Janet M. Hergt

A simple method for the precise determination of ≥ 40 trace elements in geological samples by ICPMS using enriched isotope internal standardisation

The combination of enriched isotopes and conventional elemental internal standards permits the precise determination of > 40 trace elements by ICPMS in a broad spectrum of geological matrixes. Enriched isotopes expand the suite of available reference isotopes spaced through the mass spectrum, so that the complex mass-dependent variations in sensitivity encountered during ICPMS analysis can be monitored and deconvolved. The method we have developed is straightforward, entailing simple sample preparation, instrument calibration, and data reduction procedures, as well as providing extended element coverage, improved precision, and both time and cost benefits compared to alternative analytical strategies. Analytical precision near or better than 1% RSD (relative standard deviation) is achieved for most elements with mass > 80 amu and between 1% and 4% (RSD) for elements with mass 80 amu and < 10 ng g−1 to 1 μg g−1 for elements with mass < 80 amu). The subtle geochemical differences which can be resolved using this method are demonstrated by analyses of Nb, Ta, Zr, and Hf in magmas from ocean islands and subduction zones. These data reveal significant departures from chondritic Zr/Hf and Nb/Ta values, and systematic trends which are consistent with greater incompatibility of Zr relative to Hf and also of Nb relative to Ta during melting of the upper mantle. The occurrence of significantly subchondritic Zr/Hf and Nb/Ta ratios in Nb-poor subduction zone magmas, supports the notion that the depletion of high-field strength elements in subduction magmas is due to their removal from the mantle wedge by prior melting events.

Iolite: Freeware for the visualisation and processing of mass spectrometric data

Iolite is a non-commercial software package developed to aid in the processing of inorganic mass spectrometric data, with a strong emphasis on visualisation versus time of acquisition. The goal of the software is to provide a powerful framework for data processing and interpretation, while giving users the ability to implement their own data reduction protocols. It is intended to be highly interactive, providing the user with a complete overview of the data at all stages of processing, and allowing the freedom to change parameters and reprocess data at any point. The program presents a variety of windows for the selection and viewing of data versus time, as well as features for the generation of X-Y plots, summary reports and export of data. In addition, it is capable of generating X-Y images from laser ablation rasters, and combining information from up to four separate elemental concentrations (intensities of red, green and blue, and the z-axis) in a false-colour three-dimensional image. By virtue of its underlying computing environment—Igor Pro—Iolite is capable of processing very large datasets (i.e., millions of timeslices) rapidly, and is thus ideal for the interrogation of multi-hour sessions of laser ablation data that can not be easily manipulated in conventional spreadsheet applications, for example. It is also well suited to multi-day sessions of solution-mode inductively-coupled plasma mass spectrometer (ICPMS) or thermal ionisation mass spectrometer (TIMS) data. A strong emphasis is placed on the interpolation of parameters that vary with time by a variety of user selectable methods including smoothed cubic splines. Data are processed on a timeslice-by-timeslice basis, allowing outlier rejection and calculation of statistics to be employed directly on calculated results. This approach can reduce the risk of processing biases associated with the manipulation of integrated datasets, while also allowing the implementation of more complex data reduction methods.

Hafnium isotope evidence for ‘conservative’ element mobility during subduction zone processes

The high field strength elements (HFSE) play a critical role in the interpretation of chemical variation within subduction-related magmas by providing an assumed mantle-dominated ‘baseline’ from which enrichments in many other slab-derived elements may be gauged. Of the HFSE, hafnium is unique in combining the characteristics of HFSE chemistry with a powerful isotopic tracer and should, in theory, allow the delineation of mantle domains and help constrain the timing of melt depletion processes. A detailed Hf isotope study of oceanic arc lavas and paired arc/back-arc settings has been conducted. Here we show, contrary to expectations, that the Hf isotopic compositions of arc lavas are always displaced significantly from their co-existing back-arc spreading centres which can be considered to sample the local mantle. This is true not only of those arcs in which direct sediment melting or AFC-like processes within the crust are implicated, but also in low-K tholeiitic arcs where hydrous fluids are believed to be the dominant medium of slab-to-mantle transport. This observation calls into question the concept of ‘conservative’ or ‘immobile’ elements and suggests that some transfer of material from the subducting slab into the sub-arc mantle wedge probably occurs for almost all elements. These conclusions have significant implications for models of arc geochemistry.

