劳盆地与马努斯盆地俯冲熔体与流体组分的识别——Sr-Nd-Pb同位素与独立成分分析
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摘要
为了探讨俯冲沉积物与蚀变洋壳组分对弧后盆地岩浆作用的影响,利用独立成分分析从劳盆地、马努斯盆地,以及太平洋与印度洋共634组Sr-Nd-Pb同位素数据中提取了5个独立成分(IC1—IC5)。其中:IC1与_87Sr/_86Sr、Th/Nb值为负相关关系,表明其能够代表弧下地幔中的俯冲沉积物熔体组分的影响,能够很好地区别弧后盆地与洋中脊;弧后盆地IC5与Ba/Th值为负相关关系,暗示其能够反映弧下地幔中的俯冲蚀变洋壳脱水流体组分的影响;IC5与κ值(1.03Th/U)之间的关系,也说明洋中脊IC5可能与地幔中再循环蚀变洋壳的年龄有关。进而通过对比马努斯和劳盆地岩石中IC1与IC5间的差异,发现马努斯盆地东部裂谷(ER)表现出绝对值更高的IC1负异常以及与劳盆地相似的IC5特征,而马努斯盆地扩张中心与扩张转换带(MSC&ETZ)则表现出类似劳盆地的IC1特征以及更高的IC5正异常特征。两个盆地间的差异表明,ER更多地受到沉积物熔体组分影响,MSC&ETZ更少地受到俯冲洋壳脱水流体组分影响。其中,沉积物熔体组分影响可能与New Britain trench与Tonga trench巨大的沉积物厚度差异有关,而俯冲洋壳脱水流体组分影响可能受到了弧后扩张中心与俯冲带间距离的控制。
The effects of subducted sediments and altered oceanic crust(AOC)on back-arc magmatism were studied using independent component analysis on multi-isotope dataset.Sr-Nd-Pb isotope data of magmatic rocks from Lau and Manus basins,as well as the Pacific and Indian oceanic ridges,were analyzed,and five independent components(ICs)were identified.The negative correlation between IC1 and 87 Sr/86 Sr and Th/Nb suggests that the IC1 represents the influence of hydrous melt derived from subducted sediments on the sub-arc mantle,and can be used to distinguish back-arc basin from midocean ridge settings.The negative correlation between IC5 and Ba/Th suggests that IC5 may reflect the influence of aqueous fluid derived from the AOC on the sub-arc mantle.Furthermore,the correlation between IC5 and Th/U indicates that the IC5 values may be related to the age of recycled oceanic crust in the mantle.A comparison between the IC1 and IC5 values of Lau and Manus basins shows that the eastern rift(ER)of Manus basin exhibits similar IC5 but lower IC1 relative to that of Lau basin,whereas the manus spreading center(MSC)and extensional transform zone(ETZ)of Manus basin show similar IC1 but higher IC5 compared with that of Lau basin.These differences suggest that the influences of sediment melts are the strongest in the ER,while AOC fluid exerts similar influence on ER and Lau basin,but much less on MSC and ETZ.We believe that the difference in the degree of sediment melt influence may be associated with variable sediment thickness being subducted into the New Britain and Tonga trenches,while the extent of AOC fluid influence is controlled by the distance of back-arc ridges away from the trench.
The effects of subducted sediments and altered oceanic crust(AOC)on back-arc magmatism were studied using independent component analysis on multi-isotope dataset.Sr-Nd-Pb isotope data of magmatic rocks from Lau and Manus basins,as well as the Pacific and Indian oceanic ridges,were analyzed,and five independent components(ICs)were identified.The negative correlation between IC1 and 87 Sr/86 Sr and Th/Nb suggests that the IC1 represents the influence of hydrous melt derived from subducted sediments on the sub-arc mantle,and can be used to distinguish back-arc basin from midocean ridge settings.The negative correlation between IC5 and Ba/Th suggests that IC5 may reflect the influence of aqueous fluid derived from the AOC on the sub-arc mantle.Furthermore,the correlation between IC5 and Th/U indicates that the IC5 values may be related to the age of recycled oceanic crust in the mantle.A comparison between the IC1 and IC5 values of Lau and Manus basins shows that the eastern rift(ER)of Manus basin exhibits similar IC5 but lower IC1 relative to that of Lau basin,whereas the manus spreading center(MSC)and extensional transform zone(ETZ)of Manus basin show similar IC1 but higher IC5 compared with that of Lau basin.These differences suggest that the influences of sediment melts are the strongest in the ER,while AOC fluid exerts similar influence on ER and Lau basin,but much less on MSC and ETZ.We believe that the difference in the degree of sediment melt influence may be associated with variable sediment thickness being subducted into the New Britain and Tonga trenches,while the extent of AOC fluid influence is controlled by the distance of back-arc ridges away from the trench.
