, George R.0000-0002-4571-6884
Additional Information:© 2020 Elsevier Ltd. Received 29 January 2019, Accepted 1 March 2020, Available online 1 April 2020. SEM, EBSD and EPMA analyses were carried out at the Caltech GPS Division Analytical Facility, which is supported, in part, by NSF Grants EAR-0318518 and DMR-0080065. The meteorite sections studied are from Caltech, the Smithsonian Institution’s National Museum of Natural History, the Field Museum, the Johnson Space Center, the University of Copenhagen and the University of Münster. SBS acknowledges NASA grant NNX17AI43G. SIMS measurements were supported by NASA grant NNX17AE22G (PI A.N. Krot). The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funders:
Funding AgencyGrant Number
NSFEAR-0318518
NSFDMR-0080065
NASANNX17AI43G
NASANNX17AE22G
Subject Keywords:Warkite; Ca2Sc6Al6O20; New mineral; Carbonaceous chondrites; Ultrarefractory inclusions
Record Number:CaltechAUTHORS:20200403-125805955
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20200403-125805955
Official Citation:Chi Ma, Alexander N. Krot, John R. Beckett, Kazuhide Nagashima, Oliver Tschauner, George R. Rossman, Steven B. Simon, Addi Bischoff, Warkite, Ca2Sc6Al6O20, a new mineral in carbonaceous chondrites and a key-stone phase in ultrarefractory inclusions from the solar nebula, Geochimica et Cosmochimica Acta, Volume 277, 2020, Pages 52-86, ISSN 0016-7037, https://doi.org/10.1016/j.gca.2020.03.002. (http://www.sciencedirect.com/science/article/pii/S0016703720301630)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:102330
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:03 Apr 2020 20:11
Last Modified:03 Apr 2020 20:11

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