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1.
Mantle xenoliths from Paleogene basanites of East Serbia were studied using EMP and LA-ICP-MS techniques in order to better understand mantle characteristics in this region. Five different mantle lithologies have been distinguished: a dunite/harzburgite/lherzolite (D/HZ/L) group, clinopyroxene-rich lherzolites (Cpx-L), clinopyroxene megacrysts (Cpx-M), spinel-rich olivine websterites (OWB1) and spinel-poor olivine websterites (OWB2). D/HZ/L xenoliths are the most common and represent normal mantle composed of typical anhydrous spinel peridotites with well equilibrated, unzoned silicates characterized by high Mg# s. Negative correlations between Mg# and TiO2, Al2O3 and CaO wt% in clinopyroxenes (cpx) and orthopyroxenes (opx) and the Cr–Al trend in spinel (sp) suggest depletion via extraction of basaltic melts. The modal composition of D/HZ/L xenoliths and unusual low-Al opx suggest that the lithospheric mantle underneath East Serbia is more depleted than normal European lithosphere. D/HZ/L xenoliths contain numerous pockets and veins filled by Cr-rich cpx, Ti-rich spinel, altered glass, apatite and rare ilmenite and phlogopite. Petrographic observations, supported by major element contents in sp and cpx, and modelling using trace element contents in cpx, indicate that the pockets and veins formed from infiltration of alkaline melts and reaction with peridotite wall-rock causing opx and spinel replacement. The same alkaline melt-related metasomatism gave rise to the Cpx-L and OWB1 mantle xenoliths and Cpx-M xenocrysts. Trace element contents of cpx in these xenoliths show a distinctively concave downwards REE pattern with a HFSE depletion, very similar to cpx megacrysts from the Pannonian Basin and to vein cpx from Eifel. In contrast, the OWB2 xenoliths show evidence of precipitation from subduction-related mafic to ultramafic melts, as inferred from their opx-rich lithology and unusual Cr-rich spinels. They are probably related to subduction magmatism during the Late Cretaceous.Milivoje Jovanovi: deceased in April 2004  相似文献   

2.
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   

3.
We report Lithium (Li) concentrations and isotopic compositions for co-existing olivine, orthopyroxene (opx), and clinopyroxene (cpx) mineral separates from depleted and metasomatised peridotite xenoliths hosted by basaltic lavas from northwestern Ethiopian plateau (Gundeweyn area). The peridotites contain five lherzolites and one harzburgite and are variably depleted and enriched in LREE relative to HREE. In both depleted and enriched lherzolites, Li is preferentially incorporated into olivine (2.4-3.3 ppm) compared to opx (1.4-2.1 ppm) and cpx (1.4-2.0 ppm) whereas the Li contents of olivines (5.4 ppm) from an enriched harzburgiteare higher than those of lherzolites. Olivines from the samples show higher Li abundances than normal mantle olivines (1.6-1.9 ppm) indicating the occurrence of Li enrichments through melt-preroditite interaction. The average δ7 Li values range from +2.2 to +6.0‰ in olivine, from -0.1 to +2.0‰ in opx and from -4.4 to -0.9‰ in cpx from the lherzolites. The Li isotopic composition (3.5‰) of olivines from harzburgite fall within the range of olivine from lherzolites but the opxs show low in δ7Li (-2.0‰). Overall Li isotopic compositions of olivines from the peridotites fall within the range of normal mantle olivine, δ7Li values of ~+4±2‰ within uncertainty, reflecting metasomatism (enrichment) of the peridotites by isotopically heavy Li-rich asthenospheric melt. Li isotope zonation is also observed in most peridotite minerals. Majority of olivine grains display isotopically heavy cores and light rims and the reverse case is observed for some olivine grains. Orthopyroxene and clinopyroxene grains show irregular distribution in δ7Li. These features of Li isotopic compositions within and between grains in the samples reflect the effect of diffusion-driven isotopic fractionation during meltperidotite interaction and cooling processes.  相似文献   

