Na-Al-rich Chondrule Glasses

Details Na-Al-rich Chondrule Glasses Na-Al-rich Chondrule Glasses

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.512g&link_type=abstract

Meteoritics, vol. 30, no. 5, page 512

Computer Science

1

Chondrites, Unequilibrated, Chondrule, Electron Microprobe, Glass, Silicate Melts

Scientific paper

Na-Al rich chondrules are rare components of ordinary chondrites [1] and their origin and relationship to the ferromagnesium chondrules is a controversial subject. Na-Al rich chondrules have been hypothesized to be extreme differentiates of the common ferromagnesium chondrules that have been separated by collision and splashing [2,3]. In contrast, an origin for these chondrules by flash-heating of precursor materials rich in refractory and moderately volatile components admixed with relict olivine and spinel has also been proposed [4]. We have begun an investigation of the Na2O-rich glass in these chondrules from unequilibrated ordinary chondrites to further elucidate their origin. At this time, we have analyzed the glass of three Na-Al rich chondrules and the glass from one barred olivine chondrule. We have used the Na decay-curve method [5] to analyze these glasses with the electron microprobe. The four glasses range between 12.5-14.5 wt% Na2O and 18.3-22.2 wt% Al2O3. They are all peralkaline and highlight the problems with analyzing Na2O-rich glasses with electron beams. Previous analyses of Na-Al rich chondrules employed broad-beam techniques [1] and found that the Na/Al ratios fell between CI and albite ratio lines (Fig. 7 of [1]). These results were probably affected by Na-loss, as the four chondrules studied here all lie above these ratio lines. The existence of peralkaline chondrules in unequilibrated ordinary chondrites suggests that these chondrules did not experience significant volatilization of alkalies during heating and crystallization, and is consistent with an origin by flash heating. The observation that some of these chondrules also contain relict forsteritic olivine [4] limits the maximum temperature they experienced to 1901 degrees C, the melting point of forsterite [6]. The glass in the barred olivine chondrule mentioned above is compositionally indistinguishable from glass in the Na-Al rich chondrules, suggesting that they may be closely related. References: [1] Bischoff A. and Keil K. (1984) GCA, 48, 693-709. [2] Bischoff A. and Palme H. (1988) LPS XIX, 86-87. [3] Bischoff et al. (1989) EPSL, 93, 170-180. [4] Krot A. and Rubin A. (1994) Meteoritics, 29, 697-707. [5] Devine et al. (1995) Am. Mineral., 80, 319-328. [6] Greenwood J. and Hess P. (1994) LPS XXV, 471-472.

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