ABSTRACTThe preservation of metastable diamond in ultrahigh-pressure metamorphic (UHPM) complexes challenges our understanding of the processes taking place during exhumation of these subduction zone complexes. The presence of diamonds in UHPM rocks implies that diamonds remained metastable during exhumation, and within thermodynamic stability of graphite for an extended period. This work studies the influence of pressure on the surface graphitization rate of diamond monocrystals in carbonate systems to understand the preservation of microdiamond during exhumation of UHP subduction complexes. Experiments were performed with 2–3 mm synthetic diamond monocrystals at 2–4 GPa in СаСО3 (1550°С) and К2СО3 (1450°С) melts using a high-pressure multi-anvil apparatus. The highest rate of surface graphitization took place at 2 GPa; diamond crystals were almost completely enveloped by a graphite coating. At 4 GPa, only octahedron-shaped pits formed on flat {111} diamond crystal faces. Our results demonstrate that the surface graphitization rate of diamonds in the presence of carbonate melts at 1450–1550°C increases with decreasing pressure. Decreased pressure alone can graphitize diamond regardless of exhumation rate. Metastable diamond inclusions survive exhumation with little or no graphitization because of excess pressure up to 2 GPa acting on them, and because inclusions are protected from interaction with C-O-H fluid. 相似文献
The 10Be method was used to investigate the effect of mining activities on the natural denudation rates in alluvial sediments from catchments of the Southern Espinhaço Range (SER) in Minas Gerais State (Brazil). In this region, which is predominantly composed of quartzites, the 10Be concentrations were measured in alluvial sediments from catchments in a preserved natural area of the Serra do Cipó National Park and on the Diamantina Plateau, which was subjected to diamond extraction from beginnings of XVIII century until the end of the XX. Two types of drainage were identified in the Diamantina Plateau area: (i) reworked drainage (alluvial sediments reworked by panning) and (ii) overloaded drainage (alluvial sediments originating from panning processes on saprolites located upstream). The mean denudation value for the natural drainages (∼4.4 m.My-1) is similar to that of the reworked drainages (∼4.3 m.My-1) However, the denudation rates obtained for eleven samples from three sites in overloaded basins range from ∼6.4 m My-1 to ∼22.8 m My-1 and are thus higher than those determined for the reworked and natural basins. These results show that despite the alluvium deposits have been intensely reworked by panning, the values of denudation rates were not changed, they are similar to denudation rates from the natural drainages. However, the natural rates are lower than those affected by panning processes on saprolites. 相似文献
The petrology and geochemistry of some new occurrences of Mesoproterozoic diamondiferous hypabyssal-facies kimberlites from
the Chigicherla, Wajrakarur-Lattavaram and Kalyandurg clusters of the Wajrakarur kimberlite field (WKF), Eastern Dharwar craton
(EDC), southern India, are reported. The kimberlites contain two generations of olivine, and multiple groundmass phases including
phlogopite, spinel, calcite, dolomite, apatite, perovskite, apatite and rare titanite, and xenocrysts of eclogitic garnet
and picro-ilmenite. Since many of the silicate minerals in these kimberlites have been subjected to carbonisation and alteration,
the compositions of the groundmass oxide minerals play a crucial role in their characterisation and in understanding melt
compositions. While there is no evidence for significant crustal contamination in these kimberlites, some limited effects
of ilmenite entrainment are evident in samples from the Kalyandurg cluster. Geochemical studies reveal that the WKF kimberlites
are less differentiated and more primitive than those from the Narayanpet kimberlite field (NKF), Eastern Dharwar craton.
Highly fractionated (La/Yb = 108–145) chondrite-normalised distribution patterns with La abundances of 500–1,000 × chondrite
and low heavy rare earth elements (HREE) abundances of 5–10 × chondrite are characteristic of these rocks. Metasomatism by
percolating melts from the convecting mantle, rather than by subduction-related processes, is inferred to have occurred in
their source regions based on incompatible element signatures. While the majority of the Eastern Dharwar craton kimberlites
are similar to the Group I kimberlites of southern Africa in terms of petrology, geochemistry and Sr–Nd isotope systematics,
others show the geochemical traits of Group II kimberlites or an overlap between Group I and II kimberlites. Rare earth element
(REE)-based semi-quantitative forward modelling of batch melting of southern African Group I and II kimberlite source compositions
involving a metasomatised garnet lherzolite and very low degrees of partial melting demonstrate that (1) WKF and NKF kimberlites
display a relatively far greater range in the degree of melting than those from the on-craton occurrences from southern Africa
and are similar to that of world-wide melilitites, (2) different degrees of partial melting of a common source cannot account
for the genesis of all the EDC kimberlites, (3) multiple and highly heterogeneous kimberlite sources involve in the sub-continental
lithospheric mantle (SCLM) in the Eastern Dharwar craton and (4) WKF and NKF kimberlites generation is a resultant of complex
interplay between the heterogeneous sources and their different degrees of partial melting. These observations are consistent
with the recent results obtained from inversion modelling of REE concentrations from EDC kimberlites in that both the forward
as wells as inverse melting models necessitate a dominantly lithospheric, and not asthenospheric, mantle source regions. The
invading metasomatic (enriching) melts percolating from the convecting (asthenosphere) mantle impart an OIB-like isotopic
signature to the final melt products. 相似文献
We studied diamonds and barren and diamondiferous eclogite xenoliths from the Jericho kimberlite (Northern Slave craton).
