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1.
Oxygen fugacities are calculated for olivine—spinel ±orthopyroxene assemblages recovered from diamonds and the concentrate of the Dokolwayo kimberlite, Swaziland. In addition thermobarometric oxygen fugacities are obtained for chrome spinel-garnet peridotites and diamonds from several other southern African kimberlites. The southern African lithosphere appears to be laterally homogeneous with respect to oxygen fugacity. Vertically the oxygen fugacity of the lithospheric upper mantle decreases with an increase in pressure. Locally, oxygen fugacities calculated for Dokolwayo mineral assemblages are indicative of an upper mantle characterised by diverse redox conditions within the range FMQ-IW. Reduced oxygen fugacities, calculated for the majority of the Dokolwayo samples, suggest that CH4 may be the dominant carbon volatile species in the lower lithosphere. These reduced conditions also suggest that the Dokolwayo kimberlite is unlikely to be a product of redox melting, but may be the product of a thermal anomaly. Calculated equilibrium temperatures for olivine-spinel pairs from Dokolwayo diamonds and concentrate indicate that the upper mantle in the vicinity of Dokolwayo was characterised by cool subsolidus conditions.  相似文献   

2.
The oxygen fugacities of 48 mantle xenoliths from 5 localities in southern Siberia (USSR) and Mongolia have been determined. Ferric iron contents of spinels were measured by 57Fe Mössbauer spectroscopy and oxygen fugacities calculated from spinel-olivineorthopyroxene equilibrium. The samples studied represent the major types of upper mantle lithologies including spinel and garnet peridotites and pyroxenites, fertile and depleted peridotites and anhydrous and metasomatized samples which come from diverse tectonic settings. Extensive geochemical and isotope data are also available for these samples. Oxygen fugacity values for most central Asian xenoliths fall within the range observed in peridotite xenoliths from other continental regions at or slightly below the FMQ buffer. However, xenoliths from the Baikal rift zone are the most reduced among xenoliths for which Mössbauer data on spinels are available. They yield fO2 values similar to those in oceanic peridotites and MORBs, while xenoliths in other occurrences have higher fO2s. In general, the continental lithosperic mantle is more oxidized than MORB-like oceanic mantle. This difference seems to be due to incorporation of oxidized material into some parts of the subcontinental mantle as a result of subduction of oceanic crust. Garnet- and garnet-spinel lherzolites from the Baikal rift area have slightly higher oxygen fugacities than shallower spinel lherzolites. Oxygen fugacity does not appear to be correlated with the degree of depletion of peridotites, and its values in peridotites and pyroxenites are very much alike, suggesting that partial melting (at least at moderate degrees) takes place at essentially the same fO2s that are now recorded by the residual material. Modally (amphibole- and phlogopitebearing) and cryptically metasomatized xenoliths from the Baikal rift zone give the same fO2 values as depleted anhydrous peridotites, suggesting that solid-melt-fluid reactions in the continental rift mantle also take place without substantial change in redox state. This is in contrast to other tectonic environments where metasomatism appears to be associated with oxidation.  相似文献   

3.
Cr-poor and Cr-rich megacryst suites, both comprising of varying proportions of megacrysts of orthopyroxene, clinopyroxene, garnet, olivine, ilmenite and a number of subordinate phases, coexist in many kimberlites, with wide geographic distribution. In rare instances, the two suites occur together on the scale of individual megacryst hand specimens. Deformation textures are common to both suites, suggesting an origin related to the formation of the sheared peridotites that also occur in kimberlites. Textures and compositions of the latter are interpreted to reflect deformation and metasomatism within the thermal aureole surrounding the kimberlite magma in the mantle. The megacrysts crystallized in this thermal aureole in pegmatitic veins representing small volumes of liquids derived from the host kimberlite magma, which were injected into a surrounding fracture network prior to kimberlite eruption. Close similarities between compositions of Cr-rich megacryst phases and those in granular lherzolites are consistent with early crystallization from a primitive kimberlite liquid. The low-Cr megacryst suite subsequently crystallized from residual Cr-depleted liquids. However, the Cr-poor suite also reflects the imprint of contamination by liquids formed by melting of inhomogeneously distributed mantle phases with low melting temperatures, such as calcite and phlogopite, present within the thermal aureole surrounding the kimberlite magma reservoir. Such carbonate-rich melts migrated into, and mixed with some, but not all, of the kimberlite liquids injected into the mantle fracture network. Contamination by the carbonate-rich melts changed the Ca–Mg and Mg–Fe crystal–liquid distribution coefficient, resulting in the crystallization of relatively Fe-rich and Ca-poor phases. The implied higher crystal-melt Mg–Fe distribution coefficient for carbonate-rich magmas accounts for the generation of small volumes of Mg-rich liquids that are highly enriched in incompatible elements (i.e. primary kimberlite magmas). The inferred metasomatic origin for the sheared peridotites implies that this suite provides little or no information regarding vertical changes in the thermal, chemical and mechanical characteristics of the mantle.  相似文献   

