Mass-dependent molybdenum isotope variations in high temperature systems

Abstract

Over the last few years, the molybdenum (Mo) isotope system has gained considerable momentum as a novel tracer for petrogenetic processes in magmatic rocks. Yet several aspects of Mo isotope systematics in high temperature systems remain poorly constrained. In this contribution, I examine mass-dependent Mo isotope variations in magmatic rocks from a diversity of petrogenetic systems as well as over different length scales in order to identify and characterise the mechanisms controlling their Mo isotope composition during important geological processes. The systems investigated here include the Izu oceanic island arc (Western Pacific), the mafic-ultramafic Rum layered intrusion (NW Scotland) and the continental arc of the Central Andes (northern Chile and southern Peru). These examples allow the study of the control of processes such as fluid release during subduction, magma-crust interaction(s) and open system magma replenishment, and formation and evolution of the continental crust, respectively, on the Mo isotope variations of the magmas produced. At the Izu arc, the high d98/95Mo values of mafic arc lavas are associated with indices for fluid input in the magma source and indicate that slab-derived fluids transfer isotopically heavy Mo from the slab to the sub-arc mantle wedge. In the Izu system, fluids acquire a high d98/95Mo (0.1 - 0.25 per mil) due to Mo isotope fractionation during their passage through the subducted slab. The Mo isotope signature of slab-derived fluids is imprinted onto arc magmas via fluid induced melting, exerting a greater control on the d98/95Mo of magmas derived from highly depleted mantle components. The study of the Rum layered intrusion, despite analytical difficulties that precluded the acquisition of high-quality Mo isotope data, revealed the promising potential of Mo isotopes as a tracer for magma-crust interactions and their importance for sulphide saturation in mafic-ultramafic intrusions. However, distinguishing between mechanisms for crustal input (e.g., assimilation versus fluid migration), as well as the potential controls of fractional crystallisation, require further research. In the case of the Central Andes, Mo isotope variations and relative Mo enrichments compared to similarly incompatible elements in evolved ignimbrites and lavas suggest an important role for intra-crustal differentiation processes in controlling the Mo isotope composition of crustal rocks, likely associated with Mo transport and re-distribution via late-stage fluid exsolution processes. In conjunction with literature data, I provide new estimates for the d98/95Mo of the upper continental crust at ~0.05 - 0.10 per mil, and the first constraint at the Mo isotope composition of the lower continental crust, the latter based on a small set of high-temperature metamorphic samples from the Andean basement. Throughout this study, fluid activity is found to play an important role in generating the variations in d98/95Mo observed in magmatic rocks in a diversity of geological systems. Accessory phases such as oxides and sulphides may also be relevant drivers of Mo isotope fractionation in high temperature environments. While further research is needed to better constrain the control of these mechanisms on the Mo isotope compositions of crustal rocks, the findings of this thesis provide valuable new insights into the Mo isotope systematics in high temperature systems and showcase the potential of the Mo isotope system to trace mass transfer processes in a variety of settings and over different length scales. (Please refer to the full text for the formatted version of this abstract)

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Brian O'Driscoll (Supervisor) & Romain Tartese (Supervisor)