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Processes of deformation and uptake of volatile elements in large oceanic fault zones

Dr Andrew McCaig (SEE), Dr Ivan Savov (SEE), Dr Jason Harvey (SEE) and Dr Thomas Mueller (SEE)

Contact email: a.m.mccaig@leeds.ac.uk

Project synopsis:

Fig. 1: Cross-section through the Atlantis Massif oceanic core complex (Boschi et al., 2006)

Fig. 2: Typical fault breccia from IODP Expedition 357, containing clasts of higher temperature amphibole schist. 357_68B_15-20. PPL

We propose an integrated study of deformation and alteration in major fault zones cutting the lower oceanic crust and upper mantle. Volatile elements such as boron are key indicators of the extent to which seawater is recycled into the deep mantle, and these elements are introduced into the oceanic lithosphere by seawater alteration, often focused along fault zones. Work will focus on new IODP sample sets of fault rocks from Expedition 345 (Hess Deep), expedition 357 (Atlantis Massif core complex) and hopefully ICDP drilling in the Oman ophiolite. We will combine traditional isotopic methods (Sr and O) with boron contents and boron isotopic analysis to assess the mechanisms and extent of hydration and the contribution that early alteration of the ocean floor may make to isotopic and elemental fluxes in subduction zones. The combination of Sr-O-B isotopes offer a wealth of opportunities for the unravelling of the extent and history of hydration of the lithosphere at a mid-ocean ridges and will provide invaluable information regarding the nature of material entering into subduction zones, and perhaps even further into the deep mantle.

Background

Subduction zone volcanism could not occur without release of water from subducting slabs. However the extent of hydration of the oceanic lithosphere is uncertain, and this is one of the biggest uncertainties in modelling how much water may be recycled into the deep mantle (eg. Konrad-Schmolke et al., 2016). A major source of water is serpentinite, but to hydrate the upper mantle almost certainly requires fluid influx along faults, for example transform faults, large normal faults/detachment faults and bend faults at subduction zones.  A key tracer of release of water from serpentinite and other hydrous phases is the volatile element boron, which has a distinctive isotopic signature in seawater and is easily fixed in serpentine and other hydrous minerals. Recent work suggests a correlation between high boron content in arc volcanics and subduction of transform faults (Manea et al., 2014). Hence it is important to study in detail the uptake of boron and other elements during deformation, comparing this with more static hydration. 

Fig. 3: Cataclasite from IODP Expedition 345 to Hess Deep, strongly overprinted by prehnite and cut by prehnite veins

Two recent IODP expeditions have recovered excellent samples of highly altered fault rocks:

  • Expedition 357 in 2015 used sea bottom corers to sample a detachment fault capping the Atlantic Massif (Fig. 1) at 30° N on the mid-Atlantic Ridge (Früh-Green et al., 2016). Fault rocks include breccias and cataclasites overprinting ductile fabrics in mafic rocks (Fig. 2), and talc-tremolite-chlorite schists overprinting serpentinite. The serpentinites and talc schists are known to have high 87Sr/86Sr ratios (McCaig et al., 2010) high boron concentrations, and high values of δ11B (Boschi et al. 200…), but recovery in Expedition 357 is much better than previously, allowing sampling of overprinting assemblages in detail.  Active magmatism occurred during deformation, with basaltic melt invading fault breccias.
  • Expedition 345 to Hess Deep sampled lower crustal gabbros from a major fault scarp cutting right through fast spread ocean crust (Gillis et al., 2013). Cataclasites are often overprinted by assemblages rich in prehnite and chlorite (Fig. 3). These rocks are our only current samples of fault zones in the lower ocean crust in the Pacific, and are therefore the best constraint we have on altered ocean crust entering subduction zones at trenches. Our initial analyses show that the altered rocks are significantly enriched in boron and have high δ11B and 87Sr/86Sr values.

In addition, samples of fault zones in the Oman ophiolite will be studied in collaboration with Jurgen Koepke (Hannover), and it is hoped that new samples may be collected in the ICDP drilling programme in Oman over the next 2 years.

