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1.
A new type of podiform chromitite was found at Wadi Hilti in the northern Oman ophiolite. It is within a late-intrusive dunite body, possibly derived from olivine-rich crystal mush, between the sheeted dike complex and upper gabbro. This chromitite forms small (<30 cm in thickness) pods with irregular to lenticular shapes. Neither layering nor graded bedding is observed within the pods. The chromitite is in the upper crust, by far shallower in ophiolite stratigraphy than the other podiform chromitites that have ever been found in the Moho transition zone to the upper mantle. It is distributed along a small felsic to gabbroic melt pool within the dunite body, which was formed by melting of gabbroic blocks captured by the mush. Chromian spinel was precipitated due to mixing of two kinds of melt, a basaltic interstitial melt from the mush and an evolved, possibly felsic, melt formed by the melting of gabbro blocks. The podiform chromitite reported here is strikingly similar in petrography and spinel chemistry to the stratiform chromitite from layered intrusions. The former contains plagioclase and clinopyroxene as matrix silicates instead of olivine as well as includes euhedral and fine spinel with solid mineral inclusions. Chromian spinel of the upper crustal podiform chromitite from Oman has relatively low content of (Cr2O3 + Al2O3), the Cr/(Cr + Al) atomic ratio of around 0.6, and the relatively high TiO2 content ranging from 1 to 3 wt%. We conclude that assimilation of relatively Si-rich materials (crustal rocks or mantle orthopyroxene) by olivine-spinel saturated melts can explain the genesis of any type of chromitite.Editorial responsibility: V. Trommsdorff 相似文献
2.
The bimodal suite (BMS) comprises leucotonalitic and trondhjemitic gneisses interlayered with amphibolites. Based on geochemical parameters three main groups of siliceous gneiss are recognized: (i) SiO2 < 73%, Al2O3 > 14%, and fractionated light rare-earth element (REE) and flat heavy REE patterns; (ii) SiO2 and Al2O3 contents similar to (i) but with strongly fractionated REE patterns with steep heavy REE slopes; (iii) SiO2 > 73%, Al2O3 < 14%, Zr ～ 500 ppm and high contents of total REE having fractionated light REE and flat heavy REE patterns with large negative Eu anomalies. The interlayered amphibolites have major element abundances similar to those of basaltic komatiites, Mg-tholeiites and Fe-rich tholeiites. The former have gently sloping REE patterns, whereas the Mg-tholeiites have non-uniform REE patterns ranging from flat (～ 10 times chondrite) to strongly light REE-enriched. The Fe-rich amphibolites have flat REE patterns at 20–30 times chondrite.The Dwalile metamorphic suite, which is preserved in the keels of synforms within the BMS, includes peridotitic komatiites that have depleted light REE patterns similar to those of compositionally similar volcanics in the Onverwacht Group, Barberton, basaltic komatiites and tholeiites. The basaltic komatiites have REE patterns parallel to those of the BMS basaltic komatiites but with lower total REE contents. The Dwalile tholeiites have flat REE patterns.The basic and ultrabasic liquids were derived by partial melting of a mantle source which may have been heterogeneous or the heterogeneity may have resulted from sequential melting of the mantle source. The Fe-rich amphibolites were derived either from liquids generated at shallow levels or from liquids generated at depth which subsequently underwent extensive fractionation. 相似文献
3.
Eight clinopyroxenes from wehrlites and clinopyroxenites and three clinopyroxenes of crystal lapilli in tuff of Dreiser Weiher in Eifel, Germany, have been separated and chemically analysed. One hornblende and two phlogopites from a wehrlite and clinopyroxenites have also been analysed. The rocks enclosing these inclusions are alkali basalts of basanite composition. The analysed clinopyroxenes contain considerable amounts of Al2O3 (3.87–10.84 wt%). The calculated Tschermak's component ranges from 5.9 to 18.4 mol per cent. All of the analysed clinopyroxenes are clearly different from chromian diopsides in lherzolite inclusions in basaltic rocks in Dreiser Weiher and other localities; the former has higher contents of total FeO, CaO and TiO2 and lower contents of MgO and Cr2O3 than the latter. Two clinopyroxenes separated from apatite-bearing clinopyroxenites show high contents of Fe2O3 with about 2 per cent of Na2O, indicating the presence of considerable amounts of acmite component in addition to Tschermak's component. The relative proportions of Al in the tetrahedral site and that in the octahedral site in the analysed clinopyroxenes are clearly different from those of the common igneous clinopyroxenes and eclogites, and similar to those of the clinopyroxenes from other inclusions in basaltic rocks and granulites. It is suggested that all the analysed clinopyroxenes and their host inclusions have crystallized from alkali basalt magmas in relatively deep levels of the continental crust. 相似文献
4.
