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1.
The mineral chemistry, major and trace element, and Sr–Nd isotopic composition of Cr-diopside, spinel peridotite xenoliths from the Estancia Lote 17 locality in southern Patagonia document a strong carbonatitic metasomatism of the backarc continental lithosphere. The Lote 17 peridotite xenolith suite consists of hydrous spinel lherzolite, wehrlite, and olivine websterite, and anhydrous harzburgite and lherzolite. Two-pyroxene thermometry indicates equilibration temperatures ranging from 870 to 1015 °C and the lack of plagioclase or garnet suggests the xenoliths originated from between ˜40 and 60 km depth. All of the xenoliths are LILE- and LREE-enriched, but have relatively low 87Sr/86Sr (0.70294 to 0.70342) and high ɛNd (+3.0 to +6.6), indicating recent trace element enrichment (∼25 Ma, based on the low 87Sr/86Sr and high Rb concentrations of phlogopite separates) in the long-term, melt-depleted Patagonian lithosphere. Lote 17 peridotite xenoliths are divided into two basic groups. Group 1 xenoliths consist of fertile peridotites that contain hydrous phases (amphibole ± phlogopite ± apatite). Group 1 xenoliths are further subdivided into three groups (a, b, and c) based on distinctive textures and whole-rock chemistry. Group 1 xenolith mineralogy and chemistry are consistent with a complex metasomatic history involving variable extents of recent carbonatite metasomatism (high Ca/Al, Nb/La, Zr/Hf, low Ti/Eu) that has overprinted earlier metasomatic events. Group 2 xenoliths consist of infertile, anhydrous harzburgites and record cryptic metasomatism that is attributed to CO2-rich fluids liberated from Group 1 carbonatite metasomatic reactions. Extremely variable incompatible trace element ratios and depleted Sr–Nd isotopic compositions of Lote 17 peridotite xenoliths indicate that the continental lithosphere was neither the primary source nor an enriched lithospheric contaminant for Neogene Patagonian plateau lavas. Neogene plateau magmatism associated with formation of asthenospheric slab windows may have triggered this occurrence of “intraplate-type” carbonatite metasomatism in an active continental backarc setting. Received: 26 January 2000 / Accepted: 1 March 2000  相似文献   

2.
Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ57Fe are in a range of −0.25 to 0.14‰ for olivine, −0.17 to 0.17‰ for orthopyroxene, −0.21 to 0.27‰ for clinopyroxene, and −0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.  相似文献   

3.
Mantle xenoliths hosted by the historic Volcan de San Antonio, La Palma, Canary Islands include veined spinel harzburgites and spinel dunites. Glasses and associated minerals in the vein system of veined xenoliths show a gradual transition in composition from broad veins to narrow veinlets. Broad veins contain alkali basaltic glass with semi-linear trace element patterns enriched in strongly incompatible elements. As the veins become narrower, the SiO2-contents in glass increase (46 → 67 wt% SiO2 in harzburgite, 43 → 58 wt% in dunite) and the trace element patterns change gradually to concave patterns depleted in moderately incompatible elements (e.g. HREE, Zr, Ti) relative to highly incompatible ones. The highest SiO2-contents (ca. 68% SiO2, low Ti-Fe-Mg-Ca-contents) and most extreme concave trace element patterns are exhibited by glass in unveined peridotite xenoliths. Clinopyroxenes shift from LREE-enriched augites in basaltic glass, to REE-depleted Cr-diopside in highly silicic glass. Estimates indicate that the most silicic glasses represent melts in, or near, equilibrium with their host peridotites. The observed trace element changes are compatible with formation of the silicic melts by processes involving infiltration of basaltic melts into mantle peridotite followed by reactions and crystallization. The Fe-Mg interdiffusion profiles in olivine porphyroclasts adjacent to the veins indicate a minimum period of diffusion of 600 years, implying that the reaction processes have taken place in situ in the upper mantle. The CaO-TiO2-La/Nd relationships of mantle rocks may be used to discriminate between metasomatism caused by carbonatitic and silicic melts. Unveined mantle xenoliths from La Palma and Hierro (Canary Islands) show a wide range in La/Nd ratios with relatively constant, low-CaO contents which is compatible with metasomatism of “normal” abyssal peridotite by silicic melts. Peridotite xenoliths from Tenerife show somewhat higher CaO and TiO2 contents than those from the other islands and may have been affected by basaltic or carbonatitic melts. The observed trace element signatures of ultramafic xenoliths from La Palma and other Canary Islands may be accounted for by addition of small amounts (1–7%) of highly silicic melt to unmetasomatized peridotite. Also ultramafic xenoliths from other localities, e.g. eastern Australia, show CaO-TiO2-La/Nd relationships compatible with metasomatism by silicic melts. These results suggest that silicic melts may represent important metasomatic agents. Received: 15 November 1998 / Accepted: 17 May 1999  相似文献   

