『Abstract
The origin of cratonic diamonds is reviewed on the basis of nearly
5000 analyses of silicate, oxide and sulphide inclusions in diamonds.
Compositional fields are defined for common minerals of the peridotitic,
eclogitic and websteritic inclusion suites and used to establish
the characteristics of diamond source rocks in the subcratonic
lithospheric mantle. Peridotitic inclusion compositions overlap
with the record established from cratonic garnet peridotite xenoliths
and zenocrysts but reflect overall higher levels of depletion
in basaltic components. The interior of the Kaapvaal Block (Kalahari
Craton) is by far the best studied diamond source region in the
world but appears to be unique rather than representative because
of extreme levels of chemical depletion preserved in the peridotitic
inclusion suite. Major and trace element characteristics of peridotitic
diamond sources indicate polybaric melt extraction proceeding
from the garnet into the spinel stability fields, most readily
explained by protolith depletion in Archean mid-oceanic ridge
environments. Eclogitic mineral inclusions broadly reflect basaltic
source compositions and show chemical rends that are indicative
of igneous fractionation and cumulate enrichment in magmatic precursors.
In agreement with mounting evidence from xenolith studies, eclogitic
diamond sources are linked to subducted oceanic protoliths. A
more mafic character relative to present-day MORB may relate to
(i) higher degrees of partial melting in Archean or early Proterozoic
spreading centres and (ii) secondary melt depletion during subduction,
or after emplacement in the subcratonic lithosphere. In line with
a subduction origin of eclogitic diamond source rocks, mines with
predominantly eclogitic diamond populations are generally (but
not invariably) associated with craton margin settings or lithosphere
with a post-Archean tectonothermal history. The websteritic suite
is poorly defined and reflects a range of broadly pyroxenitic
source rocks intermediate between peridotite and eclogite.
Geothermometry, based on inclusions and nitrogen aggregation
in diamonds, indicates that crystallization and mantle storage
of peridotitic, eclogitic and websteritic diamonds occurred under
the same thermal conditions. Geobarometry for peridotitic inclusions
shows that the majority of diamonds formed at depths of less than
200 km along model geotherms corresponding to 38 to 42 mW/m2
surface heat flow. Lower geothermal gradients observed for diamonds
from the Kalahari and Slave cratons likely represent re-equilibration
of touching inclusion pairs to cooling ambient conditions, suggesting
that diamond formation was accompanied by transient heating events.
Diamond precipitation is interpreted to have occurred during metasomatic
events under super-solidus (melt dominated) and dub-solidus (CHO-fluid
dominated) conditions. Increasing evidence for a reduced character
of the subcratonic lithospheric mantle implies that diamond precipitation
through redox reactions requires upward migration of carbonate-bearing
melts/fluids. In such a redox scenario high solubility of sulphate
relative to sulphide in melts/fluids may provide an explanation
for a high abundance of sulphide inclusions as a consequence of
co-precipitation with diamond in response to decreasing oxygen
fugacity. Such comparatively oxidized metasomatic agents cannot
derive from the reduced deep upper mantle and, therefore, likely
relative to recycling of oceanic lithosphere.
Keywords: Diamond formation; Peridotitic; Eclogitic; Websteritic;
subcratonic lithospheric mantle』
1. Introduction
1.1 Database
2. Inclusions in diamonds
2.1. Inclusion suites and parageneses
2.2. Inclusions
2.2.1. Garnet
2.2.2. clinopyroxene
2.2.3. Orthopyroxene
2.2.4. Olivine
2.2.5. Mg-chromite
2.2.6. Sulphides
2.3. Evolution of the major element composition of diamond sources
in the subcratonic lithospheric mantle
2.3.1. Peridotitic diamonds
2.3.2. Eclogitic diamonds
2.4. Trace elements in garnet and clinopyroxene inclusions
2.4.1. Peridotitic suite
2.4.2. Eclogitic suite
3. Geothermobarometry
4. Radiometric dating of inclusions
4.1. Uranium-lead (U-Pb), lead-lead (Pb-Pb)
4.2. Samarium-neodymium (Sm-Nd)
4.3. Rhenium-osmium (Re-Os)
4.4. Argon-argon (Ar-Ar)
4.5. What are we dating - Diamond or source ages?
5. Variations within and among cratons
6. Model
Acknowledgements
Appendix A. Supplementary data
References