The Neogene-Quaternary Harrat Rahat volcanic field is part of the major intercontinental Harrat fields in western Saudi Arabia.It comprises lava flows of olivine basalt and hawaiite,in addition to mugearite,benmorite,and trachyte that occur mainly as domes,tuff cones and lava flows.Based on opaque mineralogy and mineral chemistry,the Harrat Rahat volcanic varieties are distinguished into Group I(olivine basalt and hawaiite) and Group II(mugearite,benmorite and trachyte).The maximum forsterite content(~85) is encountered in zoned forsteritic olivine of Group I,whereas olivine of Group II is characterized by intermediate(Fo=50),fayalitic(Fo=25) and pure fayalite in the mugearite,benmorite and trachyte,respectively.The more evolved varieties of Group II contain minerals that show enrichment of Fe2+,Mn2+and Na+that indicates normal fractional crystallization.The common occurrence of coarse apatite with titanomagnetite in the benmorite indicates that P5+becomes saturated in this rock variety and drops again in trachyte.Cr-spinel is recorded in Group I varieties only and the Cr#(0.5) suggests lherzolite as a possible restite of the Harrat Rahat volcanics.The plots of Cr# vs.the forsterite content(Fo) suggest two distinct trends,which are typical of mixing of two basaltic magmas of different sources and different degrees of partial melting.The bimodality of Harrat Rahat Cr-spinel suggests possible derivation from recycled MORB slab in the mantle as indicated by the presence of high-Al spinel.It is believed that the subcontinental lithospheric mantle was modified by pervious subduction process and played the leading role in the genesis of the Harrat Rahat intraplate volcanics.The trachytes of the Harrat Rahat volcanic field were formed most probably by melting of a lower crust at the mantle-crust boundary.The increase in fO2 causes a decrease in Cr2 O3,and Al2 O3,and a strong increase in the proportion of Fe3+and Mg# of spinel crystallizing from the basaltic melt at T ~1200°C.The olivine-pyroxene and olivine-spinel geothermometers yielded equilibrium temperature in the range of 935-1025°C,whereas the range of <500-850°C from single-pyroxene thermometry indicates either post crystallization reequilibrium of the clinopyroxene,or the mineral is xenocrystic and re-equilibrated in a cooling basaltic magma. 相似文献
5.
Petrography, mineral and bulk chemistry of upper mantle-derived eclogites (garnet and clinopyroxene) from the Koidu Kimberlite Complex, Sierra Leone, are presented in the first comprehensive study of these xenoliths from West Africa. Although peridotite-suite xenoliths are generally more common in kimberlites, the upper mantle sample preserved in Pipe Number 1 at Koidu is exclusively eclogitic, making this the fifth locality in which eclogite is the sole polymineralic xenolith in kimberlite. Over 2000 xenoliths were collected, of which 47 are described in detail that include diamond, graphite, kyanite, corundum, quartz after coesite, and amphibole eclogites. Grossular-pyrope-almandine garnets are chromium-poor (<0.72 wt% Cr2O3) and fall into two distinct groups based on magnesium content. High-MgO garnets have an average composition of Pyr67Alm22Gross11, low-MgO garnets are grossular- and almandine-rich with an average composition of Gross34Pyr33Alm33. Clinopyroxenes are omphacitic with a range in jadeite contents from 7.7 to 70.1 mol%. Three eclogites contain zoned and mantled garnets with almandine-rich cores and pyrope-rich rims, and zoned clinopyroxenes with diopside-rich cores and jadeite-rich rims, and are among a very rare group of eclogites reported on a world-wide basis. The bulk compositions of eclogites have ranges comparable to that of basalts. High-MgO eclogites (16–20 wt% MgO) have close chemical affinities to picrites, whereas low-MgO eclogites (6–13 wt% MgO) are similar to alkali basalts. High-MgO eclogites contain high-MgO garnets and jadeiterich clinopyroxenes. Low-MgO eclogites contain low-MgO garnets, diopside and omphacite, and the group of primary accessory phases (diamond, graphite, quartz after coesite, kyanite, and corundum); grospydites are peraluminous. Estimated temperatures and pressures of equilibration of diamond-bearing eclogites, using the diamond-graphite stability curve and the Ellis and Green (1979) geothermometer, are 1031°–1363° C at 45–50 kb.K
D values of Fe-Mg in garnet and clinopyroxene range from 2.3 to 12.2. Diamonds in eclogites are green, yellow, and clear, and range from cube to octahedral morphologies; the entire spectrum in color and morphology is present in a single metasomatized eclogite with zoned garnet and clinopyroxene. Ages estimated from Sm-Nd mineral isochrons range from 92–247 Ma. Nd values range from +4.05 to 5.23. Values of specific gravity range from 3.06–3.60 g/cc, with calculated seismic Vp of 7.4–8.7 km/s. Petrographie, mineral, and bulk chemical data demonstrate an overall close similarity between the Koidu xenolith suite and upper mantle eclogites from other districts in Africa, Siberia and the United States. At least two origins are implied byP-T, bulk chemistry and mineral compositions: low-MgO eclogites, with diamond and other accessory minerals, are considered to have formed from melts trapped and metamorphically equilibrated in the lithosphere; high-MgO eclogites are picritic and are the products of large degrees of partial melting, with equilibration in the asthenosphere. Fluid or diluted melt metasomatism is pervasive and contributed here and elsewhere to the LIL and refractory silicate incompatible element signature in kimberlites and lamproites, and to secondary diamond growth. 相似文献
6.