4.
Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ57Fe are in a range of ?0.25 to 0.14‰ for olivine, ?0.17 to 0.17‰ for orthopyroxene, ?0.21 to 0.27‰ for clinopyroxene, and ?0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.  相似文献   

5.
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6.
Summary Mantle-derived xenoliths from Baarley in the Quaternary West Eifel volcanic field contain six distinct varieties of glass in veins, selvages and pools. 1) Silica-undersaturated glass rich in zoned clinopyroxene microlites that forms jackets around and veins within the xenoliths. This glass is compositionally similar to groundmass glass in the host basanite. 2) Silica-undersaturated alkaline glass that contains microlites of Cr-diopside, olivine and spinel associated with amphibole in peridotites. This glass locally contains corroded primary spinel and phlogopite. 3) Silica-undersaturated glass associated with diopside, spinel ± olivine and rh?nite microlites in partly to completely broken down amphibole grains in clinopyroxenites. 4) Silica-undersaturated to silica-saturated, potassic glass in microlite-rich fringes around phlogopite grains in peridotite. 5) Silica-undersaturated potassic glass in glimmerite xenoliths. 6) Silica-rich glass around partly dissolved orthopyroxene crystals in peridotites. Geothermometry of orthopyroxene–clinopyroxene pairs (P = 1.5 GPa) gives temperatures of ∼ 850 °C for unveined xenoliths to 950–1020 °C for veined xenoliths. Clinopyroxene – melt thermobarometry shows that Cr-diopside – type 2 glass pairs in harzburgite formed at 1.4 to 1.1 GPa and ∼ 1250 °C whereas Cr-diopside – type 2 glass pairs in wehrlite formed at 0.9 to 0.7 GPa and 1120–1200 °C. This bimodal distribution in pressure and temperature suggests that harzburgite xenoliths may have been entrained at greater depth than wehrlite xenoliths. Glass in the Baarley xenoliths has three different origins: infiltration of an early host melt different in composition from the erupted host basanite; partial melting of amphibole; reaction of either of these melts with xenolith minerals. The composition of type 1 glass suggests that jackets are accumulations of relatively evolved host magma. Mass balance modelling of the type 2 glass and its microlites indicates that it results from breakdown of disseminated amphibole and reaction of the melt with the surrounding xenolith minerals. Type 3 glass in clinopyroxenite xenoliths is the result of breakdown of amphibole at low pressure. Type 4 and 5 glass formed by reaction between phlogopite and type 2 melt or jacket melt. Type 6 glass associated with orthopyroxene is due to the incongruent dissolution of orthopyroxene by any of the above mentioned melts. Compositional gradients in xenolith olivine adjacent to type 2 glass pools and jacket glass can be modelled as Fe–Mg interdiffusion profiles that indicate melt – olivine contact times between 0.5 and 58 days. Together with the clinopyroxene – melt thermobarometry calculations these data suggest that the glass (melt) formed over a short time due to decompression melting of amphibole and infiltration of evolved host melt. None of the glass in these xenoliths can be directly related to metasomatism or any other process that occurred insitu in the mantle. Received November 23, 1999; revised version accepted September 5, 2001  相似文献   

7.
Quaternary basanitic to nephelinitic volcanoes from Tahalra (western Ahaggar, southern Algeria) contain numerous Mg-ilmenite and amphibole-rich inclusions (±olivine, ±salite) and spinel lherzolite (±pargasite) inclusions associated with kaersutite megacrysts. On the basis of petrological, geochemical and Sr isotopic study of representative xenoliths (including a composite nodule defined as a vein cross-cutting peridotite) and lavas, we attribute the series of amphibole-rich xenoliths and megacrysts to segregation under upper mantle conditions from a hydrous high Ti and LREE melt geochemically similar to the Quaternary basanite but isotopically different. Amphibole-rich rocks and megacrysts are the results of magmatic events (less than 40 Ma) probably contemporaneous with the various pre-Quaternary volcanic phases recognized in Ahaggar. The amphibole-rich veins and the Quaternary lavas have a garnet lherzolitic source enriched in REE (7 to 9 times chondritic in LREE, 2 times in HREE). This enrichment probably results from former metasomatic events unrelated to the recent magmatic history. Melts from which these veins precipitated within upper mantle peridotite also account for mantle enrichment processes; they induced a local partial melting and contact metasomatism (pargasitization). The upper mantle beneath the volcanic areas of Ahaggar is veined and hydrous, and consequently lightened: thus, the uplift of basement may be the isostatic response to magmatism and related metasomatism and therefore the result of the Cenozoïc igneous activity.  相似文献   