4.

Five compositional-textural types of ilmenite can be distinguished in nine kimberlites from the Eastern Dharwar craton of southern India. These ilmenite generations record different processes in kimberlite history, from mantle to surface. A first generation of Mg-rich ilmenite (type 1) was produced by metasomatic processes in the mantle before the emplacement of the kimberlite. It is found as xenolithic polycrystalline ilmenite aggregates as well as megacrysts and macrocrysts. All of these ilmenite forms may disaggregate within the kimberlite. Due to the interaction with low-viscosity kimberlitic magma replacement of pre-existing type 1 ilmenite by a succeeding generation of geikielite (type 2) along grain boundaries and cracks occurs. Another generation of Mg-rich ilmenite maybe produced by exsolution processes (type 3 ilmenite). Although the identity of the host mineral is unclear due to extensive alteration and possibility includes enstatite. Type 4 Mn-rich ilmenite is produced before the crystallization of groundmass perovskite and ulvöspinel. It usually mantles ilmenite and other Ti-rich minerals. Type 5 Mn-rich ilmenite is produced after the crystallization of the groundmass minerals and replaces them. The contents of Cr and Nb in type 2, 4 and 5 ilmenites are highly dependent on the composition of the replaced minerals, they may not be a good argument in exploration. The highest Mg contents are recorded in metasomatic ilmenite that is produced during kimberlite emplacement, and cannot be associated with diamond formation. The higher Mn contents are linked to magmatic processes and also late processes clearly produced after the crystallization of the kimberlite groundmass, and therefore ilmenite with high Mn contents cannot be considered as a reliable diamond indicator mineral (DIM) and kimberlite indicator mineral (KIM).

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5.
Criteria are suggested for distinguishing xenocrystic ilmenites from those indigenous to the host kimberlite. For instance, in contrast to groundmass grains, ilmenite xenocrysts usually are larger, have reaction rims of leucoxene and perovskite, exhibit strong magnesium enrichment outward, and sometimes have exsolution lamellae and deformation features. Most of the abundant ilmenite macrocrysts found in kimberlite appear to have been phenocrysts in a crystal mush unrelated to kimberlite. On the other hand, kimberlitic groundmass ilmenite is rare, but consistently more magnesian than the cores of macrocrysts. Strong Mg-enrichment patterns evident in the ilmenite macrocrysts probably developed during their attempt to equilibrate with the more magnesian, fractionating kimberlitic liquid. The hypothesis of extensive reaction of ilmenite with kimberlite melt/ fluid has implications with regard to the following: (1) the degree of differentiation of kimberlite melts; (2) the genesis of mantle megacrysts; (3) the reactivity of kimberlite; and (4) the usefulness of groundmass ilmenite as a petrogenetic indicator.  相似文献   

