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1.
Three groups of ultramafix xenoliths were collected from alkali basalt in the island of Hierro, Canary Islands: (1) Cr-diopside series (spinel harzbugite, lherzolite, dunite); (2) Al-augite series xenoliths (spinel wherlite, olivine clinopyroxenite, dunite, olivine websterite); (3) gabbroic xenoliths. The main textures are granoblastic, porphyroclastic and granular, but poikilitic textures, and symplectitic intergrowths of clinopyroxene (cpx) + spinel (sp)±orthopyroxene (opx)±olivine (ol) (in rare cases cpx+opx), occur locally. Textural relations and large inter- and intra-sample mineral chemical variations testify to a complex history of evolution of the mantle source region, involving repeated heating, partial melting, and enrichment associated with infiltration by basaltic melts. The oldest assemblage in the ultramafic xenoliths (porphyroclasts of ol+opx±sp±cpx) represents depleted abyssal mantle formed within the stability field of spinel lherzolite. The neoblast assemblage [ol+cpx+ sp±opx±plagioclase (plag)±ilmenite (il)±phlogopite (phlog)] reflect enrichment in CaO+Al2O3+Na2O+ FeO±TiO2±K2O±H2O through crystal/liquid separation processes and metasomatism. The Al-augite-series xenoliths represent parts of the mantle where magma infiltration was much more extensive than in the source region of the Cr-diopside series rocks. Geothermometry indicates temperature fluctuations between about 900–1000 and 1200°C. Between each heating event the mantle appears to have readjusted to regional geothermal gradient passing 950°C at about 12 kbar. The gabbroic xenoliths represent low-pressure cumulates.  相似文献   

2.
The paper discusses the results of mineralogical and petrographic studies of spinel lherzolite xenoliths and clinopyroxene megacrysts in basalt from the Jixia region related to the central zone of Cenozoic basaltic magmatism of southeastern China. Spinel lherzolite is predominantly composed of olivine (Fo89.6–90.4), orthopyroxene (Mg# = 90.6–92.7), clinopyroxene (Mg# = 90.3–91.9), and chrome spinel (Cr# = 6.59–14.0). According to the geochemical characteristics, basalt of the Jixia region is similar to OIB with asthenospheric material as a source. The following equilibrium temperatures and pressures were obtained for spinel peridotite: 890–1269°C and 10.4–14.8 kbar. Mg# of olivine and Cr# of chrome spinel are close to the values in rocks of the enriched mantle. It is evident from analysis of the textural peculiarities of spinel lherzolite that basaltic melt interacted with mantle rocks at the xenolith capture stage. Based on an analysis of the P–T conditions of the formation of spinel peridotite and clinopyroxene megacrysts, we show that mantle xenoliths were captured in the course of basaltic magma intrusion at a significantly lower depth than the area of partial melting. However, capture of mantle xenoliths was preceded by low-degree partial melting at an earlier stage.  相似文献   

