Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.329b&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 329
Computer Science
Cristobalite, Isotopes, Oxygen, Meteorites, Carlisle Lake-Type, Parnallee, Petrology
Scientific paper
Three alpha-cristobalite-bearing clasts up to 1.0cm in diameter (CB1, CB2, CB3) have been identified in Parnallee (LL3). CB1 and CB2 consist of <30 micrometer cristobalite grains rimmed by minor augite in a 'roof tile' mosaic texture [1]. In CB3 cristobalite and augite together form 0.2-1.5mm long blebs enclosed by clinoenstatite, as in a cristobalite-bearing clast from Jilin [1]. CB3 contains glassy areas beside some of the cristobalite- bearing blebs. SEM examination and EDS analysis of the glassy areas show sub-micron high-Ca pyroxene grains in feldspathic mesostasis. The glassy areas have a bulk composition including 52-53 wt% SiO2, 12-13 wt% CaO and 22wt% Al2O3. The surrounding clinoenstatite is En95.7-98.5 with CaO <0.12 wt% and Al2O3 <0.15 wt%. It contains occasional lamellae of high-Ca pyroxene (Wo33- 45, En54-61) with 3.6-7.1wt% Al2O3. Augite, which rims the cristobalite is Wo31-40, En45-52. The Parnallee clasts do not contain the secondary high-Ca pyroxene, which surrounds a cristobalite grain in ALHA 76003 (L6) [2]. Replicate analyses gave delta^17O and delta^18O values of 8.60, 11.50 for CB1, and 8.56, 11.48 for CB2 (Fig. 1). The corresponding Delta^17O values [3] are 2.62 and 2.59. These unusually high delta^17O excesses preclude an origin through planetary [4] or impact-induced differentiation of ordinary chondrite material. CB1 and CB2 plot on an extension of a mixing line defined between the cristobalite grain from ALHA 76003 and its ordinary chondrite host [2]. The Parnallee clasts have escaped the metamorphism associated with secondary pyroxene crystallization in ALHA 76003 and so have undergone less isotopic exchange. Our values are probably close to the endmember of the mixing line. The Delta^17O values do fall within the range of the Carlisle Lakes-type chondrites, whole samples of which have Delta^17O values of 2.47 to 2.91 [5]. A line of slope 0.5 through the Carlisle Lakes field (Fig. 1) passes close to the Parnallee clasts. It has been suggested that early crystallized olivine in chondrules from Carlisle Lakes-type meteorites reacted with nebular vapors leading to the nucleation of a second generation of olivine [5]. Such extended olivine condensation would enhance the likelihood of SiO2 saturation in the residual nebular gas [6]. Gas-solid separation models predict that when nebular gas starts condensing SiO2 it should be extremely depleted in Al and Ca [1]. However their petrography suggests that the Parnallee clasts crystallized from an Al2O3 and CaO-bearing melt, which was presumably derived from SiO2-rich solid containing these oxides. The Parnallee clasts' origin is unclear but might be related through unknown fractionation processes to a larger O-isotope reservoir with which the Carlisle Lakes-type meteorites are associated. References: [1] Brigham C. A. et al. (1986) GCA, 50, 1655-1666. [2] Olsen E. J. et al. (1981) EPSL, 56, 82-88. [3] Clayton R. N. et al. (1991) GCA, 55, 2317-2337. [4] Ruzicka A. and Boynton W. V. (1992) Meteoritics, 27, 283. [5] Weisberg M. K. et al. (1991) GCA, 55, 2657-2669. [6] Grossman J. N. and Wasson J. T. (1983) GCA, 47, 759-771.
Bridges John C.
Franchi Ian A.
Hutchison Robert
Pillinger Colin T.
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