6.
The petrography, major element, and trace element (TE) compositions of minerals from two types of modal metasomatites (metasomatized peridotites and pyroxenites) from kimberlite pipes Udachnaya and Komsomol'skaya-Magnitnaya, Yakutia, have been studied. It is shown that texturally and chemically equilibrated metasomatites A consist of a set of superimposed minerals: phlogopite + diopside ± ilmenite ± apatite ± sulfides ± graphite. Their major and trace element compositions have specific features. The contents of TEs in garnet and clinopyroxene from these metasomatites are close to those in garnet and clinopyroxene from low-temperature coarse-grained peridotites richest in TEs. The distribution of a significant portion of TEs between garnet and clinopyroxene from A-type metasomatites and from coarse-grained lherzolites rich in TEs is close to experimental values reported for minerals coexisting with carbonatitic and basaltic fluids. We assume that this metasomatic process was nearly synchronous with the global metamorphism and cratonization of the mantle lithosphere and that high-density silicate–carbonate fluidmelts were metasomatizing agents.Another large mantle metasomatism process in the lithosphere of the Siberian craton was associated with the Middle Paleozoic kimberlite magmatic event, induced by the Yakutian thermochemical plume. Metasomatic minerals (Mg phlogopite + Cr diopside + chromite ± sulfides ± graphite) intensely replaced the minerals of the primary paragenesis, particularly, garnet. These reaction metasomatites show a sine-shaped REE pattern in garnet and disequilibrium between garnet and clinopyroxene. It is supposed that the reaction metasomatism in the mantle lithosphere of the Siberian craton was associated with ingress of reduced asthenospheric fluids at early stages of the kimberlite formation cycle. Metasomatic graphite formed in metasomatites of both types, and this fact evidences for two diamond formation epochs in the history of the mantle lithosphere of the Siberian craton.  相似文献   

7.

Kimberlites from the Diavik and Ekati diamond mines in the Lac de Gras kimberlite field contain abundant large (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) crystals. We present the first extensive mineral chemical dataset for these megacrysts from Diavik and Ekati and compare their compositions to cratonic peridotites and megacrysts from the Slave and other cratons. The Diavik and Ekati Cr-diopside and Cr-pyrope megacrysts are interpreted to belong to the Cr-rich megacryst suite. Evidence for textural, compositional, and isotopic disequilibrium suggests that they constitute xenocrysts in their host kimberlites. Nevertheless, their formation may be linked to extensive kimberlite magmatism and accompanying mantle metasomatism preceding the eruption of their host kimberlites. It is proposed that the formation of megacrysts may be linked to failed kimberlites. In this scheme, the Cr-rich megacrysts are formed by progressive interaction of percolating melts with the surrounding depleted mantle (originally harzburgite). As these melts percolate outwards, they may contribute to the introduction of clinopyroxene and garnet into the depleted mantle, thereby forming lherzolite. This model hinges on the observation that lherzolitic clinopyroxenes and garnets at Lac de Gras have compositions that are strikingly similar to those of the Cr-rich megacrysts, in terms of major and trace elements, as well as Sr isotopes. As such, the Cr-rich megacrysts may have implications for the origin of clinopyroxene and garnet in cratonic lherzolites worldwide.

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8.
《International Geology Review》2012,54(12):1104-1120
A suite of samples, including kaersutite and ilmenite megacrysts, spinel peridotites, garnet pyroxenites, and the alkali basalts that host them, have been studied in an effort to better constrain the mineralogy and chemistry of the subcontinental mantle beneath the central portion of the Arabian plate. Kaersutite megacrysts are classified as Type-A high-pressure precipitates of the alkali basalt host, which transported these xenoliths to the surface and extruded them during formation of the Tell-Danun volcano, southwestern Syria. Ilmenite megacrysts are classified as Type-B megacrysts and could not have precipitated from the alkali basalts presently sampled. Instead, they were derived from a magma that was enriched in the rare-earth elements (REE) by ca. four times and depleted in Zr and Hf, compared to the alkali basalts.

Garnet pyroxenites from the Tell-Danun volcanic field yield temperatures and pressures of 946-1045° C and 8-10 kbar, respectively. These xenoliths likely were precipitated as dikes or along walls of conduits at depths of 24-30 km in the lower crust and/or upper mantle beneath the Arabian plate. Spinel peridotites last were equilibrated at temperatures of 755-1080° C and pressures from 10-20 kbar (30-60 km depth) and could represent samples of a mantle that has been depleted by a prior partial melting event. Many spinel peridotites also contain evidence (specifically in concave-upward REE patterns) of a subsequent enrichment event. However, the age and timing of this depletion, and of the subsequent enrichment event, are not known. This event could have occurred as a consequence of the entrainment of the xenoliths in the LREE- enriched alkali basalts or could have occurred prior to alkali basalt volcanism via metasomatic processes.  相似文献   

