Jon D. Woodhead

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.

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

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.

High field strength and transition element systematics in island arc and back-arc basin basalts: Evidence for multi-phase melt extraction and a depleted mantle wedge

Abstract The geochemical character of the mantle wedge in convergent margin settings is investigated using the high field strength and transition elements Ti, Zr, V, Sc and Y in basaltic rocks from island arcs and associated back-arc basins. Systematic differences are observed in the composition of each arc relative to the adjacent back-arc basin. The arcs have lower average abundances of incompatible high field strength elements and Y, and extend to higher ratios of Ti/Zr, V/Ti and Sc/Y. Partial melting of a normal mid-ocean ridge basalt source accounts for the back-arc basin basalt compositions, but cannot explain the higher element ratios observed in many arc basalts. These arc magmas are consistent with derivation from sources which are more depleted in incompatible elements those that of the back-arc basin basalts. Source depletion by melt extraction, prior to arc magma genesis, explains the similar degree of Y depletion and, by analogy, the heavy REE depletion, to that of the high field strength elements in island arc basalts. The observations are not compatible with the presence of residual titanate phases (e.g. rutile). However, amphibole may be required in the source of a subgroup of arc basalts which have relatively high Zr abundances and anomalously low Ti/Zr and high V/Ti values. These results have implications for subduction zone geochemical budgets, as the influence of any slab-derived component may be enhanced in regions of extensive mantle wedge depletion.

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.

Geochemistry of the Mariana arc (western Pacific): Source composition and processes

Abstract The northern Mariana islands in the western Pacific form a prime example of an intra-oceanic island arc system, free from the effects of continental crustal contamination. Radiogenic (Sr, Nd, Pb, Hf) and stable (O, H, S) isotope data are presented for a suite of lavas from these islands, together with rare-earth and other trace-element compositional data for the same samples. This geochemical synthesis provides an opportunity to address a number of outstanding questions concerning oceanic arc petrogenesis, in particular the nature of the mantle source and processes occurring within this region. Much debate surrounds the origin of the commonly observed enrichments in island arc lavas relative to mid-ocean ridge basalt (MORB), in particular whether these result from incorporation of slab-derived components into a MORB-type mantle or whether they reflect preferential melting of a source with affinities to oceanic island basalt (OIB). Although some geochemical data are ambiguous, the majority appear to favour the slab involvement hypothesis, with contributions from both subducted sediment and altered oceanic crust. The trace-element enrichment patterns of the Mariana arc volcanics suggest that the agent of mass transfer between the subducting slab and overlying mantle wedge is a fluid, which has leached elements from the slab, rather than a direct partial melt of the latter. Although the exact composition of this fluid is unknown, the data indicate that a large proportion of it may be aqueous. Elemental partitioning into this fluid is highly selective (i.e. mass transfer between the slab and mantle wedge is not an iso-chemical process), a conclusion which has considerable implications for studies of crust and mantle evolution. The commonly cited depletions of high-field-strength elements (HFSE) in island arcs relative to MORB are thought to be largely a product of the data normalization process; in reality substantial overlap exists between HFSE concentrations in arc lavas and MORB. Regional geochemical variations between the islands in the arc are small and are thought most likely to be related to heterogeneity in the nature of the subducting assemblage.

A simple method for obtaining highly accurate Pb isotope data by MC-ICP-MS

Many recent studies of isotope ratio determination by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) have highlighted the excellent levels of analytical precision obtainable using this newly emergent technology. Unfortunately, it is often a considerably more difficult matter to obtain highly accurate analyses by MC-ICP-MS. This is particularly the case for isotope systems in which internal normalisation for instrumental mass bias effects is not possible, e.g., Pb, Cu, Zn, B, Li. Given the fact that extremely accurate Pb isotope data can now be obtained by double-spike thermal ionisation mass spectrometry (TIMS), this is seemingly a major impediment to the widespread adoption of MC-ICP-MS methods for the analysis of Pb. It is demonstrated in this contribution, however, that a few simple modifications to existing protocols can produce MC-ICP-MS Pb isotope data of comparable accuracy to the best double spike TIMS results. This method also preserves many of the inherent advantages of ICP-MS analysis, such as rapid sample processing, and avoids many of the stringent analytical requirements of double spiking. The method is highly reliable and a summary of results from our laboratory over the past two years is presented, together with suggestions for further development.

Extreme HIMU in an oceanic setting: the geochemistry of Mangaia Island (Polynesia), and temporal evolution of the Cook—Austral hotspot

Abstract New isotope and trace-element data are presented for volcanic rocks from the island of Mangaia in Polynesia, which forms the most extreme example of the HIMU mantle endmember known at the present time. Observations from these data, coupled with published results, provide strong support for an origin of the HIMU signature related to recycling of oceanic crust. Evidence exists for carbonate metasomatism within the mantle beneath Polynesia; however, it is likely that this is also related to the recycling process, rather than being responsible itself for generating HIMU. The elevated U/Pb ratios observed in the HIMU source appear to be largely a result of Pb loss rather than U uptake. The apparent rarity of “pure” HIMU and its relation to other OIB types is presented within a simple recycling framework; to a first approximation, the spectrum of OIB compositions between HIMU and EM may reflect simple mixing between subducted oceanic crust and sediment sources. Temporal/genetic relationships with the other islands in the Cook-Austral archipelago are investigated. Within the Macdonald-Mangaia segment relatively well-defined age progressions require a common heat source. In contrast, isotopic compositions do not follow a simple trend—there is a decrease of the HIMU influence with time, but it is not clear whether this change was abrupt or gradual. The southern Cook islands appear to be largely unrelated to volcanism within the Macdonald-Mangaia segment.

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.

Geochemical and isotopic variations in the calc-alkaline rocks of Aeolian arc, southern Tyrrhenian Sea, Italy: constraints on magma genesis

New geochemical and isotopic data are reported for calc-alkaline (CA) volcanics of the Aeolian arc. Three main groups are recognized: the Alicudi and Filicudi volcanics in the western part of the arc; the Panarea, Salina and Lipari (henceforth termed PSL) volcanics in the central part of the arc and the Stromboli suite which makes up the eastern part of the arc. Each group is characterized by distinctive isotopic ratios and incompatible element contents and ratios. 87Sr/86Sr values (0.70352–0.70538) increase from west to northeast, and are well correlated with 143Nd/144Nd (ɛNd from +4.8 to -1.5). Pb isotope ratios are fairly high (6/4=19.15–19.54; 7/4=15.61–15.71; 8/4=38.97–39.36), with a general increase of 7/4 and 8/4 values from Alicudi to PSL islands and Stromboli. LILE contents and some incompatible element ratios (e.g. Ba/La, La/Nb, Zr/Nb, Rb/Sr) increase from the western to the central part of the arc, whereas HFSE and REE abundances decrease. Opposite variations are often observed in the volcanics toward the north-east from PSL islands. To account for these features and the decoupling observed between isotopic compositions and incompatible element abundances and ratios, it is suggested that a mantle source with affinities to the MORB source is “metasomatized” by slab-derived, crustal components. The proportion of crustal material entrained in the mantle source increases from Alicudi to Stromboli, according to the Sr and Nd isotope variations. It is also proposed that slab derived hydrous fluids play an important role, but which is variable in different sectors of the arc. This is attributed to the metasomatizing agent having variable fluid/melt ratios, reflecting different types of mass transfer from the subducted contaminant (probably pelagic sediments) to the mantle wedge. Thus, it is suggested that the slab derived end-member has a high hydrous fluid/melt ratio in the PSL mantle source and a correspondingly lower ratio in the Alicudi and Stromboli sources.