School of Earth and Environment

Institute of Geophysics and Tectonics (IGT) PhD Projects

The paradoxes associated with mantle reservoirs and basalt petrogenesis

Supervisors: Dr Jason Harvey and Dr Daniel Morgan

Introduction

The Earth’s crust is the part of the planet with which we are, as geologists, the most familiar. But the crust itself was formed from the mantle, a region about which we know surprisingly little.  Volumetrically, the crust is tiny by comparison to the mantle, yet direct samples of mantle material are rare, requiring very particular circumstances to preserve slivers of mantle material as orogenic massifs, ophiolites and, at the smaller scale, as inclusions in volcanic products.  This project  will investigate in detail the composition of peridotite and pyroxenite mantle samples from intra-plate localities worldwide (including Kilbourne Hole, USA, The Eifel volcanic field (Germany) and the French Massif central) with the aim to:

Figure 1. Basaltic cones in the French Massif Central. Intraplate volcanism such as this brings occasional mantle samples to the surface which allow us to test the hypotheses that link mantle composition to basalt petrogenesis.
Figure 1. Basaltic cones in the French Massif Central. Intraplate volcanism such as this brings occasional mantle samples to the surface which allow us to test the hypotheses that link mantle composition to basalt petrogenesis.
  1. better understand the elemental and isotopic   composition of the two largest mantle reservoirs
  2. determine how these two reservoirs contribute to a range      of basaltic melts and in what proportions
  3. examine the interaction between basaltic melts and the        sub-continental lithospheric mantle.

Background

Despite its enormous volume, the mantle is inaccessible, and obscured by the overlying crust, meaning that representative samples of the mantle can be difficult to come by. Apart from a few instances where the mantle is exposed, the majority of what we think we know about the composition of the mantle has, to date, been derived indirectly. Geochemical models tell us about incompatible trace elements during partial melting in the mantle, predicting that peridotite and the lavas that it produces should have complimentary elemental compositions. Moreover, isotopic ratios of elements should be conserved, such that the isotopic fingerprint of a mantle reservoir is reproduced in the melts that it produces.

The problem is that as analytical developments have progressed, and study has become more detailed, (e.g. Warren et al., 2009; Harvey et al., 2011; Burton et al., 2012), it has been revealed that some initial assumptions made regarding links between mantle isotopic signatures and their extrusive counterparts may not be as robust as previously thought. For example, it is not possible to reconcile the osmium (Os) and lead (Pb) isotopic signatures of basalts with those of the mantle using current models of mantle melting. Even Neodymium (Nd) isotopes, long thought to be a robust measure of mantle reservoir compositions, do not always correspond between peridotite and the igneous crust it is inferred to have produced. This is compounded by a relative lack of information concerning pyroxenites in the mantle. Although thought to be volumetrically minor, under prevailing mantle pressure-temperature conditions pyroxenites are predicted to melt much more easily than peridotites, with the potential to create large volumes of basaltic melt (e.g. Sobolev et al., 2007), yet comparatively little is known about their isotopic character and how relative degrees of peridotite- and pyroxenite-melting affect the composition of basaltic melts.

Figure 2. Typical examples of ultramafic rocks of the type to be examined during this project. Left: Peridotite from the San Carlos Volcanic Field, Arizona, USA. Right: pyroxenite, preserved as a dark band in peridotite, from the French Massif Central. Field of view 20 cm.

 

 

Project outline

The project will focus on characterisation of pyroxenite endmembers from various classical world localities for xenolith collection such as Kilbourne Hole (New Mexico), the French Massif Central and the volcanic fields of Eifel, Germany.  Initial study will focus on modal mineralogy, composition and conditions of formation, as well as a textural assessment to constrain possible metasomatic events that could have affected the samples over geological time.  The analytical geochemistry side of the project will constitute the backbone of the project. Data will be obtained using a variety of techniques such as X-Ray Fluorescence (XRF) for major element analysis, Inductively-Coupled Plasma Mass Spectrometry (ICPMS) for trace element determinations, and isotopic ratio analysis of parent-daughter systems such as Re-Os, Sm-Nd, U-Pb and Rb-Sr, in order to constrain at high resolution the isotopic character of the samples.  This work will be complementary to a major research initiative at Leeds being headed by Jason Harvey.  Full training in the analytical component of the PhD will be offered, with subsidiary training in data modelling and data processing to undertake the modelling component of the PhD.  Field collection of additional samples is a possibility and would likely focus on the Northern domain of the French Massif Central and Eifel, Germany.

Applications

Applicants will be expected to hold a good degree in geology, geochemistry, or a closely related discipline. The project will involve aspects of geochemical modelling, petrological study and isotopic analysis in a clean-chemistry laboratory environment. 

References

Burton et al (2012) Unradiogenic lead in Earth’s upper mantle. Nature Geoscience 5, 570-573.doi: 10.1038/NGEO1531

Harvey et al., (2011) Osmium mass balance in peridotite and the effects of mantle-derived sulphides on basalt petrogenesis Geochimica et Cosmochimica Acta 75, 5574-5596. doi:10.1016/j.gca.2011.07.001

Lustrino and Wilson (2007) The circum-Mediterranean anorogenic Cenozoic igneous province. Earth Science Reviews 81, 1-65. doi:10.1016/j.earscirev.2006.09.002

Sobolev et al., (2007) The Amount of Recycled Crust in Sources of Mantle-Derived Melts. Science 316, 412-417. doi: 10.1126/science. 1138113

Warren et al., (2009) An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions. Journal of Geophysical Research 114, 1-36. doi:10.1029/2008JB006186

Wittig et al., (2007) U–Th–Pb and Lu–Hf isotopic constraints on the evolution of sub-continental lithospheric mantle, French Massif Central. Geochimica et Cosmochimica Acta 71, 1290-1311. doi:10.1016/j.gca.2006.11.025