9.
Mineralogical and chemical relationships indicate that the majority of ilmenites recovered from Group I kimberlites crystallized directly from the kimberlite magma in two contrasting P-T regimes: Ilmenites of the discrete nodule association formed in pegmatitic veins and apophyses surrounding the kimberlite magma at depth. Compositional ranges of the discrete nodule assemblage reflect essentially isobaric crystallization across the thermal aureole about the magma reservoir. Early crystallization of high pressure Cr-rich phases (garnet, clinopyroxene and possibly spinel) could result in later forming megacryst ilmenites being Cr-poor. During ascent of the kimberlite magma (essentially identical to the liquid injected into the pegmatitic veins), crystallization of garnet and clinopyroxene would be inhibited as a result of the expansion of the olivine phase field. The magma would not undergo Crdepletion, with the result that later crystallizing (ground-mass) ilmenites would be Cr-rich relative to associated ilmenite megacrysts.Rare ilmenite inclusions in diamonds show chemical affinities with those of the discrete nodule suite. It is proposed that large Type IIa diamonds may be late-crystallizing members of the discrete nodule assemblage. They are in other words related to the kimberlite event itself, and would represent a third diamond paragenesis, distinctly younger than those related to peridotites and eclogites.The mode of formation of rare MARID suite and metasomatized mantle xenoliths is not clearly understood, although mineralogical and chemical evidence point to a direct or indirect link to the host kimberlite.  相似文献   

10.
Oxygen fugacities have been estimated for a wide distribution of samples from the granulite facies terrane (region C) of the Bamble Sector, SE Norway using both the titaniferous magnetite–ilmenite and orthopyroxene–titaniferous magnetite–quartz oxygen barometers. These oxygen fugacities are estimated using temperatures calculated from the titaniferous magnetite–ilmenite thermometer of Ghiorso and Sack (1991) and are both internally consistent with each other as well with the thermometer. In samples for which the estimated temperature is high, the two oxygen barometers show good agreement whereas agreement is poor for low temperature samples. In these low temperature samples, oxygen fugacities estimated from titaniferous magnetite–ilmenite are considerably less than those estimated from orthopyroxene–titaniferous magnetite–quartz. An increase in this discrepancy with decrease in temperature appears to reflect preferential resetting of the hematite component in the ilmenite grains without significant alteration of the more numerous titaniferous magnetite grains. This is due, in part, to greater re-equilibration of the ilmenite grains during retrograde interoxide resetting between the ilmenite grains and the titaniferous magnetite grains. The mean temperature for the non-reset samples, 791?±?17?°C (1σ), is in good agreement with temperatures obtained from garnet–orthopyroxene KD exchange thermometry in the same region, 785–795?°C (1σ) (Harlov 1992, 2000a). Most non-reset oxygen fugacities range from log10?f?O2=?14 to ?11.8 or approximately 0.5–1.5?log units above quartz–fayalite–magnetite at 7.5?kbar. Both these temperatures and the range of oxygen fugacities are in good agreement with those estimated using the titaniferous magnetite–ilmenite thermometer/oxygen barometer of Andersen et?al. (1991). The QUIlP equilibrium (quartz–ulvöspinel–ilmenite–pyroxene) is used to project self-consistent equilibrium temperatures and oxygen fugacities for samples reset due to hematite loss from the ilmenite grains. These projected temperatures and oxygen fugacities agree reasonably well with the non-reset samples. The mean projected QUIlP temperature is 823?±?6?°C (1σ). This result supports the conclusion that low titaniferous magnetite–ilmenite temperatures (down to 489?°C) and accompanying low oxygen fugacities are the result of hematite loss from the ilmenite grains. Non-reset oxygen fugacities lie approximately 1.5?log10 units above the upper graphite stability curve indicating that the stable C–O–H fluid phase interacting with these gneisses, whether regionally or locally, was CO2. This is borne out by the presence of numerous CO2-rich fluid inclusions in these rocks.  相似文献   

