Stephen M. Eggins

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.

Deposition and element fractionation processes during atmospheric pressure laser sampling for analysis by ICP-MS

Abstract We have used an ArF excimer laser coupled to a quadrupole inductively coupled plasma mass spectrometry (ICP-MS) for the measurement of a range of elements during excavation of a deepening ablation pit in a synthetic glass (NIST 612). Analyte behaviour shows progressive volatile element enrichment at shallow hole depths, with a change to refractory element enrichment as the ablation pit deepens further. Examination of ablation pit morphology and the surface condensate deposited around the ablation site reveals the importance of sequential condensation of refractory, then volatile phases from the cooling plasma plume after the end of the laser pulse. We interpret the observed element fractionation behaviour to reflect a change in ablation processes from photothermal dominated to plasma dominated mechanisms. The development of the surface deposit is greatly reduced by ablating in an ambient atmosphere of He instead of Ar and is accompanied by a two- to four-fold increase in ICP-MS sensitivity.

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 COMPOSITION OF PERIDOTITES AND THEIR MINERALS : A LASER-ABLATION ICP-MS STUDY

We have analyzed the major- and trace-element contents of minerals and whole rocks for two peridotite xenoliths from SE Australia (one fertile, the other infertile) using both solution chemistry (ICP–MS) and microbeam techniques (EMP, LA–ICP–MS). The incompatible lithophile trace elements are contained mainly in solid mineral phases at upper-mantle conditions, with no significant concentrations occurring on grain boundaries or in fluid inclusions. No exotic accessory minerals are required hosts for elements such as Nb and Ta. Mass balance is achieved for the highly incompatible trace elements only with inclusion of data for precursor amphibole, now present as pockets of alkali-, alumina- and silica-rich glass containing euhedral olivine, clinopyroxene, and spinel daughter crystals. The glass in these pockets is enriched in incompatible trace elements, particularly Ba, Nb and Ta, reflecting the preferential partitioning of these elements into amphibole at upper-mantle conditions. Orthopyroxene–clinopyroxene partition coefficients are similar for most incompatible elements (except Ti) between the two peridotites, suggesting that compositional differences do not significantly affect subsolidus partitioning for these elements. Comparison of our data with those from the literature reveals large variations in Dopx/cpx for all incompatible trace elements. For the REE, Zr (Hf) and Sr, these variations correlate with reciprocal equilibration temperature, reflecting the substitution of these elements for Ca in the M2 site. In contrast, poor correlation exists for V and no correlations exist for Ti and Nb between Dopx/cpx and temperature, which may reflect their substitution into multiple sites in the pyroxene structure and the influence of mineral composition on their partitioning. These features highlight the need for caution in inferring crystal–liquid D-values from subsolidus partitioning relations.

Zirconium abundance in granulite-facies minerals, with implications for zircon geochronology in high-grade rocks

Zircon growth in high-grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of a phase bearing zirconium (Zr). We have measured the concentration of Zr in the major minerals in granulite-facies rocks of differing bulk composition. Both garnet and hornblende contain tens of parts per million Zr, and no other major phase contains significant Zr. Simple calculations show that reaction of either garnet or hornblende to form non–Zr-bearing phases will release sufficient Zr to account for at least some new zircon growth. U-Pb ages from new zircon, grown as a result of either hornblende or garnet breakdown, are not expected to record the time of peak metamorphism, but rather will record the time of particular metamorphic reactions, allowing direct correlation of zircon ages with petrologically derived pressure-temperature-time paths. This approach offers the potential for more rigorous interpretation of the metamorphic significance of zircon ages than has previously been possible.

Mg/Ca variation in planktonic foraminifera tests: implications for reconstructing palaeo-seawater temperature and habitat migration

The nature of compositional variability within the tiny calcitic shells (tests) that are precipitated by planktonic foraminifera has been investigated using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Systematic large and correlated variation of Mg/Ca, Mn/Ca, Ba/Ca and Zn/Ca but relatively uniform Sr/Ca are observed through the test walls of analysed species (Globigerinoides sacculifer, Globigerinoides ruber, Neogloboquadrina pachyderma and Neogloboquadrina dutertrei). Distinct chamber and chamber-wall layer compositions can be resolved within individual tests, and Mg/Ca compositional differences observed in sequentially precipitated test components of the different species analysed are consistent with seawater temperature changes occurring with habitat migration during their adult life-cycle stages. Estimated test calcification temperatures are in keeping with available seawater temperature constraints, indicating the potential for accurate seawater temperature reconstruction using LA-ICP-MS. Mg-rich (<1–6 mol% Mg) surface veneers that are also enriched in Mn, Ba, and Zn have been found on all species and all fossil tests, as well as on live-sampled tests of G. ruber, with the latter suggesting a possible biogenic origin. These Mg-rich surfaces bias bulk test compositions toward higher Mg/Ca values by between 5 and 35%.

