D.G. Pearson

The continental lithospheric mantle: characteristics and significance as a mantle reservoir

The continental lithospheric mantle (CLM) is a small–volumed (ca. 2.5% of the total mantle), chemically distinct mantle reservoir that has been suggested to play a role in the source of continental and oceanic magmatism. It is our most easily identifiable reservoir for preserving chemical heterogeneity in the mantle. Petrological and geophysical constraints indicate that the maximum depth of the CLM is ca. 250 km. There is a clear secular variation of CLM composition, such that CLM formed in the last 2 Gyr is less depleted and therefore less dynamically stable than ancient CLM formed in the Archean. We present new trace–element data for kimberlite–hosted lithospheric peridotites and metasomites. These data, combined with other data for spinel peridotites from non–cratonic regions, show that neither hydrous nor anhydrous lithospheric mantle xenoliths make suitable sources for continental or oceanic basalts. Addition of a hydrous phase, either amphibole or phlogopite, to depleted peridotite results in positive Nb and Ti anomalies that are the opposite of those predicted for some flood–basalt sources on the basis of their trace–element abundances. Overall, the Sr and Nd isotopic composition of cratonic and non–cratonic CLM is close to bulk Earth, with cratonic CLM showing small numbers of extreme compositions. Thus, while the CLM is certainly ancient in many locations, its average composition is not significantly ‘enriched’ over primitive upper mantle, in terms of either radiogenic isotopes or trace elements. These characteristics, plus a change in lithospheric chemistry with depth, indicate that the elemental and isotopic composition of lithospheric mantle likely to be re–incorporated into convecting mantle via delamination/thermal erosion processes is probably not very distinct from that of the convecting mantle. These observations lead us to question the requirement for CLM participation in the source of oceanic magmas and to promote consideration of a mantle that is chemically heterogeneous on all scales. Hf and Nd isotope compositions identify a distinctive source component in deeply derived alkaline volcanics associated with continents. This component cannot be constrained to the CLM but may originate from a deeper reservoir of ancient, subducted oceanic crust stored in the mantle.

Late Accretion on the Earliest Planetesimals Revealed by the Highly Siderophile Elements

Coming Late to the Planetesimal Highly siderophile (iron-loving) elements (Re, Os, Ir, Ru, Rh, Pt, Pd, and Au) must have been added to the mantles of Earth, the Moon, and Mars after their iron cores formed; otherwise the mantles would be devoid of these elements, which tend to be segregated to the core. Dale et al. (p. 72) report data on highly siderophile elements in rocks from different planetary bodies, including asteroid 4 Vesta and other differentiated asteroids, which are representative of the planetesimals from which the solar system planets formed. Like the larger planetary bodies, differentiated asteroids, which formed over the first few million years of the solar system, bear the evidence of the late addition of highly siderophile elements to their mantles. Thus, this process was not unique to Earth, the Moon, and Mars and happened over an extended period of time in the inner solar system. Analysis of meteorites shows that unprocessed material was accreted to both planets and asteroids 150 million years after the start of the solar system. Late accretion of primitive chondritic material to Earth, the Moon, and Mars, after core formation had ceased, can account for the absolute and relative abundances of highly siderophile elements (HSEs) in their silicate mantles. Here we show that smaller planetesimals also possess elevated HSE abundances in chondritic proportions. This demonstrates that late addition of chondritic material was a common feature of all differentiated planets and planetesimals, irrespective of when they accreted; occurring ≤5 to ≥150 million years after the formation of the solar system. Parent-body size played a role in producing variations in absolute HSE abundances among these bodies; however, the oxidation state of the body exerted the major control by influencing the extent to which late-accreted material was mixed into the silicate mantle rather than removed to the core.

Adaptive dosing and platinum-DNA adduct formation in children receiving high-dose carboplatin for the treatment of solid tumours

A pharmacokinetic–pharmacodynamic study was carried out to investigate the feasibility and potential importance of therapeutic monitoring following high-dose carboplatin treatment in children. High-dose carboplatin was administered over 3 or 5 days, with the initial dose based on renal function, to achieve target area under the plasma concentration–time curve (AUC) values of 21 or 20u2009mgu2009ml−1.min, respectively. Dose adjustment was carried out based on observed individual daily AUC values, to obtain the defined target exposures. Platinum–DNA adduct levels were determined in peripheral blood leucocytes and toxicity data were obtained. Twenty-eight children were studied. Based on observed AUC values, carboplatin dose adjustment was performed in 75% (21 out of 28) patients. Therapeutic monitoring resulted in the achievement of carboplatin exposures within 80–126% of target AUC values, as compared to estimated exposures of 65–213% of target values without dose adjustment. The carboplatin AUC predicted with no dose modification was positively correlated with pretreatment glomerular filtration rate (GFR) values. Higher GFR values were observed in those patients who would have experienced AUC values >25% above the target AUC than those patients attaining AUC values >25% below the target AUC, following renal function-based dosing. Platinum–DNA adduct levels correlated with observed AUC values on day 1 of carboplatin and increased over a 5-day course of treatment. Real-time monitoring of carboplatin pharmacokinetics with adaptive dosing is both feasible and necessary for the attainment of consistent AUC values in children receiving high-dose carboplatin treatment. Pharmacodynamic data suggest a strong correlation between carboplatin pharmacokinetics and the drug–target interaction.