11.
Accessory, homogeneous ilmenite and rutile are important oxide phases in amphibole-rich high-pressure cumulate veins which crosscut the Lherz orogenic lherzolite massif. Those veins crystallized from alkaline melts at P = 1.2–1.5 GPa within the uppermost lithospheric mantle. Transitional basalts contaminated by peridotitic wall-rocks and then uncontaminated alkali basalts (basanites) reused the same vein conduits. Petrographic observations give evidence that Fe–Ti oxide saturation depends on the silica contents of each parental melt. The water-poor silica-rich transitional melts that generated websterites and plagioclase-rich clinopyroxenites reached early Ti-oxide saturation (1,200°C; 1.5 GPa). Rutile is as abundant as ilmenite. It is enriched with Nb–Zr–Hf by a factor of 10–100 relative to either amphibole or ilmenite. The amphibole pyroxenites and hornblendites crystallized from basanites reached late Fe–Ti oxide saturation after precipitation of amphibole, with ilmenite crystallizing along with phlogopite in the latter. The Lherz ilmenites are devoid of exsolution and contain very little trivalent iron. This compositional feature indicates more reducing crystallization conditions than usually inferred for alkali lavas and their megacrysts (FMQ ± 1). The veins incompletely equilibrated for redox conditions with their wall-rock peridotites which record more oxidizing conditions (FMQ ± 1). The veins also exchanged magnesium and chromium, as suggested by Cr-bearing, Mg-rich ilmenite (up to 44 mol% MgTiO3) in veins less than 3–4 cm thick. Mg-rich ilmenite megacrysts occurring in alkali basalts could be actually xenocrysts from veins similar in thickness to those occurring at the Lherz massif, although crystallized from more oxidized magmas.  相似文献   

12.
In the present work we studied Mg-ilmenite megacrysts from the Arkhangelsk kimberlites (the Kepino kimberlite field and mantle xenoliths from the Grib pipe). On the basis of isotopic (Rb/Sr, Sm/Nd, δ18O) and trace-element data we argue that studied Mg-ilmenite megacrysts have a genetic relation to the “protokimberlitic” magma, which was parental to the host kimberlites. Rb-Sr ages measured on phlogopite from ilmenite-clinopyroxenite xenoliths and the host Grib kimberlite overlap within the error (384 Ma and 372 ± 8 Ma, respectively; Shevchenko et al., 2004) with our estimation of the Kotuga kimberlite emplacement (378 ± 25 Ma). Sr and Nd isotopic compositions of megacrysts are close to the isotopic composition of host kimberlites (Mg-ilmenites from kimberlites have 87Sr/86Sr(t = 384) = 0.7050–0.7063, ?Nd(t = 384) = + 1.7, +1.8, ilmenite from ilmenite-garnet clinopyroxenite xenolith has 87Sr/86St(t = 384) = 0.7049, ?Nd(t = 384) = +3.5). Oxygen isotopic composition of ilmenites (δ18O = +3.8–+4.5‰) is relatively “light” in comparison with the values for mantle minerals (δ18O = +5–+6‰). Taking into account ilmenite-melt isotope fractionation, these values of δ18O indicate that ilmenites could crystallize from the “protokimberlitic” melt. Temperatures and redox conditions during the formation of ilmenite reaction rims were estimated using ilmenite-rutile and titanomagnetite-ilmenite thermo-oxybarometers. New minerals within the rims crystallized at increasing oxygen fugacity and decreasing temperature. Spinels precipitated during the interaction of ilmenite with kimberlitic melt at T = 1000–1100°C and oxygen fugacity $\Delta \log f_{O_2 }$ [QFM] ≈ 1. Rims comprised with rutile and titanomagnetite crystallized at T ≈ 1100°C, $\Delta \log f_{O_2 }$ [NNO] ≈ 4 and T = 600–613°C, $\Delta \log f_{O_2 }$ [QFM] ≈ 3.7, respectively. Rutile lamellae within ilmenite grains from clinopyroxenitic xenolith were formed T ≥ 1000–1100°C and oxygen fugacity $\Delta \log f_{O_2 }$ [NNO] = ?3.7. Since the pressure of clinopyroxene formation from this xenolith was estimated to be 45–53 kbar, redox conditions at 135–212 km depths could be close to $\Delta \log f_{O_2 }$ [NNO] = ?3.7.  相似文献   

