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Behavior of molybdenum and its isotopes during alteration of the oceanic crust

Final Report Summary - MOLYALOC (Behavior of molybdenum and its isotopes during alteration of the oceanic crust)

During movements of the oceanic crust away from mid-ocean ridges, hydrothermal alteration of the oceanic crust imparts distinct elemental and isotopic signals. The change in isotopic composition of the altered oceanic crust compared to unaltered MORB provides important tracers to identify the involvement of fluids/melts released from the subducted slab in the formation of volcanic arc basalts and ultimately of deep recycling of subducted material. Over the last decade, many studies demonstrated the great potential of Mo isotopes in the understanding of ocean processes and continental weathering (e.g. Barling and Anbar, 2004; Archer and Vance, 2008; Voegelin et al., 2009; Wand et al., 2009). These studies showed that Mo is a redox-sensitive tracer. Although recent studies suggest that diffusion can play an important role in isotope fractionation (e.g. Li; Richter et al., 2003), the heavy element Mo, forming large oxyanion complexes, is unlikely to be substantially isotopically fractionated by diffusion. During continental weathering, the Mo isotopic composition is mainly controlled by the bedrock compositions (Neubert et al. 2011). Recently, work has made the most of improved analytical precision to document more subtle variations in the Mo isotopic composition of basaltic rocks in ocean island basalts and arc volcanoes (e.g. Willbold et al., 2009, Freymuth et al., 2011), likely as a consequence of recycled inputs. However, published Mo isotopic data are still scarce and interpreting subtle Mo isotopic variations in the mantle are difficult (Fig. 1). This Marie-Curie research project undertook a detailed study, in its first part, to construct a Mo isotope profile through the altered, mafic oceanic crust to better constrain this key input into the subduction zone. Additionally the potential role of serpentine in carrying Mo into the mantle was examined. In a second part, the study managed to examine the consequences of the subduction process in fractionating Mo isotopes, using exhumed eclogites. In combination these new data provide dramatically improved constraints on the Mo isotopic composition that gets mixed into the deep mantle and thus the signature expected of recycled components.

In the first part of the project, samples from the deepest drill hole into the Pacific Oceanic crust (IODP Site1256) were analysed for Mo isotopes and Mo concentrations. In addition, a large number of samples from shallower ODP holes were also measured to understand the behavior of Mo during alteration more generally. To maximise the unique resource of samples obtained by deep sea-drilling, many samples were analysed that had already been prepared and studied by others. In trying to understand some anomalously high Mo concentrations in these samples, it was discovered that some had been powdered in a steel mill. This process had clearly compromised the Mo contents and isotope ratios, making it difficult to use any of these earlier analyses. The study on the altered oceanic crust then became focussed on a new set of samples from ODP Site 1256, completely prepared by the fellow. This smaller, but still significant dataset is shown in Fig 2.

A profile through the complete upper altered oceanic crust (passing pillow basalts, sheeted dyke complex, and gabbros) at ODP Site 1256D has been sampled and Mo isotopes and Mo concentrations have been measured. The studied samples show high variability down-hole (over 1.5‰ d98/95Mo), with a tendency to increase with depth. The heterogeneous dehydration of the oceanic crust might lead to extraction of the heavier isotope ratio in the source of arc volcanoes while the lighter isotope ratio might be the source for the light isotope ratio found in some ocean islands basalts. To further constrain the importance of Mo isotopes for recycling, altered ultramafic samples from ODP Site 1272A, as representatives of the hydrated upper mantle that may be subducting in many locations. Both altered crust and serpentine reservoirs are important players in the fluid/melt regions of arc volcanoes, but the work in the project showed that Mo is not abundantly incorporated into serpentine. In general it seems the altered oceanic crust has an isotopic ratio slightly heavier then average continental crust and mantle, highlighting the potential of Mo to fingerprint recycling of subducted material, depending on the consequences of processing beneath the arc. This issue was addressed in the second part of the project.

Eclogites are the high temperature/ high pressure dehydrated equivalent of the altered oceanic crust, thus are the indication of the isotopic composition of material transported into the deep mantle after a full range of subduction zone processes have acted upon it. In additions, some subduction related metasediments have been analyzed. Both data sets are show in Figure 3. The samples studied are from Syros and the Alps, both identified as exhumed fragments of subuducted oceanic crust. The samples show, that there is a significant change in d98/95Mo as lithologies change from hydrated blueschists to anhydrous eclogites. The changes in isotopic compositions indicate that heavy Mo leaves the subducted slab, resulting in an isotopically light residue. This is in-line with the data found in arc volcanoes, initial data measured in Bristol appear to be isotopically heavy. This is also consistent with the light Mo isotopic ratios observed in some ocean island basalts (e.g. the Canaries) previously implicated to contain large amounts of recycled oceanic crust.
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