H. M. Williams

Age and composition of dikes in Southern Tibet: New constraints on the timing of east-west extension and its relationship to postcollisional volcanism

Controversy exists over whether east-west extension in southern Tibet is related to plateau uplift or to the accommodation of plate boundary forces. Relationships between the onset of extension, plateau uplift, and the thermal state of the lithosphere are critical to this debate. We present new data on postcollisional, north-south–trending dikes in southern Tibet. Their ages range from 18.3 ± 2.7 Ma to 13.3 ± 0.8 Ma, and define the onset of regional east-west extension in southern Tibet. Dikes are compositionally indistinguishable from postcollisional lavas in southern Tibet, being either ultrapotassic, having a source in the subcontinental lithospheric mantle, or calc-alkaline with a dominantly crustal origin. The ultrapotassic dikes demonstrate that east-west extension and subcontinental lithospheric mantle–derived magmatism were temporally and spatially linked, supporting models that relate the latest phase of plateau uplift to subcontinental lithospheric mantle thinning. Thus, the onset of extension by 18.3 ± 2.7 Ma represents the time at which the potential energy of the plateau exceeded convergent boundary forces. This places a new age limitation on the attainment of high elevation in southern Tibet, with implications for models that relate Cenozoic monsoon intensification to plateau uplift.

Determination of mass-dependent variations in nickel isotope compositions using double spiking and MC-ICPMS

We present a new technique for the accurate and precise determination of mass-dependent variations in nickel isotope compositions in geological materials. Our method involves an ion-exchange procedure comprising three columns and utilising the ability of Ni to form strong complexes with both ammonia and dimethylglyoxime. The separation procedure is independent of sample pH and works even for samples with large matrix to analyte ratios. Processed Ni solutions are free of matrix elements and direct isobars of Ni, and the yield is normally 85–95%. The purified Ni solutions were analysed using a Nu Plasma, multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS), where instrumental mass fractionation—together with potential isotopic fractionation during chemical separation due to incomplete yield—was corrected for by a double-spike technique, where samples were spiked prior to column chemistry. Tests performed on both mixtures of synthetic and natural terrestrial standards demonstrates that the method is accurate. Replicate measurements of USGS reference materials (peridotite PCC-1, basalt BHVO-2, and shale SCo-1) yield a long-term external reproducibility (2 s.d.) of typically ± 0.07‰, ± 0.1‰, and ± 0.14‰ for 60Ni/58Ni, 61Ni/58Ni, and 62Ni/58Ni respectively.

Isotopic evidence for iron mobility during subduction

Subduction zones are one of the most important sites of chemical interchange between the Earth’s surface and interior. One means of explaining the high Fe3+/ΣFe ratios and oxidized nature of primary arc magmas is the transfer of sulfate (SOX), carbonate (CO3–), and/or iron (Fe3+) bearing fluids from the slab to the overlying mantle. Iron mobility and Fe stable isotope fractionation in fluids are influenced by Fe redox state and the presence of chlorine and/or sulfur anions. Here we use Fe stable isotopes (δ56Fe) as a tracer of iron mobility in serpentinites from Western Alps metaophiolites, which represent remnants of oceanic lithosphere that have undergone subduction-related metamorphism and devolatilization. A negative correlation (R2 = 0.72) is observed between serpentinite bulk δ56Fe and Fe3+/ΣFe that provides the first direct evidence for the release of Fe-bearing fluids during serpentinite devolatilization in subduction zones. The progressive loss of isotopically light Fe from the slab with increasing degree of prograde metamorphism is consistent with the release of sulfate-rich and/or hypersaline fluids, which preferentially complex isotopically light Fe in the form of Fe(II)-SOX or Fe(II)-Cl2 species. Fe isotopes can therefore be used as a tracer of the nature of slab-derived fluids.

Re-Os isotope characteristics of postorogenic lavas: Implications for the nature of young lithospheric mantle and its contribution to basaltic magmas

Re-Os isotopes have been measured on postorogenic potassic lavas from the Tibetan Plateau, the Betic domain of southeastern Spain, and the Colorado Plateau of the southwestern United States. Previous work has established that these lavas were all derived from parts of the subcontinental lithospheric mantle that had undergone metasomatic enrichment in incompatible elements, following various degrees of melt depletion. Cratonic depleted subcontinental lithospheric mantle peridotites typically have subchondritic 187 Os/ 188 Os; however, the postorogenic lavas are characterized by radiogenic 187 Os/ 188 Os ratios (0.139‐0.559). Simple modeling shows that only very large degrees of melt depletion (>25%‐30%) can lower source Re/Os ratios sufficiently to permit timeintegrated development of subchondritic 187 Os/ 188 Os ratios. Such processes may have been largely restricted to the older Precambrian, and the peridotite component of the postorogenic lavas source was probably depleted by <25%. The more radiogenic values may reflect increasing contributions from metasomatic components or possibly crustal contamination. Our findings imply the need for caution in the use of Os isotopes as a diagnostic tracer of subcontinental lithospheric mantle contributions to lavas erupted through younger Proterozoic and Phanerozoic lithosphere.