13.
The oxygen isotope ratios of Phanerozoic zircons from kimberlite pipes in the Kaapvaal Craton of southern Africa and the Siberian Platform vary from 4.7 to 5.9‰ VSMOW. High precision, accurate analyses by laser reveal subtle pipe-to-pipe differences not previously suspected. These zircons have distinctive chemical and physical characteristics identifying them as mantle-derived megacrysts similar to zircons found associated with diamond, coesite, MARID xenoliths, Cr-diopside, K-richterite, or Mg-rich ilmenite. Several lines of evidence indicate that these 18O values are unaltered by kimberlite magmas during eruption and represent compositions preserved since crystallization in the mantle, including: U/Pb age, large crystal size, and the slow rate of oxygen exchange in non-metamict zircon. The average 18O of mantle zircons is 5.3‰, ∼0.1 higher and in equilibrium with values for olivine in peridotite xenoliths and oceanic basalts. Zircon megacrysts from within 250 km of Kimberley, South Africa have average 18O=5.32±0.17 (n=28). Small, but significant, differences among other kimberlite pipes or groups of pipes may indicate isotopically distinct reservoirs in the sub-continental lithosphere or asthenosphere, some of which are anomalous with respect to normal mantle values of 5.3±0.3. Precambrian zircons (2.1–2.7 Ga) from Jwaneng, Botswana have the lowest values yet measured in a mantle zircon, 18O=3.4 to 4.7‰. These zircon megacrysts originally crystallized in mafic or ultramafic rocks either through melting and metasomatism associated with kimberlite magmatism or during metamorphism. The low 18O zircons are best explained by subduction of late Archean ocean crust that exchanged with heated seawater prior to underplating as eclogite and to associated metasomatism of the mantle wedge. Smaller differences among other pipes and districts may result from variable temperatures of equilibration, mafic versus ultramafic hosts, or variable underplating. The narrow range in zircon compositions found in most pipes suggests magmatic homogenization. If this is correct, these zircons document the existence of significant quantities of magma in the sub-continental mantle that was regionally variable in 18O and this information restricts theories about the nature of ancient subduction. Received: 8 August 1997 / Accepted: 6 May 1998  相似文献   

14.
Using the secondary spinel standard, the authors have precisely measured the Fe3+/∑ Fe values of spinels in mantle xenoliths from Cenozoic basalts in eastern China, and estimated the oxygen fugacities recorded by 63 mantle xenoliths through olivine-orthopyroxene-spinel oxygen barometry. The results indicate that the oxygen fugacities of the lithospheric mantle in eastern China are higher in the south than in the north. Among them, the oxygen fugacity of the North China craton lithospheric mantle is the lowest, similar to that of the oceanic mantle, while that of Northeast and South China are the same as that of the global continental mantle. The variations of mantle redox state in eastern China are mainly controlled by the C-O-H fluids derived from the asthenospheric mantle. According to the mantle oxidation state, it can be concluded that the C-O-H fluids in the lithospheric mantle of eastern China consist mainly of CO2 and minor H2O, but CH4-rich fluids should come from the asthenosphere where the ox  相似文献   

15.
Type I spinel peridotite xenoliths from Simcoe Volcano, southern Washington (USA), are from lithospheric mantle approximately 65 km inboard from the axis of the subduction-related Cascade Range. Oxygen fugacities calculated from contents of Fe3+/ΣFe in Simcoe spinels, determined by Mössbauer spectroscopy, are up to 1.4 log units more oxidizing than the FMQ buffer. These are among the most oxidized mantle xenoliths reported, with fugacities substantially higher than those calculated for mantle beneath most of western North America. These results, together with those from amphibole-bearing spinel peridotites from Ichinomegata, Japan (Wood and Virgo, 1989), provide evidence that the mantle above subduction zones is more oxidized than is oceanic or ancient cratonic mantle. We suggest that oxidation was accomplished by an agent ranging in composition from solute-rich hydrous fluid to water-bearing silicate melt. A qualitative model relating extent of oxidation, duration of the oxidation process, and proportion of the available water (derived from subducting slabs) that oxidizes Fe in subarc mantle peridotite, suggests that such an agent can easily produce the observed extents of oxidation over timescales similar to the typical lifespans of subduction zones. For the Cascade arc with a duration of 50 Ma, the observed oxidation in the Simcoe peridotites can be achieved by reacting about 6–11 % of the available water with the mantle. These results demonstrate that water can make an efficient oxidizing agent, and because of the comparatively low ferric iron contents reported for mantle peridotites from other tectonic settings, oxidation of the mantle by water is mostly restricted to subduction zones where water is recycled from the surface and transferred into the mantle wedge.  相似文献   