The evolution of calcite-bearing kimberlites by melt-rock reaction: evidence from polymineralic inclusions within clinopyroxene and garnet megacrysts from Lac de Gras kimberlites, Canada

Megacrystic (>1xa0cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) xenocrysts within kimberlites from Lac de Gras (Northwest Territories, Canada) contain fully crystallized melt inclusions. These ‘polymineralic inclusions’ have previously been interpreted to form by necking down of melts at mantle depths. We present a detailed petrographical and geochemical investigation of polymineralic inclusions and their host crystals to better understand how they form and what they reveal about the evolution of kimberlite melt. Genetically, the megacrysts are mantle xenocrysts with peridotitic chemical signatures indicating an origin within the lithospheric mantle (for the Cr-diopsides studied here ~4.6xa0GPa, 1015xa0°C). Textural evidence for disequilibrium between the host crystals and their polymineralic inclusions (spongy rims in Cr-diopside, kelyphite in Cr-pyrope) is consistent with measured Sr isotopic disequilibrium. The preservation of disequilibrium establishes a temporal link to kimberlite eruption. In Cr-diopsides, polymineralic inclusions contain phlogopite, olivine, chromite, serpentine, and calcite. Abundant fluid inclusion trails surround the inclusions. In Cr-pyropes, the inclusions additionally contain Al-spinel, clinopyroxene, and dolomite. The major and trace element compositions of the inclusion phases are generally consistent with the early stages of kimberlite differentiation trends. Extensive chemical exchange between the host phases and the inclusions is indicated by enrichment of the inclusions in major components of the host crystals, such as Cr2O3 and Al2O3. This chemical evidence, along with phase equilibria constraints, supports the proposal that the inclusions within Cr-diopside record the decarbonation reaction: dolomitic meltxa0+xa0diopsidexa0→xa0forsteritexa0+xa0calcitexa0+xa0CO2, yielding the observed inclusion mineralogy and producing associated (CO2-rich) fluid inclusions. Our study of polymineralic inclusions in megacrysts provides clear mineralogical and chemical evidence for an origin of kimberlite that involves the reaction of high-pressure dolomitic melt with diopside-bearing mantle assemblages producing a lower-pressure melt that crystallizes a calcite-dominated assemblage in the crust.

The North America mid-Cretaceous kimberlite corridor: Wet, edge-driven decompression melting of an OIB-type deep mantle source

Thirty new high precision U-Pb perovskite and zircon ages from kimberlites in central North America delineate a corridor of mid-Cretaceous (115 to 92 Ma) magmatism that extends ∼4000 km from Somerset Island in Arctic Canada through central Saskatchewan to Kansas, U.S.A. The least contaminated whole rock Sr, Nd and Hf isotopic data, coupled with Sr isotopic data from groundmass perovskite indicates an exceptionally limited range in Sr-Nd-Hf isotopic compositions, clustering at the low eNd end of the OIB array. These isotopic compositions are distinct from other studied North American kimberlites and point to a sub-lithospheric source region. This mid-Cretaceous kimberlite magmatism cannot be related to mantle plumes associated with the African or Pacific large low-shearwave velocity province (LLSVP). All three kimberlite fields are adjacent to strongly attenuated lithosphere at the edge of the North American craton. This facilitated edge-driven convection, a top-down driven processes that caused decompression melting of the transition zone or overlying asthenosphere. The inversion of ringwoodite and/or wadsleyite and release of H2O, with subsequent metasomatism and synchronous wet partial melting generates a hot CO2- and H2O-rich proto-kimberlite melt. Emplacement in the crust is controlled by local lithospheric factors; all three kimberlite fields have mid-Cretaceous age, re-activated major deep-seated structures that facilitated kimberlite melt transit through the lithosphere.

