Harte,B.(2010): Diamond formation in the deep mantle: the record of mineral inclusions and their distribution in relation to mantle dehydration zones. Mineralogical Magazine, 74(2), 189-215.

『深部マントルにおけるダイヤモンド生成:鉱物包有物の記録およびマントル脱水帯に関連したそれらの分布』


Abstract
 Studies of the inclusions contained in natural diamonds have shown the occurrence of minerals which must have formed at depths below the lithosphere and which may be closely matched with the silicate mineral assemblages determined by high pressure and temperature experimental studies for depths of 300 to 800 km in the Earth's mantle. The inclusions come principally from two main depth zones: (1) the lower asthenosphere and upper transition zone; (2) the Upper Mantle/Lower Mantle (UM/LM) boundary region and the uppermost LM. The inclusions from zone 1 are very largely majoritic garnets (with or without clinopyroxene) which indicate bulk compositions of eclogitic/metabasic affinity. The minerals from zone 2 include Ca-Si and Mg-Si perovskites and ferropericlase and are dominantly of metaperidotitic bulk composition, but include some possible metabasite assemblages. In many of these natural assemblages, the tetragonal almandine pyrope phase occurs rather than the garnet found in experiments.
 As natural diamonds are believed to crystallize in fluids/melts, the hypothesis is developed that the restriction of diamonds and inclusions of particular compositions to the above two depth intervals is because they are controlled by loci of fluid/melt occurrence. Attention is focused on subduction zones because both suites of inclusions show some evidence of subducted protoliths. The lower zone (600-800 km) coincides with the region where dehydration may be expected for hydrous ringwoodite and dense hydrous Mg-silicates formed in subducted peridotites. The dehydration of lawsonite in subducted metabasites provides a particular location for melt formation and the inclusion of the shallower (〜300 km) majoritic inclusions. For the deeper majoritic inclusions in the region of the upper transition zone, melt development may occur as a consequence of the hydrous wadsleyite-to-olivine transformation, and such melt may then interact with the upper crustal portion of a subducting slab. These suggestions offer an explanation of the depth restrictions and the compositional restrictions of the inclusions. The differences in δ13C values in the host diamonds for the two suites of inclusions may also be explained on this basis.

Keywords: majoritic garnet; Ca-Si perovskite; Mg-Si perovskite; hydrous wadsleyite; hydrous ringwoodite; dense hydrous Mg-silicates; dehydration reactions.

Introduction and background
 General features of natural diamonds
 The spectrum of inclusions in diamonds
 Sources of diamonds with sublithospheric mineral inclusions
Experimental data on expected mantle mineral assemblages in the depth region 300-800 km
Depth zones indicated by inclusions in deep diamonds
Inclusions of metabasite affinity from the uppermost TZ and asthenosphere
Inclusions of metaperidotite and metabasite affinity from the lowermost TZ, the UM/LM boundary region and uppermost LM
Protoliths and evidence of subducted rock compositions
Diamond characteristics
Discussion
 Diamonds from two principal depth zones
 Dehydration of subducting oceanic lithosphere and diamond formation in the region of the lowermost TZ and upper LM
 Dehydration zones and the formation of lower asthenosphere and upper TZ inclusions
 Diamond carbon isotope compositions in relation to dehydration zone models
Conclusions
Acknowledgements
References


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