Shoji Arai Yohei Shimizu Tomoaki Morishita Yoshito Ishida 《Contributions to Mineralogy and Petrology》2006,152(3):387-398
We found fine-grained Fe-rich orthopyroxene-rich xenoliths (mainly orthopyroxenite) containing partially digested dunite fragments of Group I from Takashima, Southwest Japan. Orthopyroxenite veinlets, some of which contain plagioclase at the center, also replace olivine in dunite and wehrlite xenoliths of Group I. This shows high reactivity with respect to olivine of the melt involved in orthopyroxenite formation, indicating its high SiO2 activity. The secondary orthopyroxene of this type is characterized by low Mg# [= Mg/(Mg + total Fe) atomic ratio] (down to 0.73) and high Al2O3 contents (5–6 wt%). It is different in chemistry from other secondary orthopyroxenes found in peridotite xenoliths derived from the mantle wedge. Clinopyroxenes in the Fe-rich orthopyroxenite show a convex-upward REE pattern with a crest around Sm. This pattern is strikingly similar to that of clinopyroxenes of Group II pyroxenite xenoliths and of phenocrystal and xenocrystal clinopyroxenes, indicating involvement of similar alkali basaltic melts. The Fe-rich orthopyroxenite xenoliths from Takashima formed by reaction between evolved alkali basalt melt and mantle olivine; alkali basalt initially slightly undersaturated in silica might have evolved to silica-oversaturated compositions by fractional crystallization at high-pressure conditions. The Fe-rich orthopyroxenites occur as dikes within the uppermost mantle composed of dunite and wehrlite overlying pockets of Group II pyroxenites. The orthopyroxene-rich pyroxenites of this type are possibly common in the uppermost mantle beneath continental rift zones where alkali basalt magmas have been prevalent. 相似文献
7.
Bor-Ming Jahn 《Geochimica et cosmochimica acta》1974,38(6):873-885
Some rocks of the Onverwacht Group, South Africa, have been analyzed for Rb and Sr concentrations and Sr isotopie composition. These rocks include volcanic rocks, layered ultramafic differentiates and cherty sediments. Whole rock data indicate that the Rb-Sr isotopie systems in many samples were open and yield no reasonable isochron relationships. However, the data of mineral separates from a basaltic komatiite define a good isochron of (2δ) b.y. with an initial Sr87/Sr86 ratio of (2δ). The orthodox interpretation of this age is the time of the low grade metamorphism. Since the basaltic komatiite is stratigraphically lower than the Middle Marker Horizon (dated as b.y. Hurley et al., 1972), and since it is commonly found that volcanism, sedimentary deposition, metamorphism and igneous intrusion in many Archean greenstone-granite terrain all took place in a relatively short time interval (less than 100 m.y.), it is reasonable to assume that the age of 3.50 b.y. might also represent the time of initial Onverwacht volcanism and deposition. The initial Sr87/Sr86 ratio obtained above is important to an understanding of the Sr isotopic composition of the Archean upper mantle. If the komatiite represents a large degree of partial melt (40–80 per cent) of the Archean upper mantle material, then the initial ratio obtained from the metamorphic komatiite should define an upper limit for the Sr isotopic composition of the upper mantle under the African crustal segment. 相似文献
8.
