School of Earth and Environment

Caroline Barnett Caroline Barnett

Postgraduate Researcher

Email address: eecjb@leeds.ac.uk
Room: SCR/8.18 d

Affiliation: Institute of Geophysics and Tectonics

Qualifications

BSc.(Hons.) Geosciences, The Open University, 2010

MSc. Geosciences, University College London, 2013

Memberships/Fellowships

The Mineralogical Society of Great Britain and Ireland, 2013 -

American Geophysical Union, 2013 -

Teaching Interests

I have demonstrated on undergraduate level modules SOEE 1580 Petrology (Year 1) and SOEE 2590 Mineralogy and Petrology (Year 2).

Project details

Project title

The paradoxes associated with mantle reservoirs and basalt petrogenesis

Supervisors

Jason Harvey

Dan Morgan

Start date: 1st October 2013

Funding

This research is funded by NERC studentship NE/L50154/1.

Project outline

Despite its enormous volume, the mantle is largely inaccessible to direct sampling, except when volcanically erupted as xenoliths or tectonically exposed in abyssal peridotites, ophiolites or orogenic massifs. Most of what we know about the composition of the mantle has therefore been derived indirectly from basalts, the products of mantle melting.

Geochemical models of partial melting predict that peridotite and the melts it produces should have complementary elemental compositions, and isotopic signatures should be preserved. However, examination of direct samples from the mantle, and increased analytical precision, have revealed that some initial assumptions regarding links between mantle isotopic signatures and their extrusive counterparts may not be as robust as previously thought; peridotites show small-scale chemical heterogeneity that is not apparently reflected in basalt compositions. For example, it is not possible to reconcile the osmium (Os) isotopic signatures of basalts with those of the mantle using current models of mantle melting.

Examination of peridotites xenoliths reveals two main processes acting to determine their composition: partial melting / melt extraction - which forms the crust and sub-continental lithospheric mantle (SCLM) - and secondary metasomatism. Mass balance calculations further demonstrate that the incompatible trace element budget cannot always be reconciled by the major phases alone, and imply an important role for minor or trace phases. If the chemical budget of peridotite xenoliths can be altered by small volume melts or fluids, and possibly concentrated in minor or trace phases, particularly interstitial components, then not only may the mantle be metasomatised by chemical interaction with percolating melts, but ascending asthenospheric melts may themselves be 'contaminated' by chemical interaction with previously metasomatised mantle.

Osmium is highly chalcophile and partitions strongly into sulphide minerals in peridotite. Peridotites appear to contain at least two geochemically and texturally distinct sulphide populations, one enclosed in silicate minerals, the other interstitial. The existence of an interstitial population of sulphides potentially mobilised by infiltrating melts or fluids introduces the possibility that asthenospheric melts ascending through the SCLM may interact with these sulphides and be chemically altered by them, particularly in relation to their Os-isotopic compositions.

This project will investigate in detail the chemical composition of peridotite mantle xenoliths from intraplate localities worldwide - San Carlos (USA), the Massif Central (France) and the Eifel (Germany) - to better understand the elemental and isotopic composition of this mantle reservoir, and to examine the interactions between basaltic melts and the sub-continental lithospheric mantle.