ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract A grospydite from Roberts Victor contains the most Ca-rich garnets yet found in South African kimberlite xenoliths and also sub-micron sized sodic nepheline in melted and quenched clinopyroxene. Three stages can be recognised in the textural evolution of the grospydite. The first is the development of a layering of large kyanite laths. Kyanite together with complex aluminous clinopyroxene precipitated and accumulated from an evolved residual eclogitic liquid which has penetrated across the garnet join so that garnet no longer precipitated. Solidus conditions for the Roberts Victor grospydite are estimated as T = 1350–1550 ° C, P = 27–39 kbars. Adjacent layers in the grospydite have slightly different mineral compositions suggesting that the small-scale layering (1–5 cm) in this, and associated rocks, may be related to varying activities of R2O3 components and possibly to f o 2. The second stage is represented by a necklace texture in which all the garnet and some kyanite developed along grain boundaries of clinopyroxenes with triple-point textures. This is interpreted as an example of incoherent, grain-boundary exsolution resulting from large subsolidus volume changes. The conditions for subsolidus equilibration are estimated to be T= 1120–1320 ° C, P = 42–56 kbars. Moderate Ca-contents in garnet and excess Al[6] in clinopyroxene may be subsolidus indicators of eclogite samples evolving towards grospydite at the solidus. The third stage is represented by the melting of jadeite-rich clinopyroxenes and quenching to glass, nepheline and plagioclase. Most of the glass has a composition similar to clinopyroxene, except for K2O, though local areas of different glass, possibly the result of phase separation, also occur. The melting process seems to be a low-pressure feature involving limited addition of H2O at temperatures between 900–1000 ° C. Water-absent melting could indicate temperatures up to 1500 ° C. The temperatures and pressures assigned to the three-stage evolution of this grospydite imply formation at moderate pressures and subsolidus equilibration at higher pressures. This is equated with downgoing mantle/asthenosphere tectonic processes. After entrainment in a kimberlite magma the grospydite fragment apparently ascended rapidly, thus allowing low-pressure melting and quenching.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00373407
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