This paper presents new major and trace element data from 150 garnet xenocrysts from the V. Grib kimberlite pipe located in the central part of the Arkhangelsk diamondiferous province (ADP). Based on the concentrations of Cr2O3, CaO, TiO2 and rare earth elements (REE) the garnets were divided into seven groups: (1) lherzolitic “depleted” garnets (“Lz 1”), (2) lherzolitic garnets with normal REE patterns (“Lz 2”), (3) lherzolitic garnets with weakly sinusoidal REE patterns (“Lz 3”), (4) lherzolitic garnets with strongly sinusoidal REE patterns (“Lz 4”), (5) harzburgitic garnets with sinusoidal REE patterns (“Hz”), (6) wehrlitic garnets with weakly sinusoidal REE patterns (“W”), (7) garnets of megacryst paragenesis with normal REE patterns (“Meg”). Detailed mineralogical and geochemical garnet studies and modeling results suggest several stages of mantle metasomatism influenced by carbonatite and silicate melts. Carbonatitic metasomatism at the first stage resulted in refertilization of the lithospheric mantle, which is evidenced by a nearly vertical CaO-Cr2O3 trend from harzburgitic (“Hz”) to lherzolitic (“Lz 4”) garnet composition. Harzburgitic garnets (“Hz”) have probably been formed by interactions between carbonatite melts and exsolved garnets in high-degree melt extraction residues. At the second stage of metasomatism, garnets with weakly sinusoidal REE patterns (“Lz 3”, “W”) were affected by a silicate melt possessing a REE composition similar to that of ADP alkaline mica-poor picrites. At the last stage, the garnets interacted with basaltic melts, which resulted in the decrease CaO-Cr2O3 trend of “Lz 2” garnet composition. Cr-poor garnets of megacryst paragenesis (“Meg”) could crystallize directly from the silicate melt which has a REE composition close to that of ADP alkaline mica-poor picrites. P-T estimates of the garnet xenocrysts indicate that the interval of ～60–110 km of the lithospheric mantle beneath the V. Grib pipe was predominantly affected by the silicate melts, whereas the lithospheric mantle deeper than 150 km was influenced by the carbonatite melts. 相似文献
9.
《Chemie der Erde / Geochemistry》2016,76(2):299-308
The ultramafic member of the Variscan Ślęża Ophiolite (SW Poland) consists of heavily serpentinised, refractory harzburgites. Those located down to 1.5 km below paleo-Moho contain scarce grains or aggregates of olivine, clinopyroxene and spinel. Non-serpentine phases occur in various assemblages: M1—olivine (Fo 90.2–91.0%, NiO 0.38–0.47 wt.%) and rounded or amaeboidal aluminous chromite, rimmed by Al poor chromite and magnetite; M2—olivine (Fo 90.5–91.5, NiO 0.32–0.44 wt.%), olivine with magnetite inclusions (Fo 87.1–92.5, NiO 0.01–0.68 wt.%), rounded, cleavaged clinopyroxene I (Mg# 91.1–93.2, Al2O3 3.00–4.00 wt.%, Cr2O3 1.00–1.40 wt.%) and elongated clinopyroxene II and clinopyroxene from symplectites with magnetite (Mg# = 92.2–94.1, Al2O3 2.20–3.20 wt.% and Cr2O3 0.80–1.20 wt.%). Clinopyroxene is depleted in REEs relative to chondrite. The M3 assemblage consists of intergrown olivine (Fo 90.8–92.7, NiO 0.20–0.38 wt.%) and clinopyroxene (Mg# = 96.0–98.1, Al2O3 0.00–1.00 wt.% and Cr2O3 0.20–0.60 wt.%).The M1 assemblage contains chromite which records greenschist-facies metamorphism. Textural relationships and chemical composition of clinopyroxene occurring in the M2 assemblage are similar to those formed in oceanic spreading centres by LREE depleted basaltic melt percolation. Olivine occurring in M1 assemblage and part of that from M2 have composition typical of residual olivine from the abyssal harzburgites and of olivine formed in those rocks by melt percolation. The olivine with magnetite inclusions (M2 assemblage) and that from M3 record later deserpentinization event, which supposedly produced also M3 clinopyroxene. The non-serpentine phases from the Ślęża ophiolite mantle member, albeit very poorly preserved, document depleted basaltic melt percolation in the Variscan oceanic spreading centre. 相似文献
10.
Mica kimberlite and alkali picrite were identified in the northwestern Urik-Iya Graben of the eastern Sayan region. Typomorphism
of Cr-diopside and high-Cr (up to 55.22 wt % Cr2O3) spinel from kimberlite of the Bushkanai dike indicate that the melt was generated in the mantle, composed of spinel peridotite.