引文
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[25]Brenan J M,Shaw H F,Ryerson F J,et al.MineralAqueous Fluid Partitioning of Trace Elements at 900℃and 2.0 GPa:Constraints on the Trace Element Chemistry of Mantle and Deep Crustal Fluids[J].Geochimica et Cosmochimica Acta,1995,59(16):3331-3350.
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[27]Elliott T,Zindler A,Bourdon B.Exploring the Kappa Conundrum:The Role of Recycling in the Lead Isotope Evolution of the Mantle[J].Earth&Planetary Science Letters,1999,169:129-145.
[28]Hanyu T,Dosso L,Ishizuka O,et al.Geochemical Diversity in Submarine HIMU Basalts from Austral Islands,French Polynesia[J].Contributions to Mineralogy and Petrology,2013,166(5):1285-1304.
[29]Hanyu T,Kawabata H,Tatsumi Y,et al.Isotope Evolution in the HIMU Reservoir Beneath St.Helena:Implications for the Mantle Recycling of U and Th[J].Geochimica et Cosmochimica Acta,2014,143(1):232-252.
[30]Whitmore G P,Crook K A W,Johnson D P.Sedimentation in a Complex Convergent Margin:The Papua New Guinea Collision Zone of the Western Solomon Sea[J].Marine Geology,1999,157:19-45.
[2]Class C,Miller D M,Goldstein S L,et al.Distinguishing Melt and Fluid Subduction Components in Umnak Volcanics,Aleutian Arc[J].Geochemistry Geophysics Geosystems,2000,1(6):927-928.
[3]Elliott T,Plank T,Zindler A,et al.Element Transport from Slab to Volcanic Front at the Mariana Arc[J].Journal of Geophysical Research Atmospheres,1997,1021:14991-15020.
[4]Plank T,Langmuir C H.The Chemical Composition of Subducting Sediment and Its Consequences for the Crust and Mantle[J].Chemical Geology,1998,145:325-394.
[5]Pearce J A,Stern R J.Origin of Back-Arc Basin Magmas:Trace Element and Isotope Perspectives[J].Washington Dc American Geophysical Union Geophysical Monograph,2006,166:63-86.
[6]Pearce J A,Stern R J,Bloomer S H,et al.Geochemical Mapping of the Mariana Arc-Basin System:Implications for the Nature and Distribution of Subduction Components[J].Geochemistry Geophysics Geosystems,2005,6(7):406-407.
[7]Sinton J M,Ford L L,Chappell B,et al.Magma Genesis and Mantle Heterogeneity in the Manus Back-Arc Basin,Papua New Guinea[J].Journal of Petrology,2003,44(1):159-195.
[8]Volpe A M,Macdougall J D,Hawkins J W.Mariana Trough Basalts(MTB):Trace Element and Sr Nd Isotopic Evidence for Mixing Between MORB-Like and Arc-Like Melts[J].Earth&Planetary Science Letters,1987,82(3):241-254.
[9]Volpe A M,Macdougall J D,Lugmair G W,et al.Fine-Scale Isotopic Variation in Mariana Trough Basalts:Evidence for Heterogeneity and a Recycled Component in Backarc Basin Mantle[J].Earth&Planetary Science Letters,1990,100(1):251-264.
[10]Manning C E.The Chemistry of Subduction-Zone Fluids[J].Earth&Planetary Science Letters,2004,223(1/2):1-16.
[11]Hermann J,Spandler C J.Sediment Melts at SubArc Depths:An Experimental Study[J].Journal of Petrology,2008,49(4):717-740.
