Geoff Nowell

Bio: Geoff Nowell is an academic researcher from Durham University. The author has contributed to research in topics: Mantle (geology) & Kimberlite. The author has an hindex of 42, co-authored 118 publications receiving 6600 citations. Previous affiliations of Geoff Nowell include University of Nottingham & Instituto Superior Técnico.

Hf-Nd Element and Isotope Perspective on the Nature and Provenance of Mantle and Subduction Components in Western Pacific Arc-Basin Systems

Abstract: This paper develops methods for using the integrated study of Hf-Nd element and isotope covariations to define the nature and provenance of the mantle and subduction inputs to subduction systems. In particular, it can be demonstrated that (1) Hf-Nd isotope space permits discrimination between mantle of Pacific and Indian provenance, (2) displacements from mantle arrays on Hf-Nd isotope and trace element projections can be related to the magnitude, source and composition of the subduction input, and (3) Hf-Nd isotope and trace element covariations can be used to interpret high field strength element (HFSE) anomalies [specifically, Hf anomalies on extended rare earth element (REE) patterns] in subduction-related magmas. These methods are tested using published volcanic arc data coupled with new data from the many components of the Izu-Bonin-Mariana (IBM) subduction system, namely the pre-subduction marginal basins, the Eocene to Recent volcanic arcs, and the crust, volcanogenic sediments and pelagic sediments of the subducting Pacific plate. The results of the IBM study show that the mantle that fed the IBM system was always of Indian provenance and that Pacific volcanogenic sediments make the most significant, though variable, contribution to the subduction component. Modelling demonstrates that the Nd/Hf ratio of the subduction component probably lay between 40 and infinity and thus was probably the main cause of the negative HFSE anomalies that characterize much of the Recent arc. This result may further indicate that the subducting sediment lost elements to the mantle wedge mostly by dehydration rather than fusion. In contrast, the data also show that the positive Hf anomalies that characterize much of the Protoarc cannot be attributed directly to subduction. One option consistent with Hf-Nd systematics is that the positive Hf anomalies in the Protoarc boninites were caused by fusion of mafic veins in their shallow mantle sources. Comparison with published data on other arcs shows significant inter-arc variations. For example, the subduction components in near-continent arcs (Banda, Lesser Antilles) appear to have lower Nd/Hf ratios more consistent with sediment fusion, and at least one arc (Tonga-Fiji) carries evidence of temporal variations in mantle provenance.

Common-Pb corrected in situ U–Pb accessory mineral geochronology by LA-MC-ICP-MS

Abstract: LA-MC-ICP-MS is shown to be a rapid, precise and accurate method for determination of U–Pb ages of accessory minerals. For the protocol described, total analysis time is <3 min with a main acquisition sequence of only 30 s. Using a raster ablation protocol, within-run inter-element fractionation can be effectively eliminated and an external ablation standard used to quantify an overall error for the analysis. Reproducibilities of 206Pb/238U = ca. 3% and 207Pb/206Pb = <1% (2 σ) are achieved, with the resulting age accurate to within 1% as determined using in-house samples previously characterised by TIMS. A key control on the Pb/Pb reproducibility is shown to be the size of the 207Pb peak and an error propagation curve is determined for the accurate representation of this data. Propagation of these errors allows each individual sample analysis to be considered a stand-alone result, removing the need for statistical averaging of multiple data points. Simultaneous collection of flat-topped peaks enables precise measurement and correction of isobaric interference from 204Hg and a procedure for the consistent correction of common-Pb using 204Pb is described. Determination and correction of the common-Pb component is shown to be critical to the reliable interpretation of the data for certain minerals including those phases where a correction is often deemed unnecessary. Combined with time-resolved analysis of the data, this allows the Pb-loss history and nature of discordance within individual crystal domains to be ascertained. Successful analyses of zircons using a non-matrix matched (monazite) standard are also demonstrated suggesting that particle size distribution, ionisation efficiency and plasma loading, are more important issues in controlling inter-element fractionation in the plasma than absolute matrix matching.

Magmatism Associated with Orogenic Collapse of the Betic–Alboran Domain, SE Spain

Abstract: lithospheric mantle. Overall, the magmatic succession is more Extensional collapse of the Betic–Alboran Domain in the Miocene consistent with a geodynamic model involving convective removal of was accompanied by several discrete episodes of magmatism which the majority of the lithospheric mantle than with models invoking provide insights into the relationships between the thermal and wholesale delamination at the Moho, slab detachment or slab rollmechanical state of the lithosphere during late orogenesis. The back. tholeiitic Malaga dykes, emplaced at ~30–27 Ma, have flat rare earth element (REE) patterns and low Sr/Sr and high Nd/ Nd indicative of shallow decompression melting (5–15%) within the asthenosphere following removal of lithospheric mantle. Calcalkaline magmas, erupted between 15 and 6 Ma, have negative

Hf Isotope Systematics of Kimberlites and their Megacrysts: New Constraints on their Source Regions

Abstract: Kimberlites from Southern Africa, along with their low-Cr megacrysts, have unusual Hf–Nd isotopic characteristics. Group I and Transitional kimberlites define arrays trending oblique to, and well below, the Nd–Hf isotope ‘mantle array’, defined by oceanic basalts, i.e. they have negative Hf values. Group I kimberlites have Hf values varying from –1.2 to –10.1. Low-Cr megacryst suites from individual Group I kimberlites have compositions that overlap those of their host kimberlites. The trend for all Group I kimberlite megacrysts (Hf values –1.0 to –9.0) shows a striking correspondence to that of the Group I kimberlite field. Group II kimberlites and their low-Cr megacrysts plot on or close to the mantle Nd–Hf array (Hf values 3.6 to –2.6). The data indicate a genetic link between kimberlites and the low-Cr megacryst suite. The negative Hf characteristics of Group I kimberlites and their megacrysts require a source component that is ancient (>1 Ga), and has evolved with low time-integrated Lu/Hf relative to Sm/Nd. Our preferred option is that this component originates beneath the lithosphere, from a reservoir of ancient, deeply subducted oceanic basalt that became incorporated into the convecting mantle source region for Group I and Transitional kimberlites.

Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals

Abstract: The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition. The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet. The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources. The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.