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Previous work has shown that Mo isotopes measurably fractionate between metal and silicate liquids, even at temperatures appropriate for core formation. However, the effect of variations in the structural environment of Mo in the silicate liquid, especially as a function of valence state, on Mo isotope fractionation remained poorly explored. We have investigated the role of valence state in metal-silicate experiments in a gas-controlled furnace at 1400 °C and at oxygen fugacities between 10−12.7 and 10–9.9, i.e. between three and 0.2 log units below the iron-wüstite buffer. Two sets of experiments were performed, both with a silicate liquid in the CaO-Al2O3-SiO2 system. One set used molybdenum metal wire loops as the metal source, the other liquid gold alloyed with 2.5 wt% Mo contained in silica glass tubes. X-ray absorption near-edge spectroscopy analysis indicates that Mo6+/ΣMo in the silicate glasses varies between 0.24 and 0.77 at oxygen fugacities of 10–12.0 and 10–9.9 in the wire loop experiments and between 0.15 and 0.48 at 10–11.4 and 10–9.9 in the experiments with Au-Mo alloys. Double-spiked analysis of Mo isotope compositions furthermore shows that Mo isotope fractionation between metal and silicate is a linear function of Mo6+/ΣMo in the silicate glasses, with a difference of 0.51‰ in 98Mo/95Mo between purely Mo4+-bearing and purely Mo6+-bearing silicate liquid. The former is octahedrally and the latter tetrahedrally coordinated. Our study implies that previous experimental work contained a mixture of Mo4+ and Mo6+ species in the silicate liquid. Our refined parameterisation for Mo isotope fractionation between metal and silicate can be described as Δ98/95Mometal–silicate=[Formula presented] Molybdenum isotope ratios therefore have potential as a proxy to constrain the oxygen fugacity during core formation on planetary bodies if the parameterisation of Mo6+/ΣMo variation with oxygen fugacity is expanded, for instance to include iron-bearing systems. On Earth literature data indicate that the upper mantle is depleted in heavy Mo isotopes relative to the bulk Earth, as represented by chondrites. As previously highlighted, this difference is most likely not caused by core formation, which either enriches the mantle in heavy Mo isotopes or causes no significant fractionation, depending on temperature and, as we determined here, Mo6+ content. We reaffirm that core formation does not account for the Mo isotope composition of the modern upper mantle, which may instead reflect the effect of fractionation during subduction as part of global plate recycling.

The island of Sao Miguel, Azores piques geochemical interest due to a strikingly large range in the long-lived radiogenic isotope ratios of its lavas. The ‘enriched’ signatures (e.g., radiogenic 87Sr/86Sr and unradiogenic 143Nd/144Nd) observed in lavas found in the east of the island have been proposed to originate from recycled crustal components in their mantle sources. Such fertile lithologies should have higher melt productivities (amount of melt generated per decrement in pressure) than peridotitic mantle and this heritage ought to be evident in the U-series signatures of melts derived from such materials. Specifically, combined 238U–230Th and 235U–231Pa disequilibria in erupted lavas should identify significant differences in the melting rate and/or the mineralogical makeup of their sources (e.g., the amount of garnet). To this end, new 238U–230Th–226Ra and 235U–231Pa disequilibria are presented for sixteen mafic lavas ( 300% excesses of 226Ra) that strongly correlate with Ba/Th ratios. The longer-lived U-series nuclides are only slightly perturbed in a number of the more enriched lavas, resulting in increased noise in the dataset. In apparently uncontaminated samples, isotopically depleted lavas from the west of Sao Miguel display slightly higher 230Th and 231Pa excesses than the enriched samples from the east. The U-series signature of the enriched lavas can be modeled with a combination of higher absolute values of D ¯ U U and D ¯ Th , coupled with higher melting rates compared to the depleted western source. These parameters are consistent with a peridotite source enriched with contributions from more fusible lithologies such as eclogite and/or pyroxenite in the eastern source, but not with direct derivation from such fusible lithologies. The only subtle variations in U-series isotopes coupled with large, systematic variations in radiogenic isotope compositions suggest that enriched components and/or their melts are well-mixed with mantle peridotite, thus masking their distinctive melting behavior. Finally, basalts from Sete Cidades volcano in the west of Sao Miguel have very similar long-lived isotope compositions and U-series disequilibria to basalts from the island of Pico, Azores. The geochemical similarity of lavas from islands at the center and periphery of the archipelago argue against systematic differences in melting behavior related to sampling different portions of an underlying plume structure.