Persistence of deeply sourced iron in the Pacific Ocean

Significance The vertical supply of dissolved Fe (iron) is insufficient compared with the physiological needs of marine phytoplankton in vast swathes of the open ocean. However, the relative importance of the main sources of “new” Fe to the ocean—continental mineral dust, hydrothermal exhalations, and sediment dissolution—and their temporal evolution are poorly constrained. By analyzing the isotopic composition of Fe in marine sediments, we find that much of the dissolved Fe in the central Pacific Ocean originated from hydrothermal and sedimentary sources thousands of meters below the sea surface. As such, these data underscore the vital role of the oceans’ physical mixing in determining if any deeply sourced Fe ever reaches the Fe-starved surface-dwelling biota. Biological carbon fixation is limited by the supply of Fe in vast regions of the global ocean. Dissolved Fe in seawater is primarily sourced from continental mineral dust, submarine hydrothermalism, and sediment dissolution along continental margins. However, the relative contributions of these three sources to the Fe budget of the open ocean remains contentious. By exploiting the Fe stable isotopic fingerprints of these sources, it is possible to trace distinct Fe pools through marine environments, and through time using sedimentary records. We present a reconstruction of deep-sea Fe isotopic compositions from a Pacific Fe−Mn crust spanning the past 76 My. We find that there have been large and systematic changes in the Fe isotopic composition of seawater over the Cenozoic that reflect the influence of several, distinct Fe sources to the central Pacific Ocean. Given that deeply sourced Fe from hydrothermalism and marginal sediment dissolution exhibit the largest Fe isotopic variations in modern oceanic settings, the record requires that these deep Fe sources have exerted a major control over the Fe inventory of the Pacific for the past 76 My. The persistence of deeply sourced Fe in the Pacific Ocean illustrates that multiple sources contribute to the total Fe budget of the ocean and highlights the importance of oceanic circulation in determining if deeply sourced Fe is ever ventilated at the surface.

Zinc isotope evidence for sulfate-rich fluid transfer across subduction zones

Subduction zones modulate the chemical evolution of the Earths mantle. Water and volatile elements in the slab are released as fluids into the mantle wedge and this process is widely considered to result in the oxidation of the sub-arc mantle. However, the chemical composition and speciation of these fluids, which is critical for the mobility of economically important elements, remain poorly constrained. Sulfur has the potential to act both as oxidizing agent and transport medium. Here we use zinc stable isotopes (δ66Zn) in subducted Alpine serpentinites to decipher the chemical properties of slab-derived fluids. We show that the progressive decrease in δ66Zn with metamorphic grade is correlated with a decrease in sulfur content. As existing theoretical work predicts that Zn-SO42− complexes preferentially incorporate heavy δ66Zn, our results provide strong evidence for the release of oxidized, sulfate-rich, slab serpentinite-derived fluids to the mantle wedge.

High precision osmium stable isotope measurements by double spike MC-ICP-MS and N-TIMS

Osmium stable isotopes provide a new, potentially powerful tool with which to investigate a diverse range of geological processes including planetary formation, ore-genesis and weathering. In this paper, we present a new technique for high precision measurement of osmium (Os) stable isotope ratios by both Multiple-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) and Negative ion Thermal Ionisation Mass Spectrometry (N-TIMS). We use a 188Os–190Os double spike, composed of 61% 188Os and 39% 190Os, to correct for mass dependent fractionation resulting from sample preparation and isotope measurement, with the ideal spike to sample ratio being 55 : 45. Isotope ratios are expressed as δ190Os which is the per mil deviation in the measured 190Os/188Os ratio relative to isotope reference material DROsS. Repeated analyses of double spiked DROsS for both MC-ICP-MS (n = 80 cycles) and N-TIMS (n = 280 cycles) show that an internal precision of 0.01–0.02‰ on δ190Os (2 se) can be attained, with a long-term reproducibility of 0.016‰ and 0.029‰ (2 sd; n = 91 and 83, respectively). The better reproducibility on MC-ICP-MS than on N-TIMS is, predominantly, due to measurement at higher beam intensities (11–18 V with consumption of ∼200 ng natural Os vs. 2–18 V with consumption of 2.3–45 ng natural Os, respectively). In addition to stable isotope compositions, our method allows for simultaneous measurement of 187Os/188Os and 186Os/188Os ratios with a precision of <40 ppm (2 se; 80 cycles for MC-ICP-MS and 280 cycles for N-TIMS) and an external reproducibility of 123–268 ppm and 234–361 ppm (2 sd; n = 91 for MC-ICP-MS and n = 83 for N-TIMS), respectively. We demonstrate that a similar precision and reproducibility can be obtained for other pure Os solutions as well as for geological materials. Furthermore, with a range of of analytical tests we evaluate and demonstrate the robustness of our method with regards to residual matrix effects and interference correction, signal intensity and on-peak zero on MC-ICP-MS, and the effect of oxygen corrections and isobaric interference on N-TIMS. Finally, we report the first Os stable isotope compositions for geological reference materials, including mantle peridotites and chromitites, and one ordinary chondrite.

