Lunar and XXXIII (2002) 1948.pdf

Chromites in : C.E. Nehru, Dept. of Geology, Brooklyn College, CUNY, Brooklyn, NY 11210; Dept. of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024.

Data and discussion: Traditionally the FFM Introduction: Chromites are ubiquitous [(Fe+Mn)/(Fe+Mn+Mg)] and CRAL components of many chondritic and achondritic , including pallasites. Chromite [(Cr+V)/(Cr+V+Al)] ratios of chromites have compositions of non-chondritic meteorites have been used to illustrate the petrogenetic trends been related to the oxygen fugacities by Bunch of chromite populations. Variations in FFM and Keil [1]. Nehru et al [2] reported on and CRAL ratios from the three pallasites chromites from three primitive groups were studied. Chromite compositional groups (IAB and IIICD +), data from 30 pallasites were recalculated and (+) and and plotted on the FFM and CRAL diagram (Fig. concluded that the FFM ratios 1). It is obvious that the ratios in the pallasites [(Fe+Mn)/(Fe+Mn+Mg)] in chromites reflect occupy a surprisingly small range of values - the oxidation states of the different groups. FFM varies between 0.60 to 0.80 and CRAL Until recently two separate groups of pallasites values vary between 0.80 to 1.00. Chromites were recognized and recently one more group from Vermillion, the pyroxene , have a was added [3, 4] making three groups of narrow FFM range of 0.646-0.657 and CRAL pallasites. A majority of the pallasites belong to of 0.946- 0.947. Data collected from the Eagle what is known as the ‘Main Group Pallasites’; Station pallasite yields an FFM range of 0.755- the second is the (three 0.789 and CRAL of 0.858-0.867. In contrast known meteorites) and third is the Pyroxene the main group pallasites show a trend of Pallasite Group (only made up of two samples generally increasing FFM values (0.060 - and one of them has only very fine grained 0.075) and CRAL from 0.810-0.990. The FFM inter-growth of augite and chromite). Oxygen values do not vary as much as the CRAL isotope studies, [3], siderophile trace element values. Compared to other pallasites, the main abundances [5 ] and compositions [5,6] group pallasite chromites have a lower have been used to distinguish the three pallasite (limited) FFM range and a variable CRAL groups. range.

Present Study: This study evaluates the Eagle Station pallasites have a higher FFM systematic compositional changes of chromites range in (Fa 19-20) and also have a in the pallasites, relates them to the silicates higher FFM value in chromites. The main present in the meteorites, and uses them for group pallasites have lower FFM in olivines petrogenetic interpretation. Of the sixty known (Fa 11-13) and exhibit a lower FFM range in pallasites, about thirty have been known to chromites. However a small set of the main contain chromites. In this study I used chromite group pallasites exhibit intermediate Fa values electron microprobe data from earlier authors and have correspondingly higher FFM chromite [1,4,6,7]; new probe data collected from 10 values. Thus there is a positive correlation chromite bearing pallasites from the AMNH between the Fa contents of the pallasite collections; and verified the chromite olivines (FFM values) and the FFM ranges of compositions from several meteorites which the chromites. have been analyzed earlier. It is noted that the new data is in agreement with the data reported The FFM and CRAL relationships of the by Bunch and Keil [1]. chromites for three groups are reproduced in Fig. 1 and the same values

Lunar and Planetary Science XXXIII (2002) 1948.pdf

CHROMITES IN PALLASITES: C. E. Nehru for the pallasite chromites are shown in Fig. 2. The two figures are separated to avoid confusion due to overlap of the acapulcoites and the pallasites. The pallasite chromites occupy the total and incorporated into the chromites. Olivine does chromite area and part of the (IAB+WIN) not take up Al and there is no other phase that group. Al could go into except chromite with the exception of the pyroxene pallasites. The Conclusion: The FFM ratios in chromites of variability of CRAL ratios is a reflection of the the main group pallasites correspond to the Fa availability of Cr and Al in the bulk ranges of the olivines and confirm the composition of the pallasite in question. Thus suggestion of Bunch and Keil [1] that there is a the FFM ratio is related to the oxidation state positive correlation between the two. The Eagle and the CRAL values are related to the bulk Station pallasite has more rich olivine and chemistry and the refractory nature of the chromite with higher FFM value. The pyroxene mineral chromite. pallasite Vermillion has a lower Fa content (Fa 11-12) and has a moderate FFM for chromites. References: The FFM ratios of chromites are related to the oxidation state of the meteorites. The FFM and [1] Bunch T. E. and Keil K. (1971) Am. Min CRAL values of the pallasites are comparable 146-157; [2] Nehru C.E. et al. (1997) to those of acapulcoites and part of the A 97-98; [3] Clayton R. N. and (IAB+WIN) group and are close to the brachina Mayeda K.M. (1996) GCA, 1999-2017; [4] group. Boesenberg J.S. et al. (2000) Meteoritics , 757- 769; [5] Scott E. R.D. (1977) GCA 349-360; The CRAL ratios in all pallasites are variable. [6] Mason B. (1963) Nova-tates 1-19; [7] It is to be noted that chromite is perhaps the Wasson J.T. et al. (1999) GCA 1219-1232; [8] most refractory mineral to crystallize and in Buseck P.R. (1977) GCA 711-740. many pallasites it is observed that chromite crystallized after olivine crystallized [8]. In the pallasite meteorites, Al follows Cr and is

Fig. 2 Chromites from Pallasites

ACA-LOD

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