We provide the first exploration of thallium (Tl) abundances and stable isotope compositions as potential tracers during arc lava genesis. We present a case study of lavas from the Central Island Province (CIP) of the Mariana arc, supplemented by representative sedimentary and altered oceanic crust (AOC) inputs from ODP Leg 129 Hole 801 outboard of the Mariana trench. Given the large Tl concentration contrast between the mantle and subduction inputs coupled with previously published distinctive Tl isotope signatures of sediment and AOC, the Tl isotope system has great potential to distinguish different inputs to arc lavas. Furthermore, CIP lavas have well-established inter island variability, providing excellent context for the examination of Tl as a new stable isotope tracer. In contrast to previous work ( Nielsen et al., 2006b ), we do not observe Tl enrichment or light e205Tl (where e205Tl is the deviation in parts per 10,000 of a sample 205Tl/203Tl ratio compared to NIST SRM 997 Tl standard) in the Jurassic-aged altered mafic ocean crust subducting outboard of the Marianas (e205Tl = − 4.4 to 0). The lack of a distinctive e205Tl signature may be related to secular changes in ocean chemistry. Sediments representative of the major lithologies from ODP Hole Leg 129 801 have 1–2 orders of magnitude of Tl enrichment compared to the CIP lavas, but do not record heavy signatures (e205Tl = − 3.0 to + 0.4), as previously found in similar sediment types (e205Tl > + 2.5; Rehkamper et al., 2004 ). We find a restricted range of e205Tl = − 1.8 to − 0.4 in CIP lavas, which overlaps with MORB. One lava from Guguan falls outside this range with e205Tl = + 1.2. Coupled Cs, Tl and Pb systematics of Guguan lavas suggests that this heavy Tl isotope composition may be due to preferential degassing of isotopically light Tl. In general, the low Tl concentrations and limited isotopic range in the CIP lavas is likely due to the unexpectedly narrow range of e205Tl found in Mariana subduction inputs, coupled with volcaniclastic, rather than pelagic sediment as the dominant source of Tl. Much work remains to better understand the controls on Tl processing through a subduction zone. For example, Tl could be retained in residual phengite, offering the potential exploration of Cs/Tl ratios as a slab thermometer. However, data for Tl partitioning in phengite (and other micas) is required before developing this application further. Establishing a database of Tl concentrations and stable isotopes in subduction zone lavas with different thermal parameters and sedimentary inputs is required for the future use of Tl as a subduction zone tracer.

The phase relations of natural volcaniclastic sediments from the west Pacific Ocean were investigated experimentally at conditions of 3–6 GPa and 800–900 °C with 10 wt.% added H2O (in addition to ~ 10 wt.% structurally-bound H2O) to induce hydrous melting. Volcaniclastic sediments are shown to produce a sub-solidus assemblage of garnet, clinopyroxene, biotite, quartz/coesite and the accessory phases rutile ± Fe–Ti oxide ± apatite ± monazite ± zircon. Hydrous melt appears at temperatures exceeding 800–850 °C, irrespective of pressure. The melt-producing reaction consumes clinopyroxene, biotite and quartz/coesite and produces orthopyroxene. These phase relations differ from those of pelagic clays and K-bearing mid ocean ridge basalts (e.g. altered oceanic crust) that contain phengite, rather than biotite, as a sub-solidus phase. Despite their relatively high melt productivity, the wet solidus for volcaniclastic sediments is found to be higher (825–850 °C) than other marine sediments (700–750 °C) at 3 GPa. This trend is reversed at high-pressure conditions (6 GPa) where the biotite melting reaction occurs at lower temperatures (800–850 °C) than the phengite melting reaction (900–1000 °C). Trace element data was obtained from the 3 GPa run products, showing that partial melts are depleted in heavy rare earth elements (REE) and high field strength elements (HFSE), due to the presence of residual garnet and rutile, and are enriched in large ion lithophile elements (LILE), except for Sr and Ba. This is in contrast to previous experimental studies on pelagic sediments at sub-arc depths, where Sr and Ba are among the most enriched trace elements in glasses. This behavior can be partly attributed to the presence of residual apatite, which also host some light REE in our supra-solidus residues. Our new experimental results account for a wide range of trace element and U-series geochemical features of the sedimentary component of the Mariana arc magmas, including imparting a substantial Nb anomaly to melts from an anomaly-free protolith.