3.
Volcanic activity started about 20 Ma before present with quartz tholeiites (QTh), had a climax with alkali olivine basalts (AOB) 13 to 14 Ma ago and ended 7 Ma ago with nepheline basanites (NB) and olivine nephelinites (ON). AOB covers 73% of the volcanic area. About 250 basalts and peridotite xenoliths were sampled for investigation. An upper mantle layer ranging from about 90 to 60 km depth has been conditioned for a preferential alkali basalt production by advection of H2O-CO2-fluids containing Si, Al, Ca, K, Na, P as major constituents beside numerous incompatible minor elements. At the onset of the geodynamically triggered mantle conditioning locally restricted diapirism into shallow depth has caused formation of olivine tholeiite magmas (OTh) at about 1,300° C by partial melting. All of these OTh primary melts intruded due to a favourable compressibility into granulites of the lower crust. The rare QTh basalts are their derivative magmas which have been slightly contaminated in the crust. Magmas of the subsequent alkali basaltic volcanism (AOB, bAOB, NB, ON, MON) formed by in-situ partial melting at about 75 to 90 km depth after depression of the peridotite solidi by fluids to temperatures 1,200° C. Except many AOB these magmas are primary melts as characterized by olivine/melt distribution coefficients of Mg/Fe2+ (K D=0.29 to 0.34), by Ni concentrations (260 to 330 ppm) and the occurrence of peridotite xenoliths. Rapid rise of gas charged melts due to saturation in CO2 prevented separation of olivine etc. and of xenoliths. The sequence of magmas from OTh to ON (or MON) is formed from decreasing proportions of orthopyroxene (opx) and increasing contributions of clinopyroxene (cpx) and phlogopite (ph) at almost equal proportions of spinel (sp). Incongruent melting of opx (and cpx) for OTh, AOB, NB and ON is correlated with precipitation of olivine. The average xenolith composition (73% ol, 18% opx, 7% cpx, 1.1% sp and 1.3/0.5% ph) was used to model the sources of the investigated melts by 9 incompatible elements and to calculate degrees of partial melting. The occurrence of garnet cannot be reliably excluded by modelling on the basis of HREE distribution coefficients. The average xenolith composition was used for modelling because of its resemblance with worldwide sampled depleted mantle inclusions. For avoiding to exhaust at least one mineral of the model mantle in the support of the norm composition of OTh, AOB, NB and MON magmas the degrees of partial melting cannot exceed 12.5%, 6%, 6% and 4% respectively. Mantle containing about 500 ppm K (and the correlated incompatible elements), like the average of 36 xenoliths, allows to explain the formation of OTh magmas. AOB, NB and ON melts require peridotite with slightly less than 1,500 ppm K, 670 ppm P and proportions of the correlated elements LREE, Sr, Ba, Zr, Rb, Cs, Ta, Th, Hf, U, which are higher than their abundance in primitive mantle rocks. About 20% of the xenoliths have this composition. Metasomatism of fluids with these elements must have been an immediate precursor of the alkali basaltic volcanism. Otherwise the preservation of a local disequilibrium in 87Sr/86Sr ratios between cpx cores and total rock at upper mantle temperatures cannot be explained.  相似文献   

4.
Mantle xenoliths hosted in Miocene-Quaternary mafic alkaline volcanic rocks from Sardinia have been investigated with electron microprobe, laser ablation microprobe-inductively coupled plasma-mass spectrometry and thermal ionization mass spectrometry techniques. The xenoliths are anhydrous clinopyroxene-poor lherzolites and harzburgites, plus very rare websterites and olivine-websterites. Glassy pods having thin subhedral to euhedral microlites of olivine, clinopyroxene and spinel have been found in harzburgites and websterites. Clinopyroxene shows trace element variability, with values of (La/Yb)N ranging from sub-chondritic (0.01) to supra-chondritic (8.6). The Sr–Nd isotopic ratios of the clinopyroxenes fall mostly in the field of the European lithospheric mantle xenoliths (87Sr/86Sr from 0.70385 to 0.70568 and 143Nd/144Nd ranging from 0.512557 to 0.512953). The geochemical characteristics of the Sardinian xenoliths testify to the variable degrees of earlier partial melt extraction, followed by metasomatic modification by alkaline melts or fluids. Websterites are considered to represent small lenses or veins of cumulitic (i.e. magmatic) origin within the mantle peridotite.  相似文献   

5.
Nominally anhydrous phases (clinopyroxene (cpx), orthopyroxene (opx), and olivine (ol)) of peridotite xenoliths hosted by the Cenozoic basalts from Beishan (Hebei province), and Fansi (Shanxi province), Western part of the North China Craton (WNCC) have been investigated by Fourier transform infrared spectrometry (FTIR). The H2O contents (wt.) of cpx, opx and ol are 30–255 ppm, 14–95 ppm and ~ 0 ppm, respectively. Although potential H-loss during xenolith ascent cannot be excluded for olivine, pyroxenes (cpx and opx) largely preserve the H2O content of their mantle source inferred from (1) the homogenous H2O content within single pyroxene grains, and (2) equilibrium H2O partitioning between cpx and opx. Based on mineral modes and assuming a partition coefficient of 10 for H2O between cpx and ol, the recalculated whole-rock H2O contents range from 6 to 42 ppm. In combination with previously reported data for other two localities (Hannuoba and Yangyuan from Hebei province), the H2O contents of cpx, opx and whole-rock of peridotite xenoliths (43 samples) hosted by the WNCC Cenozoic basalts range from 30 to 654 ppm, 14 to 225 ppm, and 6 to 262 ppm respectively. The H2O contents of the Cenozoic lithospheric mantle represented by peridotite xenoliths fall in a similar range for both WNCC and the eastern part of the NCC (Xia et al., 2010, Journal of Geophysical Research). Clearly, the Cenozoic lithospheric mantle of the NCC is dominated by much lower water content compared to the MORB source (50–250 ppm). The low H2O content is not caused by oxidation of the mantle domain, and likely results from mantle reheating, possibly due to an upwelling asthenospheric flow during the late Mesozoic–early Cenozoic lithospheric thinning of the NCC. If so, the present NCC lithospheric mantle mostly represents relict ancient lithospheric mantle. Some newly accreted and cooled asthenospheric mantle may exist in localities close to deep fault.  相似文献   

