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Metals and chlorine in the evolution of convergent margin magmas
Goldschmidt Conference Abstracts 2006
Nb/Ta fractionation in rutile from eclogites
A629
Metals and chlorine in the evolution of convergent margin magmas
W.D. SUN1, Y.L. XIAO2, J. HOEFS2, K. SIMON2, Z.M. ZHANG3, S.G. LI4
W.D. SUN1, R.J. ARCULUS2, V.S. KAMENETSKY3, R.A. BINNS2, H.Y. LIANG1
1
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, PR China (weidongsun @gig.ac.cn) 2 Geowissenschaftliches Zentrum der Universita¨t Go¨ttingen, Goldschmittstrasse 1, D-37077 Go¨ttingen, Germany ([email protected]) 3 Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, PR China 4 Department of Earth and Space Science, University of Science and Technology of China, Hefei 230026, PR China The formation of the continental crust is an important topic in Earth sciences. Many geologists believe that the continental crust started mainly from the melting of subducted slabs in the early history of the Earth, which then experienced complicated modifications, e.g., melting at the bottom of thickened oceanic crust, arc accretion and delamination of thickened lower crust, etc. Others believe that melting at the bottom of the thickened oceanic crust may have contributed more to the formation of the early continental crust and thus shaped its chemical features. Currently, the debate on the formation of the continental crust is mainly focused on the unique subchondritic Nb/Ta values of the continental crust, because Nb and Ta are ‘‘geochemical identical twins,’’ which usually do not fractionate from each other. As a result, the Nb/Ta has long been regarded as constant and chondritic in major silicate reservoirs of the Earth. By contrast, the Nb/Ta of the CC is 12–13, considerably lower than the chondritic value. Given rutile is the dominant carrier of Nb and Ta and a common minor phase in high-grade metamorphic rocks, the subchondritic Nb/Ta of the CC has been attributed to the melting of subducted slabs in the presence of rutile. Experiments however, show that rutile favors Ta over Nb, such that partial melting of rutile-bearing eclogites with chondritic Nb/Ta results in suprachondritic Nb/Ta in the melts. We find rutile grains from different depths of the slab have different Nb/Ta, indicating major fractionation of Nb and Ta that probably occurred during blueschist to amphibole–eclogite transformation before rutile appeared. Given Ta is more mobile during dehydration, the fluids have subchondritic Nb/Ta, which then forms hydrous rutile-bearing eclogite with low Nb/Ta, leaving suprachondritic Nb/Ta in dehydrated eclogite. Dehydration-melting of such eclogite results in building blocks of the continental crust (e.g., TTG) with subchondritic Nb/Ta, and variable but overall suprachondritic Nb/Ta in residual eclogites. Dehydration of hydrous rutile-bearing eclogites cannot transfer the fractionated Nb/Ta features to the mantle wedge due to their low solubilities in fluids, which explains the Nb/Ta characteristics of modern arc magmas.
1
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640, China ([email protected]) 2 Department of Earth and Marine Sciences, The Australian National University, Canberra ACT 0200, Australia 3 Center for Ore Deposit, Research and School of Earth Sciences, University of Tasmania, Hobart, Australia We have investigated the behaviors of Cu, Au, Re and Cl in subduction-related volcanic glasses belong to basalt–andesite– dacite–rhyolite fractionation series from the eastern Manus backarc basin. At the early stage of magma evolution, Cu, Au and Re increase with increasing SiO2 content. Copper and Au drop suddenly at SiO2 content of 58 wt%, concurrent with a switch in the behavior of titanium and iron from concentration increases to decreases as SiO2 rises (Sun et al., 2004). This relates to the crystallization of titanomagnetite, which reduces sulphate to sulphide, forming Au–Cu hydrosulphide complexes and thus Au and Cu are taken into comagmatic fluids. This can plausibly explain the genetic links between Au–Cu ore deposits and convergent margin magmas. By contrast, Re starts to drop at SiO2 content of 60 wt% and, shows continuous decrease (Sun et al., 2003). The solubility of Re in magmas increases with increasing oxygen fugacity (Ertel et al., 2001), therefore the observed Re behavior is controlled by oxygen fugacity buffering during titanomagnetite crystallization. Chlorine is highly incompatible during the whole evolution of the eastern Manus basin magmas. Modelling shows that the behavior of Cl in magmas is strongly influenced by pressure, initial H2O content and the degree of magmatic fractionation. For basaltic magmas with low initial H2O contents (<2.5 wt%), Cl is highly incompatible under essentially all pressures. For more evolved magmas at moderately high pressure and high H2O contents, considerable amounts of Cl can be extracted from the magma once H2O saturation is reached. Accordingly, Cl is usually highly incompatible in MORB and OIB, because of low H2O contents and relatively low degrees of fractional crystallization. The behavior of Cl in arc magmas is more complicated, ranging from highly incompatible to compatible depending on H2O contents and depths of magma chambers. It also behaves differently in intrusive rocks compared to volcanic rocks because of the different pressures involved.
