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Dating ore deposits using the Te–Xe system
Goldschmidt Conference Abstracts 2006
1
University of Oxford, Department of Earth Science, Parks Road, Oxford OX1 3PR, UK ([email protected]) 2 University of Cambridge, Department of Earth Science, Downing Street, Cambridge CB2 3EQ, UK Variation of past deep-ocean flow rate has been assessed for the North Atlantic using the sortable silt (McCave, 1995; Bianchi and McCave, 1999) and 231Pa/230Th (McManus, 2004) proxies. Changes in Southern Ocean deep-water flow rate might be expected to be anti-phased to those in the north but this scenario has not been widely investigated. Two sortable silt records, however, indicate faster flow into the Pacific during glacials (Hall, 2001) and some changes of inflow into the Indian Ocean (McCave, 2005). Here, we present the first down-core record of (231Paxs/230Thxs)0 from the southern Indian Ocean. The record spans the last 140 kyr and comprises 22 measurements of (231Paxs/230Thxs)0 in a core with an existing sortable-silt record (McCave, 2005). (231Paxs/230Thxs)0 is nearly constant at 0.055. This is significantly lower than the production ratio (0.093) indicating that the proxy is sensitive to changes in circulation and/ or productivity at this location. A simple particle scavenging model has shown that (231Paxs/230Thxs)0 in sediments is likely to record conditions in the bottommost water mass (Thomas, 2006) so this (231Paxs/230Thxs)0 record suggests little change of AABW flow into the Indian Ocean during the last glacial–interglacial cycle. This is in contrast to the sortable silt proxy for bottom-water flow which shows variation during this interval (McCave, 2005). The sortable silt variability might, instead, be attributed to a local geostrophic effect in Amirante Passage amplifying small changes in circulation, or to possible changes in sediment source during sea-level change. An absence of changes in deep-water flow into the Indian Ocean does not preclude changes in intermediate flow, which could be assessed with (231Paxs/230Thxs)0 records from shallower cores. Similarly, changes in the flow of AABW into the Pacific could result from deep water production around the coast of E. Antarctica (Ade´lie coast) rather than in the Weddell Sea, which could be confirmed with appropriate (231Paxs/230Thxs)0 records from these basins.
References Bianchi, McCave, 1999. Nature 397, 6719–7515. Hall et al., 2001. Nature 412, 809–812. McCave et al., 1995. Paleoceanography 10, 593–610. McCave et al., 2005. Proc. Roy. Soc. 363, 81–99. McManus et al., 2004. Nature 428, 834–837. Thomas et al., 2006. EPSL 241 (3–4), 493–504.
H.V. THOMAS, R. PATTRICK, J. GILMOUR S.E.A.E.S, University of Manchester, Oxford Road, Manchester M13 9PL, UK ([email protected]) The Te–Xe dating technique relies on determining the ratio between the 130Xe decay product and its parent 130Te (Kirsten et al., 1968). 130Xe accumulates in telluride minerals due to bb-decay of 130Te, while 131Xe is produced from 130Te by neutron capture, so Xe isotopic analysis of irradiated tellurium-rich minerals allows a xenon closure age to be determined (Fig. 1). The technique can be applied to sub-mg. samples, and in contrast to other methods which date deposits indirectly, bb-decay of 130Te to 130 Xe has the potential to give a direct age of ore minerals. We report data from ore deposits in Wales, Colorado, Western Australia and Uzbekistan. We employ the University of Manchester’s unique resonance ionization mass spectrometer RELAX (Refrigerator Enhanced Laser Analyser for Xenon). Samples are analysed by laser step heating. Identifying closure to Xe loss with the accepted age of mineralization of Kochbulak leads to a 130Te half-life of 9.62 ± 0.2 · 1020 yr using data from Kochbulak. Using this value, the data from Clogau Gold Mines, Wales, suggest an age of 331 ± 9 Ma; the data for Good Hope Mine, Colorado suggest an age of 490 ± 13 Ma; and the data for Kalgoorlie, Western Australia suggest an age of 2.38 ± 0.06 Ga. These ages represent closure of the minerals to xenon loss which may be affected by post-formational resetting events. 7 6 5
Xe/ 130Xe
A.L. THOMAS1, G.M. HENDERSON1, I.N. MCCAVE2
Dating ore deposits using the Te–Xe system
132
Constant flow of AABW into the Indian Ocean over the past 140 ka? Conflict between 231Pa/230Th and sortable silt records
A647
4 3 2 1 0 0
1
2 130
4
5
6
Fig. 1. Te–Xe isochron diagram for Kochbulak data (squares) reveals mixing between air xenon and a pure-tellurium component. Good Hope data (triangles) and isochron (broken line) are shown for comparison.