6.
以中国东部宽甸、汉诺坝和明溪含有幔源包体的新生代玄武岩中的单斜辉石斑（巨）晶为研究对象,采用最新的单斜辉石－熔体平衡温压计对单斜辉石斑（巨）晶－熔体进行了平衡温压计算。结果表明,碱性玄武岩中的单斜辉石斑晶结晶温度和压力高于共生的亚碱性玄武岩中的单斜辉石斑晶,单斜辉石巨晶的结晶温度和压力高于单斜辉石斑晶。这说明碱性玄武岩的形成深度大于亚碱性玄武岩,单斜辉石巨晶是更高压力下的结晶产物,单斜辉石斑晶在岩浆上升的不同深度均有晶出。回归分析表明,尽管携带幔源包体的玄武岩浆上升速度较快,但并不是绝热上升。单斜辉石斑（巨）晶的结晶温压条件与同一地点幔源包体平衡温压条件的对比表明,单斜辉石巨晶和碱性玄武岩中的部分单斜辉石斑（巨）晶的结晶温压大于幔源包体的平衡温压,表明了包体寄主岩浆的来源深度大于包体的深度。因此,幔源包体是寄主岩浆上升途中捕虏的上地幔碎块,而非寄主岩浆形成源区的残留体。  相似文献   

7.
Lavas of the Biu and Jos Plateaus, Northern Cameroon Volcanic Line (CVL), contain abundant genetically related megacrysts of clinopyroxene, garnet and subordinately plagioclase, ilmenite and amphibole. P, T-estimates of crystallization for the primitive group of cpx and gnt megacrysts are 1.7–2.3 GPa and ~1,400 °C. Because crustal thickness in these areas is only ~30 km (~0.9 GPa), megacrysts must have formed within the lithospheric mantle. Primitive Biu and Jos lavas are isotopically heterogeneous in Sr-Nd isotope space (87Sr/86Sr=0.70285–0.70360 and Nd=7.5–4.6). Biu Plateau megacrysts overlap the range of Biu lavas in Sr-Nd isotope composition, indicating that crustal contamination of Biu lavas was minor. Jos Plateau lavas are isotopically more enriched than their associated megacrysts. Therefore an additional contamination of Jos lavas due to assimilation of continental crust (~5%) or enriched shallow lithospheric mantle is indicated. Lavas of Biu and Jos Plateau do not reflect simple fractionation or equilibrium crystallization products, but instead reflect mixing between primary melts and their fractionated derivatives.Editorial Responsibility: I. Carmichael  相似文献   

8.
1. Introduction The depth of source of Cenozoic basalts and the genesis of mantle xenoliths in eastern China have been widely discussed by many geoscientists. There were basically two main opinions: one was that the relationship of basalts and the peridotite xenoliths are melt and residues (Cong et al, 1982; Liu et al, 1985; Qiu et al, 1986; E et al, 1987; Chi et al, 1988; Deng et al, 1988), and pyroxenite and gabrro enclaves are accumulation of fractional crystallization (Qiu et al, 1986;…  相似文献   