The majority of the diamonds are non-resorbed octahedral crystals, with moderately aggregated N (IaB < 50%, N < 300 ppm) and δ13C = −5 to −41‰. The diamonds belong to “eclogitic” (90% of the studied samples), “websteritic” (7%) and “peridotitic” (3%)
assemblages. The Jericho diamonds differ from the majority of “eclogitic” diamonds worldwide in magnesian compositions of
associated minerals and extremely light C isotopic compositions (δ13C = −24 to −41‰). We propose that metasomatism triggered by H2O fluids may have been involved in the diamond formation. Multiple episodes of the metasomatism and associated melt extraction
of various ages are evident in Jericho eclogite xenoliths where primary garnet and clinopyroxene have been recrystallized
to more magnesian minerals with higher contents of some incompatible trace elements and to hydrous secondary phases. The model
is supported by the general similarity of mineral compositions in diamondiferous eclogites to those in diamond inclusions
and to secondary magnesian garnet and clinopyroxene in recrystallized barren eclogites. The ultimate products of the metasomatism
could be “websteritic” diamond assemblages sourced from magnesian eclogites.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
High-pressure and high-temperature experiments using a laser-heated diamond anvil cell (LHDAC) and synchrotron X-ray diffraction have revealed a phase transition in MgAl2O4. CaTi2O4-type MgAl2O4 was found to be stable at pressures between 45 and at least 117 GPa. The transition pressure of CaTi2O4-type phase in MgAl2O4 is much lower than that in the natural N-type mid-oceanic ridge basalt composition. The Birch–Murnaghan equation of state for CaTi2O4-type MgAl2O4 was determined from the experimental unit cell parameters with K0=219(±6) GPa, K0′=4(constrained value), and V0=238.9(±9) Å3. The observed compressibility was in agreement with the theoretical compressibility calculated in a previous study. ε-MgAl2O4 was observed at pressures between 40 and 45 GPa, which has not been reported in natural rock compositions. The gradient (dP/dT slope) of the transition from the ε-type to CaTi2O4-type MgAl2O4 had a positive value. These results should resolve the dispute regarding the stable high-pressure phase of MgAl2O4, which has been reported in earlier studies using both the multi-anvil press and the diamond anvil cell. 相似文献
Over 10,000 discontinuity orientations from various sources at the EKATI Diamond Mine in northern Canada were collected and amalgamated into a common database. A new technique in which structural domains were determined in the area of three closely spaced active pits by quantitatively comparing stereonets from different regions was developed. The frequencies of discontinuity poles plotting in orientation windows on stereonets were determined and compared using a correlation coefficient to quantify the degree of similarity between different stereonets. Structural boundaries between regions were established wherever their associated stereonets display low correlation coefficients. The technique was able to determine boundaries in the vertical direction as well as radially in a number of pie-shaped zones extending around the kimberlite pipes. 相似文献
About half the diamonds studied from the Cenozoic placer deposits along the Namibian coast belong to the peridotitic suite. The peridotitic mantle source is heterogeneous ranging from lherzolitic to strongly Ca depleted (down to 0.24 wt.% CaO in garnet) and shows large variations in Cr/Al ratio, illustrated by very low to very high Cr2O3 contents in garnet (2.6–17.3 wt.%). The Cr-rich end of this range includes exceptionally high Cr2O3 contents in Mg-chromite (70.7 wt.%) and clinopyroxene (3.6 wt.%). Garnet-olivine thermometry appears to indicate two groups, one that equilibrated at temperatures between 1200 and 1220°C and a second between 960 and 1100°C. Combined estimates of pressure and temperature based on garnet-orthopyroxene pairs indicate a large variance in geothermal gradients, corresponding to 38–42 mW/m2 surface heat flow.
The trace-element composition of peridotitic garnet inclusions (determined by SIMS) also indicates large diversity. Two principal groups, corresponding to different styles of metasomatic source enrichment, are recognized. The first group ranges from extremely LREEN-depleted patterns, through trough-shaped REEN to sinusoidal patterns with the position of the first peak gradually moving from the LREEN to the MREEN. This series of REE patterns is interpreted to reflect a range of metasomatic agents with decreasing LREE/HREE. Only in the case of the two garnets with REEN peaking at Sm–Eu is this process connected with enrichment in Zr, without significant introduction of Y and Ti. The metasomatism responsible is interpreted as reflecting percolation of CHO-fluids through harzburgite under sub-solidus conditions. A second group of garnets shows an increase from LREEN–MREEN and almost flat (lherzolitic garnet) to moderately declining MREEN–HREEN at super-chondritic levels. This second style of metasomatism is caused by an agent carrying HFSE and showing only moderate enrichment in LREE over HREE, which points towards silicate melts. 相似文献