Christopher W. Dale

Community differentiation and kinship among Europe’s first farmers

Community differentiation is a fundamental topic of the social sciences, and its prehistoric origins in Europe are typically assumed to lie among the complex, densely populated societies that developed millennia after their Neolithic predecessors. Here we present the earliest, statistically significant evidence for such differentiation among the first farmers of Neolithic Europe. By using strontium isotopic data from more than 300 early Neolithic human skeletons, we find significantly less variance in geographic signatures among males than we find among females, and less variance among burials with ground stone adzes than burials without such adzes. From this, in context with other available evidence, we infer differential land use in early Neolithic central Europe within a patrilocal kinship system.

Late Accretion on the Earliest Planetesimals Revealed by the Highly Siderophile Elements

Coming Late to the Planetesimal Highly siderophile (iron-loving) elements (Re, Os, Ir, Ru, Rh, Pt, Pd, and Au) must have been added to the mantles of Earth, the Moon, and Mars after their iron cores formed; otherwise the mantles would be devoid of these elements, which tend to be segregated to the core. Dale et al. (p. 72) report data on highly siderophile elements in rocks from different planetary bodies, including asteroid 4 Vesta and other differentiated asteroids, which are representative of the planetesimals from which the solar system planets formed. Like the larger planetary bodies, differentiated asteroids, which formed over the first few million years of the solar system, bear the evidence of the late addition of highly siderophile elements to their mantles. Thus, this process was not unique to Earth, the Moon, and Mars and happened over an extended period of time in the inner solar system. Analysis of meteorites shows that unprocessed material was accreted to both planets and asteroids 150 million years after the start of the solar system. Late accretion of primitive chondritic material to Earth, the Moon, and Mars, after core formation had ceased, can account for the absolute and relative abundances of highly siderophile elements (HSEs) in their silicate mantles. Here we show that smaller planetesimals also possess elevated HSE abundances in chondritic proportions. This demonstrates that late addition of chondritic material was a common feature of all differentiated planets and planetesimals, irrespective of when they accreted; occurring ≤5 to ≥150 million years after the formation of the solar system. Parent-body size played a role in producing variations in absolute HSE abundances among these bodies; however, the oxidation state of the body exerted the major control by influencing the extent to which late-accreted material was mixed into the silicate mantle rather than removed to the core.

Mantle flow, volatiles, slab-surface temperatures and melting dynamics in the north Tonga arc―Lau back-arc basin

The Fonualei Spreading Center affords an excellent opportunity to evaluate geochemical changes with increasing depth to the slab in the Lau back-arc basin. We present H2O and CO2concentrations and Sr, Nd, Pb, Hf and U-Th-Ra isotope data for selected glasses as well as new Hf isotope data from boninites and seamounts to the north of the Tonga arc. The Pb and Hf isotope data are used to show that mantle flow is oriented to the southwest and that the tear in the northern end of the slab may not extend east as far as the boninite locality. Along the Fonualei Spreading Center, key geochemical parameters change smoothly with increasing distance from the arc front and increasing slab surface temperatures. The latter may range from 720 to 866°C, based on decreasing H2O/Ce ratios. Consistent with experimental data, the geochemical trends are interpreted to reflect changes in the amount and composition of wet pelite melts or super-critical fluids and aqueous fluids derived from the slab. With one exception, all of the lavas preserve both238U excesses and 226Ra excesses. We suggest that lavas from the Fonualei Spreading Center and Valu Fa Ridge are dominated by fluid-fluxed melting whereas those from the East and Central Lau Spreading Centers, where slab surface temperatures exceed ∼850–900°C, are largely derived through decompression. A similar observation is found for the Manus and East Scotia back-arc basins and may reflect the expiry of a key phase such as lawsonite in the subducted basaltic crust.

Generation and Modification of the Mantle Wedge and Lithosphere beneath the West Bismarck Island Arc: Melting, Metasomatism and Thermal History of Peridotite Xenoliths from Ritter Island

