NWA 13784 (8,514 g)

86th Annual Meeting of the Meteoritical Society 2024 (LPI Contrib. No. 3036)             6430.pdf

 

REEVALUATING THE EXTENT OF METAMORPHISM IN THE “CV4” CHONDRITES T.L. Dunn1 and E.G. Alevy1 Department of Geology, Colby College, Waterville, ME 04901 (tldunn@colby.edu). Introduction: The Vigarano-like CV chondrites are a heterogeneous group of chondrites characterized by the presence of large chondrules (~45 vol%), refractory Ca-Al-rich inclusions and amoeboid olivine aggregates (~10 vol%) [1], and relatively fine-grained matrix with FeO-rich olivine [1,2]. Attempts to determine the petrologic subtypes of the CV chondrites have produced inconsistent results. Thermoluminescence sensitivity [3] and Cr2O3 content of olivine [4] indicate that CVs are petrologic subtype 3.3 or lower. However, Raman spectroscopy of organic materials suggests that CV chondrites have experiences a wider range of metamorphic conditions (from petrologic type 3.1 to >3.6) [5]. Recent studies suggest that Allende-type oxidized CVs are more metamorphosed (> type 3.6) than Balitype oxidized CVs and reduced CVs [6]. Northwest Africa (NWA) 8418 (CVred) was recognized as the first type 4 CV chondrite based on its homogeneous, Mg-rich matrix olivine (Fa38), CAI mineralogy, and presence of secondary chloroapatite [7,8]. Since then, we have identified two additional samples that appear to be more metamorphosed than typical CV chondrites: NWA 13784 (CV3) and “lithology C” in CV/CK breccia Camel Donga (CD) 003 [9,10]. Both samples have petrographies that are consistent with NWA 8418 (i.e., chondrules sizes and types, chondrule/matrix ratio, etc.) and are reduced CV chondrites, as evidenced by the lack of Ni-rich sulfides and oxides and the presence of metal [9,10]. Like NWA 8418, the matrices of NWA 13784 and lithology C in CD 003 are comprised of equilibrated, Mg-rich olivine (Fa39±0.5 and Fa42±1, respectively) [9,10]. Thus, all three samples appear to be metamorphosed CVred chondrites. NWA 8418 was classified as a CV4 based in part on its equilibrated matrix olivine. However, because matrix equilibrates faster than chondrules, chondrule olivine compositions must also be considered when establishing petrologic type. In an effort to better understand the maximum extent of metamorphism in the CV chondrites, we compare the homogeneity of chondrule olivine in these three samples to the CK3 and CK4 chondrites. Petrologic Type: Homogeneity of olivine is one of the most useful geochemical indicators of metamorphism in chondrites. Fa content of chondrule and matrix olivine become increasing homogenous during metamorphism, eventually converging on equilibrated values at petrologic type 4 (e.g., Fa31 in the CK chondrites [11]). Because matrix equilibrates faster than chondrules, olivine in the matrix may be fully equilibrated prior to reaching petrologic type 4 conditions. For example, average matrix olivine compositions in NWA 4425 (CK3.8) and NWA 4423 (CK3.9) (Fa31.7±0.5 and Fa32.4±0.7, respectively) are nearly indistinguishable from type 4 CKs Dhofar 015 and NWA 4422 (Fa32±0.6 and Fa30.5±0.7) [12]. Therefore, the equilibrated matrices of NWA 8418, 13784, and CD 003 (Fa38, Fa39, and Fa42, respectively) may not be indicative of petrologic type 4 conditions. In NWA 13784, compositions of chondrule olivine range from Fa1-41, with an average of Fa19.6±10.3 (n=260) [9]. Chondrule olivine in lithology C of CD 003 is more Fe-rich than in NWA 13784 (Fa26±10.7; n=66), but the range of compositions and the extent of equilibration (as indicated by standard deviation) are nearly identical [10]. The average composition and standard deviation of chondrule olivine in NWA 8418 was not reported. However, a histogram of chondrule olivine in NWA 8418 shows that compositions range from Fa1-38 [8]. Thus, the homogeneity and composition of chondrule olivine in NWA 8418 is similar to NWA 13784 and lithology C in CD 003. In all samples, chondrule olivine is not equilibrated. The wide range of chondrule olivine compositions suggest that the samples examined here have not reached petrologic type 4 conditions. Rather, the extent of equilibration in chondrule olivine is similar to what we have observed in CK3 chondrites (e.g., average chondrule olivine composition in NWA 4425 and NWA 4423 is Fa24.6±7.8 and Fa17.9±10.5, respectively) [12]. Conclusion: Though all three samples examined here are metamorphosed, the homogeneity of chondrule olivine suggest that they are not fully equilibrated. Based on comparisons with CK3 chondrites, these samples appear to be metamorphosed to petrologic subtype 3.8 or 3.9 conditions. However, this is inconsistent with CAI mineralogy in NWA 8418, which suggests that type 4 conditions were achieved [8]. These conclusions also contradict results from Raman spectroscopy studies of CVs, which indicate that reduced CVs are the least metamorphosed CV chondrites. Thus, more work is necessary to fully understand the extent of metamorphism in the CV chondrites. References: [1] Rubin A. and Wasson J. (1987) GCA 51:1923-1937. [2] Krot A. et al. (1995) Meteoritics 30:748- 775. [3] Guimon P.K. (1995) Meteoritics 30:704-714. [4] Grossman J.N. and Brearley A.J. (2005) MAPS 40:87-122. [5] Bonal et al. (2006) GCA 70:1849-1863. [6] Bonal L. et al. (2020) GCA 276:363-383. [7] Mallozzi et al. (2018) 49th LPSC, Abstract 2555. [8] MacPherson G.J. et al. (2023) MAPS 58:135-157. [9] Alevy E.G. and Dunn T.L. (2023) 85th MetSoc Meeting, Abstract 6078. [10] Alevy E.G. et al. (2024) LPSC LLV, Abstract 1711. [11] Kallemeyn G.W. et al. (1991) GCA 55:881-892 [12] Dunn T.L. et al. (2016) 79th MetSoc Meeting, Abstract 6429.