The behaviour of iron and zinc stable isotopes accompanying the subduction of mafic oceanic crust: A case study from Western Alpine Ophiolites

Arc lavas display elevated Fe3+/ΣFe ratios relative to MORB. One mechanism to explain this is the mobilization and transfer of oxidised or oxidising components from the subducting slab to the mantle wedge. Here we use iron and zinc isotopes, which are fractionated upon complexation by sulfide, chloride and carbonate ligands, to remark on the chemistry and oxidation state of fluids released during prograde metamorphism of subducted oceanic crust. We present data for metagabbros and metabasalts from the Chenaillet massif, Queyras complex and the Zermatt-Saas ophiolite (Western European Alps), which have been metamorphosed at typical subduction zone P-T conditions and preserve their prograde metamorphic history. There is no systematic, detectable fractionation of either Fe or Zn isotopes across metamorphic facies, rather the isotope composition of the eclogites overlaps with published data for MORB. The lack of resolvable Fe isotope fractionation with increasing prograde metamorphism likely reflects the mass balance of the system, and in this scenario Fe mobility is not traceable with Fe isotopes. Given that Zn isotopes are fractionated by S- and C-bearing fluids, this suggests that relatively small amounts of Zn are mobilised from the mafic lithologies in within these types of dehydration fluids. Conversely, metagabbros from the Queyras that are in close proximity to metasediments display a significant Fe isotope fractionation. The covariation of δ56Fe of these samples with selected fluid mobile elements suggests the infiltration of sediment derived fluids with an isotopically light signature during subduction.

Service user and carer feedback: simply pass/fail or a genuine learning tool?

ABSTRACT Social Work Students in Britain are required to submit feedback from Service Users and Carers as part of their Practice Learning portfolios. The purpose of this is twofold—to demonstrate evidence of the student’s ability and to provide information for the student to reflect on and learn from. Members of the Service User and Carer Advisory group attached to a Social Work degree course believe this represents an opportunity for users and carers to influence future social work practice. In reality however, does this feedback actually make a difference? Group members reviewed user and carer feedback contained within student portfolios submitted during one academic year. They found that whilst there were some excellent examples of constructive criticism, the feedback was overwhelmingly positive towards students. The findings raised questions about how gathering Service User and Carer feedback is viewed and undertaken. Is the feedback seen more as a pass/fail issue or as an opportunity to learn from? Could there be better ways to gain constructive feedback for students? What conditions enable students to learn from the feedback that is gathered? This paper starts to explore these questions and suggests ideas for further research and training.

Carbonate Transfer during the Onset of Slab Devolatilization: New Insights from Fe and Zn Stable Isotopes

Long-term carbon cycling is a subject of recent controversy as new mass balance calculations suggest that most carbon is transferred from the slab to the mantle wedge by fluids during subduction, limiting the efficiency of carbon recycling to the deep mantle. Here, we examine the large scale mobility of carbon during subduction using new isotopic tracers sensitive to H–C–O–S–Cl fluids, namely iron and zinc stable isotopes, in samples interpreted to represent residual slab (Queyras, Western Alps) and sub-arc mantle (Kohistan, Himalaya). We show that during subduction there are several stages of carbonate precipitation and dissolution at metasomatic interfaces between metasedimentary and ultramafic rocks in the slab. During the early stages of subduction, before the slab reaches the 300–400C isotherms, the infiltration of sediment-derived fluids into ultramafic lithologies enhances carbonate precipitation in antigorite-bearing serpentinites. Carbonate storage in serpentinites, therefore, acts as a temporary reservoir of carbon in subduction zones. This episode is accompanied by a decrease in serpentinite iron isotope composition (d56Fe), due to interaction with low-d56Fe sediment-derived fluids, and an increase in the concentrations of fluid-mobile elements (e.g. B, Li, As). At higher temperatures (>400C), carbonate is leached from the serpentinites by fluids. This is accompanied by a decrease in serpentinite zinc isotope composition (d66Zn) which we interpret as the release of a carbonate-bearing fluid with an isotopically heavy d66Zn signature. Thermodynamic modelling shows that the sudden change in fluid carbon mobility is due to a decrease in the aCO2 of the fluids released during slab prograde metamorphism, which shifts from sediment- to serpentinite-dominated dehydration. This demonstrates that slab fluids bearing oxidized carbon (e.g. CO2), associated with isotopically light Fe, heavy Zn and fluid-mobile elements, can be released before the slab reaches eclogite facies P-T conditions. These observations provide strong evidence for the mobility of carbon in fluids during the early stages of subduction. Moreover, the fluids released will act as a potential metasomatic agent for the fore-arc mantle (or slab/mantle interface). The observation of carbonate-bearing metamorphic veins in the Himalayan sub-arc mantle with complementary light d56Fe and heavy d66Zn signatures provides further support for the large scale transfer of both sulphate- and carbonate-bearing fluids during the early stages of subduction. This suggests that the fore-arc may have an important role in delivering water, sulfur and carbon to the source of arc-magmas.