[12]Stolper E,Newman S.The Role of Water in the Petrogenesis of Mariana Trough Magmas[J].Earth&Planetary Science Letters,1994,121:293-325.
[13]Iwamori H,Albarède F.Decoupled Isotopic Record of Ridge and Subduction Zone Processes in Oceanic Basalts by Independent Component Analysis[J].Geochemistry Geophysics Geosystems,2008,9(4):366-389.
[14]Iwamori H,Albaréde F,Nakamura H.Global Structure of Mantle Isotopic Heterogeneity and Its Implications for Mantle Differentiation and Convection[J].Earth&Planetary Science Letters,2010,299(3):339-351.
[15]Iwamori H,Nakamura H.East-West Geochemical Hemispheres Constrained from Independent Component Analysis of Basalt Isotopic Compositions[J].Geochemical Journal,2012,46(4):39-46.
[16]Iwamori H,Nakamura H.Isotopic Heterogeneity of Oceanic Arc and Continental Basalts and Its Implications for Mantle Dynamics[J].Gondwana Research,2015,27(3):1131-1152.
[17]Yasukawa K,Nakamura K,Fujinaga K,et al.Tracking the Spatiotemporal Variations of Statistically Independent Components Involving Enrichment of Rare-Earth Elements in Deep-Sea Sediments[J].Scientific Reports,2016,6:29603.
[18]Martinez F,Taylor B.Mantle Wedge Control on Back-Arc Crustal Accretion.[J].Nature,2002,416:417-420.
[19]Taylor B.Bismarck Sea:Evolution of the Back-Arc Basin[J].Geology,1979,7(4):1190.
[20]Taylor B,Crook K,Sinton J.Extensional Transform Zones and Oblique Spreading Centers[J].Journal of Geophysical Research Solid Earth,1994,99(B10):19707-19718.
[21]Yang J,Cheng Q.A Comparative Study of Independent Component Analysis with Principal Component Analysis in Geological Objects Identification:Part II:A Case Study of Pinghe District,Fujian,China[J].Journal of Geochemical Exploration,2015,149:136-146.
[22]Othman D B,White W M,Patchett J.The Geochemistry of Marine Sediments,Island Arc Magma Genesis,and Crust-Mantle Recycling[J].Earth&Planetary Science Letters,1989,94:1-21.
[23]DupréB,Allègre C J.Pb-Sr Isotope Variation in Indian Ocean Basalts and Mixing Phenomena[J].Nature,1983,303:142-146.
[24]Mper M R,Hofmann A W.Recycled Ocean Crust and Sediment in Indian Ocean MORB[J].Earth&Planetary Science Letters,1997,147:93-106.
[25]Brenan J M,Shaw H F,Ryerson F J,et al.MineralAqueous Fluid Partitioning of Trace Elements at 900℃and 2.0 GPa:Constraints on the Trace Element Chemistry of Mantle and Deep Crustal Fluids[J].Geochimica et Cosmochimica Acta,1995,59(16):3331-3350.
[26]Hart S R.The DUPAL Anomaly:A Large-Scale Isotopic Anomaly in the Southern Hemisphere[J].Nature,1984,309:753-757.
[27]Elliott T,Zindler A,Bourdon B.Exploring the Kappa Conundrum:The Role of Recycling in the Lead Isotope Evolution of the Mantle[J].Earth&Planetary Science Letters,1999,169:129-145.
[28]Hanyu T,Dosso L,Ishizuka O,et al.Geochemical Diversity in Submarine HIMU Basalts from Austral Islands,French Polynesia[J].Contributions to Mineralogy and Petrology,2013,166(5):1285-1304.
[29]Hanyu T,Kawabata H,Tatsumi Y,et al.Isotope Evolution in the HIMU Reservoir Beneath St.Helena:Implications for the Mantle Recycling of U and Th[J].Geochimica et Cosmochimica Acta,2014,143(1):232-252.
[30]Whitmore G P,Crook K A W,Johnson D P.Sedimentation in a Complex Convergent Margin:The Papua New Guinea Collision Zone of the Western Solomon Sea[J].Marine Geology,1999,157:19-45.