8.
Metasomatic oxidation of upper mantle periodotite   总被引：1，自引：0，他引：1  
Examination of Fe3+ in metasomatized spinel peridotite xenoliths reveals new information about metasomatic redox processes. Composite xenoliths from Dish Hill, California possess remnants of magmatic dikes which were the sources of the silicate fluids responsible for metasomatism of the peridotite part of the same xenoliths. Mössbauer spectra of mineral separates taken at several distances from the dike remnants provide data on Fe3+ contents of minerals in the metasomatized peridotite. Clinopyroxenes contain 33% of total iron (FeT) as Fe3+ (Fe3+/FeT=0.33); orthopyroxenes contain 0.06–0.09 Fe3+/FeT; spinels contain 0.30–0.40 Fe3+/FeT; olivines contain 0.01–0.06 Fe3+/FeT; and metasomatic amphibole in the peridotite contains 0.85–0.90 Fe3+/FeT. In each mineral, Fe3+ and Fe2+ cations per formula unit (p.f.u.) decrease with distance from the dike, but the Fe3+/FeT ratios of each mineral do not vary. Clinopyroxene, spinel, and olivine Fe3+/FeT ratios are significantly higher than in unmetasomatized spinel peridotites. Metasomatic changes in Fe3+/FeT ratios in each mineral are controlled by the oxygen fugacity of the system, but the mechanism by which each phase accommodates this ratio is affected by crystal chemistry, kinetics, rock mode, fluid composition, fluid/rock ratio, and fluid-mineral partition coefficients. Ratio increases in pyroxene and spinel occur by exchange reactions involving diffusion of Fe3+ into existing mineral grains rather than by oxidation of existing Fe2+ in peridotite mineral grains. The very high Fe3+/FeT ratio in the metasomatic amphibole may be a function of the high Fe3+/FeT of the metasomatic fluid, crystal chemical limitations on the amount of Fe3+ that could be accommodated by the pyroxene, spinel, and olivine of the peridotite, and the ability of the amphibole structure to accommodate large amounts of 3 + valence cations. In the samples studied, metasomatic amphibole accounts for half of the bulk-rock Fe2O3. This suggests that patent metasomatism may produce a greater change in the redox state of mantle peridotite than cryptic metasomatism. Comparison of the metasomatized samples with unmetasomatized peridotites reveals that both Fe2+ and Fe3+ cations p.f.u. were increased during metasomatism and 50% or more of iron added was Fe3+. With increasing distance from the dike, the ratio of added Fe3+ to added Fe2+ increases. The high Fe3+/FeT of amphibole and phlogopite in the dikes and in the peridotite, and the high ratios of added Fe3+/added Fe2+ in pyroxenes and spinel suggest that the Fe3+/FeT ratio of the metasomatic silicate fluid was high. As the fluid perolated through and reacted with the peridotite, Fe3+ and C–O–H volatile species were concentrated in the fluid, increasing the fluid Fe3+/FeT.  相似文献   

