RONDA PYROXENITES · Melting of a pyroxenite-peridotite marble cake mantle, Ronda Massif-southern Spain: implications for the compositional heterogeneity of mantle-derived melts

Melting of a pyroxenite-peridotite marble cake mantle, Ronda Massif-southern Spain

Project context and objectives The major aim of mantle geochemistry is to clarify the composition and provide constraints on the structure of the Earth's interior. The chemical heterogeneities found in mantle-derived basalts imply the existence of similar or even greater heterogeneities in their source regions in the mantle. The Ronda ultramafic massif (southern Spain) is an excellent natural laboratory to carry out a geochemical investigation on melting processes in a heterogeneous mantle as it constitutes the largest exposure of subcontinental lithospheric mantle in the world and is characterised by the presence of a partial melting domain. In this project, the major trace element and isotopic compositions of different types of mantle rocks in Ronda (i.e. pyroxenites and peridotites) were investigated in order to get an insight into how chemical heterogeneities are spatially arranged in the Earth's mantle and how they affect the composition of mantle-derived melts. Mineral compositions of pyroxenites and peridotites were obtained by Electron Microprobe Analyses (EMPA). These analyses were carried out on both main minerals (clinopyroxene, orthopyroxene, olivine, spinel) and accessory phases (sulphides). For the latter, back-scattered images and compositional maps were also acquired. The trace element compositions of clinopyroxene from chromium (Cr)-rich websterites and of sulphides from different types of pyroxenites were obtained by Laser Ablation Inductively-Coupled Mass Spectrometry (LA-ICP-MS). Isotopic compositions were acquired by Secondary Ion Mass Spectrometry (SIMS) in the same sulphide grains analysed by LA-ICP-MS. Clinopyroxene separates from Cr-rich pyroxenites were washed, acid leached and strontium-neodymium-lead (Sr-Nd-Pb) isotopic ratios of clinopyroxene residues and leachates were analysed by Multi Collector Inductively-Coupled Mass Spectrometry (MC-ICP-MS) and Thermal Ionization Mass Spectrometry (TIMS). The trace element composition of the leachates was obtained by Inductively-Coupled Mass Spectrometry (ICP-MS). ICP-MS was also used to analyse the contents in Platinum-Group Elements (PGE) and rhenium (Re) in a composite pyroxenite/peridotite layer. In these samples, whole-rock osmium (Os) isotopic ratios were acquired by TIMS. The trace element and Sr-Nd-Pb isotopic compositions of clinopyroxene from Cr-rich pyroxenitic dykes in Ronda clearly show their subduction-related origin. These results thus constrain the tectonic scenario of the western Mediterranean in Cenozoic time, strongly supporting slab roll-back as the tectonic mechanism responsible for the Miocene extension in the region instead of delamination or convective removal of the thick lithosphere. The trace element and isotopic analyses of clinopyroxene-leachate pairs permit to assess the effectiveness of leaching methods to remove secondary material from mantle minerals. In particular, these data show that leaching is critical to obtain accurate analyses of Pb isotopes that may be significantly affected by anthropogenic contamination. These results thus contribute to the definition of the correct geochemical processing for isotopic analyses of mantle minerals. The Rare Earth Element (REE) compositions of clinopyroxene in Cr-rich websterites are highly variable and are related to the petrological zoning of the Ronda massif. The numerical modelling of these REE variations demonstrates how rock-melt interaction controls the composition of the mantle during melt percolation and may generate chemical heterogeneities at different scales. The PGE and Os isotopic compositions of pyroxenites and peridotites from an approximately 10 metre-thick composite layer in Ronda show how the percolation of the parental melts of pyroxenites affects the composition of peridotites in terms of siderophile elements. These results also permit to assess the length scale of trace element and isotopic re-homogenisation in the mantle during melt percolation. The data on the major trace elements and sulphur isotopic ratios in sulphide grains from different types of pyroxenites highlight how these phases control the composition of the mantle in terms of siderophile elements and sulphur isotopes. Moreover, these results contribute to estimate the distribution of compositional heterogeneities in the mantle and the role of crustal recycling in the generation of mantle-derived melts. The results of this project provide important new knowledge about mantle processes that advance our understanding of the Earth's global evolution over its 4.5 billion year history. These results furnish new information on the way in which a range of major trace elements and isotopic systems behave in the mantle. They contribute to clarify how compositional heterogeneities are distributed in mantle rocks and between minerals and how they influence the composition of mantle-derived melts. Such information can ultimately provide better understanding of the long-term Earth system, its evolution, the relative contributions of primitive and recycled source material contributing to mantle-derived melts, and fundamental issues of the proportion and distribution of basaltic compositions in the deep mantle. These data are important to correctly interpret the nature of seismic tomography signals from the deep Earth. The results of this project will thus help to maintain the worldwide profile of European research institutions in unravelling the nature of Earth's mantle and explain how this fundamental reservoir of our planet works.

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