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silicate-melt partitioning
A386
Goldschmidt Conference Abstracts 2006
Os isotope systematics of Os-bearing alloys and sulfides from continental and oceanic mantle: New data K.N. MALITCH All-Russia Geological Research Institute, Sredny pr. 74, St. Petersburg, 199106, Russia ([email protected]) The advantage of the Re–Os system, applied to Os-rich platinum-goup minerals (PGMs), is that PGMs contain Os as a main or trace element, while at the same time almost lack Re. Consequently, primary Os-rich PGMs (e.g., laurite-erlichmanite series (RuS2–OsS2) and Os–Ir–Ru alloys), frequently the earliest minerals in ultramafic mantle rocks, are (1) the best tracers of mantle melting events, and (2) promising targets to constrain initial 187 Os/188Os ratios of different mantle environments. This contribution summarizes the extensive data set of Os isotopic compositions of bedrock and detrital Ru–Os sulfides and Os–Ir–Ru alloys derived from continental and oceanic ultramafic complexes. Os isotopic composition of PGMs was evaluated by negative thermal ionization mass-spectrometry (NTI-MS) and laser ablation (LA) attached to multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). PGMs were extracted from bedrock samples using the innovative non-destructive concentration teqnique at NATI Research JSC (St. Petersburg, Russia). The range of 187Os/188Os values in Os-bearing alloys and sulfides from chromitite and dunite at Kondyor, Inagli and Guli (Siberian Craton, Russia) show a relatively narrow range of ‘unradiogenic’ 187Os/188Os values, indicative of a subchondritic mantle source of PGE (e.g., 0.1230–0.1253, n = 135, NTI-MS and LA MC-ICP-MS data). In contrast, Ru–Os sulfides of podiform chromitites from ophiolitic mantle sections at Shetland (UK), Kraubath and Hochgro¨ssen (Austria) yield a very wide range of subchondritic 187Os/188Os values (0.1158 ± 0.0015 to 0.1244 ± 0.0005, n = 27, NTI-MS and LA MC-ICP-MS data), which is almost identical to that of Os–Ir–Ru alloys (e.g. 0.1094–0.1252, n = 45, NTI-MS data) derived from the Kunar and Ust’-Bel’sky dunite-harzburgite complexes (Russia). ‘Radiogenic’ 187Os/188Os (0.1302–0.1321) values, indicative of a suprachondritic source, are characteristic of osmium alloy from clinopyroxenite at Kondyor and Ru–Os sulfides from banded chromitite within the Moho transition zone at Kraubath. The Os isotopic system of Os-bearing alloys and sulfides from the mantle sections of ophiolites provides isotopic evidence for locally restricted but temporally extended melting events in parent ultramafic protoliths. This variability is probably controlled by deep-geodynamic processes implying that source rocks in the residual mantle have a more complex geological history that it is frequently assumed. doi:10.1016/j.gca.2006.06.780
The redox control upon the rhenium crystal/silicate-melt partitioning GUILHERME MALLMANN, HUGH ST.C. O’NEILL Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia The full development of the potential of the Re–Os isotopic system is currently hindered by the poorly understood behaviour of Re and Os during mantle melting and magma genesis. Experimental studies have appeared to show that Re is incompatible in olivine, pyroxenes, and spinel (Watson et al., 1987; Righter and Hauri, 1998; Brenan et al., 2003; Righter et al., 2004), and compatible in garnet (Righter and Hauri, 1998), but they have underestimated the effect of oxygen fugacity (fO2). Rhenium is a heterovalent element, and its solubility in silicate melts shows that it dissolves as both Re4+ and Re6+ (Ertel et al., 2001), with the changeover from one oxidation state to the other occurring over the range of fO2s pertinent to the Earth’s mantle. We have studied the partitioning behaviour of Re between the common upper mantle minerals (garnet, spinel, clinopyroxene, orthopyroxene, and olivine) and Fe-free silicate melts under temperature (1275–1450 °C) and pressure (1.5–3.2 GPa) conditions relevant for basaltic magma genesis, over a range of fO2s large enough (QFM +5.6 to 1.3) to demonstrate the effects of changing the oxidation state of Re from 4+ to 6+. Experiments were run in the piston–cylinder apparatus, with fO2 controlled either by equilibrium with Re metal, or using the Ru + RuO2 buffer. Run products were analysed by electron microprobe, and for Re and some other trace elements, by laser-ablation ICP-MS. Rhenium crystal/silicate-melt partition coefficients for pyroxenes, garnet and spinel vary by four to five orders of magnitude, from compatible to highly incompatible, as the oxidation state of Re changes from 4+ to 6+ as a function of fO2, but Re in either oxidation state is incompatible in olivine. Whole-rock crystal/silicate-melt partition coefficients are averages of both oxidation states, and are therefore expected to vary significantly according to the oxidation state of the system. For instance, assuming average fO2s of QFM -0.9 and +1.1 for mid-ocean ridge and island arc basalts, respectively, a difference of one order of magnitude is predicted for bulk Re partition coefficients. Our results also demonstrate that Re4+ has a partitioning behaviour similar to Ti4+ rather than to Yb, and as such is not a sensitive indicator of garnet in the source.