4.
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   

5.
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   

6.
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   

7.
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   

8.
The anhydrous melting behaviour of two synthetic peridotite compositions has been studied experimentally at temperatures ranging from near the solidus to about 200° C above the solidus within the pressure range 0–15 kb. The peridotite compositions studied are equivalent to Hawaiian pyrolite and a more depleted spinel lherzolite (Tinaquillo peridotite) and in both cases the experimental studies used peridotite –40% olivine compositions. Equilibrium melting results in progressive elimination of phases with increasing temperature. Four main melting fields are recognized; from the solidus these are: olivine (ol)+orthopyroxene (opx)+clinopyroxene (cpx)+Al-rich phase (plagioclase at low pressure, spinel at moderate pressure, garnet at high pressure)+liquid (L); ol+opx+cpx+Cr-spinel+L; ol+opx+Cr-spinel +L: ol±Cr-spinel+L. Microprobe analyses of the residual phases show progressive changes to more refractory compositions with increasing proportion of coexisting melt i.e. increasing Mg/(Mg+Fe) and Cr/(Cr+Al) ratios, decreasing Al2O3, CaO in pyroxene.The degree of melting, established by modal analysis, increases rapidly immediately above the solidus (up to 10% melting occurs within 25°–30° C of the solidus), and then increases in roughly linear form with increasing temperature.Equilibrium melt compositions have been calculated by mass balance using the compositions and proportions of residual phases to overcome the problems of iron loss and quench modification of the glass. Compositions from the melting of pyrolite within the spinel peridotite field (i.e. 15 kb) range from alkali olivine basalt (<15% melting) through olivine tholeiite (20–30% melting) and picrite to komatiite (40–60% melting). Melting in the plagioclase peridotite field produces magnesian quartz tholeiite and olivine-poor tholeiite and, at higher degrees of melting (30–40%), basaltic or pyroxenitic komatiite. Melts from Tinaquillo lherzolite are more silica saturated than those from pyrolite for similar degrees of partial melting, and range from olivine tholeiite through tholeiitic picrite to komatiite for melting in the spinel peridotite field.The equilibrium melts are compared with inferred primary magma compositions and integrated with previous melting studies on basalts. The data obtained here and complementary basalt melting studies do not support models of formation of oceanic crust in which the parental magmas of common mid-ocean ridge basalts (MORB) are attributed to segregation from source peridotite at shallow depths ( 25 km) to leave residual harzburgite. Liquids segregating from peridotite at these depths are more silica-rich than common MORB.  相似文献   

9.
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   

10.
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   

11.
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   

12.
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   

13.
The Cenozoic volcanicity of eastern China is entirely basalticand occurred as relatively small eruptions widely dispersedin space and time, closely associated with graben basins andtheir regional bounding faults. Samples (157) from over 30 sitesin eastern China have been studied. They are predominantly alkalinebasalts, but vary in composition from olivine nephelinites andleucitites to quartz tholeiites. The majority are aphyric butsome contain olivine and clinopyroxene phenocrysts. Whole-rockanalyses (X-ray fluorescence) of all samples for the major and13 trace elements are used, as are the compositions of all themajor mineral phases determined by electron microprobe. It is argued that the most primitive basanites, alkali olivinebasalts, and olivine tholeiites represent primary or near-primarymagmas which were formed by different degrees of partial meltingof the upper mantle at different depths. The olivine tholeiitesrepresent larger degrees of partial melting (8–9%) ofa spinel peridotite at depths of <66 km. The alkalic basaltscarry xenoliths of spinel and garnet peridotite and appear tohave been derived by 1–7% partial melting of a garnetlherzolite (50% ol, 25% opx, 15% cpx, 10% garnet) at depths> 79 km. The olivine nephelinite may have formed by evensmaller degrees of partial melting. Most flows are not primary; the variations in their compositionsare consistent with fractional crystallization from the spectrumof primary parents created by varying degrees of partial meltingof a mineralogically heterogeneous source. The tholeiites havefractionated by the removal of clinopyroxene and some olivine;the alkali basalts by the removal of clinopyroxene with a smallerproportion of olivine. The incompatible behavior of Sr impliesthe absence of plagioclase from any of the fractionating assemblagesand, together with the high Al content of the pyroxene phenocrysts,suggests that much of the fractionation occurred at mantle depthsand pressures. The Cenozoic magmatism of eastern China is seen as a typicalexample of volcanism associated with continental extension.That is, small volumes of predominantly alkalic basalts andolivine tholeiites erupted over a prolonged period and associatedwith extensional basins and their bounding faults. As such,the province is distinct from continental flood basalt provinces.  相似文献   

14.
应用离子探针技术对山东蓬莱新生代玄武岩中橄榄岩包体的主要组成矿物进行了原位测定,结果显示了强烈偏离正 常地幔矿物组成和平衡的Li 含量和同位素比值。橄榄石、斜方辉石和单斜辉石的Li 含量和δ7Li 分别为（2.0~10.4）×10-6, （2.8~23.8）×10-6 和（3.3~59.7）×10-6;-28.6‰ ~+0.5‰,-12.7‰ ~+12.8‰和-41.5‰ ~+22.6‰。这种强烈的非平衡体系反 映了最近强烈和复杂的地幔交代作用。辉石的高Li（~ 60×10-6）和δ7Li（~ +20‰）反映了最近富Li地幔熔体/ 流体的特征, 而橄榄石的中等Li 富集（<5×10-6）和强烈的δ7Li 亏损（~ -20‰）则可能记录了更早一期低δ7Li 的地幔交代。同位素剖面 的扩散反演以及Li 强烈富集与LREE 相对亏损之间的解耦显示了两期地幔交代作用的短暂性和近期性。因此,我们认为华 北东部克拉通上地幔中存在与再循环有关的锂同位素异常组分,它们的形成应该与中生代以来太平洋板块向东亚大陆下的 俯冲有关. 这些组分能够在地幔中较长的保存直至最近被热扰动所释放,并被橄榄岩包体所记录。  相似文献   