The high content of Cr-spinel (45–55 wt % Cr2O3) microlites in the groundmass of kimberlite and small amounts of ulvospinel and titanomagnetite in the absence of perovskite
testifies to the diamond potential of this kimberlite. Picroilmenite, manganoilmenite with an anomalously high MnO content
(11.37–17.78 wt %), and barium titanate with (wt %) 62.21 TiO2, 0.61 Cr2O3, 15.89 FeO, 4.05 MnO, 1.71 CaO, and 11.13 BaO close in composition to a new mineral species from the Murun pluton were identified
in the groundmass for the first time. Kimberlite from the Bushkanai dike belongs to the Zolotitsa low-Ti geochemical type
of kimberlites derived from the slightly enriched lithospheric mantle EM1. The distribution of trace elements, including REE,
in picrite from the same dike corresponds to the slightly depleted asthenospheric mantle. Different mantle sources of kimberlite
and picrite from the same dike indicate that these rocks are related to independent melts rather than to products of fractionation
of a common parental alkaline ultramafic magma. 相似文献
11.
Tatjana Rehfeldt Stephen F. Foley Richard W. Carlson 《Geochimica et cosmochimica acta》2008,72(23):5722-5756
Dunite, wehrlite and websterite are rare members of the mantle xenolith suite in the Kimberley kimberlites of the Kaapvaal Craton in southern Africa. All three types were originally residues of extensive melt extraction and experienced varying amounts and types of melt re-enrichment. The melt depletion event, dated by Re-Os isotope systematics at 2.9 Ga or older, is evidenced by the high Mg# (Mg/(Mg + Fe)) of silicate minerals (olivine (0.89-0.93); pyroxene (0.88-0.93); garnet (0.72-0.85)), high Cr# (Cr/(Cr + Al)) of spinel (0.53-0.84) and mostly low whole-rock SiO2, CaO and Al2O3 contents. Shortly after melt depletion, websterites were formed by reaction between depleted peridotites and silica-rich melt (>60 wt% SiO2) derived by partial melting of eclogite before or during cratonization. The melt-peridotite interaction converted olivine into orthopyroxene.All three xenolith types have secondary metasomatic clinopyroxene and garnet, which occur along olivine grain boundaries and have an amoeboid texture. As indicated by the preservation of oxygen isotope disequilibrium in the minerals and trace-element concentrations in clinopyroxene and garnet, this metasomatic event is probably of Mesozoic age and was caused by percolating alkaline basaltic melts. This melt metasomatism enriched the xenoliths in CaO, Al2O3, FeO and high-field-strength-elements, and might correspond to the Karoo magmatism at 200 Ma. The websterite xenoliths experienced both the orthoyproxene-enrichment and clinopyroxene-garnet metasomatic events, whereas dunite and wehrlite xenoliths only saw the later basaltic melt event, and may have been situated further away from the source of melt migration channels. 相似文献
12.
The results of the geological and petrologic study of eudialyte-bearing phonolite from the Late Devonian Kontozero carbonatite paleovolcano are discussed in the paper. This eudialyte-bearing phonolite corresponds to the primary melt derived from the metasomatized upper mantle source composed of phlogopite-amphibole eclogite. The average ZrO2 content in the Kontozero phonolite (1.02 wt %) is considered to be cotectic (Kogarko et al., 1988). Similar REE patterns of the Kontozero eudialyte-bearing phonolite and porphyritic lujavrite from the chilled contact zone of the Lovozero eudyalite complex and close REE and Zr concentrations in the compared rocks show that this complex could have been formed from a melt similar in composition to the phonolite melt of Kontozero. 相似文献
13.
Monomineral universal clinopyroxene and garnet barometers for peridotitic,eclogitic and basaltic systems 总被引：1，自引：1，他引：0
I.V. Ashchepkov T. Ntaflos A.M. Logvinova Z.V. Spetsius H. Downes N.V. Vladykin 《地学前缘(英文版)》2017,8(4):775-795
New versions of the universal Jd-Di exchange clinopyroxene barometer for peridotites,pyroxenites and eclogites,and also garnet barometer for eclogites and peridotites were developed.They were checked using large experimental data sets for eciogitic(~530) and peridotitic systems(650).The precision of the universal Cpx barometer for peridotites based on Jd-Di exchange is close to Cr-Tschermalite method produced by Nimis and Taylor(2000).Cpx barometer was transformed by the substitution of major multiplier for K_D by the equations dependent from Al-Na-Fe.Obtained equation in combination with the thermometer of Nimis and Taylor(2000) allow to reconstruct position of the magma feeder systems of the alkali basaltic magma within the mantle diapirs in modern platforms like in Vitim plateau and other Southern Siberia localities and several localities worldwide showing good agreement of pressure ranges for black and green suites.These equations allow construct PTX diagrams for the kimberlite localities in Siberia and worldwide calculating simultaneously the PT parameters for different groups of mantle rocks.They give very good results for the concentrates from kimberlite lamproites and placers with mantle minerals.They are useful for PT estimates for diamond inclusions.The positions of eclogite groups in mantle sections are similar to those determined with new Gar—Cpx barometer produced by C.Beyer et al.(2015).The Fe rich eclogites commonly trace the boundary between the lower upper parts of subcontinental lithospheric mantle(SCLM) at 3-4 CPa marking pyroxenite eclogites layer.Ca-rich eclogites and especially grospydires in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts.The diamondiferous Mg Cr-less group eclogites referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5-6 GPa.Commonly eclogites in lower apart of mantle sections are remelted and trace the high temperature convective branch.The Mg-and Fe-rich pyroxenites also show the extending in pressure trends which suggest the anatexic melting under the influence of volatiles or under the interaction with plums. 相似文献
14.
