『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