9.
 Lherzolite xenoliths in Miocene to Pleistocene basalts from five sites in the Hamar-Daban range in southern Siberia provide sampling of the mantle close to the axis of the Baikal rift. These anhydrous spinel lherzolites commonly have foliated fabrics and spongy rims around clinopyroxene, and many contain accessory feldspar. The feldspar occurs in reaction zones adjacent to spinel and orthopyroxene (where it appears to have been formed by the reaction: spl+opx+cpx+fluid →fs+ol) and less commonly as thin, irregular veins. The feldspars have variable compositions but are generally alkali-rich; their K2O content ranges from 0.3 to 11.2% and is much higher than in plagioclase from orogenic lherzolites (usually <0.1% K2O). The temperature range for the Hamar-Daban xenolith suite (950–1010° C) is more restricted than for spinel peridotite xenoliths from other occurrences in the Baikal area. The feldspar-bearing lherzolites yield equilibration temperatures similar to or slightly lower than feldspar-free ones. The majority of the Hamar-Daban lherzolites are fertile and clinopyroxene-rich, as for most other occurrences in the Baikal region. Trace element compositions of selected xenoliths and their clinopyroxenes were determined by ICP-MS, INAA and proton microprobe. Feldspar-bearing xenoliths are enriched in alkalies indicating that feldspar formation is associated with addition of material and is not simply due to isochemical phase changes. Most xenoliths and their clinopyroxenes studied are depleted in light REE and have contents of Sr, Zr and Y common for fertile or moderately depleted mantle peridotites. Few are moderately enriched in LREE, Sr, Th and U. Sr-Nd isotope compositions of clinopyroxenes indicate long-term depletion in incompatible elements similar to unmetasomatised xenoliths from other occurrences south and east of Lake Baikal. The formation of feldspar and of spongy aggregates after clinopyroxene, and the enrichment in alkalies appear to be recent phenomena related to infiltration of an alkali-rich, H2O-poor fluid into spinel peridotites. Received: 20 March 1995 / Accepted: 26 June 1995  相似文献   

10.
Ion probe investigations on mineral phases forming the Al-Di pyroxenites from the Zabargad peridotite body indicate that porphyroclastic pyroxenes in composite mafic layers record an unusual HREE, Zr, Sc enrichment not registered by pyroxenes in spinel websterites. Orthopyroxene in the opx+sp clusters forming the inner, cpx-free zone of layered pyroxenites shows strongly fractionated REE patterns (HREEN/LREEN>1000; Yb>100xch) and very high Zr, Sc and Y abundances (up to 30,672 and 60ppm, respectively). In the outer, cpx-rich zone porphyroclastic clinopyroxene is strongly HREE enriched (HREEN/LREEN29; Yb 269xch) and displays very high Sc and Zr abundances (up to 819 and 164 ppm, respectively). It is suggested that the unusual trace element abundances are inherited from a precursor garnet. Composite pyroxenite layers are interpreted as former garnet clinopyroxenites characterized by gnt/cpx modal zoning. The sp+opx(cpx-free) assemblage in the inner part is a product of the break-down reaction of garnet upon decompression, with Ca of the original garnet completely entering the enstatite solid solution. The temperature at which the breakdown reaction occurred is estimated to be higher than 1000°C (P in the range 20–30 kbar). In the outer part, decompression caused the garnet to form a sp+opx assemblage; however, the grossularite component participated in the formation of new clinopyroxene which reacted with the clinopyroxene present in the original mode before the decompression reaction, thus forming a cpx2+sp+opx assemblage. As a result of garnet breakdown, pyroxenes have peculiar HFSE anomalies. Progressive upwelling during the Red Sea rifting produced incomplete reaction under pl-facies conditions. The geochemical signatures of precursor garnet in pyroxenes were partially crased during the recrystallization from granular spincl-bearing to granoblastic plagioclase-bearing assemblages, being preserved only in a few porphyroclast relies. The finding of pyroxenes with trace element characteristics of precursor garnet has important geodynamic and geochemical implications. Al-Di pyroxenite layers had a long history within the mantle, before the continental lithosphere rifting and thinning took place in the region. It is suggested that Al-Di pyroxenites were formed by deep-seated tholeiitic magmatism unrelated to the Red Sea evolution, thus representing the earliest event in the Zabargad upper mantle. Garnet breakdown significantly preceded the metasomatism induced by hydrous fluids (crystallization of Ti-rich pargasite) and the later intrusion of hydrous (Cr-Di) pyroxenite dykes. During the stages of mantle evolution, the HFSE anomalies in pyroxenes varied significantly. We note that the study of HFSE anomalies in mineral phases reveals complex geochemical histories which are not recorded by the whole-rock system.  相似文献   

