In chondrules of unequilibrated ordinary chondrites (UOC's), mesostases sometimes have compositions that are not in equilibrium with the co-existing minerals. Examples include highly silica-oversaturated mesostases in porphyritic olivine chondrules and feldspathoid-bearing mesostases in radiating pyroxene (enstatite) chondrules . As part of a larger study of chondrules and clasts in UOC's, we report the results of a survey of the mineral assemblages in mesostases. Silica enrichment can manifest itself in mesostasis glass, with SiO2 contents up to 73wt%, or as silica polymorphs. Neither of these assemblages are in equilibrium with the olivine phenocrysts which comprise the bulk of the chondrules. Chondrule CC35 (type IIA ) separated from Chainpur (LL3.4) is an example of the latter. Mesostasis comprising 10% of CC35 contains An(sub)69-83, Ca-px (En(sub)44-52, Fs(sub)17-18, Wo(sub)31-39) and a silica polymorph. Silica oversaturation in chondrules can readily be attributed to abundant metastable olivine crystallisation, which drives residua towards quartz-, diopside- and feldspar-rich normative compositions. This could occur in chondrules crystallising as closed systems, so sampling of a larger, chemically fractionated reservoir need not necessarily be invoked, although silica-rich clasts provide evidence that an analogous process occurred in larger, open igneous systems . Some silica undersaturated mineral assemblages in mesostases may also be explained by closed system crystallisation within chondrules. A radiating pyroxene chondrule in Chainpur (Chr1) contains interstitial nepheline and scapolite. Metastable crystallisation of enstatite from an initially chondritic melt composition, at low pressure, can create silica undersaturated residua. The LREE-enriched abundances of the Chr1 mesostasis minerals are consistent with this, having up to 19 x OC La and Eu/Eu* = 10 . Similarly nepheline-bearing mesostasis identified in two Parnallee (LL3.6) chondrules (P6, P22), may have crystallised from residual chondrule liquid. Other feldspathoid occurrences in chondrule mesostases cannot be explained in this way. Chondrule CC1 (type IIAB, Chainpur) consists of sodalite (<= 7wt% Cl), nepheline, An(sub)88-91 and pyroxene (En(sub)75, Fs(sub)16, Wo(sub)9) dendrites in a texture suggesting devitrification. This alkaline assemblage cannot simply be a residuum following crystallisation of the phenocrysts, because they are predominantly olivine. Mobilisation of alkali fluids within UOC parent bodies, after the formation of chondrules, is probably responsible for "white matrix"  but low temperature metasomatism cannot be invoked for CC1 because its texture indicates that the mesostasis assemblage is derived from a melt. Instead, there could have been a late influx of alkali elements into the precursor melt. A similar conclusion was reached for the FELINE nepheline-rich clast  and the feldspathoid-bearing SA-1 basaltic clast . If true, the CC1 precursor melt may, like that of FELINE, have originated within an open igneous system on a planetary body. Oxygen isotopic data is being collected on these and other samples in order to help ascertain whether the alkali-enriched melts envisaged are from normal OC reservoirs or exotic sources . References:  Alexander C. M. O'D. et al. (1994) LPS XXV, 11-12.  Scott E. R. D. et al. (1994) GCA, 58, 1203-1209.  Bridges J. C. et al. (1995) Meteoritics, submitted.  Hutchison R. et al. (1994) Meteoritics, 29, 476-477.  Bridges J. C. et al. (1995) Proc. NIPR Symp. Antarct. Met., 8, in press.  Kennedy A. K. and Hutcheon I. D. (1992) Meteoritics, 27, 539-554.
- Pub Date:
- September 1995