9.
New data on metasomatic processes in the lithospheric mantle in the central part of the Arkhangelsk diamondiferous province (ADP) are presented. We studied the major- and trace-element compositions of minerals of 26 garnet peridotite xenoliths from the V. Grib kimberlite pipe; 17 xenoliths contained phlogopite. Detailed mineralogical, petrographic, and geochemical studies of peridotite minerals (garnet, clinopyroxene, and phlogopite) have revealed two types of modal metasomatic enrichment of the lithospheric-mantle rocks: high temperature (melt) and low-temperature (phlogopite). Both types of modal metasomatism significantly changed the chemical composition of the peridotites. Low-temperature modal metasomatism manifests itself as coarse tabular and shapeless phlogopite grains. Two textural varieties of phlogopite show significant differences in chemical composition, primarily in the contents of TiO2, Cr2O3, FeO, Ba, Rb, and Cs. The rock-forming minerals of phlogopite-bearing peridotites differ in chemical composition from phlogopite-free peridotites, mainly in higher FeO content. Most garnets and clinopyroxenes in peridotites are the products of high-temperature mantle metasomatism, as indicated by the high contents of incompatible elements and REE pattern in these minerals. Fractional-crystallization modeling gives an insight into the nature of melts (metasomatic agents). They are close in composition to picrites of the Izhmozero field, basalts of the Tur’ino field, and carbonatites of the Mela field of the ADP. The REE patterns of the peridotite minerals make it possible to determine the sequence of metasomatic enrichment of the lithospheric mantle beneath the V. Grib kimberlite pipe.  相似文献   

10.
Iron isotope and major- and minor-element compositions of coexisting olivine, clinopyroxene, and orthopyroxene from eight spinel peridotite mantle xenoliths; olivine, magnetite, amphibole, and biotite from four andesitic volcanic rocks; and garnet and clinopyroxene from seven garnet peridotite and eclogites have been measured to evaluate if inter-mineral Fe isotope fractionation occurs in high-temperature igneous and metamorphic minerals and if isotopic fractionation is related to equilibrium Fe isotope partitioning or a result of open-system behavior. There is no measurable fractionation between silicate minerals and magnetite in andesitic volcanic rocks, nor between olivine and orthopyroxene in spinel peridotite mantle xenoliths. There are some inter-mineral differences (up to 0.2‰ in 56Fe/54Fe) in the Fe isotope composition of coexisting olivine and clinopyroxene in spinel peridotites. The Fe isotope fractionation observed between clinopyroxene and olivine appears to be a result of open-system behavior based on a positive correlation between the Δ56Feclinopyroxene-olivine fractionation and the δ56Fe value of clinopyroxene and olivine. There is also a significant difference in the isotopic compositions of garnet and clinopyroxene in garnet peridotites and eclogites, where the average Δ56Feclinopyroxene-garnet fractionation is +0.32 ± 0.07‰ for six of the seven samples. The one sample that has a lower Δ56Feclinopyroxene-garnet fractionation of 0.08‰ has a low Ca content in garnet, which may reflect some crystal chemical control on Fe isotope fractionation. The Fe isotope variability in mantle-derived minerals is interpreted to reflect subduction of isotopically variable oceanic crust, followed by transport through metasomatic fluids. Isotopic variability in the mantle might also occur during crystal fractionation of basaltic magmas within the mantle if garnet is a liquidus phase. The isotopic variations in the mantle are apparently homogenized during melting processes, producing homogenous Fe isotope compositions during crust formation.  相似文献   

11.
Orthopyroxene porphyroblasts zoned to interiors abnormally low in Al and Cr and containing numerous inclusions of olivine occur in some spinel peridotite xenoliths from the Colorado Plateau. Rims of these orthopyroxene grains contain 2.5–3.0 wt% Al2O3, consistent with equilibration in spinel peridotite at temperatures near 850 °C, but interiors contain as little as 0.20 wt% Al2O3 and 0.04 wt% Cr2O3. The Al-poor compositions are inferred to have equilibrated in chlorite peridotite, before porphyroblast growth during heating and consequent reactions that eliminated talc, tremolite, and chlorite. The distinctive orthopyroxene textures are inferred to have formed during reaction of talc and olivine. Rare intergrowths of orthopyroxene plus diopside are attributed to olivine-tremolite reaction. Al and Cr have gradients at grain rims that appear little modified by diffusion, but divalent elements are almost homogeneous throughout the porphyroblasts. Judging from the relative gradients, diffusion of Ca was at least 100 times faster than that of Al and Cr at the temperatures near and below 850 °C. Diffusion of Al and Cr was most effective along subgrain boundaries, and along these boundaries it appears to have been at least ten times faster than within the lattice: diffusion along such boundaries may be a dominant mechanism for re-equilibration of orthopyroxene at low mantle temperatures. Orthopyroxene with similar low Al and Cr occurs in chlorite peridotite xenoliths from the Navajo field, 300 km east of the Grand Canyon localities, and in spinel peridotite xenoliths from the Sierra Nevada, 500 km west across the extended Basin and Range province. Chlorite peridotite may therefore have been a significant minor component in much of the mantle lithosphere of western North America, although evidence for it would be erased at the higher temperatures recorded by xenoliths from the Basin and Range. Chemical changes during hydration may have been important in the evolution of these mantle volumes, and the case for addition of Sr is particularly strong. Dehydration reactions during heating could have influenced patterns of extension and crustal magmatism. Received: 1 July 1996 / Accepted: 2 December 1996  相似文献   

