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Published June 2003 | public
Journal Article

Alkalic magmas generated by partial melting of garnet pyroxenite


Many oceanic-island basalts (OIBs) with isotopic signatures of recycled crustal components are silica poor and strongly nepheline (ne) normative and therefore unlike the silicic liquids generated from partial melting of recycled mid-oceanic-ridge basalt (MORB). High-pressure partial-melting experiments on a garnet pyroxenite (MIX1G) at 2.0 and 2.5 GPa produce strongly ne-normative and silica-poor partial melts. The MIX1G solidus is located below 1350 and 1400 °C at 2 and 2.5 GPa, respectively, slightly cooler than the solidus of dry peridotite. Chemographic analysis suggests that natural garnet pyroxenite compositions straddle a thermal divide. Whereas partial melts of compositions on the silica-excess side of the divide (such as recycled MORB) are silica saturated, those from silica-deficient garnet pyroxenites can be alkalic and have similarities to low-silica OIB. Although the experimental partial melts are too rich in Al_2O_3 to be parental to highly undersaturated OIB suites, higher-pressure (4–5 GPa) partial melting of garnet pyroxenite is expected to yield more appropriate parental liquids for OIB lavas. Silica-deficient garnet pyroxenite, which may originate by mixing of MORB with peridotite, or by recycling of other mafic lithologies, represents a plausible source of OIB that may resolve the apparent contradiction of strongly alkalic composition with isotopic ratios characteristic of a recycled component.

Additional Information

© 2003 Geological Society of America. Manuscript received 17 October 2002. Revised manuscript received 5 March 2003. Manuscript accepted 7 March 2003. We thank John Shervais for samples of Balmuccia pyroxenite and Laura Wasylenki for assistance with the experiments. This work was supported by National Science Foundation grants EAR-9219899 and OCE-9314505 (to Stolper) and OCE-9706526 and OCE-9876255 (to Hirschmann).

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