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Journal of Petrology | Volume 43 | Number 6 | Pages 981-1001 | 2002
© Oxford University Press 2002
Lamproites from Gaussberg, Antarctica: Possible Transition Zone Melts of Archaean Subducted Sediments
ADVANCED CENTRE FOR QUEENSLAND UNIVERSITY ISOTOPE RESEARCH EXCELLENCE (ACQUIRE), DEPARTMENT OF EARTH SCIENCES, STEELE BUILDING, ST. LUCIA CAMPUS, UNIVERSITY OF QUEENSLAND, ST. LUCIA, BRISBANE, QLD. 4072, AUSTRALIA
Petrogenetic models for the origin of lamproites are evaluated using new major element, trace element, and Sr, Nd, and Pb isotope data for Holocene lamproites from the Gaussberg volcano in the East Antarctic Shield. Gaussberg lamproites exhibit very unusual Pb isotope compositions (206Pb/204Pb = 17·44–17·55 and 207Pb/204Pb = 15·56–15·63), which in common Pb isotope space plot above mantle evolution lines and to the left of the meteorite isochron. Combined with very unradiogenic Nd, such compositions are shown to be inconsistent with an origin by melting of sub-continental lithospheric mantle. Instead, a model is proposed in which late Archaean continent-derived sediment is subducted as K-hollandite and other ultra-high-pressure phases and sequestered in the Transition Zone (or lower mantle) where it is effectively isolated for 2–3 Gyr. The high 207Pb/204Pb ratio is thus inherited from ancient continent-derived sediment, and the relatively low 206Pb/204Pb ratio is the result of a single stage of U/Pb fractionation by subduction-related U loss during slab dehydration. Sr and Nd isotope ratios, and trace element characteristics (e.g. Nb/Ta ratios) are consistent with sediment subduction and dehydration-related fractionation. Similar models that use variable time of isolation of subducted sediment can be derived for all lamproites. Our interpretation of lamproite sources has important implications for ocean island basalt petrogenesis as well as the preservation of geochemically anomalous reservoirs in the mantle.
KEY WORDS: lamproites; Pb isotopes; mantle Transition Zone; subducted sediment; anomalous mantle reservoirs
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