The petrogenesis of Mesozoic Gondwana low-Ti flood basalts

Abstract New major, trace element and isotopic data for Jurassic basalts from SE Australia indicate that they are strikingly similar to the Jurassic tholeiitic rocks of Tasmania and the Transantarctic Mountains. These rocks are all characterised by low TiO2, P2O5, Na2O, Fe2O3, Ti/Zr, Ti/Y and eNd, and high SiO2, Rb/Ba, Rb/Sr, 87Sr/86Sr and 207Pb/204Pb, relative to oceanic basalts. They therefore comprise a major province, termed the Ferrar magmatic province, which extended for 3000–4000 km across the Gondwana supercontinent. A review of the other Mesozoic low-Ti CFBs suggests that the Ferrar rocks are an extreme example of these magma types. It is striking that both the major and trace element compositions are different from oceanic basalts, which suggests that these features are linked, and it is argued that they were derived from distinctive source regions in the sub-continental mantle. Such source regions were variably depleted in major and minor elements, and then relatively enriched in highly incompatible elements and Sr and Pb isotopes, which is best explained by the introduction of a small amount of subducted sediment. The tectonic setting of the Ferrar magmatism is poorly constrained, but at present there is no clear geochemical evidence for the involvement of asthenospheric plume material in the petrogenesis of these low-Ti CFBs.

Remobilisation of the continental lithosphere by a mantle plume: major-, trace-element, and Sr-, Nd-, and Pb-isotope evidence from picritic and tholeiitic lavas of the Noril'sk District, Siberian Trap, Russia

The Late Permian to Early Triassic Siberian Traps have been sampled by drill core (core SG-9) and from surface exposure (section 1F) in the Norilsk region of the Siberian Platform, Russia. Combined major, trace element, and Nd-, Sr-, and Pb-isotope data on selected samples through the Siberia Trap, offer new chemostratigraphic criteria for the identification and characterisation of two fundamentally different magma types and 9 of the 11 formations of lava developed near Norilsk. A Lower Sequence of sub-alkalic basalts, tholeiites, and picritic basalts (upwards these are the Ivakinsky, Syverminsky, and Gudchichinsky formations) are overlain by an Upper Sequence of picritic basalts and tholeiites interbedded with tuffs (upwards, these are the Khakanchansky, Tuklonsky, Nadezhdinsky, Morongovsky, Mokulaevsky and Kharayelakhsky formations).The Gudchichinsky and Tuklonsky formations contain both picritic and tholeiitic lavas. The Tuklonsky formation tholeiites and picrites have moderate Gd/Yb (1.6–1.8), low TiO2 (0.45–0.95 wt%), a significant negative Ta and Nb anomaly (Nb/La =0.42–0.57) and unradiogenic Nd (ɛNdCHUR = to -4.6). In contrast, both the Gudchichinsky formation tholeiites and picrites have high Gd/Yb (2.3–3.1), and TiO2 (1.2–2.3 wt%), no significant Nb or Ta anomaly (Nb/La =0.8–1.1), and radiogenic Nd (ɛNdCHUR = to 7.3). The low-Ti and Nb/La, high La/Sm, and unradiogenic Nd-isotope signatures of the picritic Tuklonsky formation lavas and the tholeiitic lavas of the Upper Sequence are characteristic of magmas strongly influenced by material from the continental lithosphere, whereas the high-Ti and Nb/La, low La/Sm and radiogenic Nd-isotope signatures of the Lower Sequence are more comparable to deeper asthenospheric mantle-plume generated lavas similar to oceanic island basalts. The lavas overlying the Tuklonsky formation have mg-numbers of 0.63 to 0.68, and are more evolved than the Tuklonsky (Mg-number < 0.62) and have more radiogenic ɛNdCHUR (Tuklonsky:-0.03 to-4.66; Mokulaevsky: + 0.60 to + 1.61), but have many of the incompatible trace element features of the Tuklonsky sky type magma. These lavas show a progressive upwards decline in SiO2 (55–49 wt%), La/Sm (4.6–2.0), and ɛURSr ( + 67 to + 13) which has previously been attributed to a decrease in the proportion of crustal material contributed to the magma. This paper explores and alternative model where a component of the crustal contribution might be derived from within an ancient region of the mantle lithosphere as recycled sediment rather than from the overlying continental crust.