12.
Spinel peridotite xenoliths from the Atsagin-Dush volcanic centre, SE Mongolia range from fertile lherzolites to clinopyroxene(cpx)-bearing harzburgites. The cpx-poor peridotites typically contain interstitial fine-grained material and silicate glass and abundant fluid inclusions in minerals, some have large vesicular melt pockets that apparently formed after primary clinopyroxene and spinel. No volatile-bearing minerals (amphibole, phlogopite, apatite, carbonate) have been found in any of the xenoliths. Fifteen peridotite xenoliths have been analysed for major and trace elements; whole-rock Sr isotope compositions and O isotope composition of all minerals were determined for 13 xenoliths. Trace element composition and Sr-Nd isotope compositions were also determined in 11 clinopyroxene and melt pocket separates. Regular variations of major and moderately incompatible trace elements (e.g. heavy-rare-earth elements) in the peridotite series are consistent with its formation as a result of variable degrees of melt extraction from a fertile lherzolite protolith. The Nd isotope compositions of LREE (light-rare-earth elements)-depleted clinopyroxenes indicate an old (≥ 1 billion years) depletion event. Clinopyroxene-rich lherzolites are commonly depleted in LREE and other incompatible trace elements whereas cpx-poor peridotites show metasomatic enrichment that can be related to the abundance of fine-grained interstitial material, glass and fluid inclusions in minerals. The absence of hydrous minerals, ubiquitous CO2-rich microinclusions in the enriched samples and negative anomalies of Nb, Hf, Zr, and Ti in primitive mantle-normalized trace element patterns of whole rocks and clinopyroxenes indicate that carbonate melts may have been responsible for the metasomatic enrichment. Low Cu and S contents and high δ34S values in whole-rock peridotites could be explained by interaction with oxidized fluids that may have been derived from subducted oceanic crust. The Sr-Nd isotope compositions of LREE-depleted clinopyroxenes plot either in the MORB (mid-ocean-ridge basalt) field or to the right of the mantle array, the latter may be due to enrichment in radiogenic Sr. The LREE-enriched clinopyroxenes and melt pockets plot in the ocean island-basalt field and have Sr-Nd isotope signatures consistent with derivation from a mixture of the DMM (depleted MORB mantle) and EM (enriched mantle) II sources. Received: 18 January 1996 / Accepted: 23 August 1996  相似文献   

13.
A suite of ultramafic mantle xenoliths from the TUBAF and EDISONseamounts in the Bismarck Archipelago NE of Papua New Guineawas sampled by video-guided grab. The xenoliths, which weretransported to the sea floor by rift-related, Quaternary trachybasalts,mainly represent part of the oceanic mantle. Mineral zoningin peridotite xenoliths testifies to slow cooling after mantleformation at a mid-ocean ridge. Cooling rates in the range of1°C/Ma were calculated from zoning of Ca in olivine usingthe Lasaga algorithm. Subsequent to this cooling, a strong metasomatismaffected the mantle peridotites when metasomatic agents emergedfrom the underlying slab of a subduction zone, which was stalledabout 15 my ago. This resulted in the formation of orthopyroxene-,clinopyroxene-, phlogopite- and hornblende-bearing veins crosscuttingspinel peridotites and olivine clinopyroxenites, as well aspervasively metasomatized plagioclase lherzolites. The metasomaticxenoliths reveal strong chemical disequilibria between the metasomaticminerals and the adjacent, unaltered host rock minerals, whichare especially prominent in the veined samples. Temperaturesduring the metasomatic overprint, estimated using spinel–olivinethermometry, range between 660 and 950°C. Oxygen barometryreveals an elevated oxygen fugacity, with  相似文献   