Earliest evidence of modern human life history in North African early Homo sapiens

Recent developmental studies demonstrate that early fossil hominins possessed shorter growth periods than living humans, implying disparate life histories. Analyses of incremental features in teeth provide an accurate means of assessing the age at death of developing dentitions, facilitating direct comparisons with fossil and modern humans. It is currently unknown when and where the prolonged modern human developmental condition originated. Here, an application of x-ray synchrotron microtomography reveals that an early Homo sapiens juvenile from Morocco dated at 160,000 years before present displays an equivalent degree of tooth development to modern European children at the same age. Crown formation times in the juveniles macrodont dentition are higher than modern human mean values, whereas root development is accelerated relative to modern humans but is less than living apes and some fossil hominins. The juvenile from Jebel Irhoud is currently the oldest-known member of Homo with a developmental pattern (degree of eruption, developmental stage, and crown formation time) that is more similar to modern H. sapiens than to earlier members of Homo. This study also underscores the continuing importance of North Africa for understanding the origins of human anatomical and behavioral modernity. Corresponding biological and cultural changes may have appeared relatively late in the course of human evolution.

The effect of melt composition on trace element partitioning: an experimental investigation of the activity coefficients of FeO, NiO, CoO, MoO 2 and MoO 3 in silicate melts

Abstract The thermodynamic theory describing the partitioning of trace elements between crystals and silicate melt implies that partition coefficients should depend on the major-element composition of the melt from two different causes, namely (1) the activity coefficient of the trace-element oxide component in the melt, and (2) the activities of all the major-element components needed to balance the trace-element substitution in the crystal (the “stoichiometric control”). Partition coefficients are also expected to vary with the composition of the crystal, and temperature and pressure. Because these variables cannot be controlled independently in direct crystal/melt partitioning studies, it has not been possible to disentangle their effects, or to determine their relative importance. In order to explore the effects of melt composition on activity coefficients of trace-element oxide components, the activity coefficients of five such components, MoO 2 , MoO 3 , FeO, NiO and CoO, were measured in 18 different melt compositions in the system CaO–MgO–Al 2 O 3 –SiO 2 plus one composition in CaO–MgO–Al 2 O 3 –SiO 2 –TiO 2 at 1400 °C, by equilibration with the metal under controlled oxygen fugacity. MoO 2 and MoO 3 are expected to have geochemical properties similar to the High Field Strength Elements (HFSEs). The activity coefficients of MoO 2 and MoO 3 vary by factors of 20 and 60, respectively, over the range of compositions investigated. Their variation is highly correlated, and mainly depends on the amount of CaO in the melt, suggesting the influence of CaMoO 3 and CaMoO 4 complexes. The analogy between Mo and HFSEs implies that melt composition can be expected to have an important influence on HFSE partition coefficients. The activity coefficients of FeO, NiO and CoO vary by a factor of two over the same range of melt compositions, but show no simple dependence on any particular major-element oxide component. However, the activity coefficients of all three components are very highly correlated with each other. This means that the effect of melt composition can be largely eliminated if the ratios of two activity coefficients are taken, as, for example, when two-element distribution coefficients are used.