Objectives

  1. Thorough characterisation of deformation fabrics and overprinting events using SEM and electron microprobe to establish the sequence of events and deformation conditions.
  2. Small whole rock sampling to capture changes in whole rock composition (ICP-OES/ICPMS), and O, Sr, B and Li isotopes (TIMS and MC-ICPMS)
  3. In situ study of redistribution of light elements during deformation and alteration using TOF-SIMS and nanoSIMS techniques (in Manchester).
  4. Prediction of the devolatilisation of alteration assemblages during subduction using pseudosection analysis.

The student will resample IODP core in the College Station (Texas) and Bremen core repositories.   There may be the opportunity to be involved in the International Continental Drilling Programme in the Oman ophiolite, or in IODP expeditions. Work in Oman will focus on outcrop-scale sampling of altered fault zones cutting the lower crust (eg. Coogan et al., 2006). An initial suite of IODP samples is already available in Leeds and will be characterised using SEM and electron probe to assess the conditions of alteration and select domains for isotopic analysis (eg. Fig. 2, 3). Sr isotope and ICPMS work will be undertaken at Leeds, oxygen isotopes at SUERC East Kilbride, and boron analysis in collaboration with Samuele Agostini (CNR - B isotope facility, Pisa), and the Bristol University MC-ICPMS facility.

This project will give the student a strong grounding in characterisation of deformation and alteration in mafic and ultramafic rocks in outcrop, hand specimen, optical microscope and SEM/electron probe, and in analytical methods such as Thermal Ionisation Mass Spectrometry (TIMS), MC-ICPMS and LA-ICPMS.

The student will join the High Temperature Geochemistry Group. Details of group members and current projects are here:

References

Coogan L. A., Howard, K.A., Gillis, K.M., Bickle M.J., Chapman, H. Boyce A.J., Jenkin G.R.T. & Wilson R.N. (2006) Chemical and thermal constraints on focussed fluid flow in the lower oceanic crust A. J. Sci. 306, 389-427;

Boschi, C., Früh-Green, G.L., Delacour, A., Karson, J.A., and Kelley, D.S., (2006). Mass transfer and fluid flow during detachment faulting and development of an oceanic core complex, Atlantis Massif (MAR 30°N). Geochemistry, Geophysics, Geosystems, 7:Q01004. http://dx.doi.org/10.1029/2005GC001074

Früh-Green, G.L., Orcutt, B.N., Green, S., Cotterill, C., and the Expedition 357 Scientists, (2016). Expedition 357 Preliminary Report: Atlantis Massif Serpentinization and Life. International Ocean Discovery Program. http://dx.doi.org/10.14379/iodp.pr.357.2016

Gillis, K.M., et al, (2013) Primitive layered gabbros from fast-spreading lower oceanic crust, Nature, 505, pp.204-207. doi: 10.1038/nature12778

Harvey J; Savov IP; Agostini S; Cliff R; Walshaw R (2014), Si-metasomatism in serpentinized peridotite: the effects of talc-alteration on strontium and boron isotopes in abyssal peridotites from Hole 1268a, ODP Leg 209. Geochimica et Cosmochimica Acta, 126, pp.30-48. doi: 10.1016/j.gca.2013.10.035;

Konrad-Schmolke, M., R. Halama, and V. C. Manea (2016), Slab mantle dehydrates beneath Kamchatka—Yet recycles water into the deep mantle, Geochem. Geophys. Geosyst., 17, doi:10.1002/2016GC006335.

McCaig AM; Cliff RA; Escartin J; Fallick AE; MacLeod CJ (2007) Oceanic detachment faults focus very large volumes of black smoker fluids. Geology, 35, pp.935-938;

McCaig AM; Delacour A; Fallick AE; Castelain T; Früh-Green GL (2010) Detachment Fault Control on Hydrothermal Circulation Systems: Interpreting the Subsurface Beneath the TAG Hydrothermal Field Using the Isotopic and Geological Evolution of Oceanic Core Complexes in the Atlantic, In: Rona PA; Devey CW; Dyment J; Murton BJ (Ed) Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges, AGU Geophysical Monograph, 108, AGU, pp.207-240.

Schmidt, M.W. & Poli. S., Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation (1998) EPSL, 163: 361-379;

Till, C.B., Grove, T.L. & Withers, A.C. The Beginnings of Hydrous Mantle Wedge Melting, (2011) Contrib. Mineral. Petrol., DOI 10.1007/s00410-011-0692-6

Related undergraduate subjects:

  • Geology