ALBERT

Description: When continents collide, continental crust of the lower plate may be subducted to mantle depth and return to the surface to form eclogite facies metamorphic terranes, as typified by the Western Gneiss Complex of the Scandinavian Caledonides. Proterozoic basement of the Lofoten Islands, located northeast and along strike of the Western Gneiss Complex, contains Caledonian eclogite, although Caledonian deformation is only minor. Previous dating suggested that Lofoten eclogites formed at ca. 480 Ma, i.e., ~50 Ma before the collision between the major continents Baltica and Laurentia, and that the Lofoten basement may not originate from Baltica but rather represents a stranded microcontinent. Newly discovered kyanite eclogites from the Lofoten Islands record deep subduction of continental crust during the main (Scandian) stage of Baltica-Laurentia collision ca. 400 Ma. Thermobarometry and thermodynamic modeling yield metamorphic conditions of 2.5–2.8 GPa and ~650 °C. Lu-Hf geochronology yields 399 ± 10 Ma, corresponding to the time of garnet growth during subduction. Our results demonstrate that the Lofoten basement belonged to Baltica, was subducted to ~90 km depth during the collision with Laurentia, and was exhumed at an intermediate to high rate (〉6 mm/yr) while thrusting of a Caledonian allochthon (Leknes Group) was still ongoing. This supports the challenging conclusions that (1) subducted continental crust may stay rigid down to a depth of ~90 km, and (2) it may be exhumed during ongoing collision and crustal shortening.

Description: The origin of vast accumulations of nickel and platinum in some continental magmatic rocks is still enigmatic, but ultimately linked to silicate-sulfide liquid immiscibility. The exact composition of pristine sulfide melts has proved extremely difficult to document and understand, largely because of the ephemeral, reactive qualities and small quantities of such melts. Here we report the discovery of Fe-Ni sulfide melt globules highly enriched in noble metals (Pt, Pd, Au; 120 ppm total platinum group elements [PGE]) within an unusual high-Mg andesitic glass (8.2 wt% MgO, 57.3 wt% SiO 2 ) dredged from the southern Mid-Atlantic Ridge, near the Bouvet triple junction. The composition of this glass indicates derivation of its parental silicate melt from a garnet pyroxenite mantle source with pronounced "continental" isotopic (Pb, Sr, Nd, Hf, Os, O) signatures. We infer that the chemical properties of this high-temperature (1250 °C) melt, notably high SiO 2 and Ni (310 ppm) contents, promoted sulfide saturation at low pressures in a purely oceanic setting, and propose that this unique example, with its likely origin in the continental lithospheric mantle, may be a useful analogue for incipient Ni-PGE-sulfide melt generation and magmatic PGE enrichment.

Description: Two Fe-Ni sulfides, resembling the chemical composition of a typical mantle base metal sulfide (BMS), were synthesized and used to test micro-analytical procedures (sample digestion and chemical separation) to determine Os-187/Os-188 and highly siderophile element (HSE) mass fractions in single BMS grains. The bulk Os-187/Os-188 and HSE mass fractions of the synthetic sulfides were independently determined after high pressure asher (HPA) digestion and conventional HSE separation (Os solvent extraction and Ir, Ru, Pt, Pd, Re separation via anion exchange chromatography), while the homogenous distribution of HSE was assessed via LA-ICP-MS. Tests were performed following the protocol of Pearson et al. (1998), where sulfides are digested in H2SO4/CrO3 and Os is simultaneously extracted as OsO4. Additional tests were performed adding a pre-digestion step in HBr and/or HCl. The duration and the temperature of the pre-digestion and micro-distillation steps, as well as different chromatographic separations of the HSE were also evaluated. While the majority of the tests yielded Os-187/Os-188 in agreement with that obtained after HPA digestion, HSE mass fractions show large deviations from the reference content, depending on the used procedure. Such variations are interpreted as the result of incomplete sulfide digestion, Os spike loss, and the possible presence of undigested sub-micrometric platinum group minerals (PGM). Overall, the bulk sulfide HSE mass fractions obtained after HPA digestion are best reproduced (mean deviations 〈= 10%) using a pre-digestion step in HBr+HCl at 120 degrees C. This study highlights the need for matrix-matched sulfide reference materials for routine use in laboratories determining HSE mass fractions and Os isotope ratios on single BMS grains. Such an approach is fundamental for comparing and compiling Re-Os ages collected on BMS in many different laboratories. This study also demonstrates that there is need for further testing and for methodological developments/improvement of the analytical procedure(s) used for Re-Os dating and HSE mass fraction determinations in single BMS grains.