Element fluxes associated with subduction related magmatism

Destructive plate margin magmas may be subdivided into two groups on the basis of their rare earth element (REE) ratios. Most island arc suites have low Ce/Yb, and remarkably restricted isotope ratios of 87Sr/86Sr = 0.7033, 143Nd/144Nd = 0.51302, 206Pb/204Pb = 18.76 , 207Pb/204Pb = 15.57, and 208Pb/204Pb = 38.4. However, they also have Rb/Sr (0.03), Th/U (2.2) and Ce/Yb (8.5) ratios which are significantly less than accepted estimates for the bulk continental crust. The high Ce/Yb suites have higher incompatible element contents, more restricted heavy REE, and much more variable isotope ratios. Such rocks are found in the Aeolian Islands, Grenada, Indonesia and Philippines, and their isotope and trace element features have been attributed both to contributions from subducted sediment, and/or old trace element enriched material in the mantle wedge. It is argued that for isotope and trace element models the slab component can usefully be taken to consist of subducted sediment and altered mid-ocean ridge basalts, since these may contain ca. 80% of the water in the subducted slab, and the distinctive trace element features of arc magmas are generally attributed to the movement of material in hydrous fluids. The isotope data indicate that not more than 15% of the Sr and Th in an average arc magma were derived from subducted material, and that the rest were derived from the mantle wedge. The fluxes of elements which cannot be characterized isotopically are more difficult to constrain, but for most minor and trace elements the slab derived contribution in arc magmas is too small to have a noticeable effect on the residual slab.

In situ Sr-isotope analysis of carbonates by LA-MC-ICP-MS: interference corrections, high spatial resolution and an example from otolith studies

In situ Sr-isotope analysis by laser ablation multi-collector ICP-MS is a potentially powerful tracer technique with widespread application to many fields of study. The usefulness of the method, however, depends very strongly upon the quality of data that can be obtained (compared with conventional ‘solution-based’ analyses), and the spatial resolution, particularly in samples with strong compositional zonation or fine-scale growth banding. In this contribution we show that highly accurate (∼50 ppm) and precise (external precision ∼125 ppm) analyses of carbonate materials can be obtained in situ and further demonstrate that, by utilising the aperture-imaging optics of an excimer laser system with appropriate time-resolved software, isotopic variations on the scale of tens of micrometres can be resolved. An example is shown using relatively small (∼500 μm diameter) otoliths from a diadromous fish species, Galaxias maculatus.

