Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle

被引:159
作者
Berry, Andrew J. [1 ]
Danyushevsky, Leonid V. [2 ]
O'Neill, Hugh St C. [3 ]
Newville, Matt [4 ]
Sutton, Stephen R. [4 ,5 ]
机构
[1] Univ London Imperial Coll Sci Technol & Med, Dept Earth Sci & Engn, London SW7 2AZ, England
[2] Univ Tasmania, CODES, Hobart, Tas 7001, Australia
[3] Australian Natl Univ, Res Sch Earth Sci, Canberra, ACT 0200, Australia
[4] Univ Chicago, Ctr Adv Radiat Sources, Chicago, IL 60637 USA
[5] Univ Chicago, Dept Geophys Sci, Chicago, IL 60637 USA
基金
美国国家科学基金会; 英国自然环境研究理事会;
关键词
D O I
10.1038/nature07377
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
Komatiites are volcanic rocks mainly of Archaean age that formed by unusually high degrees of melting of mantle peridotite. Their origin is controversial and has been attributed to either anhydrous melting of anomalously hot mantle(1-3) or hydrous melting at temperatures only modestly greater than those found today(4,5). Here we determine the original Fe(3+)/Sigma Fe ratio of 2.7- Gyr- old komatiitic magma from Belingwe, Zimbabwe(6), preserved as melt inclusions in olivine, to be 0.10+/-0.02, using iron K- edge X- ray absorption near- edge structure spectroscopy. This value is consistent with near- anhydrous melting of a source with a similar oxidation state to the source of present- day mid- ocean- ridge basalt. Furthermore, this low Fe(3+)/Sigma Fe value, together with a water content of only 0.2-0.3 wt% ( ref. 7), excludes the possibility that the trapped melt contained significantly more water that was subsequently lost from the inclusions by reduction to H(2) and diffusion. Loss of only 1.5 wt% water by this mechanism would have resulted in complete oxidation of iron ( that is, the Fe(3+)/Sigma Fe ratio would be similar to 1). There is also no petrographic evidence for the loss of molecular water. Our results support the identification of the Belingwe komatiite as a product of high mantle temperatures (similar to 1,700 degrees C), rather than melting under hydrous conditions ( 3-5-wt% water), confirming the existence of anomalously hot mantle in the Archaean era.
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页码:960 / U42
页数:5
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