Complexities in the Interpretation of Melt Inclusions in Aluminous Spinel-bearing Plagioclase-rich MORB and Island Arc Magmas.
Abstract
The simplest relationship to be expected for diverse melt inclusions in co-precipitated phases is that the reconstructed equilibrium melt compositions should be on the same liquid line-of-descent, reflecting the crystallization path potentially prior to and during co-precipitation. Interpretation of trends in melt inclusion compositions is generally predicated on simple "cotectic" behavior. However, such a relationship between melt inclusions in co-existing mineral phases or even within a single phase may not be applicable when the co-existing crystalline phases have a peritectic relationship, as is commonly the case for the assemblage aluminous spinel + anorthitic plagioclase + olivine. This assemblage, with rounded/corroded spinel, is commonly seen associated with anorthite megacrysts in subduction zone magmas and associated with anorthitic plagioclase in plagioclase ultra-phyric lavas in MORs. In both cases, the relationship of this assemblage to the host lava remains uncertain. Peritectic behavior involving first formation of spinel+L during source melting and then dissolution of spinel back into the melt during precipitation of plagioclase and/or olivine upon cooling or decompression suggests that (i) spinel can have a long and varied history and (ii) its melt inclusions should not lie on a simple co-precipitation path with melt inclusions in plagioclase and olivine. As our experimental work is showing, the peritectic relationship between liquid, aluminous spinel, and plagioclase and the liquid line of descent this produces is made more complex by the presence of a pseudo-azeotrope in the olivine-anorthitic plagioclase system. The spinel+L composition can be either more or less anorthitic than the coexisting plagioclase because of the presence of a pseudo-azeotrope in this system and spinel can either dissolve or crystallize during decompression. In the case of the spinel-consuming peritectic crystallization reaction, the liquid composition evolution before and after entrapment in plagioclase hosts is not dictated by co-precipitation of plagioclase and co-existing olivine and spinel. This complex peritectic behavior will also influence trace element evolution and the Mg# of precipitating olivine and co-existing melt.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V11A..05N
- Keywords:
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- 1042 Mineral and crystal chemistry;
- GEOCHEMISTRY;
- 1043 Fluid and melt inclusion geochemistry;
- GEOCHEMISTRY;
- 1065 Major and trace element geochemistry;
- GEOCHEMISTRY;
- 3630 Experimental mineralogy and petrology;
- MINERALOGY AND PETROLOGY