Multiple Growth Episodes or Prolonged Formation of Diamonds? Inferences from Infrared Absorption Data

The infrared characteristics of 21 sulphide inclusion-bearing diamonds from Finsch Mine, 1 sulphide inclusion-bearing diamond from Udachnaya and 18 silicate inclusion-bearing diamonds from Premier were examined and modelled to investigate the complexity of diamond genesis. Internal heterogeneities in N-abundance and aggregation state within individual diamonds at Finsch range from 5 to 336 at.ppm and 2–60 % of B-defects, respectively. The Udachnaya diamond 3648 displays a steep decrease from the core to the rim of N-abundance from 482 to 10 at.ppm. Nitrogen aggregation state describes the same trend with value of 86 %B in the core down to 14 %B in the rim. Internal variations in N-abundance and aggregation state within diamonds from Premier are 93–654 at.ppm and of 7–62 %B, respectively. These variations reflect more likely multiple growth episodes of diamond at distinct ages rather than steady changes in temperature conditions during prolonged diamond growth. Modelling of infrared characteristics indicates that some diamonds have experienced distinct growth episodes over extended time periods with estimates up to 2,387 ± 931 Ma. There are implications for dating studies, indicating that isochron ages may be flawed as there appears to be no single formation age for a single diamond. N-abundance and aggregation state mapping by FTIR provide the opportunity to constrain diamond growth history for selecting diamonds for dating.

High precision rhenium and platinum isotope dilution analyses by plasma ionisation multicollector mass spectrometry

The platinum group elements (PGE; Os, Ir, Ru, Rh, Pt, Pd) and rhenium, which is considered with the PGEs due to its similar chemical behaviour, are powerful tools for understanding several fundamental aspects of the origin and evolution of the Earth such as core segregation, late accretion histories, 1 2 and core-mantle exchange. 4 Abundance measurements for the PGE are useful geochemical tracers in a variety of terrestrial and extraterrestrial materials. 5 6 7 8 In addition the ability to accurately analyze Pt at very low levels is being applied to studies of cancer therapy drugs 9 10 and environmental studies. The information obtained from abundance measurements of the PGE and Re, can be complemented by two geologically important isotopic decay schemes;

Variable H and O isotopes in Tongan basaltic glasses: Source or degassing?

New H and O isotope data are presented for submarine basaltic and basaltic-andesite glasses dredged from the Tongan arc (north of Tongatapu), the Fonualei Rifts (FR, a nascent backarc spreading centre) and the Mangatolu Triple Junction (MTJ). These data complement a comprehensive trace element and Nd-Sr-Pb-Hf isotope dataset. The highest δD values (-50‰ to -30‰) occur in the MTJ and FR. There is a positive co-variation between δD and H2O abundance, initially suggesting addition of a δD-rich H2O component derived from the subducted slab to the upper mantle source. In this scenario, the most negative values (≥-85‰), which have lower water contents, would represent uncontaminated mantle wedge. This is consistent with a MORB source component (typically -80‰ ± 10‰). However, H2O concentration and δD also increase with depth of eruption, suggesting that degassing may explain the fractionation observed. In this scenario the highest δD (-40‰ to -30‰) may be typical of the whole arc-backarc mantle source, while the lower values are produced by degassing (whereby D is preferentially incorporated into H2O rather than melt). Extrapolation to infinite water for the whole arc-FR dataset gives a δD value of ca. -26‰. Samples from the MTJ (n=2) are offset to more elevated δD at similar H2O contents. Ba/La ratios are highest at the central Tongan volcanoes, indicating that these have the greatest fluid-mobile trace element flux, but they do not possess complementary high δD. Thus, either the D-rich signature is masked by fractionation during degassing or the trace element flux is partially decoupled from the water flux. δO values range from 4.6‰ to 6.2‰. There is a poor negative co-variation of δO and δD but this is most likely masked by the effects of H fractionation during degassing. Such a co-variation requires the transfer of an O-depleted and D-rich fluid from high-T altered oceanic crust, rather than low-T altered crust or sediment. Trace element analysis and petrology of Martian meteorite RBT04262 H.A. DALTON*, C.-T.A. LEE, A.H. PESLIER, A.D. BRANDON AND T. LAPEN Rice Univ., Dept of Earth Science, MS 126, PO Box 1892, Houston, TX 77251, USA (*correspondence: heather.a.dalton@rice.edu) (ctlee@rice.edu) ARES, NASA JSC, Houston, TX 77058, USA (anne.h.peslier@nasa.gov, alan.d.brandon@nasa.gov) Jacobs Tech., E.S.C.G., Houston, TX 77058, USA Univ. of Houston, Dept of Geosciences, Houston, TX 77204, USA (tjlapen@uh.edu)