D. Graham Pearson

Bio: D. Graham Pearson is an academic researcher from Durham University. The author has contributed to research in topics: Mantle (geology) & Kimberlite. The author has an hindex of 32, co-authored 55 publications receiving 3209 citations. Previous affiliations of D. Graham Pearson include University of Calgary & Open University.

Physical, chemical, and chronological characteristics of continental mantle

Abstract: [1] Unlike in the ocean basins where the shallow mantle eventually contributes to the destruction of the overlying crust, the shallow mantle beneath continents serves as a stiff, buoyant “root” whose presence may be essential to the long-term survival of continental crust at Earth's surface. These distinct roles for subcrustal mantle come about because the subcontinental mantle consists of a thick section of material left behind after extensive partial melt extraction, possibly from the wedge of mantle overlying a subducting oceanic plate. Melt removal causes the continental mantle to be cold and strong but also buoyant compared to oceanic mantle. These characteristics allow thick sections of cold mantle to persist beneath continental crust in some cases for over 3 billion years. If the continental mantle becomes gravitationally unstable, however, its detachment from the overlying crust can cause major episodes of intracontinental deformation and volcanism.

The Origin and Evolution of the Kaapvaal Cratonic Lithospheric Mantle

Abstract: A detailed petrological and geochemical study of low-temperature peridotite xenoliths from Kimberley and northern Lesotho is presented to constrain the processes that led to the magmaphile element depletion of the Kaapvaal cratonic lithospheric mantle and its subsequent re-enrichment in Si and incompatible trace elements. Whole-rocks and minerals have been characterized for Re^Os isotope compositions, and major and trace element concentrations, and garnet and clinopyroxene for Lu^Hf and Sm^Nd isotope compositions. Most samples are characterized by Archaean Os model ages, low Al, Fe and Ca contents, high Mg/Fe, low Re/Os, very low (50 1 chondrite) heavy rare earth element (HREE) concentrations and a decoupling between Nd and Hf isotope ratios. These features are most consistent with initial melting at 3 2 Ga followed by metasomatism by hydrous fluids, which may have also caused additional melting to produce a harzburgitic residue. The low HREE abundances of the peridotites require that extensive melting occurred in the spinel stability field, possibly preceded by some melting in the presence of garnet. Fractional melting models suggest that 30% melting in the spinel field or 20% melting in the garnet field followed by 20% spinel-facies melting are required to explain the most melt-depleted samples. Garnet Nd^Hf isotope characteristics indicate metasomatic trace element enrichment during the Archaean.We therefore suggest a model including shallow ridge melting, followed by metasomatism of the Kaapvaal upper mantle in subduction zones surrounding cratonic nuclei, probably during amalgamation of smaller pre-existing terranes in the Late Archaean ( 2 9 Ga). The fluid-metasomatized residua have subsequently undergone localized silicate melt infiltration that led to clinopyroxene garnet enrichment. Calculated equilibrium liquids for clinopyroxene and their Hf^Nd isotope compositions suggest that most diopside in the xenoliths crystallized from an infiltrating kimberlite-like melt, either during Group II kimberlite magmatism at 200^110Ma (Kimberley), or shortly prior to eruption of the host kimberlite around 90Ma (northern Lesotho).

Highly Siderophile Element Constraints on Accretion and Differentiation of the Earth-Moon System

Abstract: A new combined rhenium-osmium- and platinum-group element data set for basalts from the Moon establishes that the basalts have uniformly low abundances of highly siderophile elements. The data set indicates a lunar mantle with long-term, chondritic, highly siderophile element ratios, but with absolute abundances that are over 20 times lower than those in Earth's mantle. The results are consistent with silicate-metal equilibrium during a giant impact and core formation in both bodies, followed by post-core-formation late accretion that replenished their mantles with highly siderophile elements. The lunar mantle experienced late accretion that was similar in composition to that of Earth but volumetrically less than (approximately 0.02% lunar mass) and terminated earlier than for Earth.

Ophiolite genesis and global tectonics: Geochemical and tectonic fingerprinting of ancient oceanic lithosphere

Abstract: Ophiolites, and discussions on their origin and significance in Earth's history, have been instrumental in the formulation, testing, and establishment of hypotheses and theories in earth sciences. The definition, tectonic origin, and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century. Here, we present a review of these ideas as well as a new classification of ophiolites, incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological, geochemical, and tectonic processes during formation in different geodynamic settings. We define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments. Their geochemical characteristics, internal structure, and thickness vary with spreading rate, proximity to plumes or trenches, mantle temperature, mantle fertility, and the availability of fluids. Subduction-related ophiolites include suprasubduction-zone and volcanic-arc types, the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle, melting of the subducting sediments, and repeated episodes of partial melting of metasomatized peridotites. Subduction-unrelated ophiolites include continental-margin, mid-ocean-ridge (plume-proximal, plume-distal, and trench-distal), and plume-type (plume-proximal ridge and oceanic plateau) ophiolites that generally have mid-ocean-ridge basalt (MORB) compositions. Subduction-related lithosphere and ophiolites develop during the closure of ocean basins, whereas subduction-unrelated types evolve during rift drift and seafloor spreading. The peak times of ophiolite genesis and emplacement in Earth history coincided with collisional events leading to the construction of supercontinents, continental breakup, and plume-related supermagmatic events. Geochemical and tectonic fingerprinting of Phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in Earth history, and it can be extended into Precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the Archean.