14.
南美洲南部的Pali Aike火山岩区第四纪碱性玄武岩中普遍发育含石榴石的斜方辉石岩包体。这种斜方辉石岩既作为独立的捕掳体存在又以细脉的形式穿插于橄榄岩捕掳体中。斜方辉石岩普遍含富Ti矿物,并且次生斜方辉石含橄榄石和单斜辉石残晶。与含石榴石橄榄岩中的斜方辉石相比,这种次生的斜方辉石以高TiO2、中等含量的Al2O3以及低Mg#为特征,表明它是在一种高度分异演化的富Ti熔体交代作用下通过消耗橄榄石和单斜辉石方式形成的。斜方辉石岩全岩的Co、Ni略低,Cr和铂族元素(PGE)含量与地幔橄榄岩相当,表明这些元素在交代作用过程中相对稳定,而交代介质带入的组分以碱质(K2O+Na2O)、Ti、Si、Al和S为主。交代的斜方辉石在现代活动岛弧和古克拉通的地幔橄榄岩捕掳体中多有报道。与这些环境中地幔样品的斜方辉石相比,PaliAike地区的次生斜方辉石含有相对高的Ti和Al,以及相对低的Mg。高Ti低Mg属性反映了交代介质可能来源于下伏的软流圈地幔并且经历了高度的分异和演化过程。Pali Aike地区所见到的这种交代斜方辉石和斜方辉石岩在其他被上涌软流圈影响的陆下岩石圈地幔中可能普遍存在。这些研究对了解中国华北-东北中生代以来的岩石圈地幔减薄机制有着重要的借鉴意义。  相似文献   

15.
阿尔山—柴河第四纪碱性玄武岩中地幔捕掳体为尖晶石相的二辉橄榄岩和方辉橄榄岩,方辉橄榄岩数量略多于二辉橄榄岩。采用激光剥蚀等离子体质谱(LA--ICP--MS)对研究区地幔橄榄岩中的单斜辉石和橄榄石等矿物进行了成分分析,结合橄榄岩包体的岩相学、岩石化学的特征,重点探讨了研究区所经历的部分熔融作用和地幔交代作用。结果显示,少数样品的熔融程度5%,大多数样品熔融程度范围为10%~20%,研究区陆下岩石圈地幔性质以难熔、亏损为主要特征。同时也经历了复杂的交代作用改造,交代介质为富挥发组分的硅酸盐熔/流体。与华北克拉通东北缘陆下岩石圈地幔比较,推测研究区遭受破坏和改造的程度较小,并保留有相当量的古老地幔残余。  相似文献   

16.
 K-richterite/phlogopite-bearing peridotite xenoliths and MARID inclusions have been found in Late Cretaceous (67±0.2 million years) monchiquites and an olivine nephelinite from North Eastern Morocco. It is the first evidence of MARID rocks and K-richterite/phlogopite-bearing peridotites outside the kimberlitic context. In the hydrous xenoliths, textural features suggest that K-richterite, phlogopite and Al-poor diopside are replacement minerals. K-richterites contain 2–5 wt% FeO, 0.1–1.5 wt% TiO2 and <0.5 wt% Cr2O3. Micas contain 5.4–7.4 wt% FeO and 0.3–2.2 wt% TiO2, with Cr and Ni contents <0.2 wt%. Diopsides are Al-poor (<0.2 wt% Al2O3) and contain 0.1–0.2 wt% TiO2, 0.9–1.1 wt% Na2O and 1.3–1.7 wt% Cr2O3. Compared to known K-richterites and micas from metasomatised peridotite nodules (PKP types), the Moroccan minerals are more Fe rich, K-richterites have higher Ti and micas less Cr and Ni. They are thus closer to MARID than to PKP minerals. K-richterites and mica from the MARID inclusions show typical characteristics, e.g. high FeO (4.3–4.7 wt% in richterite and 7.2 wt% in mica), low NiO and/or Cr2O3 and the incomplete filling of the tetrahedral site by Si+Al. Ion probe D/H analyses of amphiboles and micas from both xenolith types give high δD values ranging from –8 to –73, with large variations within single grains (up to 50‰). Both the D-enrichment and the δD variations are inherited from the mantle. The similar chemical composition and δD values of K-richterite/phlogopite from the hydrous peridotites and MARID minerals suggest a genetic link between the two types of xenoliths. The conditions required for producing MARIDs and K-richterite/phlogopite-bearing peridotites may thus exist in contexts other than stable cratonic settings. MARID rocks and the associated metasomites may result from a hydrous fluid interaction with a peridotite, the metasomatic agent being characterised by a high K and low Al signature and a high δD value. A D-rich source is involved in the metasomatic event producing the hydrous minerals, and the scatter observed in the δD values suggests a mixing between this source and another one with typical upper mantle D/H composition. As indicated by the low δD (–74) values of micas from the host lava, metasomatism predated and is unrelated to the alkaline volcanism. Received: 9 March 1995 / Accepted: 4 April 1996  相似文献   

