Boron uptake in oceanic gabbros; a key tracer of subduction processes

Summary:

This project is an exciting opportunity to take part in a major international science program, IODP (International Ocean Discovery Program). As a student, you will enter the field of marine geology, gaining an understanding of ocean floor processes of tectonics and seawater-lithosphere interaction, with implications for processes of devolatilisation and volcanism at subduction zones. You will be part of a team of over 25 international scientists working on IODP Expedition 399, with the opportunity to visit IODP repositories, work in the Pisa CNR-IGG labs, present your results at an IODP post-cruise meeting and international conferences, and conduct fieldwork in Cyprus. It is hoped that you will be able to participate either in an IODP Expedition or related research cruise during the studentship.

Background:

Boron is among the most important proxies for dewatering of subducted slabs and sediments underneath subduction zones (DeHoog and Savov, 2018). It shows systematic variations in both boron content and boron isotopic ratio (δ¹¹B) with distance away from the trench occurring in erupted and plutonic rocks of many volcanic arcs.

Serpentinites collected by dredging or shallow drilling so far analysed on the mid-Atlantic Ridge (MOR) are invariably enriched in boron and with high to very high δ¹¹B ratios (Boschi et al., 2008; Harvey et al., 2014). This signal indicates alteration by seawater(DeHoog and Savov, 2018). These data have been increasingly used to infer that the hydrated upper mantle section in downgoing subducting slabs will also be enriched in boron, and that this is the principal reservoir for boron found at all arc volcanoes (Konrad-Schmolke et al., 2016). However, the subcrustal mantle in a downgoing slab cannot be sampled directly, and the samples from exposed peridotites on the seafloor are atypical, because by definition they are exposed by faulting. McCaig et al. (2018) showed that troctolites and olivine gabbros exposed at Hess Deep in the Pacific Ocean are significant sinks for seawater derived boron, and suggested that significant boron resides in the lower crustal (gabbroic) section of the oceanic crust. Importantly, such enrichments might occur before downgoing fluids can reach the mantle section of the subducted oceanic crust, for example in bend fault environments, where large scale serpentinisation of the slab mantle has been suggested (Van Avendonk et al., 2011). In these circumstances the oceanic slab mantle might be hydrated but not enriched in boron, and the distinct boron enrichments and the very large isotope fractionation may come instead from significantly hydrated lower crustal gabbros.

Aims and Objectives:

1. To test the hypothesis that boron enrichment decreases with distance from the seafloor by analyses of gabbroic and serpentinised rocks recovered from IODP deep drill holes in gabbros.
2. To compare oceanic data with sections through gabbros and mantle peridotites in ophiolites
3. To study the processes of boron uptake by microanalysis of alteration minerals.
4. To apply the results to modelling of boron release from subducting slabs

Methods:

Central to this project is the Atlantis Massif oceanic core complex (on the MAR), where Hole U1309D penetrates 1415m below the seafloor (Blackman et al., 2011). This Hole will be deepened during IODP Expedition 399 in April-June 2023,

( https://iodp.tamu.edu/scienceops/expeditions/deepening_hole_u1309d.html )

for which Dr McCaig is co-chief scientist. Alteration reduces in intensity downwards in the existing Hole, but partially serpentinised olivine-rich troctolites are nevertheless found at a range of depths (Beard et al., 2009; Frost et al., 2008). If boron contents decrease downhole, that will be a sign of removal of boron from the downward moving B-rich fluids and the formation of a little-explored boron reservoir in the lower oceanic crust.  In addition, a new sample suite of serpentinised and deformed peridotites from a 200 m Hole close to the Lost City Hydrothermal Field will be collected.

As an exposed analogue we will collect and analyse selected gabbroic and ultramafic samples from the Troodos ophiolite, Cyprus.

This proposal will thoroughly characterise hydrous alteration minerals and borehole fluids for boron and other fluid-mobile elements in the Atlantis Massif at a range of depths below seafloor. Additional data will be collected from deep boreholes 735b and U1473A on the SW Indian Ridge, a gabbro massif with a much higher temperature history of alteration and deformation. Methods will include whole rock analysis of B and δ¹¹B, in situ measurement of boron by LA-ICPMS and SIMS measurements at the Edinburgh ion probe facility (subject to application), SEM and EPMA characterisation of alteration phases, and measurement of Sr and oxygen isotopes. The student will have opportunities for fieldwork in Cyprus and it is hoped ocean-going experience. Although direct involvement in Exp 399 is unlikely, the student will attend the post-cruise meeting as a participant in post-cruise research.