School of Earth Sciences - Theses
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ItemIsotope and trace element systematics in subduction systems and the concept of ambient mantle( 2018)This project employs Hf - Nd isotope ratio and trace element concentration data to investigate aspects of magmatism in subduction systems. The primary goal is to use the compositions of back-arc basin lavas as a means to investigate the nature of the pre-subduction mantle wedge ( ‘ambient mantle’). New Sr, Nd, Pb and Hf isotope data for the Lau Basin back arc spreading centres and Hf – Nd data for the Manus Basin back arc are reported. Both areas have been extensively studied using major, trace element and Sr-Pb isotope data but many of these elements are highly mobile in aqueous fluids released from the subducting slab and so are unfavourable for studying the mantle wedge itself; here the behaviour of the less mobile elements Hf and Nd can provide additional insights. Hf is a high field strength element (HFSE) and believed to be relatively immobile in subduction zone fluids until slab melting occurs whereas Nd is slightly mobile in aqueous fluids, and also mobile in melts. The new Hf and Nd data from both the Lau and Manus back-arc basins plot broadly in the Indian Mid-Ocean Ridge Basalt(MORB) domain with the exception of two lavas from the Eastern Lau Spreading Centre (ELSC). Two geochemical filters (Ba/Nb < 7 and Th/Ta < 3), used in a previous study of the back arc region to the Mariana arc, were modified slightly for each studied site and applied to the new data to remove major influences from subduction components in the mantle wedge. These filtered data for the Lau and Manus Basins confirm that the true mantle affinity in these regions is indeed Indian MORB. When compared to existing data from the Mariana Trough and Scotia Sea, it is clear that each mantle wedge has its own unique ambient mantle character and that these site-specific compositions should be used in preference to global average MORB compositions for the purpose of modelling. Despite the geochemical filtering methods employed, it is clear that all of the samples still retain a small subduction input in terms of their LILE contents. A subsidiary part of this project re-evaluates existing HFSE concentration data for lavas of the Tongan arc, previously determined by laser ablation methods. New solution ICP-MS data suggest that the older data are likely influenced by analytical artifacts, a conclusion that dramatically changes the genetic model required for their interpretation.