11.
《Geochimica et cosmochimica acta》1999,63(23-24):3967-3982
The partitioning of transition elements (Sc, Ti, V, Cr, Mn, Co, Ni) between orthopyroxene (opx) and clinopyroxene (cpx) in carefully selected garnet peridotite, spinel peridotite and garnet websterite xenoliths was determined by electron probe microanalyses (EPMA) and secondary ion mass spectrometry (SIMS). Xenoliths studied cover a wide compositional range and equilibrated under variable upper mantle conditions at temperatures between about 760 and 1370°C (two-pyroxene thermometer based on the enstatite-diopside solvus) and pressures between about 0.8 and 3.6 GPa (Al-in-opx and Ca-in-olivine barometers). We found that the partitioning of transition elements between opx and cpx (expressed as DM = concentration of element M in opx [cations per formula unit]/concentration of M in cpx [cations per formula unit]) is mainly controlled by temperature and to a much lesser degree by pressure. Variations in major element compositions of pyroxenes (e.g., variable XMg, AlIV or Na) have no influence on DM. For Sc, V, Cr, Mn, and Co, our data result in good correlations between ln DM and reciprocal absolute temperature, with correlation coefficients (r) between 0.950 and 0.981. It is therefore possible to use the partitioning of these elements between opx and cpx from peridotites and websterites as geothermometers. On the basis of our data, we suggest the following empirical thermometer equations: TSc = [(17.64 · P + 5663)/(3.25 − ln DSc)], TV = [(18.06 · P + 3975)/(2.27 − ln DV)], TCr = [(11.00 · P + 2829)/(1.56 − ln DCr)], TMn = [(−0.20 · P − 2229)/(−1.37 − ln DMn)], TCo = [(−4.31 · P − 2358)/(−0.98 − ln DCo)], where T is the absolute temperature in Kelvin and P the pressure in kilobars. For Ti and Ni observed correlations between ln DM and 1/T are less well defined.  相似文献   

12.
Geothermometric equations for spinel peridotites by Fujii (1976), Gasparik and Newton (1984), and Chatterjee, and Terhart (1985) based on the reaction enstatite (en)+spinel (sp)Mg–Tschermaks (mats)+forsterite (fo) were tested using a nearly isothermal suite of mantle xenoliths from the Eifel, West Germany. In spite of using activities of MgAl2O4, en, and mats to allow for the non-ideal solution behaviour of the constituent phases, temperatures calculated from these equations systematically change as a function of Cr/(Cr+AL+Fe3+) in spinel. We propose an improved version of the empirical geothermometer for spinel peridotites of Sachtleben and Seck (1981) derived from the evaluation of the solubilities of Ca and Al in orthopyroxene from more than 100 spinel peridotites from the Rhenish Volcanic Province. A least squares regression yielded a smooth correlation between
  相似文献   

13.
Electron microprobe analyses sensitive to 20ppmw (2σ) were made for Na, P, K and Ti in garnet, pyroxenes and olivine from peridotite and eclogite xenoliths from African kimberlites and volcanic rocks in Tanzania. Average concentrations (ppmw) in peridotite (mostly garnet lherzolite) are: Na2O gt 340 ol 90 opx 1070 cpx 2.1 (wt.%); P2O5 gt 460 ol 130 opx 50 cpx 350; K2O gt <20 ol <20 opx 30 cpx 170; TiO2 gt 1470 ol 130 opx 480 cpx 1630. For eclogites and a cpx megacryst with gt inclusions: Na2O gt 610 cpx 4.3 (wt.%); P2O5 gt 530 cpx 300; K2O gt <20 cpx 370; TiO2 gt 1990 cpx 1980.In garnet, Na can be explained by coupled substitution with P and Ti, and there is no need to invoke six-coordinated silicon. The Na distribution between garnet and clinopyroxene correlates with the Fe/Mg distribution for both eclogites and peridotites, and for the peridotites correlates with estimates of pressure and temperature from pyroxene composition. When calibrated experimentally, the Na distribution may be a useful indicator of physical conditions at depths for which the Fe/Mg distribution is insensitive; furthermore the Na distribution may be less sensitive to oxidation state.  相似文献   

