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Science 19 May 2000:
Vol. 288. no. 5469, pp. 1215 - 1223
DOI: 10.1126/science.288.5469.1215
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Research Articles

Rocks from the Mantle Transition Zone: Majorite-Bearing Xenoliths from Malaita, Southwest Pacific

Kenneth D. Collerson, 1* Sarath Hapugoda, 1 Balz S. Kamber, 1 Quentin Williams 2

Rocks containing high-pressure mineral assemblages derived from the mantle transition zone between depths of about 400 and 670 kilometers occur as xenoliths and megacrysts on the island of Malaita in the southwest Pacific on the Ontong Java Plateau. Observed ultrahigh pressure mineral chemistries include majorite, calcium- and magnesium-perovskite, aluminous silicate phases, and microdiamond. Based on an empirical barometer, majoritic garnets in these xenoliths record pressures of up to 22 gigapascal. The occurrence of material with perovskite chemistry and several enigmatic aluminous phases indicates pressures of up to 27 gigapascal. Samples were brought to the surface at about 34 million years ago by potassic ultramafic magmas, which evidently originated in the lower mantle.

1 Department of Earth Sciences, The University of Queensland, Brisbane, Qld 4072, Australia.
2 Department of Earth Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA.
*   To whom correspondence should be addressed. E-mail: k.collerson{at}mailbox.uq.edu.au

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