References Brenan et al., 2003. EPSL 212, 135–150. Ertel et al., 2001. Geochim. Cosmochim. Acta 65, 2161–2170. Righter, Hauri, 1998. Science 280, 1737–1741. Righter et al., 2004. Geochim. Cosmochim. Acta 68, 867–880. Watson et al., 1987. Chem. Geol. 62, 191–208. doi:10.1016/j.gca.2006.06.781
Goldschmidt Conference Abstracts 2006
Os isotope systematics of Os-bearing alloys and sulfides from continental and oceanic mantle: New data K.N. MALITCH All-Russia Geological Research Institute, Sredny pr. 74, St. Petersburg, 199106, Russia ([email protected]) The advantage of the Re–Os system, applied to Os-rich platinum-goup minerals (PGMs), is that PGMs contain Os as a main or trace element, while at the same time almost lack Re. Consequently, primary Os-rich PGMs (e.g., laurite-erlichmanite series (RuS2–OsS2) and Os–Ir–Ru alloys), frequently the earliest minerals in ultramafic mantle rocks, are (1) the best tracers of mantle melting events, and (2) promising targets to constrain initial 187 Os/188Os ratios of different mantle environments. This contribution summarizes the extensive data set of Os isotopic compositions of bedrock and detrital Ru–Os sulfides and Os–Ir–Ru alloys derived from continental and oceanic ultramafic complexes. Os isotopic composition of PGMs was evaluated by negative thermal ionization mass-spectrometry (NTI-MS) and laser ablation (LA) attached to multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). PGMs were extracted from bedrock samples using the innovative non-destructive concentration teqnique at NATI Research JSC (St. Petersburg, Russia). The range of 187Os/188Os values in Os-bearing alloys and sulfides from chromitite and dunite at Kondyor, Inagli and Guli (Siberian Craton, Russia) show a relatively narrow range of ‘unradiogenic’ 187Os/188Os values, indicative of a subchondritic mantle source of PGE (e.g., 0.1230–0.1253, n = 135, NTI-MS and LA MC-ICP-MS data). In contrast, Ru–Os sulfides of podiform chromitites from ophiolitic mantle sections at Shetland (UK), Kraubath and Hochgro¨ssen (Austria) yield a very wide range of subchondritic 187Os/188Os values (0.1158 ± 0.0015 to 0.1244 ± 0.0005, n = 27, NTI-MS and LA MC-ICP-MS data), which is almost identical to that of Os–Ir–Ru alloys (e.g. 0.1094–0.1252, n = 45, NTI-MS data) derived from the Kunar and Ust’-Bel’sky dunite-harzburgite complexes (Russia). ‘Radiogenic’ 187Os/188Os (0.1302–0.1321) values, indicative of a suprachondritic source, are characteristic of osmium alloy from clinopyroxenite at Kondyor and Ru–Os sulfides from banded chromitite within the Moho transition zone at Kraubath. The Os isotopic system of Os-bearing alloys and sulfides from the mantle sections of ophiolites provides isotopic evidence for locally restricted but temporally extended melting events in parent ultramafic protoliths. This variability is probably controlled by deep-geodynamic processes implying that source rocks in the residual mantle have a more complex geological history that it is frequently assumed. doi:10.1016/j.gca.2006.06.780
The redox control upon the rhenium crystal/silicate-melt partitioning GUILHERME MALLMANN, HUGH ST.C. O’NEILL Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia The full development of the potential of the Re–Os isotopic system is currently hindered by the poorly understood behaviour of Re and Os during mantle melting and magma genesis. Experimental studies have appeared to show that Re is incompatible in olivine, pyroxenes, and spinel (Watson et al., 1987; Righter and Hauri, 1998; Brenan et al., 2003; Righter et al., 2004), and compatible in garnet (Righter and Hauri, 1998), but they have underestimated the effect of oxygen fugacity (fO2). Rhenium is a heterovalent element, and its solubility in silicate melts shows that it dissolves as both Re4+ and Re6+ (Ertel et al., 2001), with the changeover from one oxidation state to the other occurring over the range of fO2s pertinent to the Earth’s mantle. We have studied the partitioning behaviour of Re between the common upper mantle minerals (garnet, spinel, clinopyroxene, orthopyroxene, and olivine) and Fe-free silicate melts under temperature (1275–1450 °C) and pressure (1.5–3.2 GPa) conditions relevant for basaltic magma genesis, over a range of fO2s large enough (QFM +5.6 to 1.3) to demonstrate the effects of changing the oxidation state of Re from 4+ to 6+. Experiments were run in the piston–cylinder apparatus, with fO2 controlled either by equilibrium with Re metal, or using the Ru + RuO2 buffer. Run products were analysed by electron microprobe, and for Re and some other trace elements, by laser-ablation ICP-MS. Rhenium crystal/silicate-melt partition coefficients for pyroxenes, garnet and spinel vary by four to five orders of magnitude, from compatible to highly incompatible, as the oxidation state of Re changes from 4+ to 6+ as a function of fO2, but Re in either oxidation state is incompatible in olivine. Whole-rock crystal/silicate-melt partition coefficients are averages of both oxidation states, and are therefore expected to vary significantly according to the oxidation state of the system. For instance, assuming average fO2s of QFM -0.9 and +1.1 for mid-ocean ridge and island arc basalts, respectively, a difference of one order of magnitude is predicted for bulk Re partition coefficients. Our results also demonstrate that Re4+ has a partitioning behaviour similar to Ti4+ rather than to Yb, and as such is not a sensitive indicator of garnet in the source.
References Brenan et al., 2003. EPSL 212, 135–150. Ertel et al., 2001. Geochim. Cosmochim. Acta 65, 2161–2170. Righter, Hauri, 1998. Science 280, 1737–1741. Righter et al., 2004. Geochim. Cosmochim. Acta 68, 867–880. Watson et al., 1987. Chem. Geol. 62, 191–208. doi:10.1016/j.gca.2006.06.781