Values of δ11B reported in the literature for Holocene samples of the same foraminiferal species show large variability (6‰) relative to cited precision (

We report a total evaporation negative ion thermal mass spectrometry (TE-NTIMS) technique that enables precise and accurate (± 0.7‰; 2 s.d.) measurements of boron isotope ratios. The fundamental advantage of TE-NTIMS is that the effect of instrumental mass fractionation is minimised and sample signal maximised by analysing samples to exhaustion. We can analyse as little as 300 pg of B, which enables repeat analyses of dissolutions of small numbers of foraminifera (as little as 0.1 mg or ∼10 individual foraminfera). This represents a several fold reduction in the number of tests required compared to previous NTIMS studies and brings the amount of sample into line with other commonly used paleo-proxies. Standard addition experiments indicate that the 11B / 10B ratio of the NIST SRM 951 standard is not biased by differing amounts of seawater or carbonate matrix and yield an 11B / 10B within error of the certified value. We also show that our sample preparation induces no additional variations (e.g. blank contribution) beyond our analytical uncertainty. We obtain 11B / 10B ratios for seawater within error of values obtained using plasma ionisation, positive and negative thermal ionisation mass-spectrometry. Our measurements of core-top G. sacculifer from three ocean basins yield δ11B within analytical error (23.3–24.3‰) and fall within the range of published values. This study, however, further highlights significant interlaboratory biases in isotopic compositions of core-top foraminifera. Significantly, we show that our approach is not influenced by processing blank nor systematic differences in mass bias between measurements of sample and standard, which has yet to be documented for some other laboratories.

We have studied the sedimentary and basaltic inputs of lithium to subduction zones. Various sediments from DSDP and ODP drill cores in front of the Mariana, South Sandwich, Banda, East Sunda and Lesser Antilles island arcs have been analysed and show highly variable Li contents and δ7Li values. The sediment piles in front of the Mariana and South Sandwich arcs largely consist of pelagic sediments (clays and oozes). The pelagic clays have high Li contents (up to 57.3 ppm) and Li isotope compositions ranging from +1.3‰ to +4.1‰. The oozes have lower Li contents (7.3–16 ppm) with δ7Li values of the diatom oozes from the South Sandwich lower (+2.8‰ to +3.2‰) than those of the radiolarian oozes from the Mariana arc (+8.1‰ to +14.5‰). Mariana sediment also contains a significant portion of volcanogenic material, which is characterised by a moderate Li content (14 ppm) and a relatively heavy isotope composition (+6.4‰). Sediments from the Banda and Lesser Antilles contain considerable amounts of continental detritus, and have high Li contents (up to 74.3 ppm) and low δ7Li values (around 0‰), caused by weathering of continental bedrock. East Sunda sediments largely consist of calcareous oozes. These carbonate sediments display intermediate to high Li contents (2.4–41.9 ppm) and highly variable δ7Li values (−1.6‰ to +12.8‰). Basaltic oceanic crust samples from worldwide DSDP and ODP drill cores are characterised by enrichment of Li compared to fresh MORB (6.6–33.1 vs. 3.6–7.5 ppm, respectively), and show a large range in Li isotope compositions (+1.7‰ to +11.8‰). The elemental and isotopic enrichment of Li in altered basalts is due to the uptake of isotopically heavy seawater Li during weathering. However, old oceanic crust samples from Sites 417/418 exhibit lighter Li isotope compositions compared to young basaltic crust samples from Sites 332B and 504B. This lighter Li isotope signature in old crust is unexpected and further research is needed to explore this issue.

