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Extensive basaltic magmatism on the naturaliste plateau, offshore SW Australia
A116
Goldschmidt Conference Abstract 2006
Extensive basaltic magmatism on the naturaliste plateau, offshore SW Australia
Re–Os elemental and isotopic systematics in petroleum: A potentially powerful tracer
ANTHONY J. CRAWFORD1, NICHOLAS G. DIREEN2, MILLARD F. COFFIN3, BENJAMIN COHEN4, BENCE PAUL5, LESJA MITROVIC1
R.A. CREASER1, D. SELBY2
1
ARC Centre of Excellence in Ore Deposits, University of Tasmania, Australia ([email protected]) 2 School of Earth and Environmental Sciences, University of Adelaide, Australia 3 Ocean Research Institute, University of Tokyo, Japan 4 School of Earth Sciences, University of Queensland, Australia 5 School of Earth Sciences, University of Melbourne, Australia The large submarine Naturaliste Plateau (NP), lying off the southwestern tip of Australia, is a poorly known part of Australia’s offshore jurisdiction. Only two dredges, one from the northern margin and one from the southern margin, have recovered rocks from this feature, which is approximately the size of Tasmania. Based on the presence of distinctive 130 m.y. old basalts at Bunbury, on the adjacent coast of Western Australia, it has been claimed that the NP may form the easternmost part of the massive Kerguelen Plateau Large Igneous Province (LIP). In this case, the NP would be underlain by thickened oceanic-type crust, and dredged rocks will provide substantial spatial and age constraints on one of the largest LIPs on Earth. However, a recent Geoscience Australia assessment suggested that much of the NP may be underlain by continental rocks. A recently completed swath mapping and dredging cruise using R/V Southern Surveyor investigated the nature and origin of the NP. Pillowed, but often highly vesicular and autobrecciated basalts were sampled along the entire length of the S margin of the NP, and at several locations on its N and NW margins. Quartz + feldspar-phyric glassy rhyolitic lavas were present in several dredges with basalts, suggesting they are part of the same igneous event. Crustal rocks, including garnet-bearing gneisses and granitic rocks, were also dredged along the S margin, including at the most westerly extent of the NP. We will present new age- and geochemical data for the extensive volcanic rocks of the Naturaliste Plateau, and evaluate possible links with the Kerguelen plume. doi:10.1016/j.gca.2006.06.145
1
Department of Earth and Atmospheric Sciences, 1-26 ESB, University of Alberta, Edmonton, Alta., Canada T6G 2E3 ([email protected]) 2 Department of Earth Sciences, Durham University, Durham DH1 3LE, UK ([email protected]) The recent discovery that residues of migrated hydrocarbons can yield Re–Os oil generation ages requires fundamental information regarding the nature of Re and Os in oil to be established. Re and Os abundances in whole oil are found to correlate positively with asphaltene content, and a significant enrichment in Re and Os abundance in asphaltene, relative to the whole oil, is observed for all samples that contain measurable asphaltene. Relative to the whole oil, >83% Re and Os are present in the asphaltene fraction, with most samples being >90%. The Re and Os abundances in the maltene fraction are very low, being 62 ppb Re and 20 ppt Os. The enrichment of Re and Os in the asphaltene fraction relative to the maltene fraction is similar to other trace metals such as Ni, V and Mo. Given the abundance of Re and Os in the asphaltene component and that heteroatomic ligands are concentrated in the asphaltene, this may suggest these elements are bound predominantly by such ligands. The 187Re/188Os, 187Os/188Os, and 187Os/188Os values calculated at the estimated time of oil generation from asphaltene are similar to those of the whole oil, as expected from the elemental results. This suggests that the asphaltene fraction can be used to approximate the Re–Os isotopic compositions of the whole oil. In addition, to first order, the Os isotopic composition in oils today reflects source rock age. For example, Ordovician sourced oils show the highest 187Os/188Os (5.5) and Cretaceous sourced oils have low 187Os/188Os (2). This relationship is also seen for Alberta Oil Sands (187Os/188Os = 2.2–4.4), which are likely sourced from the Devonian Exshaw Formation (187Os/188Os = 2.4–4), and for oil (187Os/188Os = 2.5) sourced from the Jurassic Nordegg Formation (187Os/188Os = 2–6). However, one Devonian sourced oil with very low 187Os/188Os (1.4) has been analyzed. However, the Os isotopic composition of the whole oil and asphaltene fraction is similar to the identified source rock (Duvernay Formation, 1.6–1.7)—this source rock is characterized by unusually low 187Re/188Os ratios (220) and Re and Os abundances. Such a relationship between 187Os/188Os compositions in oil and the age of the source rock must result from the Os isotopes in oil reflecting those of the source rather than, for example, putative interaction between oil and reservoir or pathway rocks. This demonstrates that Re–Os isotopes in petroleum are potentially powerful tracers. doi:10.1016/j.gca.2006.06.146