14.
Li concentrations and isotopic compositions of coexisting minerals (ol, opx, and cpx) from peridotite xenoliths entrained in the Hannuoba Tertiary basalts, North China Craton, provide insight into Li isotopic fractionation between mantle minerals during melt-rock interaction in the considerably thinned lithospheric mantle. Bulk analyses of mineral separates show significant enrichment of Li in cpx (2.4-3.6 ppm) relative to olivine (1.2-1.8 ppm), indicating that these peridotites have been affected by mantle metasomatism with mafic silicate melts. Bulk olivine separates (δ7Li ∼ +3.3‰ to +6.4‰) are isotopically heavier than coexisting pyroxenes (δ7Li ∼ −3.3‰ to −8.2‰ in cpx, and −4.0‰ to −6.7‰ in opx). Such large variation suggests Li elemental and isotopic disequilibrium. This conclusion is supported by results from in situ SIMS analyses of mineral grains where significant Li elemental and isotopic zonations exist. The olivine and opx have lower Li concentrations and heavier Li isotopes in the rims than in the cores. This reverse correlation of δ7Li with Li concentrations indicates diffusive fractionation of Li isotopes. However, the zoning patterns in coexisting cpx show isotopically heavier rims with higher Li abundances. This positive correlation between δ7Li and Li concentrations suggests a melt mixing trend. We attribute Li concentration and isotope zonation in minerals to the effects of two-stage diffusive fractionation coupled with melt-rock interaction. The earliest melts may have been derived from the subducted oceanic slab with low δ7Li values produced by isotopic fractionation during the dehydration of the seawater-altered slab. Melts at later stages were derived from the asthenosphere and interacted with the peridotites, producing the Li elemental and isotopic zoning in mineral grains. These data thus provide evidence for multiple-stage peridotite-melt interaction in the lithospheric mantle beneath the northern North China Craton.  相似文献   

15.
Spinel lherzolite and pyroxenite xenoliths from the Rio Puerco Volcanic Field, New Mexico, were analyzed for oxygen isotope ratios by laser fluorination. In lherzolites, olivine δ18O values are high (+5.5‰), whereas δ18O values for pyroxenes are low (cpx=+5.1‰; opx=+5.4‰) compared to average mantle values. Pyroxenite δ18O values (cpx=+5.0‰; opx=+5.3‰) are similar to those of the lherzolites and are also lower than typical mantle oxygen isotope compositions. Texturally and chemically primary calcite in pyroxenite xenoliths is far from isotopic equilibrium with other phases, with δ18O values of +21‰. The isotopic characteristics of the pyroxenite xenoliths are consistent with a petrogenetic origin from mixing of lherzolitic mantle with slab-derived silicate and carbonatite melts. The anomalously low δ18O in the pyroxenes reflects metasomatism by a silicate melt from subducted altered oceanic crust, and high δ18O calcite is interpreted to have crystallized from a high δ18O carbonatitic melt derived from subducted ophicarbonate. Similar isotopic signatures of metasomatism are seen throughout the Rio Puerco xenolith suite and at Kilbourne Hole in the southern Rio Grande rift. The discrete metasomatic components likely originated from the subducted Farallon slab but were not mobilized until heating associated with Rio Grande rifting occurred. Oxygen diffusion modeling requires that metasomatism leading to the isotopic disequilibrium between calcite and pyroxene in the pyroxenites occurred immediately prior to entrainment. Melt infiltration into spinel-facies mantle (xenoliths) prior to eruption was thus likely connected to garnet-facies melting that resulted in eruption of the host alkali basalt.  相似文献   