We have measured the major and trace element compositions of a suite of olivine-hosted melt inclusions in basaltic lavas from the islands of Agrigan and Guguan (one sample each) from the Mariana arc, part of the larger Izu–Bonin–Mariana system. The two lava samples examined show distinctly different chemical signatures that are considered to represent the dominance of sediment melt (Agrigan) or fluid (Guguan) components derived from the subducting slab and transferred to the mantle wedge, and thus provide an opportunity to examine melt inclusions from both fluid- and sediment melt-dominated arc melting systems. Un-homogenized melt inclusions from both samples examined, as well as other lavas from the same islands, contain unusual subsilicic amphiboles as mineral inclusions within melt inclusions. Textural evidence, as well as the compositions of laboratory-melted inclusions, suggests that these amphiboles may have been present as an early-formed phenocryst phase and trapped alongside melt within inclusions. Major element compositions of homogenized melt inclusions vary substantially, and in part are attributable to ‘accumulative’ amphibole contributions. Inclusions that appear to contain little or no amphibole component, however, typically have higher MgO and FeOT and lower CaO and Al2O3 contents than their host and associated lavas. Melt inclusions also display a large range of incompatible element abundances. Although the relative abundances of incompatible elements are generally consistent with the composition of lavas from Agrigan and Guguan, melt inclusion compositions are also substantially more variable than lava compositions from these islands. Melt inclusions from the two samples analyzed have Ba/La and [La/Sm]N ratios (chemical indicators of addition of slab-fluid and slab-derived melt components, respectively) similar in magnitude to the entire field of Mariana lava compositions. In general inclusions from Agrigan have higher La/Nb, Th/Nb, U/Nb, [La/Sm]N, [La/Yb]N, K2O/TiO2 and lower Cl/K2O, Ba/La, Ba/Nb, Ba/Th and Ce/Ce* than Guguan melt inclusions. These signatures are consistent with addition of slab-sediment-derived melt and slab-derived fluid to the melting systems beneath Agrigan and Guguan, respectively. We suggest that the large variations apparent in melt inclusion compositions largely represent differences in the flux of slab-derived components to the mantle wedge beneath individual arc melting systems, although small-scale variations in the depletion of the mantle wedge could also play a role. Our data also indicates that the composition of the slab-derived sediment melt supplied to the mantle wedge beneath Agrigan may be compositionally heterogeneous, although a larger data set is required to examine this in further detail. The presence of melt inclusions of highly variable composition in relatively evolved olivine phenocrysts (Fo64–Fo83) show that, regardless of the ultimate cause of the incompatible element variations in melt inclusions, discrete melts of distinctly different compositions persist without substantial intermixing within the magmatic plumbing systems beneath Agrigan and Guguan (and presumably other arc volcanoes) during long intervals of magmatic differentiation. This suggests a model of small batches of magma migrating and differentiating beneath arc volcanoes, with mixing occurring only at a very late and probably shallow stage.

Volcanism in the Canary Islands is notable for its highly alkalic character even in the shield building lavas of the currently most active island, La Palma. In order to understand better the processes responsible for this alkalic end of the compositional spectrum of ocean island basalts (OIB), we have studied primary magmatic inclusions in olivine and clinopyroxene from La Palma. The investigated primary melt and fluid inclusions are hosted by olivine and clinopyroxene from lavas of two ∼700-ka, continuous stratigraphic sections in the Fagundo and Izcagua barrancos of the main shield volcano Taburiente. Lavas from the two barrancos show contrasting whole-rock compositions: Izcaqua lavas are highly alkalic or “basanitic”, typical of La Palma lavas in general, whilst the Fagundo lavas are characterised by more silica-rich, “transitional”, compositions. Although whole-rock compositions of the lavas from the two barrancos show different degrees of silica undersaturation, they show a similar range of elemental abundances such as MgO. In contrast, the Fagundo melt inclusions show a wider range, to more mafic compositions, than those from the Izcagua samples. This is in keeping with more variable partial homogenisation temperatures and host mineral compositions in the Fagundo samples, which are also generally higher (1130–1260 °C) and more primitive (Fo 76–87) than in the Izcagua suite. Indeed, the Izcagua melt inclusions are hosted in relatively evolved liquidus mineral assemblages (Fo

Techniques have been developed for the measurement of low 230Th232Th ratios (∼ 5 · 10−6) in young volcanic rocks using a conventional single-focussing, multi-collector mass spectrometer without any hardware modifications to improve abundance sensitivity. The external precision, checked by replicate analyses of the Table Mountain Latite (TML) rock standard over a 2-yr period is better than ± 1.5% (2σ). This technique therefore offers a three- to five-fold improvement in precision compared with α-counting, simple chemical separation techniques and a short analysis time (∼ 1.5 hr). The enhanced precision and reduced analytical time arises from the significantly more favourable counting statistics achieved by mass spectrometry, which in turn permits analysis of smaller samples (∼ 300 ng instead of ∼ 1 μg Th) than required by α-spectrometry.