Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.469z&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 469
Mathematics
Logic
1
Acapulcoites, Classification, Lodranites
Scientific paper
Petrological and oxygen isotopic studies suggest that acapulcoites and lodranites are closely related. Meteorites of both groups have essentially achondritic equilibrated textures and are similar in mineralogy except that lodranites are coarser grained and have lower plagioclase abundances. The Acapulco meteorite and other acapulcoites have bulk chemical compositions close to those of ordinary chondrites. Compositions of lodranites are different from acapulcoites, primarily reflecting plagioclase fractionation. We performed bulk chemical analyses by instrumental neutron activation analyses of the acapulcoites Monument Draw (M), ALHA 81261 (81), and Acapulco (A) and the lodranites Gibson (G), MAC 88177 (88), and FRO 90011 (F). Data for ALHA 81261, Monument Draw, Mac88177 and FRO 90011 are given in [1]. Additional analyses were done on a new sample of Acapulco. Published data for Lodran (L; [2]), Y-791493 (Y; [3]), ALHA 77081 (77; [4,5]), and Acapulco [5] were considered. Acapulcoites have a very narrow compositional range. Larger variations are only found for the compatible element Cr and the volatile Zn. Both elements are largely hosted in chromite. Similar variations are observed within a single meteorite. Cr in Acapulco bulk samples ranges from 3500 ppm up to 7420 ppm, reflecting inhomogeneous local distribution of chromite. High and variable U abundances and enhanced LREE contents in Acapulco indicate high and variable modal abundances of phosphates since apatite is the main U and LREE carrier. Lodranites are depleted in all plagioclase elements (K, Na, Eu, Ca) and this readily distinguishes them from Acapulcoites (see Fig. 1). Abundances for the compatible Mn are similar in both groups. Se, representing modal sulfide content, is only slightly depleted in lodranites. Similar Cr and Zn variations as in acapulcoites are observed in lodranites. Refractory elements such as Sc and V are lower in lodranites than in acapulcoites (Sc/Mg ratio is about 30% lower). The composition of metal blebs enclosed in opx and olivine of Acapulco suggests that acapulcoites and lodranites passed through a stage of partial or complete melting early in their history [1]. During this process early crystallized chromite grains may have locally accumulated in the partial melt producing an inhomogeneous Cr,Zn distribution. Lodranites suffered additional fractionation(s). Slow cooling (coarse grain size) may provide conditions favorable for removal of residual melt rich in plagioclase elements. However, equilibrium fractional crystallization would not produce the observed negative Eu anomaly in the residual solid. The negative Eu-anomaly in lodranites requires loss of solid plagioclase grains or loss of a nonequilibrium melt of plagioclase composition. Sulfides were much less effectively extracted from lodranites than plagioclase. Summary: Acapulcoites mark the transition from chondrites to differentiated achondrites. Although they experienced igneous processes they largely retained their primitive composition. Lodranites have lost a residual melt fraction. Details of this process are unclear. A common early history of acapulcoites and lodranites is possible. However, the more evolved lodranites require formation conditions different from those of acapulcoites. References: [1] Zipfel J. and Palme H. (1993) LPSC XIV, 1579-1580. [2] Fukuoka T. et al. (1978) LPSC IX, 356-358. [3] Haramura H. et al. (1983) Mem. Natl. Inst. Polar Res., Spec. Issue, 30, 109-121. [4] Schultz L. et al. (1982) EPSL, 61, 23-31. [5] Kallemeyn G. W. and Wasson J. T. (1985) GCA, 49, 261-270.
Palme Herbert
Zipfel Jutta
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