16.
The diamondiferous Letlhakane kimberlites are intruded into the Proterozoic Magondi Belt of Botswana. Given the general correlation of diamondiferous kimberlites with Archaean cratons, the apparent tectonic setting of these kimberlites is somewhat anomalous. Xenoliths in kimberlite diatremes provide a window into the underlying crust and upper mantle and, with the aid of detailed petrological and geochemical study, can help unravel problems of tectonic setting. To provide relevant data on the deep mantle under eastern Botswana we have studied peridotite xenoliths from the Letlhakane kimberlites. The mantle-derived xenolith suite at Letlhakane includes peridotites, pyroxenites, eclogites, megacrysts, MARID and glimmerite xenoliths. Peridotite xenoliths are represented by garnet-bearing harzburgites and lherzolites as well as spinel-bearing lherzolite xenoliths. Most peridotites are coarse, but some are intensely deformed. Both garnet harzburgites and garnet lherzolites are in many cases variably metasomatised and show the introduction of metasomatic phlogopite, clinopyroxene and ilmenite. The petrography and mineral chemistry of these xenoliths are comparable to that of peridotite xenoliths from the Kaapvaal craton. Calculated temperature-depth relations show a well-developed correlation between the textures of xenoliths and P-T conditions, with the highest temperatures and pressures calculated for the deformed xenoliths. This is comparable to xenoliths from the Kaapvaal craton. However, the P-T gap evident between low-T coarse peridotites and high-T deformed peridotites from the Kaapvaal craton is not seen in the Letlhakane xenoliths. The P-T data indicate the presence of lithospheric mantle beneath Letlhakane, which is at least 150 km thick and which had a 40mW/m2 continental geotherm at the time of pipe emplacement. The peridotite xenoliths were in internal Nd isotopic equilibrium at the time of pipe emplacement but a lherzolite xenolith with a relatively low calculated temperature of equilibration shows evidence for remnant isotopic disequilibrium. Both harzburgite and lherzolite xenoliths bear trace element and isotopic signatures of variously enriched mantle (low Sm/Nd, high Rb/Sr), stabilised in subcontinental lithosphere since the Archaean. It is therefore apparent that the Letlhakane kimberlites are underlain by old, cold and very thick lithosphere, probably related to the Zimbabwe craton. The eastern extremity of the Proterozoic Magondi Belt into which the kimberlites intrude is interpreted as a superficial feature not rooted in the mantle. Received: 19 March 1996 / Accepted: 16 October 1996  相似文献   

17.
Manganoan ilmenite was identified in Juina, Brazil kimberlitic rocks among other megacrysts. It forms oval, elongated, rimless grains comprising 8–30 wt.% of the heavy fraction. Internally the grains are homogeneous. The chemical composition of Mn-ilmenite is almost stoichiometric for ilmenite except for an unusually high manganese content, with MnO = 0.63–2.49 wt.% (up to 11 wt.% in inclusions in diamond) and an elevated vanadium admixture (V2O3 = 0.21–0.43 wt.%). By the composition, Mn-ilmenite megacrysts and inclusions in diamond are almost identical. The concentrations of trace elements in Mn-ilmenite, compared to picroilmenite, are much greater and their variations are very wide. Chondrite-normalized distribution of trace elements in Mn-ilmenite megacrysts is similar to the distribution in Mn-ilmenites included in diamond. This confirms that Mn-ilmenite in kimberlites is genetically related to diamond. The finds of Mn-ilmenite known before in kimberlitic and related rocks are late- or postmagmatic, metasomatic phases. They either form reaction rims on grains of picroilmenite or other ore minerals, or compose laths in groundmass. In contrast to those finds, Mn-ilmenite megacrysts in Juina kimberlites are a primary mineral phase with a homogeneous internal structure obtained under stable conditions of growth within lower mantle and/or transition zone. In addition to pyrope garnet, chromian spinel, picroilmenite, chrome-diopside, and magnesian olivine, manganoan ilmenite may be considered as another kimberlite/diamond indicator mineral.  相似文献   