16.
Li behaviour and distribution in the mantle were investigated by ion microprobe in situ measurements on co-existing olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and amphibole (amp) in xenoliths from the French Massif Central. The fertile spinel lherzolites of this study record increasing degrees of mantle metasomatism, from unmetasomatised anhydrous samples through cryptically metasomatised samples to highly metasomatised amphibole-rich samples. In anhydrous lherzolites, Li is preferentially incorporated into olivine (1.1-1.4 ppm, average values) compared to pyroxenes (0.2-0.9 ppm). The hydrous samples clearly show enrichment of Li in ol (1.5-5.0 ppm), opx (1.1-2.4 ppm) and cpx (2.4-5.4 ppm), while amphibole incorporates less Li than the co-existing phases (0.8-1.3 ppm). Average δ7Li values range from +7.6 to +14.5‰ in ol, from 5.1 to +13.7‰ in opx and from 8.8 to +10.3‰ in cpx from the anhydrous lherzolites. A layered peridotite sample (Sdi) shows higher Li content in all phases, with lighter isotopic composition in opx and cpx (−0.6 and −2‰ average δ7Li values, respectively). In the hydrous lherzolites average δ7Li values both overlap and extend beyond these ranges in ol (up to 17.5 ‰) and in opx (up to 22.9‰), and vary widely in cpx (−2.7 to +9.7‰). Low δ7Li values are observed in some opx (−10.4‰) and cpx (−13‰) from sample Sdi, and in cpx from three hydrous samples (from −9.7 to −5.3‰). The different anhydrous phases from the hydrous samples show large intra-grain variations in Li isotopic ratios (e.g., up to 18‰) compared to the same phases from the anhydrous samples (mostly less than 6‰), excepting sample Sdi which has up to 20.4‰ variation in cpx. Similar to the anhydrous silicates, amphiboles show a wide variation of δ7Li values on the intra-grain scale (2-27‰). These variations are interpreted to result from fractionation processes during metasomatism by a silicate melt undergoing compositional changes as it percolates through and reacts with the peridotite phases. Thus Li abundances and isotopic in situ measurements are useful for tracing metasomatic processes but the heterogeneities observed in the samples preclude any identification of a specific mantle source by its Li signature.  相似文献   

17.
An alkali basalt near Glen Innes, northeastern New South Wales, contains a suite of Cr-diopside group ultramafic xenoliths which includes some spinel peridotites but which is dominated by a diverse spinel pyroxenite assemblage. Pyroxenite xenoliths range from subcalcic clinopyroxenites (composed largely of unmixed prismatic subcalcic clinopyroxene megacrystals and lesser orthopyroxene megacrystals) to equant mosaic textured websterites (orthopyroxene and Ca-rich clinopyroxene ± spinel). Rare orthopyroxenite xenoliths also occur. The pyroxenite xenoliths are characterised by high 100Mg/(Mg + Fe2+) ratios (M˜ 90) and low concentrations of Ti, K, P, La, Ce and Zr. The websterites are mineralogically and chemically similar to many spinel pyroxenites occurring as layers or dykes in peridotite massifs such as those at Ronda in southern Spain and at Ariège (French Pyrénées). T / P estimates indicate crystallization temperatures of 1250–1350 °C for subcalcic clinopyroxene-orthopyroxene megacrystal pairs and 900–1000 °C for the equilibrated mosaic textured websterites and associated peridotites at pressures of 9–13 kbar. Subcalcic clinopyroxene megacrystals, websterites and orthopyroxenites have LREE-depleted chondrite-normalised REE abundances with (La/Yb)CN < 1 and their convex-upwards REE patterns are typical of subcalcic clinopyroxene-dominated cumulates. The pyroxenites are not residua from partially melted pyroxenite layers or dykes in mantle peridotites nor are they completely crystallized protobasaltic or protopicritic magmas. They are interpreted as high-pressure crystal segregations from basaltic magmas (probably mildly alkaline or transitional) flowing within narrow mantle conduits (the flow crystallization model of Irving, 1980). The parental magma(s) was Ti-poor (0.6–0.7% TiO2) and relatively Mg-rich (M˜ 74 − 70). Pyroxenite genesis was a two-stage process involving crystallization of tschermakitic subcalcic clinopyroxenes and orthopyroxenes  ±spinel as liquidus or near-liquidus phases at 1250–1350 °C and 9–13 kbar to yield “primary” subcalcic clinopyroxenites which then re-equilibrated at 900–1000 °C and similar pressures to produce the mosaic textured “secondary” websterites. The pyroxenites show a wide range of 143Nd/144Nd and 87Sr/86Sr values (0.513298–0.512473 and 0.702689–0.704659, respectively). Their isotopic ratios appear to have been variably modified by exchange with adjacent mantle peridotites or migrating basaltic melts. Received: 11 December 1995 / Accepted: 3 December 1996  相似文献   

18.