Warwick J. Sivell 《Contributions to Mineralogy and Petrology》1986,93(3):381-394
Extremely fractionated basaltic to ferrobasaltic amphibolites and granulites comprise two spatially associated mafic tholeiitic suites (?deformed sills) within the Early Proterozoic Oonagalabi basement gneiss complex, Harts Range, Central Australia. The metatholeiites are characterised by high to very high FeO, TiO2 and P2O5 contents, and variable depletion in CaO and Al2O3. Despite similar Zr/Nb ratios, the rocks from the two suites show different degrees of enrichment in LREE and other “immobile” incompatible elements. The basaltic melts which were parental to the two mafic suites were not comagmatic and the rocks cannot be related simply by fractionation of realistic assemblages of low-pressure fractionating phases. The data suggest that primary basaltic liquids for the two suites were derived by different degrees of partial melting from essentially similar undepleted mantle source regions. Clinopyroxene in the residual mantle assemblage controlled the composition of the segregating melt at lower degrees of melting. The ferrobasaltic compositions imply long residence times for the basaltic magmas in shallow-level differentiating tholeiitic sills and/or magma chambers in a mature propagating rift environment. High-grade (granulite facies) metamorphism, and subsequent restricted metasomatic reequilibration of the mafic rocks with interlayered migmatitic and quartzofeldspathic gneisses, have affected only abundances of certain highly-smobile elements (e.g. K2O and Rb), resulting in the partial disruption of inter-element correlations. However, the geochemical data do not indicate any large-scale depletion of large ion lithophile elements (LILE) in the Oonagalabi gneiss complex. 相似文献
15.
P. M. E. Tollan I. Bindeman J. D. Blundy 《Contributions to Mineralogy and Petrology》2012,163(2):189-208
In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we
have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic
blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine–gabbros
to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those
in erupted basaltic lavas from St. Vincent and in published high-pressure (4–10 kb) experimental run products of a St. Vincent
high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89–5.18‰), plagioclase (5.84–6.28‰), clinopyroxene (5.17–5.47‰) and hornblende (5.48–5.61‰) and hydrogen
isotope composition of hornblende (δD = −35.5 to −49.9‰) are all consistent with closed system magmatic differentiation of
a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic
compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this
to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by
slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to
determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite
lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude
that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at
depth within the crust, lowering MgO and Cr2O3 and raising Al2O3 and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then
ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically
distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen
isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of
magma. 相似文献
16.
The compatibility of vanadium (V) during mantle melting is a function of oxygen fugacity (fO2): at high fO2’s, V becomes more incompatible. The prospects and limitations of using the V content of peridotites as a proxy for paleo-fO2 at the time of melt extraction were investigated here by assessing the uncertainties in V measurements and the sensitivity of V as a function of degree of melt extracted and fO2. V-MgO and V-Al2O3 systematics were found to be sensitive to fO2 variations, but consideration of the uncertainties in measurements and model parameters indicates that V is sensitive only to relative fO2 differences greater than ∼2 log units. Post-Archean oceanic mantle peridotites, as represented by abyssal peridotites and obducted massif peridotites, have V-MgO and -Al2O3 systematics that can be modeled by 1.5 GPa melting between FMQ − 3 and FMQ − 1. This is consistent with fO2’s of the mantle source for mid-ocean ridge basalts (MORBs) as determined by the Fe3+ activity of peridotitic minerals and basaltic glasses. Some arc-related peridotites have slightly lower V for a given degree of melting than oceanic mantle peridotites, and can be modeled by 1.5 GPa melting at fO2’s as high as FMQ. However, the majority of arc-related peridotites have V-MgO systematics overlapping that of oceanic mantle peridotites, suggesting that although some arc mantle may melt under slightly oxidizing