Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.283r&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 283
Computer Science
3
Scientific paper
Description of Bo-1. One slab of the Bovedy (L3) chondrite contains a large (~ 4 x 7 mm) white object that in hand specimen resembles the generally chondritic lithic fragment studied by Rubin et al. (1981). However, SEM, and EMPA studies of this object, designated Bo-1, reveal an unusually Si-rich, Na-poor bulk composition that is manifested by the presence of a silica mineral, the complete absence of olivine, and feldspar that is less sodic than usual for ordinary chondrites. The clast is a highly crystalline igneous rock and exhibits a well-defined crystallization sequence. Orthopyroxene (opx) comprises 83.8 vol% of Bo-1 and is zoned from En(sub)92-85 Wo(sub)0.1-0.2 to En(sub)74 Wo(sub)1.9-2.9. The more magnesian, less calcic opx was the first phase to crystallize in Bo-1. A silica mineral (SiO2, 6.2 vol%), probably either tridymite or cristobalite, crystallized next and appears slightly "corroded" (and veined) by clinopyroxene, suggesting minor reaction between the silica mineral and its surroundings. A small amount (1 vol%) of euhedral-subhedral pigeonite (pig) also crystallized about this time. Plagioclase (plag) was the last phase to join the crystallization sequence and consists of a fine-scale, often lamellar, intergrowth of bytownite-labradorite (5.8 vol%, mainly An(sub)70- 75Or(sub)1-03) and oligoclase (3.1 vol%, mainly An(sub)15-24 Or(sub)13-6). The latter deviates from feldspar stoichiometry and may be partly amorphous. Volumetrically insignificant augite (aug) occurs as (exsolution?) patches within opx, as thin veinlets crossing SiO(sub)2 grains, and as thin rims at the interface of SiO(sub)2 and plag. The rimming and veining aug may have crystallized together with plag from the last liquid in the rock. Minor chromite (0.2 vol%) and trace metal and sulfide are also present. Bulk composition of Bo-1. The composition of the clast was determined by combining SEM modal data with EMPA data for each phase. The calculated composition (in wt%) is: SiO2, 57.05, TiO2, 0.04, Al2O3, 3.41, Cr2O3, 0.47, FeO, 11.12, MnO, 0.29, MgO, 25.4, CaO, 1.64, Na2O, 0.47, K2O, 0.07, P2O5, 0.01 and NiO, 0.02. Compared to the host chondrite and to most chondrules in ordinary chondrites, SiO2 is distinctly high and Na2O is very low. TiO2 and especially P2O5 also appear relatively low. Pseudoternary liquidus diagrams given by Morse (1980) may be used to evaluate the petrogenesis of Bo-1. The bulk clast composition projects within the protoenstatite or opx primary crystallization field at T(sub)liquidus ~ 1450-1500 degrees C. Equilibrium crystallization of this melt results in a predicted crystallization sequence that agrees with the observed texture: opx (or protoenstatite) - opx + SiO2 (+ pig) - opx + SiO2 + plag (+ aug). Cooling of the Bo-1 liquid was evidently slow enough to allow an approach to equilibrium but fast enough to prevent complete homogenization of opx. The silicic composition of the clast can be generated by extensive olivine and some opx fractionation from the silicate portion of an extensively melted L-chondrite precursor; the depletion of Na suggests that melting was accompanied by some volatile loss. The heating event involved high temperatures (T ~ 1550-1600 degrees C). Extensive olivine fractionation implies that crystallization occurred in a relatively large magma body and a substantial gravitational field. Crystallization of the residual liquids following olivine fractionation more closely approached equilibrium, probably because of an increase in melt viscosity as the magma cooled and the residual melt became more silicic. Acknowledgement: The writers wish to thank R. Farrell for the generous sample of Bovedy. Morse S.A. (1980) Basalts and Phase Diagrams. An Introduction to the Quantitative Use of Phase Diagrams in Igneous Petrology. Springer-Verlag. 493 pp. Rubin A.E., K. Keil, G.J. Taylor, M.-S. Ma, R.A. Schmitt and D.D. Bogard (1981) Derivation of a heterogeneous lithic fragment in the Bovedy L-group chondrite from impact-melted porphyritic chondrules. Geochim. Cosmochim. Acta. 45, 2213-2228.
Boynton Willam V.
Ruzicka Adam
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