References Ertel, W., O’Neill, H.S., Sylvester, P.J., Dingwell, D.B., Spettel, B., 2001. Geochim. Cosmochim. Acta 65, 2161–2170. Sun, W.D., Arculus, R.J., Bennett, V.C., Eggins, S.M., Binns, R.A., 2003. Geology 31, 845–848. Sun, W.D., Arculus, R.J., Kamenetsky, V.S., Binns, R.A., 2004. Nature 431, 975–978.
doi:10.1016/j.gca.2006.06.1167 doi:10.1016/j.gca.2006.06.1168
Nb/Ta fractionation in rutile from eclogites
A629
Metals and chlorine in the evolution of convergent margin magmas
W.D. SUN1, Y.L. XIAO2, J. HOEFS2, K. SIMON2, Z.M. ZHANG3, S.G. LI4
W.D. SUN1, R.J. ARCULUS2, V.S. KAMENETSKY3, R.A. BINNS2, H.Y. LIANG1
1
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, PR China (weidongsun @gig.ac.cn) 2 Geowissenschaftliches Zentrum der Universita¨t Go¨ttingen, Goldschmittstrasse 1, D-37077 Go¨ttingen, Germany ([email protected]) 3 Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, PR China 4 Department of Earth and Space Science, University of Science and Technology of China, Hefei 230026, PR China The formation of the continental crust is an important topic in Earth sciences. Many geologists believe that the continental crust started mainly from the melting of subducted slabs in the early history of the Earth, which then experienced complicated modifications, e.g., melting at the bottom of thickened oceanic crust, arc accretion and delamination of thickened lower crust, etc. Others believe that melting at the bottom of the thickened oceanic crust may have contributed more to the formation of the early continental crust and thus shaped its chemical features. Currently, the debate on the formation of the continental crust is mainly focused on the unique subchondritic Nb/Ta values of the continental crust, because Nb and Ta are ‘‘geochemical identical twins,’’ which usually do not fractionate from each other. As a result, the Nb/Ta has long been regarded as constant and chondritic in major silicate reservoirs of the Earth. By contrast, the Nb/Ta of the CC is 12–13, considerably lower than the chondritic value. Given rutile is the dominant carrier of Nb and Ta and a common minor phase in high-grade metamorphic rocks, the subchondritic Nb/Ta of the CC has been attributed to the melting of subducted slabs in the presence of rutile. Experiments however, show that rutile favors Ta over Nb, such that partial melting of rutile-bearing eclogites with chondritic Nb/Ta results in suprachondritic Nb/Ta in the melts. We find rutile grains from different depths of the slab have different Nb/Ta, indicating major fractionation of Nb and Ta that probably occurred during blueschist to amphibole–eclogite transformation before rutile appeared. Given Ta is more mobile during dehydration, the fluids have subchondritic Nb/Ta, which then forms hydrous rutile-bearing eclogite with low Nb/Ta, leaving suprachondritic Nb/Ta in dehydrated eclogite. Dehydration-melting of such eclogite results in building blocks of the continental crust (e.g., TTG) with subchondritic Nb/Ta, and variable but overall suprachondritic Nb/Ta in residual eclogites. Dehydration of hydrous rutile-bearing eclogites cannot transfer the fractionated Nb/Ta features to the mantle wedge due to their low solubilities in fluids, which explains the Nb/Ta characteristics of modern arc magmas.
1
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640, China ([email protected]) 2 Department of Earth and Marine Sciences, The Australian National University, Canberra ACT 0200, Australia 3 Center for Ore Deposit, Research and School of Earth Sciences, University of Tasmania, Hobart, Australia We have investigated the behaviors of Cu, Au, Re and Cl in subduction-related volcanic glasses belong to basalt–andesite– dacite–rhyolite fractionation series from the eastern Manus backarc basin. At the early stage of magma evolution, Cu, Au and Re increase with increasing SiO2 content. Copper and Au drop suddenly at SiO2 content of 58 wt%, concurrent with a switch in the behavior of titanium and iron from concentration increases to decreases as SiO2 rises (Sun et al., 2004). This relates to the crystallization of titanomagnetite, which reduces sulphate to sulphide, forming Au–Cu hydrosulphide complexes and thus Au and Cu are taken into comagmatic fluids. This can plausibly explain the genetic links between Au–Cu ore deposits and convergent margin magmas. By contrast, Re starts to drop at SiO2 content of 60 wt% and, shows continuous decrease (Sun et al., 2003). The solubility of Re in magmas increases with increasing oxygen fugacity (Ertel et al., 2001), therefore the observed Re behavior is controlled by oxygen fugacity buffering during titanomagnetite crystallization. Chlorine is highly incompatible during the whole evolution of the eastern Manus basin magmas. Modelling shows that the behavior of Cl in magmas is strongly influenced by pressure, initial H2O content and the degree of magmatic fractionation. For basaltic magmas with low initial H2O contents (<2.5 wt%), Cl is highly incompatible under essentially all pressures. For more evolved magmas at moderately high pressure and high H2O contents, considerable amounts of Cl can be extracted from the magma once H2O saturation is reached. Accordingly, Cl is usually highly incompatible in MORB and OIB, because of low H2O contents and relatively low degrees of fractional crystallization. The behavior of Cl in arc magmas is more complicated, ranging from highly incompatible to compatible depending on H2O contents and depths of magma chambers. It also behaves differently in intrusive rocks compared to volcanic rocks because of the different pressures involved.
References Ertel, W., O’Neill, H.S., Sylvester, P.J., Dingwell, D.B., Spettel, B., 2001. Geochim. Cosmochim. Acta 65, 2161–2170. Sun, W.D., Arculus, R.J., Bennett, V.C., Eggins, S.M., Binns, R.A., 2003. Geology 31, 845–848. Sun, W.D., Arculus, R.J., Kamenetsky, V.S., Binns, R.A., 2004. Nature 431, 975–978.
doi:10.1016/j.gca.2006.06.1167 doi:10.1016/j.gca.2006.06.1168