Lamproites from Gaussberg, Antarctica: Possible Transition Zone Melts of Archaean Subducted Sediments |
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| Authors: | MURPHY D T; COLLERSON K D; KAMBER B S |
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| Institution: | 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 |
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| Abstract: | Petrogenetic models for the origin of lamproites are evaluatedusing new major element, trace element, and Sr, Nd, and Pb isotopedata for Holocene lamproites from the Gaussberg volcano in theEast Antarctic Shield. Gaussberg lamproites exhibit very unusualPb isotope compositions (206Pb/204Pb = 17·44–17·55and 207Pb/204Pb = 15·56–15·63), which incommon Pb isotope space plot above mantle evolution lines andto the left of the meteorite isochron. Combined with very unradiogenicNd, such compositions are shown to be inconsistent with an originby melting of sub-continental lithospheric mantle. Instead,a model is proposed in which late Archaean continent-derivedsediment is subducted as K-hollandite and other ultra-high-pressurephases and sequestered in the Transition Zone (or lower mantle)where it is effectively isolated for 2–3 Gyr. The high207Pb/204Pb ratio is thus inherited from ancient continent-derivedsediment, and the relatively low 206Pb/204Pb ratio is the resultof a single stage of U/Pb fractionation by subduction-relatedU loss during slab dehydration. Sr and Nd isotope ratios, andtrace element characteristics (e.g. Nb/Ta ratios) are consistentwith sediment subduction and dehydration-related fractionation.Similar models that use variable time of isolation of subductedsediment can be derived for all lamproites. Our interpretationof lamproite sources has important implications for ocean islandbasalt petrogenesis as well as the preservation of geochemicallyanomalous reservoirs in the mantle. KEY WORDS: lamproites; Pb isotopes; mantle Transition Zone; subducted sediment; anomalous mantle reservoirs |
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