The Origin and Significance of Reverse Zoning in Melilite from Allende Type B Inclusions
The Journal of geology
In many Type B Allende inclusions, melilite is reversely-zoned over restricted portions of each crystal. Textural relationships and the results of dynamic crystallization experiments suggest that the reverselyzoned intervals in these Type B melilites result from the co-precipitation of melilite with clinopyroxene from a melt, prior to the onset of anorthite precipitation. When clinopyroxene begins to precipitate, the All Mg ratio of the melt rises, causing the crystallizing melilite to become
... elilite to become more gehlenitic, an effect which is negated by crystallization of anorthite. Because the equilibrium crystallization sequence in these liquids is anorthite before pyroxene, melilite reverse zoning can occur only when anorthite nucleation is suppressed relative to pyroxene. This has been achieved in our experiments at cooling rates as low as 0.5°C/hour. Our experiments further indicate, however, that reverse zoning does not form at cooling rates ~50°C/hour, probably because the clinopyroxene becomes too Al-rich to drive up the Al/Mg ratio of the liquid. Type B inclusions with reversely-zoned melilites must have cooled at rates greater than those at which anorthite begins to crystallize before clinopyroxene but <50°C/hour. Such rates are far too slow for the Type B droplets to have cooled by radiation into a nebular gas but are much faster than the cooling rate of the solar nebula itself. One possibility is that Type B's formed in local hot regions within the nebula, where their cooling rate was equal to that of their surrounding gas. Other possibilities are that their cooling rates reflect their movement along nebular temperature gradients or the influence of a heat source. The sun or viscous drag on inclusions as they moved through the nebular gas are potential candidates for such heat sources. F IG. !a.-Photomicrogra ph of a large lll~lilite (Me l) c rystal in the Type Bl inclusio n TS23F I. In this photo and in figs. 2a-4a, the lines marked A-8 are th e locat ions on the electron micro probe step-scan profiles. The melilite c ry stal is norma ll y-zoned o utwards from its core (A ; blue-white interference color) to 8' (oli ve brown) , reversely-zoned from 8 ' to the center of the anomalous blue band marked / , normall yzoned from I to the center of the o li ve brown band marked 2. reversely-zoned from 2 to the center of the band marked J , a nd normally-zoned fro m 3 o ut to the rim (B). Note that pyroxene (Pyx) inclusions are abunda nt in the outer parts of the melilite but absent from its core. This and succeeding photomicrographs taken wi th crossed polarizers. Other abbreviations: An-a northite. F IG. 2a . -Photomicrograph of a small melilite crystal , a lso in TS23F l . The melilite is reversely-zoned from its core (oli ve-brown) outwards, with only the outermost rim sho wing normal zoning. Large pyroxene inc lusio ns a re pre e nt in the core of this meli lite , in contrast to the crystal in figure I. Abbrev . as before .