Reference Kirsten, T., Shaeffer, E., Norton, E., Stoenner, R.W., 1968. Phys. Rev. Lett. 20, 1300–1303. doi:10.1016/j.gca.2006.06.1206
doi:10.1016/j.gca.2006.06.1205
3 Te/130Xe (x1012)
1
University of Oxford, Department of Earth Science, Parks Road, Oxford OX1 3PR, UK ([email protected]) 2 University of Cambridge, Department of Earth Science, Downing Street, Cambridge CB2 3EQ, UK Variation of past deep-ocean flow rate has been assessed for the North Atlantic using the sortable silt (McCave, 1995; Bianchi and McCave, 1999) and 231Pa/230Th (McManus, 2004) proxies. Changes in Southern Ocean deep-water flow rate might be expected to be anti-phased to those in the north but this scenario has not been widely investigated. Two sortable silt records, however, indicate faster flow into the Pacific during glacials (Hall, 2001) and some changes of inflow into the Indian Ocean (McCave, 2005). Here, we present the first down-core record of (231Paxs/230Thxs)0 from the southern Indian Ocean. The record spans the last 140 kyr and comprises 22 measurements of (231Paxs/230Thxs)0 in a core with an existing sortable-silt record (McCave, 2005). (231Paxs/230Thxs)0 is nearly constant at 0.055. This is significantly lower than the production ratio (0.093) indicating that the proxy is sensitive to changes in circulation and/ or productivity at this location. A simple particle scavenging model has shown that (231Paxs/230Thxs)0 in sediments is likely to record conditions in the bottommost water mass (Thomas, 2006) so this (231Paxs/230Thxs)0 record suggests little change of AABW flow into the Indian Ocean during the last glacial–interglacial cycle. This is in contrast to the sortable silt proxy for bottom-water flow which shows variation during this interval (McCave, 2005). The sortable silt variability might, instead, be attributed to a local geostrophic effect in Amirante Passage amplifying small changes in circulation, or to possible changes in sediment source during sea-level change. An absence of changes in deep-water flow into the Indian Ocean does not preclude changes in intermediate flow, which could be assessed with (231Paxs/230Thxs)0 records from shallower cores. Similarly, changes in the flow of AABW into the Pacific could result from deep water production around the coast of E. Antarctica (Ade´lie coast) rather than in the Weddell Sea, which could be confirmed with appropriate (231Paxs/230Thxs)0 records from these basins.
References Bianchi, McCave, 1999. Nature 397, 6719–7515. Hall et al., 2001. Nature 412, 809–812. McCave et al., 1995. Paleoceanography 10, 593–610. McCave et al., 2005. Proc. Roy. Soc. 363, 81–99. McManus et al., 2004. Nature 428, 834–837. Thomas et al., 2006. EPSL 241 (3–4), 493–504.
H.V. THOMAS, R. PATTRICK, J. GILMOUR S.E.A.E.S, University of Manchester, Oxford Road, Manchester M13 9PL, UK ([email protected]) The Te–Xe dating technique relies on determining the ratio between the 130Xe decay product and its parent 130Te (Kirsten et al., 1968). 130Xe accumulates in telluride minerals due to bb-decay of 130Te, while 131Xe is produced from 130Te by neutron capture, so Xe isotopic analysis of irradiated tellurium-rich minerals allows a xenon closure age to be determined (Fig. 1). The technique can be applied to sub-mg. samples, and in contrast to other methods which date deposits indirectly, bb-decay of 130Te to 130 Xe has the potential to give a direct age of ore minerals. We report data from ore deposits in Wales, Colorado, Western Australia and Uzbekistan. We employ the University of Manchester’s unique resonance ionization mass spectrometer RELAX (Refrigerator Enhanced Laser Analyser for Xenon). Samples are analysed by laser step heating. Identifying closure to Xe loss with the accepted age of mineralization of Kochbulak leads to a 130Te half-life of 9.62 ± 0.2 · 1020 yr using data from Kochbulak. Using this value, the data from Clogau Gold Mines, Wales, suggest an age of 331 ± 9 Ma; the data for Good Hope Mine, Colorado suggest an age of 490 ± 13 Ma; and the data for Kalgoorlie, Western Australia suggest an age of 2.38 ± 0.06 Ga. These ages represent closure of the minerals to xenon loss which may be affected by post-formational resetting events. 7 6 5
Xe/ 130Xe
A.L. THOMAS1, G.M. HENDERSON1, I.N. MCCAVE2
Dating ore deposits using the Te–Xe system
132
Constant flow of AABW into the Indian Ocean over the past 140 ka? Conflict between 231Pa/230Th and sortable silt records
A647
4 3 2 1 0 0
1
2 130
4
5
6
Fig. 1. Te–Xe isochron diagram for Kochbulak data (squares) reveals mixing between air xenon and a pure-tellurium component. Good Hope data (triangles) and isochron (broken line) are shown for comparison.
Reference Kirsten, T., Shaeffer, E., Norton, E., Stoenner, R.W., 1968. Phys. Rev. Lett. 20, 1300–1303. doi:10.1016/j.gca.2006.06.1206
doi:10.1016/j.gca.2006.06.1205
3 Te/130Xe (x1012)