Water and its influence on the lithosphere-asthenosphere boundary

被引:299
作者
Green, David H. [1 ,2 ,3 ]
Hibberson, William O. [3 ]
Kovacs, Istvan [3 ,4 ]
Rosenthal, Anja [3 ]
机构
[1] Univ Tasmania, Sch Earth Sci, Hobart, Tas 7001, Australia
[2] Univ Tasmania, Ctr Ore Deposit Studies, Hobart, Tas 7001, Australia
[3] Australian Natl Univ, Res Sch Earth Sci, Canberra, ACT 0200, Australia
[4] Eotvos Lorand Geophys Inst Hungary, Dept Data Management, H-1145 Budapest, Hungary
基金
澳大利亚研究理事会;
关键词
QUANTITATIVE-ANALYSIS; MIDOCEAN RIDGES; PHASE-RELATIONS; HIGH-PRESSURES; MANTLE; MELT; H2O; PERIDOTITE; AMPHIBOLE; OLIVINE;
D O I
10.1038/nature09369
中图分类号
O [数理科学和化学]; P [天文学、地球科学]; Q [生物科学]; N [自然科学总论];
学科分类号
07 ; 0710 ; 09 ;
摘要
The Earth has distinctive convective behaviour, described by the plate tectonics model, in which lateral motion of the oceanic lithosphere of basaltic crust and peridotitic uppermost mantle is decoupled from the underlying mechanically weaker upper mantle (asthenosphere). The reason for differentiation at the lithosphere-asthenosphere boundary is currently being debated with relevant observations from geophysics (including seismology) and geo-chemistry (including experimental petrology). Water is thought to have an important effect on mantle rheology, either by weakening the crystal structure of olivine and pyroxenes by dilute solid solution(1), or by causing low-temperature partial melting(2). Here we present a novel experimental approach to clarify the role of water in the uppermost mantle at pressures up to 6 GPa, equivalent to a depth of 190 km. We found that for lherzolite in which a water-rich vapour is present, the temperature at which a silicate melt first appears (the vapour-saturated solidus) increases from a minimum of 970 degrees C at 1.5 GPa to 1,350 degrees C at 6 GPa. We have measured the water content in lherzolite to be approximately 180 parts per million, retained in nominally anhydrous minerals at 2.5 and 4 GPa at temperatures above and below the vapour-saturated solidus. The hydrous mineral pargasite is the main water-storage site in the uppermost mantle, and the instability of pargasite at pressures greater than 3 GPa (equivalent to more than about 90 km depth) causes a sharp drop in both the water-storage capacity and the solidus temperature of fertile upper-mantle lherzolite. The presence of interstitial melt in mantle with more than 180 parts per million of water at pressures greater than 3 GPa alters mantle rheology and defines the lithosphere-asthenosphere boundary. Modern asthenospheric mantle acting as the source for mid-oceanic ridge basalts has a water content of 50-200 parts per million (refs 3-5). We show that this matches the water content of residual nominally anhydrous minerals after incipient melting of lherzolite at the vapour-saturated solidus at high pressure.
引用
收藏
页码:448 / U97
页数:5
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