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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19.
High temperature (1150–1250 °C), coarse-grained olivine-bearing clinopyroxenites occur in the ash-tuffs of the Dreiser Weiher maar-type volcano (West Eifel, Germany) as discrete xenoliths or as 1-5-cm-broad veins crosscutting anhydrous spinel peridotite host xenoliths. The clinopyroxenes (cpx) of these xenoliths have been analysed for trace element and Nd-Sr isotope compositions in order to document intra-suite variations and to constrain the processes involved in the formation of heterogeneities within a relatively well defined upper mantle section beneath the West Eifel. The patterns formed by cpx from the pyroxenites on multi-element diagrams are subparallel and convex-upward, showing troughs for high-field-strength elements (Nb, Zr, Hf, Ti) and Sr. Trace element modelling indicates that these pyroxenites represent high pressure precipitates of magmas that are more primitive or similar in compositions to the most undifferentiated Cenozoic alkali basaltic lavas from the West Eifel. The cpx cover the whole spectrum of Nd-Sr isotope compositions shown by the primitive lavas from the entire West Eifel volcanic field suggesting isotopic heterogeneity on the scale of an individual volcanic centre. Due to incomplete re-equilibration between the vein melts and the peridotitic wall rocks, cpx of the host peridotites of the composite xenoliths (that belong to the 1b-group of Stosch and Seck, 1980) have in some cases retained relics of a pre-vein host composition. The relic cpx range from LREE-depleted to LREE-enriched with isotope signatures indicating a time-integrated higher enrichment (lower 143Nd/144Nd and higher 87Sr/86Sr) than the cpx of the corresponding veins. The trace element and isotope compositions of the xenoliths support the perception that magmas generated from sub-lithospheric mantle sources beneath the West Eifel formed a system of narrow dike networks and differentiated during their ascent through the lithosphere (Duda and Schmincke 1985). The data provide evidence that: (1) melts parental to the Dreiser Weiher pyroxenites are genetically related to the young alkali basaltic volcanics; (2) these melts can be derived from distinct domains of the mantle beneath Dreiser Weiher ranging in Sr-Nd isotope signatures from HIMU-like to Bulk-Silicate-Earth values; (3) the enrichment process associated with the upwards migration of these magmas was spatially limited to a cm-scale in the case of the studied composite xenoliths; (4) parts of the Dreiser Weiher lithosphere have experienced an enrichment prior to the vein interaction by a metasomatic agent that is isotopically unrelated to the primitive West Eifel lavas. Received: 25 August 1997 / Accepted: 25 November 1997  相似文献   

20.
Twenty spinel peridotite xenoliths from Pliocene alkali basaltic tuffs and lavas of the western Pannonian Basin (Hungary) have been analysed for bulk rock major and trace elements, electron probe mineral compositions, and REE and Sr, Nd isotopes on separated and leached clinopyroxenes. The xenoliths are texturally diverse, including protogranular, porphyroclastic, equigranular and poikilitic textures which can generally be correlated with geochemical features. Protogranular xenoliths are relatively undepleted in Ca, Al, Ti and Na, whereas poikilitic xenoliths are more refractory. LREE-depleted patterns. and MORB-like Nd and Sr values are associated with protogranular peridotites. In contrast, xenoliths with complex textures are generally LREE-enriched. Much of the isotopic variation in the suite (Sr=–20.4 to +10.4, +Nd=+1.8 to +13.7) can be related to interaction between protogranular mantle and melts resembling the host alkali basalts, but a third (high Sr) component may be due to Miocene subduction beneath the region.  相似文献   

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