Constraints on metasomatic vein formation and implications for the petrogenesis of alkaline magmas
Abstract
Metasomatized lithosphere is a potential source for alkaline lavas in oceanic and continental settings [1, 2]. Experiments on natural mafic amphibole-rich veins ± peridotite demonstrate that high-degree melting of such veins followed by interaction of the melts with peridotite can reproduce key characteristics of the major- and trace-element compositions of alkaline lavas from intraplate volcanoes [3]. This model suggests that many geochemical features of alkaline rocks are directly inherited from the mafic veins, which are thought to form via migration, cooling, and fractional crystallization within the lithospheric mantle of low-degree melts from the asthenosphere. Such a process would generate metasomatic veins spanning a continuum of phase assemblages ranging from anhydrous to hydrous. To constrain this process, we have undertaken two studies. 1. A series of experiments at 1.5 GPa was undertaken to simulate the liquid-line-of-descent of a basanitic magma to evaluate whether the evolution of such a liquid within the lithosphere could produce cumulates similar to those seen in lithospheric veins. The sequence of cumulates predicted by the results of these experiments [anhydrous cumulates (cpx + ol) at high temperature, followed by hydrous cumulates (amphibole + minor cpx) at lower temperatures (<1130°)] support the interpretation that observed anhydrous and hydrous metasomatic veins could be produced during a continuous differentiation process. Comparing predicted cumulates from the basanitic liquid with those from a hy-normative hawaiitic liquid [4] suggests that both magma types would produce broadly similar phase assemblages during differentiation. This suggests that the formation of amphibole-bearing veins observed in the lithospheric mantle could be linked to the differentiation of hydrous parental liquids ranging from ne- to hy-normative in composition. 2. To evaluate the trace-element composition of amphibole-bearing cumulates that would be formed by the processes described above, we did Monte Carlo simulations of metasomatic vein formation in the context of a mid-ocean ridge. This model assumes that some liquid produced at depth at the periphery of a ridge is not collected to form MORB, but generates a metasomatic enrichment in the lithospheric mantle. For the calculation, we assume that initial metasomatic agents represent low-degree melts from peridotitic sources similar to the depleted sources of MORBs. The main conclusion of these simulations is that even though the parental liquids are derived from “normal” mantle peridotite (albeit small degrees of melting), the compositions of calculated cumulates can be mineralogically similar to observed lithospheric veins, they can be highly enriched in incompatible trace elements, and they can have trace-element patterns that share many similarities with the patterns of alkaline OIBs. The combination of these two complementary studies suggests that hydrous cumulates suitable to be a source of alkaline magmas observed in intraplate settings could be produced by fractional crystallisation at high pressure of low-degree melts from “normal” mantle. [1] Lloyd and Bailey ,1975; [2] Niu and O’Hara, 2003; [3] Pilet et al., 2008; [4] Nekvasil et al., 2004.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V11B2280P
- Keywords:
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- 3610 MINERALOGY AND PETROLOGY / Geochemical modeling;
- 3619 MINERALOGY AND PETROLOGY / Magma genesis and partial melting;
- 3630 MINERALOGY AND PETROLOGY / Experimental mineralogy and petrology;
- 8415 VOLCANOLOGY / Intra-plate processes