conditions, most arc mantle does not. The fact that thermobarometrically determined fO2’s in arc peridotites and lavas can be significantly higher than that inferred from V systematics, suggests that V retains a record of the fO2 during partial melting, whereas the activity of Fe3+ in arc peridotitic minerals and lavas reflect subsequent metasomatic overprints and magmatic differentiation/emplacement processes, respectively.Peridotites associated with middle to late Archean cratonic mantle are characterized by highly variable V-MgO systematics. Tanzanian cratonic peridotites have V systematics indistinguishable from post-Archean oceanic mantle and can be modeled by 3 GPa partial melting at ∼FMQ − 3. In contrast, many South African and Siberian cratonic peridotites have much lower V contents for a given degree of melting, suggesting at first glance that partial melting occurred at high fO2’s. More likely, however, their unusually low V contents for a given degree of melting may be artifacts of excess orthopyroxene, a feature that pervades many South African and Siberian peridotites but not the Tanzanian peridotites. This is indicated by the fact that the V contents of South African and Siberian peridotites are correlated with increases in SiO2 content, generating data arrays that cannot be modeled by partial melting but can instead be generated by the addition of orthopyroxene through processes unrelated to primary melt depletion. Correction for orthopyroxene addition suggests that the South African and Siberian peridotites have V-MgO systematics similar to those of Tanzanian peridotites. Thus, if the Tanzanian peridotites represent the original partial melting residues, and if the South African and Siberian peridotites have been modified by orthopyroxene addition, then there is no indication that Archean cratonic mantle formed under fO2’s significantly greater than that of modern oceanic mantle. Instead, the fO2’s inferred from the V systematics in these three cratonic peridotite suites are within range of modern oceanic mantle. This also suggests that the transition from a highly reducing mantle in equilibrium with a metallic core to the present oxidized state must have occurred by late Archean times. 相似文献
17.
Stability of phengite and biotite in eclogite is discussed using petrological data of natural eclogites, and the observational data are examined by thermodynamic calculations. Generally, phengite is a major K phase in natural eclogite and is stable in wide range of bulk composition. However, in eclogites from several localities of the Caledonides, biotite occurs as a stable eclogite-facies mineral, and is often associated with orthopyroxene. Bulk compositions of such biotite- or orthopyroxene-bearing eclogites are compared with those of eclogites from the Dabie–Sulu region, China, where phengite is a major K phase in eclogite. The biotite- or orthopyroxene-bearing eclogites from the Western Gneiss Region of the Caledonides are rich in MgO (10–15 wt%) and relatively poor in CaO (7–8 wt%) and Al2O3 (12–16 wt%). The CaO/MgO ratios of the biotite- or orthopyroxene-bearing eclogites are clearly lower than those of eclogites from the Dabie–Sulu region, indicating that MgO-rich and CaO-poor environments should be important for stabilizing of biotite and orthopyroxene in eclogite. Biotite-bearing eclogite from the North-East Greenland Eclogite Province is rich in MgO (≈16 wt%) and CaO (≈15.5 wt%) and extremely poor in Al2O3 (≈8 wt%). To stabilize biotite in eclogite, Al2O3-poor environments are also important. Bulk compositions of these biotite- or orthopyroxene-bearing eclogites are similar to picrite basaltic compositions. To examine these observational data, thermodynamic calculations were carried out in a seven-component system KH2O1.5–Na2O–CaO–FeO–MgO–Al2O3–SiO2, which includes garnet, kyanite, phengite, biotite, quartz, omphacite, orthopyroxene and olivine in conjunction with mass-balance calculations. Firstly, calculations were performed on the average bulk composition of eclogites from the Dabie–Sulu region to lherzolite (KLB-1). The calculation results confirmed that phengite should be stable in eclogite with 'ordinary' basaltic composition, whereas biotite and orthopyroxene should be stable in picrite basaltic compositions (e.g. MgO >11.0 wt%, CaO <9.8 wt%, Al2O3 <15.2 wt% at 700 °C, 2.5 GPa). Further calculations in basaltic system confirmed that increase of MgO content and decrease of CaO and Al2O3 contents were important to stabilize biotite and orthopyroxene in eclogite. Thus, mineral assemblage in picrite basalt system should be completely different from that in normal basaltic system. 相似文献
18.
