Mineral Associations in Diamonds from the Lowermost Upper Mantle and Uppermost Lower Mantle
32
Citation
54
Reference
10
Related Paper
Citation Trend
Keywords:
Ringwoodite
Silicate perovskite
Pyroxene
Ultramafic rock
Nepheline
Pyrope
A possible age of 745 ± 100m.y. (using 50b.y. = half life of 87Rb) has been determined for pyrope from the Stockdale kimberlite assuming an upper mantle initial 87Sr/86Sr ratio of 0.702 ± 0.001. Despite the large error associated with this age it clearly demonstrates that the pyrope crystallized long before its final emplacement which is set at ≤ 100 m.y. Allowance for crustal and mantle contamination and other sources of error have been carefully considered in this determination.
Pyrope
Allowance (engineering)
Cite
Citations (2)
The Tunraq diatreme is a multiple intrusion of massive kimberlite cut by a composite dike of massive and fissile micaceous kimberlite. Megacrysts common to all facies are niobian rutile, ilmenite and garnet. Ilmenites are magnesian (8.4–17.5%Mg0), the most magnesian types occurring as mantles on rutile. Garnets are pyrope, Ti-pyrope, low Cr (<2.5%Cr203) pyrope and high chrome pyrope. The latter are considered to be derived by framentation of ultramafic xenoliths and the others to be true phenocrysts. Phlogopite (2–6%Ti02) is present as laths which are rounded, distorted and kink banded, and appear to have formed prior to fluidization. Phenocrystal and groundmass olivines range in composition from Fo92 to Fo87, the euhedral groundmass olivines being in general richer in Fe0 and CaO than the rounded pre-fluidization phenocrysts. Other groundmass minerals include iron rich serpentine, calcite, perovskite and spinel. The spinel assemblage is different to that observed in other Somerset Island kimberlites in being poorer in Al203. Spinels are all suhedral post-fluidization spinels of wide compositional range. In the massive kimberlite and fissile micaceous kimberlite the spinels range from titaniferous-magnesian-chromite to magnesian ulvospinel-ulvospinel-magnetite and in the massive micaceous kimberlite from titaniferous-magnesian-chromite to Ti-free magnetite. All three facies are considered to have been derived from the same batch of mantle derived magma by differing degrees of fractional crystallization at low pressure. The generation of micaceous kimberlites is considered not to be a function of low pressure differentiation of kimberlite as the trend towards micaceous kimberlit as exemplified by the presence of Al2O3 poor spinels, is induced in the mantle; low pressure differentiation merely emphasizes that trend. Thus the occurrence of kimberlite and micaceous kimberlite within the same kimberlitic magma province probably reflects different degrees of partial melting of the mantle and/or differentiation under high pressure.
Pyrope
Phenocryst
Chromite
Ilmenite
Phlogopite
Xenolith
Lapilli
Forsterite
Diatreme
Cite
Citations (20)
Silicate perovskite
Pyrope
Ringwoodite
Discontinuity (linguistics)
Classification of discontinuities
Seismic anisotropy
Pyroxene
Cite
Citations (31)
Abstract The equation of state and elastic properties of akimotoite at simultaneously high pressures and high temperatures are obtained using first-principles calculations based on the density functional theory (DFT). The calculated results agree with the available experimental data. Combining our results with the elastic data of other minerals, we estimated the V P , V S , and density contrasts caused by the akimotoite-related transitions. The velocity contrasts between akimotoite and bridgmanite are 4.6% and 8.3% for V P and V S , respectively, which are only about half of those between majorite and akimotoite. Moreover, because both the akimotoite-bridgmanite and majorite-akimotoite transitions have broad phase boundaries, these two phase transitions may not contribute to multiple discontinuities around ∼660 km depth in subduction zones as detected by seismic studies. Instead, the decomposition of pyrope into bridgmanite and corundum, which would occur in cold subduction zones with a sharp phase boundary and a large impedance contrast due to the inhibition of the pyroxene-garnet transformation at relatively low temperatures, could be a more reasonable explanation for the discontinuity at ∼700–750 km in subduction zones. Furthermore, the transformation from high-pressure clinopyroxene to akimotoite at the depth of ∼600 km can increase the V P , V S , and density by 10.1%, 14.8%, and 9.9%, respectively, indicating that the phase transition may account for the local discontinuity at ∼600 km in some subduction zones. In addition, the anisotropies of akimotoite are significantly higher than those of other major minerals at the base of the mantle transition zone and could be the origin of the seismic anisotropies detected in some subduction zones.
Silicate perovskite
Pyrope
Ringwoodite
Discontinuity (linguistics)
Classification of discontinuities
Pyroxene
Cite
Citations (0)
Pyrope
Xenolith
Peridotite
Chromite
Cite
Citations (12)
Pyrope
Xenolith
Mahalanobis distance
Lithology
Cite
Citations (4)
Pyrope
Cite
Citations (3)
Pyrope
Pyroxene
Cite
Citations (40)
Mineralogy of diamondiferous eclogite xenolites showing metasomatosis evidence from the Udachnaya kimberlite pipe is discussed. The paper also reviews features of diamonds they contain, compositions of primary garnets and omphacites as well as alteration of structural and species compositions of original garnets and clinopyroxenes during metasomatosis. Based on pyrope structure update, two-phase garnet composition is suggested, which is mostly represented by complex pyrope associated with Ca-pyrope. In all samples, primary omphacite is replaced by another clinopyroxene variety depleted in Na2O, which is typical of partial melting products. Geothermometry results suggested that the eclogites formed within a temperature range of 1,000–1,2000 °C. Based on diamond morphology, data on total N content in diamonds and its aggregation, multiple stages of diamond formation in eclogites and the most probable growth of later diamond generations impacted by metasomatizing mantle fluids containing carbon are postulated. It is suggested that certain diamond formation stages probably had a time gap of several hundred million years.
Pyrope
Omphacite
Coesite
Cite
Citations (0)
Pyrope
Silicate perovskite
Classification of discontinuities
Stishovite
Ringwoodite
Pyroxene
Core–mantle boundary
Corundum
Discontinuity (linguistics)
Phase boundary
Grossular
Cite
Citations (117)