The origin of island arc high-alumina basalts

A detailed examination of the hypothesis that high-alumina basalts (HAB) in island arcs are primary magmas derived by 50–60% partial melting of subducted ocean crust eclogite shows that this model is unlikely to be viable. Evidence suggests that the overwhelming majority of arc HAB are porphyritic lavas, enriched in Al2O3 either by protracted prior crystallization of olivine and clinopyroxene, or by plagioclase phenocryst accumulation in magmas of basaltic andesite to dacite composition. Experimentally-determined phase relationships of such plagioclase-enriched (non-liquid) compositions have little bearing on the petrogenesis of arc magmas, and do not rule out the possibility that arc HAB can be derived by fractionation of more primitive arc lavas.Although models invoking eclogite-melting can match typical arc HAB REE patterns, calculations indicate that the Ni and Cr contents of proposed Aleutian primary HAB are many times lower than such models predict. In contrast, Ni vs Sc and Cr vs Sc trends for arc HAB are readily explained by olivine (+Cr-sp) and clinopyroxene-dominated fractionation from more primitive arc magmas. GENMIX major element modelling of several HAB compositions as partial melts of MORB eclogite, using appropriate experimentally (26–34 kb)-determined garnet and omphacite compositions yields exceptionally poor matches, especially for CaO, Na2O, MgO and Al2O3. These mismatches are easily explained if the HAB are plagioclase-accumulative.Groundmasses of arc HAB are shown to vary from basaltic andesite to dacite in composition. Crystal fractionation driving liquid compositions toward dacite involves important plagioclase separation, resulting in development of significant negative Eu anomalies in more evolved lavas. Plagioclase accumulation in such evolved liquids tends to diminish or eliminate negative Eu anomalies. Therefore, the absence of positive Eu anomaly in a plagioclase-phyric HAB does not indicate that plagioclase has not accumulated in that lava. In addition, we show that plagioclase phenocrysts in arc HAB are not in equilibrium with the liquids in which they were carried (groundmass).Exceptional volumes of picrite and olivine basalt occur in the Solomons and Vanuatu arcs; the presence in lavas from these and other arcs (Aleutian, Tonga) of olivine phenocrysts to Fo94, finds no ready explanation in the primary eclogite-derived HAB model. We suggest that most lavas in intra-oceanic arcs are derived from parental magmas with >10% MgO; fractionation of olivine (+Cr-sp) and clinopyroxene drives liquids to basalt compositions with <7% MgO, but plagioclase nucleation is delayed by their low but significant (<1%?) H2O contents. Thus evolved liquid compositions in the basaltic andesite—andesite range may achieve relatively high Al2O3 contents (<17.5%). The majority of arc basalts, however, have Al2O3 contents in excess of 18%, reflecting plagioclase accumulation.We give new experimental data to show that HAB liquids may be generated by anhydrous, low-degree (<10%) partial melting of peridotite at P<18 kb. Relative to arc HAB, these experimental melts have notably higher Mg#(69–72) and are in equilibrium with olivine Fo87–89. Only further detailed trace element modelling will show if they might be parental magmas for some arc HAB.

Origin and differentiation of picritic arc magmas, Ambae (Aoba), Vanuatu

Key aspects of magma generation and magma evolution in subduction zones are addressed in a study of Ambae (Aoba) volcano, Vanuatu. Two major lava suites (a low-Ti suite and high-Ti suite) are recognised on the basis of phenocryst mineralogy, geochemistry, and stratigraphy. Phenocryst assemblages in the more primitive low-Ti suite are dominated by magnesian olivine (mg ∼80 to 93.4) and clinopyroxene (mg ∼80 to 92), and include accessory Cr-rich spinel (cr ∼50 to 84). Calcic plagioclase and titanomagnetite are important additional phenocryst phases in the high-Ti suite lavas and the most evolved low-Ti suite lavas. The low-Ti suite lavas span a continuous compositional range, from picritic (up to ∼20 wt% MgO) to high-alumina basalts (<5 wt% MgO), and are consistent with differentiation involving observed phenocrysts. Melt compositions (aphyric lavas and groundmasses) in the low-Ti suite form a liquid-line of descent which corresponds with the petrographically-determined order of crystallisation: olivine + Cr-spinel, followed by clinopyroxene + olivine + titanomagnetite, and then plagioclase + clinopyroxene + olivine + titanomagnetite. A primary melt for the low-Ti suite has been estimated by correcting the most magnesian melt composition (an aphyric lava with ∼10.5 wt% MgO) for crystal fractionation, at the oxidising conditions determined from olivine-spinel pairs (fo2 ∼FMQ + 2.5 log units), until in equilibrium with the most magnesian olivine phenocrysts. The resultant composition has ∼15 wt% MgO and an mgFe2 value of ∼81. It requires deep (∼3 GPa) melting of the peridotitic mantle wedge at a potential temperature consistent with current estimates for the convecting upper mantle (Tp ∼1300°C). At least three geochemically-distinct source components are necessary to account for geochemical differences between, and geochemical heterogeneity within, the major lava suites. Two components, one LILE-rich and the other LILE- and LREE-rich, may both derive from the subducting ocean crust, possibly as an aqueous fluid and a silicate melt respeetively. A third component is attributed to either differnt degrees of melting, or extents of incompatible-element depletion, of the peridotitic mantle wedge.