18.
ABSTRACT

Clinopyroxene megacrysts in volcanic rocks can provide substantial information on the evolution of the magmatic system at depth. Although considerable attention has been paid to these crystals, their origin is not yet completely resolved. The clinopyroxene megacrysts worldwide can be divided into two major types in general: the green Cr-diopside type and the black Al-augite type. There is a consensus view that the Cr-diopside megacrysts are mantle xenocrysts, whereas two contrasting opinions exist regarding the origin of the black Al-augite megacrysts. One favours a cognate origin, viewing them as crystallization products of the host magmas under high-pressure; while the other argues that they are xenocrysts crystallized from previous alkali basalts or fragments of mantle peridotites, pyroxenites or pegmatite veins. A review study on the clinopyroxene megacrysts in Meso-Cenozoic volcanic rocks from the North China Craton (NCC) and their comparison with those worldwide provides new constraints on their origin, namely, the Cr-diopside megacrysts, as previously thought, are all xenocrysts, representing disaggregated clinopyroxene crystals from clinopyroxene-rich mantle rocks. Contrary to the formerly proposed cognate origin, the Al-augite megacrysts are also xenocrysts, having no direct genetic link to their host rocks. They crystallized from melts that have formed earlier than the host magmas, and probably accumulated in a magma chamber or occurred as sheets or veins filling a fracture network surrounding a magma chamber in the upper mantle. During the subsequent eruption of the host lavas, these previously formed crystals were incorporated into the magma and were brought up to the surface.  相似文献   

19.
Garnet peridotites occur in quartzofeldspathic gneisses in the Northern Qaidam Mountains, western China. They are rich in Mg and Cr, with mineral compositions similar to those in mantle peridotites found in other orogenic belts and as xenoliths in kimberlite. Garnet‐bearing lherzolites interlayered with dunite display oriented ilmenite and chromite lamellae in olivine and pyroxene lamellae in garnet that have been interpreted to indicate pressures in excess of 6 GPa. However, some garnet porphyroblasts include hornblende, chlorite and spinel + orthopyroxene symplectite after garnet; some clinopyroxene porphyroblasts include abundant actinolite/edenite, calcite and lizardite in the lherzolite; some olivine porphyroblasts (Fo92) include an earlier generation Mg‐rich olivine (Fo95–99), F‐rich clinohumite, pyroxene, chromite, anthophyllite/cummingtonite, Cl‐rich lizardite, carbonates and a new type of brittle mica, here termed ‘Ca‐phlogopite’, in the associated dunite. The pyrope content of garnet increases from core to rim, reaching the pyrope content (72 mol.%) of garnet typically found in the xenoliths in kimberlite. The simplest interpretation of these observations is that the rock association was formerly mantle peridotite emplaced into the oceanic crust that was subjected to serpentinization by seawater‐derived fluids near the sea floor. Dehydration during subduction to 3.0–3.5 GPa and 700 °C transformed these serpentinites into garnet lherzolite and dunite, depending on their Al and Ca contents. Pseudosection modelling using thermocalc shows that dehydration of the serpentinites is progressive, and involved three stages for Al‐rich and two stages for Al‐poor serpentinites, corresponding to the breakdown of the key hydrous minerals. Static burial and exhumation make olivine a pressure vessel for the pre‐subduction mineral inclusions during ultrahigh‐pressure (UHP) metamorphism. The time span of the UHP event is constrained by the clear interface between the two generations of olivine to be very short, implying rapid subduction and exhumation.  相似文献   

20.
The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo76-55 from core to rim), pyroxene (from core to rim En70Fs25Wo5, En50Fs25Wo25, and En45Fs45Wo10), and Cr-rich spinel in a matrix of maskelynite (An52Ab45), pyroxene (En30-40Fs40-55Wo10-25,), olivine (Fo50), Fe-Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30-35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo>72) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ - 2), followed by crystallization of the groundmass over a range of FMQ - 1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence.LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ − 4 to − 2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.  相似文献   

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