Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.542m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 542
Mathematics
Logic
8
Cores, Differentiation, Iie Irons, Meteorites, Colomera, Elga, Kodaikanal, Miles, Netschaevo, Techado, Watson, Weekeroo Station, Shock, Silicate Inclusions
Scientific paper
The small group of IIE irons [1] are important since many contain silicate inclusions. A wide variety of inclusions occur in Netscha vo [2], Techado [3], Watson [4], Elga, Kodaikanal, Weekeroo Station, and Colomera [5,6]. Miles contains uncharacterized silicate inclusions [7]. Most siderophile elements do not follow fractional crystallization trends, suggesting formation by impact [1]. Ages from 3.67-4.51 Ga [see 8] are taken by some authors [4] as the time of silicate-metal mixing. I have conducted petrographic studies on silicate inclusions of all silicate-bearing IIE's except Elga and propose an alternative history of heating, melting, silicate differentiation and metal-silicate mixing ~4.55 Ga ago in a core-mantle environment followed by shock heating and chronometer resetting. The chondritic IIE precursor was probably not identical to H chondrites [1,3]. Mafic silicate compositions in "primitive" IIE's (Netschaevo, Techado) and Delta 17O in all silicate-bearing IIE's (0.59+/-0.08 [3,4,9,10]) are less than or overlap H chondrites. The IIE body was heated to >=900 degrees C, metamorphosing and partially melting the mantle and forming a small Fe,Ni-FeS core ~4.55 Ga ago. Slow metallographic cooling rates (1-100 degrees C/Ma [1,3]) for IIE's with unaltered metal (e.g., Techado, Weekeroo Station, Miles) and fractional crystallization trends for some elements (e.g., Au-Ni) support a core origin, not a near-surface, impact model. Comparison of other siderophile element trends in IAB, IIICD and IIE indicate similar origins [1], but IAB and IIICD may also have originated in cores [11,12]. Silicates differentiated in the mantle and/or after mixing with metal. The degree of differentiation increases from Netschaevo (unmelted, chondritic clasts [2]) to Techado (unmelted silicates; Fe,Ni-FeS melting [3]) to Watson (nearly-total melting; no silicate differentiation; Fe,Ni-FeS lost [4]) to Miles and Weekeroo Station (opx-cpx-plag partial melts [5]) to Kodaikanal, Colomera and Elga (cpx-plag differentiated partial melts [5]; opx fractional crystallization?). No residues from partial melting exist. "Differentiated" IIE's (Miles, Weekeroo Station, Kodaikanal, Colomera, Elga) tend to have higher Fs and d18O than "primitive" IIE's, consistent with partial melting. No correlation between silicate differentiation and host iron composition suggests multiple mixing events by impact induced tectonism or gravitational smoothing of the core-mantle boundary [12]. Slow cooling produced coarse-grained, heterogeneous inclusions observed in Miles and Colomera. Reheating and melting of silicates and resetting of chronometers occurred during later shock events. Shock focused in the inclusions, although many IIE's show shock effects in the metal. "Primitive" IIE's experienced mild shock (stage S2-S4; 100-300 degrees C post-shock heating [13]). Forging of Netschaevo masks its shock history. Shock in "differentiated" IIE's was severe (S4-S6; 300-1000 degrees C), forming planar fractures and undulatory extinction in coarse pyroxene and plagioclase. Some plagioclase in Colomera, Weekeroo Station, Kodaikanal and Elga (not Miles) was melted. Low (87Sr/86Sr)o ratios [see 8] may require some differentiation during this melting. Rapid post-shock cooling yielded glassy to microcrystalline plag-SiO2 mixtures. Colomera contains coarse-grained and glassy inclusions, requiring two episodes of heating and cooling (e.g., initial formation and shock). Post-shock heating reset 39Ar-40Ar chronometers in some IIE's [8] and possibly other chronometers in heavily-shocked IIE's (e.g., Kodaikanal). Variable post-shock heating between inclusions may not allow direct comparison of chronologic studies on different inclusions. Many IIE ages reflect later shock events and not silicate-metal mixing. References: [1] Wasson J. T. and Wang J. (1986) GCA, 50, 725. [2] Olsen E. and Jarosewich E. (1971) Science, 174, 583. [3] Casanova I. et al. (1995) Science, 268, 540. [4] Olsen E. et al. (1994) Meteoritics, 29, 200. [5] Prinz M. et al. (1983) LPS XIV, 618. [6] Bunch T. E. et al. (1970) Contrib. Mineral. Petrol., 25, 297. [7] Wlotzka F. (1994) Meteoritics, 29, 893. [8] Garrison D. H. and Bogard D. D., this volume. [9] Clayton R. N. et al. (1983) EPSL, 65, 229. [10] Clayton R. N. (1995) personal communication. [11] Benedix G. K. et al., this volume. [12] McCoy T. J. et al. (1993) Meteoritics, 28, 552. [13] St"ffler D. et al. (1991) GCA, 55, 3845.
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