Peridotite xenoliths dredged from the seafloor northwest of Ritter Island in the West Bismarck Island Arc offer a rare insight into the petrogenetic processes operating in the upper mantle wedge of an active oceanic subduction zone. Harzburgitic xenoliths and subordinate dunites and pyroxenites display significant textural and compositional variability between samples, which are interpreted as fragments of heterogeneous mantle that has experienced a complex petrogenetic history. Based on textures and in situ major and trace element analyses of olivine, orthopyroxene and clinopyroxene, five significant petrogenetic stages have been deduced. The first stage was a period of partial melting at temperatures >1100 oC in a previously active arc (likely the now extinct Vitiaz West-Melanesian arc), indicated by the high Mg# and Cr#, low Al2O3 and very low concentrations of incompatible trace elements in residual phases and the absence of residual clinopyroxene. Modelling the concentrations of Y and Yb in orthopyroxene indicates depletion by high degrees (∼30 %) of wet fractional melting of a depleted mantle source. This was followed by metasomatism also related to this previous period of subduction, resulting in refertilisation of the harzburgite residues with clinopyroxene and the formation of dunite and pyroxenite channels. Three populations of secondary clinopyroxene are identified based on their trace element compositions, with both LREE-depleted and (more unusually) sinusoidal REE patterns identified, indicating that a spectrum of fluid compositions was involved in the metasomatism. The third stage involved a period of cooling and chemical re-equilibration below the wet solidus, combined with decompression to shallow lithospheric mantle depths. Geothermometers with different closure temperatures reveal large temperature discrepancies, indicating mantle cooling was both unusually extensive (down to ∼600 oC), but also very slow (∼20 oC/My). The fourth stage marked the cessation of cooling and formation of the modern mantle wedge as part of the West Bismarck Island Arc. Silicate melts percolated through networks of veins and reacted with the residual mantle, generating a variety of new disequilibrium textures, most notably orthopyroxene-clinopyroxene-glass reaction patches. This was accompanied by increases in spinel Cr#, olivine trace element concentrations and higher mineral-mineral temperatures. These disequilibrium textures were preserved through the final stage of entrainment in the host basalt and rapid transport to the seafloor. The Ritter suite thus provides a remarkably detailed insight into the broad diversity of melt/fluid compositions and fluid-rock reaction processes in oceanic sub-arc mantle. Several of these features can be found both in other samples of sub-arc mantle and also cratonic mantle, demonstrating the ubiquity of such processes beneath modern arcs and also the potential genetic relationship between subduction zone processes and the formation of cratons. In this way, sub-arc xenoliths such as the Ritter suite, whilst presently under-sampled, can provide crucial insights for understanding the relationship between the mechanisms by which modern arc systems are generated and evolve, and the nature of the upper mantle once subduction processes have ceased.

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.

Variable H and O isotopes in Tongan basaltic glasses: Source or degassing?

New H and O isotope data are presented for submarine basaltic and basaltic-andesite glasses dredged from the Tongan arc (north of Tongatapu), the Fonualei Rifts (FR, a nascent backarc spreading centre) and the Mangatolu Triple Junction (MTJ). These data complement a comprehensive trace element and Nd-Sr-Pb-Hf isotope dataset. The highest δD values (-50‰ to -30‰) occur in the MTJ and FR. There is a positive co-variation between δD and H2O abundance, initially suggesting addition of a δD-rich H2O component derived from the subducted slab to the upper mantle source. In this scenario, the most negative values (≥-85‰), which have lower water contents, would represent uncontaminated mantle wedge. This is consistent with a MORB source component (typically -80‰ ± 10‰). However, H2O concentration and δD also increase with depth of eruption, suggesting that degassing may explain the fractionation observed. In this scenario the highest δD (-40‰ to -30‰) may be typical of the whole arc-backarc mantle source, while the lower values are produced by degassing (whereby D is preferentially incorporated into H2O rather than melt). Extrapolation to infinite water for the whole arc-FR dataset gives a δD value of ca. -26‰. Samples from the MTJ (n=2) are offset to more elevated δD at similar H2O contents. Ba/La ratios are highest at the central Tongan volcanoes, indicating that these have the greatest fluid-mobile trace element flux, but they do not possess complementary high δD. Thus, either the D-rich signature is masked by fractionation during degassing or the trace element flux is partially decoupled from the water flux. δO values range from 4.6‰ to 6.2‰. There is a poor negative co-variation of δO and δD but this is most likely masked by the effects of H fractionation during degassing. Such a co-variation requires the transfer of an O-depleted and D-rich fluid from high-T altered oceanic crust, rather than low-T altered crust or sediment. Trace element analysis and petrology of Martian meteorite RBT04262 H.A. DALTON*, C.-T.A. LEE, A.H. PESLIER, A.D. BRANDON AND T. LAPEN Rice Univ., Dept of Earth Science, MS 126, PO Box 1892, Houston, TX 77251, USA (*correspondence: heather.a.dalton@rice.edu) (ctlee@rice.edu) ARES, NASA JSC, Houston, TX 77058, USA (anne.h.peslier@nasa.gov, alan.d.brandon@nasa.gov) Jacobs Tech., E.S.C.G., Houston, TX 77058, USA Univ. of Houston, Dept of Geosciences, Houston, TX 77204, USA (tjlapen@uh.edu)