Goldschmidt Conference Abstract 2006
Extensive basaltic magmatism on the naturaliste plateau, offshore SW Australia
Re–Os elemental and isotopic systematics in petroleum: A potentially powerful tracer
ANTHONY J. CRAWFORD1, NICHOLAS G. DIREEN2, MILLARD F. COFFIN3, BENJAMIN COHEN4, BENCE PAUL5, LESJA MITROVIC1
R.A. CREASER1, D. SELBY2
1
ARC Centre of Excellence in Ore Deposits, University of Tasmania, Australia ([email protected]) 2 School of Earth and Environmental Sciences, University of Adelaide, Australia 3 Ocean Research Institute, University of Tokyo, Japan 4 School of Earth Sciences, University of Queensland, Australia 5 School of Earth Sciences, University of Melbourne, Australia The large submarine Naturaliste Plateau (NP), lying off the southwestern tip of Australia, is a poorly known part of Australia’s offshore jurisdiction. Only two dredges, one from the northern margin and one from the southern margin, have recovered rocks from this feature, which is approximately the size of Tasmania. Based on the presence of distinctive 130 m.y. old basalts at Bunbury, on the adjacent coast of Western Australia, it has been claimed that the NP may form the easternmost part of the massive Kerguelen Plateau Large Igneous Province (LIP). In this case, the NP would be underlain by thickened oceanic-type crust, and dredged rocks will provide substantial spatial and age constraints on one of the largest LIPs on Earth. However, a recent Geoscience Australia assessment suggested that much of the NP may be underlain by continental rocks. A recently completed swath mapping and dredging cruise using R/V Southern Surveyor investigated the nature and origin of the NP. Pillowed, but often highly vesicular and autobrecciated basalts were sampled along the entire length of the S margin of the NP, and at several locations on its N and NW margins. Quartz + feldspar-phyric glassy rhyolitic lavas were present in several dredges with basalts, suggesting they are part of the same igneous event. Crustal rocks, including garnet-bearing gneisses and granitic rocks, were also dredged along the S margin, including at the most westerly extent of the NP. We will present new age- and geochemical data for the extensive volcanic rocks of the Naturaliste Plateau, and evaluate possible links with the Kerguelen plume. doi:10.1016/j.gca.2006.06.145
1
Department of Earth and Atmospheric Sciences, 1-26 ESB, University of Alberta, Edmonton, Alta., Canada T6G 2E3 ([email protected]) 2 Department of Earth Sciences, Durham University, Durham DH1 3LE, UK ([email protected]) The recent discovery that residues of migrated hydrocarbons can yield Re–Os oil generation ages requires fundamental information regarding the nature of Re and Os in oil to be established. Re and Os abundances in whole oil are found to correlate positively with asphaltene content, and a significant enrichment in Re and Os abundance in asphaltene, relative to the whole oil, is observed for all samples that contain measurable asphaltene. Relative to the whole oil, >83% Re and Os are present in the asphaltene fraction, with most samples being >90%. The Re and Os abundances in the maltene fraction are very low, being 62 ppb Re and 20 ppt Os. The enrichment of Re and Os in the asphaltene fraction relative to the maltene fraction is similar to other trace metals such as Ni, V and Mo. Given the abundance of Re and Os in the asphaltene component and that heteroatomic ligands are concentrated in the asphaltene, this may suggest these elements are bound predominantly by such ligands. The 187Re/188Os, 187Os/188Os, and 187Os/188Os values calculated at the estimated time of oil generation from asphaltene are similar to those of the whole oil, as expected from the elemental results. This suggests that the asphaltene fraction can be used to approximate the Re–Os isotopic compositions of the whole oil. In addition, to first order, the Os isotopic composition in oils today reflects source rock age. For example, Ordovician sourced oils show the highest 187Os/188Os (5.5) and Cretaceous sourced oils have low 187Os/188Os (2). This relationship is also seen for Alberta Oil Sands (187Os/188Os = 2.2–4.4), which are likely sourced from the Devonian Exshaw Formation (187Os/188Os = 2.4–4), and for oil (187Os/188Os = 2.5) sourced from the Jurassic Nordegg Formation (187Os/188Os = 2–6). However, one Devonian sourced oil with very low 187Os/188Os (1.4) has been analyzed. However, the Os isotopic composition of the whole oil and asphaltene fraction is similar to the identified source rock (Duvernay Formation, 1.6–1.7)—this source rock is characterized by unusually low 187Re/188Os ratios (220) and Re and Os abundances. Such a relationship between 187Os/188Os compositions in oil and the age of the source rock must result from the Os isotopes in oil reflecting those of the source rather than, for example, putative interaction between oil and reservoir or pathway rocks. This demonstrates that Re–Os isotopes in petroleum are potentially powerful tracers. doi:10.1016/j.gca.2006.06.146