17.
 Mantle xenoliths hosted by the Historic Volcan de San Antonio, La Palma, Canary Islands, fall into two main group. Group I consists of spinel harzburgites, rare spinel lherzolites and spinel dunites, whereas group II comprises spinel wehrlites, amphibole wehrlites, and amphibole clinopyroxenites. We here present data on group I xenoliths, including veined harzburgites and dunites which provide an excellent basis for detailed studies of metasomatic processes. The spinel harzburgite and lherzolite xenoliths have modal ol−opx−cpx ratios and mineral and whole rock major element chemistry similar to those found in Lanzarote and Hierro, and are interpreted as highly refractory, old oceanic lithospheric mantle. Spinel dunites are interpreted as old oceanic peridotite which has been relatively enriched in olivine and clinopyroxene (and highly incompatible elements) through reactions with basaltic Canarian magmas, with relatively high melt/peridotite ratio. Group I xenoliths from La Palma differ from the Hierro and Lanzarote ones by a frequent presence of minor amounts of phlogopite (and amphibole). Metasomatic processes are also reflected in a marked enrichment of strongly incompatible relative to moderately incompatible trace elements, and in a tendency for Fe−Ti enrichment along grain boundaries in some samples. The veins in the veined xenoliths show a gradual change in phase assemblage and composition of each phase, from Fe−Ti-rich amphibole+augite+Fe−Ti-oxides+apatite+basaltic glass, to Ti-poor phlogopite+Cr-diopside±chromite+ Si−Na−K-rich glass+fluid. Complex reaction zones between veins and peridotite include formation of clinopyroxene±olivine+glass at the expense of orthopyroxene in harzburgite, and clinopyroxene+spinel±amphibole±glass at the expense of olivine in dunite. The dramatic change in glass composition from the broadest to the narrowest veins includes increasing SiO2 from 44 to 67 wt%, decreasing TiO2/Al2O3 ratio from >0.24 to about 0.02, and increasing K2O and Na2O from 1.8 to >7.0 wt% and 3.8 to 6.7 wt%, respectively. The petrographic observations supported by petrographic mixing calculations indicate that the most silicic melts in the veined xenoliths formed as the result of reaction between infiltrating basaltic melt and peridotite wall-rock. The highly silicic, alkaline melt may represent an important metasomatic agent. Pervasive metasomatism by highly silicic melts (and possibly fluids unmixed from these) may account for the enriched trace element patterns and frequent presence of phlogopite in the upper mantle under La Palma. Received: 15 January 1996 / Accepted 30 May 1996  相似文献   

18.
Petrographic and geochemical studies of peridotites and melagabbros from the Maures massif (SE France) provide new constraints on the Early Palaeozoic evolution of the continental lithosphere in Western Europe. Peridotites occur as lenses along a unit rooted in the main Variscan suture zone. They are dominantly spinel peridotites and minor garnet–spinel peridotites. Spinel peridotites represent both residual mantle and ultramafic cumulates. Mantle-related dunites and harzburgites display high temperature textures, with olivine (Mg#0.90), orthopyroxene (Mg#0.90) and spinel (TiO2 < 0.2%; Cr#0.64–0.83) compositions typical of fore-arc upper mantle. Ultramafic cumulates are dunite adcumulates, harzburgite heteradcumulates and mesocumulates, melagabbro heteradcumulates and amphibole peridotites, with olivine (Mg#0.85–0.89), orthopyroxene (Mg#0.86–0.89) and Cr-spinel (TiO2 = 0.5–3.3%; Cr#0.7–0.98) compositions typical of ultramafic cumulates. Cr-spinel compositions of both spinel peridotite types suggest their genesis in a supra-subduction zone lithosphere. Core to rim zoning in spinel is related to the incomplete influence of regional metamorphism and serpentinisation. The covariation of major and minor elements with Al2O3 for cumulates is consistent with igneous processes involving crystal accumulation. Both mantle and cumulate dunites and harzburgites have U-shaped REE patterns and extremely low trace element contents, similar to peridotites from modern fore-arc peridotites (South Atlantic) and from ophiolites related to supra-subduction zones (Semail, Cyclops, Pindos, Troodos). Melagabbros also have U-shaped REE patterns similar to xenoliths from the Philippine island arc, but also similar to intrusive ultramafic cumulates from the Semail nappe of Oman related to a proto-subduction setting. A wehrlite has a REE pattern similar to that of amphibole peridotites reflecting metasomatism of clinopyroxene-bearing peridotites due to subduction-related fluids. The Maures spinel peridotites and melagabbros are therefore interpreted as the lowermost parts of a crustal sequence and minor residual mantle of lithosphere generated in a supra-subduction zone during Early Palaeozoic time. Garnet–spinel peridotites are chemically close to melagabbros, but have recorded high pressure metamorphism before their retrogression similar to spinel peridotites into amphibolites to greenschists facies metamorphism. They indicate burial to mantle depths of the margin of the supra-subduction lithosphere during the Early Palaeozoic continental subduction. Both peridotite types were exhumed during the Upper Palaeozoic continental collision. Comparable observations from other Variscan-related peridotites, in particular of the Speik complex of the Autroalpine basement, and a common age for the subduction stage allow extension of these regional conclusions to a broad area sharing the Cambrian suture zone, extending from the Ossa-Morena to the Bohemian massif.  相似文献   