C. M. Appleyard D. R. Bell A. P. le Roex 《Contributions to Mineralogy and Petrology》2007,154(3):309-333
Petrographic and geochemical features of a suite of eclogite xenoliths from the Rietfontein kimberlite that erupted through
probable Proterozoic crust west of the Kaapvaal Craton in the far Northern Cape region of South Africa, are described. Group
II eclogites dominate the suite both texturally and chemically, but can be subdivided into bimineralic, opx-bearing and kyanite-bearing
groups. Temperature estimates from different geothermometers range from 700 to 1,000°C, indicating derivation from relatively
shallow mantle depths. Orthopyroxene-bearing eclogites are inferred to originate from depths of 85 to 115 km and lie close
to the average cratonic thermal profile for southern Africa. These uppermost mantle temperatures during the late Cretaceous
provide evidence for equilibration of the off-craton lithosphere to craton-like thermal conditions following Namaqua-Natal
orogenesis. The kyanite eclogites are distinct from the remaining eclogites in terms of both major and trace element compositions
and their lesser degree of alteration. Garnets are richer in Ca, and are Cr-depleted relative to garnets from the bimineralic
and opx-bearing eclogites, which tend to be more magnesian. Clinopyroxenes from the kyanite eclogites are more sodic, with
higher Al2O3 and lower MgO contents than the bimineralic and opx-bearing eclogites. LREE-depletion, positive Sr and Eu anomalies, and
the Al-rich, Si-poor bulk composition suggest a plagioclase-rich, probably troctolitic protolith for the kyanite eclogites.
In contrast, the major and trace element bulk compositions of the high-MgO bimineralic and orthopyroxene-bearing eclogites
are consistent with gabbroic or pyroxenitic precursors, or high-pressure cumulates, rather than mafic to ultramafic lavas.
δ18O values for garnets do not deviate significantly from typical mantle values. The observations reported do not discriminate
unambiguously between continental and oceanic origins for the various eclogite components in the mantle lithosphere of this
region. 相似文献
19.
Atsushi Utsunomiya Bor-ming Jahn Kazuaki Okamoto Tsutomu Ota Hironao Shinjoe 《Chemical Geology》2011,280(1-2):97-114
New geochemical and Sr–Nd isotopic data for the Iratsu eclogite and surrounding metamorphic rocks of the Sanbagawa belt, Japan, show that, while the protoliths of the metamorphic rocks formed in a variety of tectonic settings, the Iratsu body represents a deeply subducted and accreted island arc. The igneous protoliths of eclogites and garnet amphibolites were probably generated from a mantle source that had components of both a depleted mantle modified by slab-released fluid (as seen in a negative Nb anomaly) and an enriched mantle, similar to that of ocean island basalts (OIB). Fractional crystallization modeling indicates that the protoliths of some garnet clinopyroxenites from the Iratsu body are cumulates from a basaltic magma that crystallized under high O2 and H2O fugacities in the middle to lower crust. The source characteristics and crystallization conditions suggest that the protoliths of the Iratsu rocks formed in an oceanic island arc. Quartz eclogites from the marginal zone of the Iratsu body have geochemical signatures similar to turbidites from the Izu–Bonin island arc (as seen in a negative Nb anomaly and a concave REE pattern). The protoliths might be volcaniclastic turbidites that formed in a setting proximal to the oceanic island arc. Geochemical and isotopic signatures of the surrounding mafic schists are similar to normal (N-) and enriched (E-) mid-ocean-ridge basalt (MORB), and distinct from the rocks from the Iratsu body. The protoliths of the mafic schists likely formed in a plume-influenced mid-ocean ridge or back-arc basin. Pelitic schists from the surrounding rocks and pelitic gneisses from the marginal zone of the Iratsu body have evolved, continental geochemical signatures (as seen in a negative εNd(t) value (~?5)), consistent with their origin as continent-derived trench-fill turbidites. 相似文献
20.
A suite of eclogites from the Roberts Victor kimberlite has been extensively characterized in terms of petrology and geochemical compositions (Gréau et al. in Geochim Cosmochim Acta 75(22):6927–6954, 2011; Huang et al. in Lithos 142–143:161–181, 2012a). In the present study, the water contents of eclogitic garnet and omphacite were analyzed by Fourier transform infrared spectrometry. Garnet does not contain measureable OH in any sample. The water content of omphacite in the studied eclogites ranges from 211 to 1,496 ppm. Mantle metasomatism has modified the water content of some of the eclogites, while others retain water contents characteristic of their original environment. The OH contents of the metasomatized eclogites may be mainly controlled by the H2O fugacity and mineral compositions. The OH contents of the non-metasomatized samples are interpreted to be more sensitive to their mantle equilibration temperature, pressure, and the local fugacities of H2O and O2. The calculated water content of the metasomatic medium is similar to that of carbonatitic–kimberlitic melts/fluids. Eclogites contain more water than peridotites recorded in the literature (341 ± 161 vs 122 ± 54 ppm) and represent an important water reservoir in the lithospheric mantle wherever they occur. This is an important parameter to be considered in the interpretation of mantle processes and geophysical data such as seismic wave speeds and electrical conductivity, and in geodynamic modeling. 相似文献