School of Earth Sciences - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemSubduction zone systematics: insights from high-precision ICP-MS radiogenic isotope analysis for the Mariana arc( 2012)Subduction zones play an essential role in the geochemical evolution of the Earth. They are sites where new continental crust is created, whilst oceanic crust and sediments are subducted and recycled into the mantle. Understanding these systems is therefore critical to our knowledge of both the crust and mantle, and the evolution of the planet as a whole. Radiogenic isotope ratios are a key tool in understanding these systems yet much of the existing literature is based upon relatively old data collected with methods that have now been superseded. The development of new analytical technologies, in particular MC-ICPMS, now provides an opportunity to re-evaluate many scientific questions in the light of ‘new generation’ analytical data with improved levels of precision and accuracy. To this end, a detailed Hf, Nd, Sr and Pb radiogenic isotope study has been performed utilising state-of-the-art analytical techniques to analyse 87 samples representing all nine islands of the active sub-aerial intra-oceanic Mariana arc. The quality of these data is shown to be a significant advancement over the bulk of previous literature data and they now represent the most comprehensive radiogenic isotope dataset available for the Mariana arc. The higher precision displayed by the current dataset allows for the observation of geochemical features that were not previously apparent. Here we show that individual islands in the arc form distinct trends in 206Pb/204Pb - 207Pb/204Pb space, a feature which has not been previously observed. It is shown that these trends may be attributed to either changes in the relative contributions of the two subducting sediment types (0.5 – 6.0% addition from a source composed of a mix of 5 – 25% pelagic and 75 – 95% volcanogenic sediment) or perhaps changes in the underlying mantle composition as was recently suggested to explain variations in Hf – Nd isotope space, or both. The new high precision data also allow for a re-examination of several key arc geochemical issues. While it is often suggested that the slab derived component dominates the Pb budget of arc lavas, it can be shown that the mantle wedge contribution is not completely overprinted, with ~ 15 % of the Pb in the arc lavas being mantle derived. There is also evidence to suggest that a sediment melt is potentially a key component in the transport of a sedimentary Pb signature from the slab to the mantle wedge. This study also casts further doubt over one of the fundamental assumptions made when using radiogenic isotopes to investigate the petrogenesis of subduction zone lavas, that of Hf immobility during subduction. It is demonstrated that a sediment derived Hf contribution most easily explains the Mariana-arc Hf isotope variation, with mixing models in Hf-Nd isotope space shown to require ~0.5 – 2.5 % sediment addition. These results demonstrate that significant new insights can be obtained from re-analysis of existing materials with improved analytical technologies.