19.
We report new textural and chemical data for 10 garnet peridotite xenoliths from the Udachnaya kimberlite and examine them together with recent data on another 21 xenoliths from the 80–220 km depth range. The samples are very fresh (LOI near zero), modally homogeneous and large (>100 g). Some coarse-grained peridotites show incipient stages of deformation with <10 % neoblasts at grain boundaries of coarse olivine. Such microstructures can only be recognized in very fresh rocks, because fine-grained interstitial olivine is strongly affected by alteration, and may have been overlooked in previous studies of altered peridotite xenoliths in the Siberian and other cratons. Some of the garnet peridotites are similar in composition to low-opx Udachnaya spinel harzburgites (previously interpreted as pristine melt extraction residues), but the majority show post-melting enrichments in Fe and Ti. The least metasomatized coarse peridotites were formed by 30–38 % of polybaric fractional melting between 7 and 4 GPa and ≤1–3 GPa. Our data together with experimental results suggest that garnet in these rocks, as well as in some other cratonic peridotites elsewhere, may be a residual mineral, which has survived partial melting together with olivine and opx. Many coarse and all deformed garnet peridotites from Udachnaya underwent modal metasomatism through interaction of the melting residues with Fe-, Al-, Si-, Ti-, REE-rich melts, which precipitated cpx, less commonly additional garnet. The xenoliths define a complex geotherm probably affected by thermal perturbations shortly before the intrusion of the host kimberlite magmas. The deformation in the lower lithosphere may be linked to metasomatism.  相似文献   

20.
Primary carbonates in peridotite xenoliths from the East African Rift in northeastern Tanzania occur as intergranular patches with accessory minerals (olivine and spinel), as patches with accessory magmatic minerals (nepheline), and as round monomineralic inclusions in primary olivine grains. All are characterized by calcitic compositions (Ca/Ca + Mg + Fe from 0.83 to 0.99), extremely low SiO2 + Al2O3 + Na2O + K2O, low trace element abundance [total rare-earth element (REE) abundance <25 ppm], uniform extinction, and lack of reaction textures with the host xenolith. Calculated Fe–Mg exchange coefficients between carbonate and primary olivine indicate disequilibrium in most samples. Combined with the lack of significant reaction textures, this suggests that the carbonates were introduced shortly before or during eruption of the host magma. A global compilation of electron microprobe analyses of mantle-derived carbonates (in xenoliths, xenocrysts, and megacrysts) reveals compositional clusters near end member calcite, end member magnesite, and stoichiometric dolomite. Eutectic liquid compositions are less common, suggesting that many carbonate inclusions reported worldwide may be crystalline precipitates. Likewise, the calcites in this study are not interpreted to represent quenched carbonatitic melts, but are interpreted instead to be crystalline cumulates from such melts. These inferences are consistent with recent experiments, which show that carbonatitic melts cannot become more calcitic than CaCO3∼80 wt%. Low trace element abundance may be a diagnostic feature of cumulate carbonate, and in combination with petrography and major element composition, serve to distinguish it from quenched carbonated liquid. Received: 30 July 1999 / Accepted: 5 February 2000  相似文献   

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