Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.267n&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 267
Mathematics
Logic
11
Scientific paper
Introduction. The term brachinite was used for Brachina [1] because it represented a new type of meteorite. Four more have now been found, constituting a new group. The brachinites are ALH84025 [2,3], Eagles Nest [4,5], LEW88763 [6], and Window Butte, a new 187-g find (1991) from Utah. Our purpose is to establish group characteristics, relate it to other groups, and determine its genesis. Results. Brachinites contain (vol%): 74-98 ol, 0-2.4 opx, 1.5-8.2 cpx, 6.7-12.9 plag, 0.5-1.2 chromite, 1.8-4.0 FeS, and trace amounts of phosphate and metal, ALH84025 and Eagles Nest have no plag. The assemblages are near-chondritic, except for the very high ol/opx ratio, and depletion of plag in some cases. Texturally, brachinites are equigranular with a grain size of 100-750 micrometers. Mineralogically, brachinites are equilibrated: ol is Fo 6768 (LEW88763, Fo 64), plag is An 22-44, metal has high Ni (19-55%), and FeS has 0.5-1.3% Ni. Olivine contains NiO (up to 0.05%). Phosphate is chlorapatite; LEW88763 has merrillite. Bulk compositions of brachinites are similar, except for plag-depleted (low Al, Na, K) ALH84025 and Eagles Nest. REE patterns [1,3,5] are: Brachina, 1x chondritic; ALH84025, subchondritic; Eagles Nest, LREE-enriched (La, 10x chondritic) indicating possible addition of an incompatible element-rich component. Further trace element data are being determined. Oxygen isotopic compositions of brachinites indicate a coherent group (figure), except LEW88763, which has a sharply different composition. Brachinites overlap parts of the HED and IAB-winonaite groups; angrites fall within the group. Individual brachinites have some variability (up to 0.5o/oo). Conclusions. (1) Brachinites are not related to HEDs (or angrites) having widely different Fe/Mn and Ga/Al ratios [1,3] and noble gases [7]. Olivine-rich HED rocks (pallasites, mean delta^18O = 3.06o/oo) lie at the low end of the HED mass fractionation line whereas brachinites are ~1o/oo heavier. (2) Brachinites are a distinct primitive achondrite (near-chondritic compositions, achondritic textures) group that is highly oxidized (equivalent to L group). (3) Brachinites (BRA) evolved from chondritic material as did the reduced (intermediate between H and E groups) IAB-winonaites (IAB-WIN) and acapulcoites (ACA). (4) They all experienced some melting (plag depletion) and recrystallization to type 6 or 7 grade. Brachinite equilibration temperatures (using three thermometers) are in the 1000-1050 degree C range. (5) The compositions and abundances of ol, opx, and metal in the IAB-WIN, ACA, and BRA groups indicates that they experienced different amounts of oxidation of metallic Fe during heating, probably promoted by an aqueous vapor derived from accreted ices. This sequence is analogous to that proposed for oxidation of ordinary chondrites [8]. (6) The high ol/opx ratio and Fe-rich mineralogy of brachinites is a result of the reaction MgSiO3 + Fe^o + 1/2 O2 --> (Mg,Fe)2 SiO4, which converted chondritic silicates to highly oxidized ol-rich rocks. References: [1] Nehru, C.E. et al. (1983) Proc. 14th LPSC, Pt. 1, JGR 88, B237-B244. [2] Prinz, M. et al. (1986) LPSC XVII, 679-680. [3] Warren, P.H. and Kallemeyn, G.W. (1989) Proc. 19th LPSC, 475-486. [4] Kring, D.A. et al. (1991) Meteoritics 26, 360. [5] Kring, D.A. and Boynton, W.V. (1992) LPSC XXIII, 727-728. [6] Mason, B. (1991) Ant. Met. News. 14, No. 2, 22. [7] Ott, U. et al. (1985) Meteoritics 20, 69-78. [8] McSween, H.Y. and Labotka, T.C. (1992) GCA, in press.
Clayton Robert N.
Ebihara Mitsuru
Mayeda Toshiko K.
Nehru Cherukupalli E.
Prinz Martin
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