Trace Element Fractionation Processes in the Generation of Island Arc Basalts

Subduction-related magmas are characterized by distinctive minor and trace element ratios which are widely attributed to the introduction of a hydrous component from the subducted crust. Island arc rocks may usefully be subdivided into high and low Ce/Yb groups, and the latter are characterized by relatively restricted radiogenic isotope ratios. In general, high LIL/ HFSE ratios are best developed in low HFSE rocks, and the variation in LILE is less than that in HFSE. A local equilibrium model is developed in which the distinctive minor and trace element feature of arc rocks are the result of fluid percolation in the mantle wedge. Peridotite/fluid distribution coefficients are inferred to vary systematically with ionic radius in the range 69—167 x 10-12 m. However, in practice the calculated olivine/fluid partition coefficients are too high to develop an arc signature in the wedge peridotite in reasonable timescales, and for acceptable fluxes from the slab. The available geochemical data would suggest that realistic distribution coefficients are 2-3 orders of magnitude less than those presently available from experimental data, presumably because the fluid compositions are different, or that local equilibrium is not appropriate. Average compositions from the low Ce/Yb arc suites exhibit a positive correlation between Ce/Sm, but not K/Sm, and crustal thickness. It is argued that the degree of melting varies with crustal thickness, but not in any simple way with the magnitude of the fluid contribution. The observed range in Ce/Sm in the low Ce/Yb rocks is consistent with 3-18% melting of slightly lree depleted source rocks.

Destructive plate margin magmatism: Geochemistry and melt generation

Abstract Subduction related magmas are characterised by distinctive minor and trace element ratios which are widely attributed to the introduction of a hydrous component from the subducted crust. In general, high LIL HFSE ratios are best developed in low HFSE rocks, and the variation in LILE is less than that in HFSE. Moreover, the size of the contributions from subducted material estimated on the basis of minor and trace element variations are consistently greater than those inferred from radiogenic isotopes. The Nd isotope ratios of island arc rocks are lower than those of N-MORB, and there is a broad negative correlation between 143 Nd 144 Nd and 208 Pb ∗ 206 Pb ∗ consistent with a small contribution (2–4%) from subducted sediment in arc magmas. However, the isotope and trace element compositions of IAB cannot be modelled by two component mixing of sediment and N-MORB from the mantle wedge, and they require an additional high LIL HFSE component which is typically attributed to the release of hydrous fluids from the subducted slab. A number of models are considered for the effects of fluid percolation in the mantle wedge. Dwedge/fluid values are not well constrained but, if local equilibrium occurs, the available data on arc rocks are consistent with Dwedge/fluid ∼ 10−2 for the LILE. Several authors have proposed that some of the trace elements in the subduction component were scavenged from the mantle wedge, in order to reconcile the different contributions from subducted materials inferred from radiogenic isotope and trace element data. In the models outlined here that suggests that Dslab/fluid for the LILE is > 0.1. Within low Ce Yb arc rocks there is a general increase in average Ce Sm , but not K Ce , with increasing crustal thickness. The observed range in Ce Sm in the low Ce Yb rocks is consistent with 3–18% melting of slightly LREE depleted source rocks, and it appears that the degree of melting varies with crustal thickness.

Geochemistry of a hydrothermal sediment core from the OBS vent-field, 21°N East Pacific Rise

Abstract We report a geochemical investigation of near-vent hydrothermal sediment from the East Pacific Rise, collected in a short (11 cm) core by submersible from the OBS vent-site at 21°N. The sediment has high concentrations of Fe (23–25%), Cu (10–22%), Zn (4–13%) and S (22–30%), but low Mn concentrations (108–279 ppm) indicating a much fresher sulfidic input than in previously reported near-vent sediments from the Mid-Atlantic Ridge. Further studies are required to fully characterise near-field hydrothermal sediments, but the data in this study allow the first comparisons with similar near-vent material collected previously from the Mid-Atlantic Ridge as well as ridge-flank data from the EPR and Juan de Fuca Ridge. Compared to ridge-flank sediments the near field sediments exhibit high ratios of Fe/Mn, Cu/Fe and Zn/Fe. Unlike Cu and Zn, Pb is not so significantly enriched. Pb isotope compositions are indistinguishable from local MORB indicating a dominant hydrothermal source for this element in these sediments. This may not necessarily be the case for ridge-flank sediments, however, where scavenging from seawater may also be important. REE distributions throughout the core exhibit low concentrations and characteristics typical of vent-fluids, with LREE enrichments and pronounced positive Eu-anomalies. These patterns are similar to those of relatively fresh sulfides, but there is also evidence for progressive oxidation downward into the core by ingressing seawater.