15.
A total of 17 alkali basalts (alkali olivine basalt, limburgite, olivine nephelinite) and quartz tholeiites, and of 10 peridotite xenoliths (or their clinopyroxenes) were analyzed for Nd and Sr isotopes. 143Nd/144Nd ratios and 87Sr/86Sr ratios of all basalts and of the majority of ultramafic xenoliths plot below the mantle array with a large variation in Nd isotopes and a smaller variation in Sr isotopes. The tholeiites were less radiogenic in Nd than the alkali basalts. Volcanics from the Eifel and Massif Central regions contain Nd and Sr, which is more radiogenic than that of the basalts from the Hessian Depression. Nd and Sr isotopic compositions of all rocks from the latter area, with the exception of one tholeiite and one peridotite plot in the same field of isotope ratios as the Ronda ultramafic tectonite (SW Spain), which ranges in composition from garnet to plagioclase peridotite. The alkali basaltic rocks are products of smaller degrees of partial melting of depleted peridotite, which has undergone a larger metasomatic alteration compared with the source rock of tholeiitic magmas. For the peridotite xenoliths such metasomatic alteration is indicated by the correlation of their K contents and isotopic compositions. We assume that the upper mantle locally can acquire isotopic signatures low in radiogenic Nd and Sr from the introduction of delaminated crust. Such granulites low in radiogenic Nd and Sr are products of early REE fractionation and granite (Rb) separation.  相似文献   

16.
Lithium elemental and isotopic disequilibrium has frequently been observed in the continental and oceanic mantle xenoliths, but its origin remains controversial. Here, we present a combined elemental and Li isotopic study on variably metasomatised peridotite xenoliths entrained in the Cenozoic basalts from Shangzhi in Northeast (NE) China that provides insight into this issue. Li concentration (0.3–2.7 ppm) and δ7Li (mostly 2‰–6‰) in olivine from the Shangzhi peridotites are similar to the normal mantle values and show roughly negative correlations with the indices of melt extraction (such as modal olivine and whole rock MgO). These features are consistent with variable degrees of partial melting. In contrast, clinopyroxene from the Shangzhi xenoliths shows significant Li enrichment (0.9–6.1 ppm) and anomalously light δ7Li (??13.8‰ to 7.7‰) relative to normal mantle values. Such features can be explained by Li diffusion from silicate melts or Li-rich fluids occurring over a very short time (several minutes to several hours). Moreover, the light Li isotopic compositions preserved in some bulk samples also indicate that these percolated melts/fluids have not had enough time to isotopically equilibrate with the bulk peridotite. We thus emphasize that Li isotopic fractionation in the Shangzhi mantle xenoliths is mainly related to Li diffusion from silicate melts or Li-rich fluids that took place shortly before or coincident with their entrainment into the host magmas.  相似文献   

17.
Spinel lherzolite and wehrlite xenoliths from the Cenozoic Calatrava volcanic field carry the geochemical imprint of metasomatic agents that have affected the subcontinental lithospheric mantle beneath Central Iberia. Some xenoliths (mainly wehrlites) were enriched in REE, Sr, P, and CO2 by silicic-carbonate-rich metasomatic melts/fluids, while others record the effects of subduction-related hydrous silicate fluids that have precipitated amphibole and induced high Ti/Eu in primary clinopyroxene. The petrographic observations and geochemical data suggest that interstitial glass in the xenoliths represent the quenched products of Si-rich melts that infiltrated the mantle peridotite shortly before the entrainment of the xenoliths in the host magmas that erupted ca 2 million years ago. During their infiltration, the metasomatic melts reacted with peridotite, resulting in silica enrichment, while remobilizing grains of iron-rich monosulfide solid solution (Fe-rich Mss) initially enclosed in, or intergranular to, primary olivine and pyroxenes. In situ laser ablation inductively coupled plasma-mass spectrometry analysis of single sulfide grains reveals that the Fe-rich Mss in glass shows platinum-group element (PGE) patterns and 187Os/188Os compositions identical to the Fe-rich Mss occurring as inclusions in, or at grain boundaries of primary silicates. Moreover, independent of its microstructural position, Fe-rich Mss exhibits PGE and 187Os/188Os signatures typical of Mss either residual after partial melting or crystallized directly from sulfide melts. Our findings reveal that young metasomatic melt(s)/fluid(s) may carry remobilized sulfides with PGE and Os-isotopic signatures identical to those of texturally older sulfides in the peridotite xenolith. These sulfides thus still provide useful information about the timing and nature of older magmatic events in the subcontinental mantle.  相似文献   

18.
The Okete Volcanics form small volume monogenetic volcanoes situated around the flanks of larger tholeiitic cones of the Plio-Pleistocene Alexandra Volcanics, in the back-arc tectonic environment of western North Island, New Zealand. The lavas and tuffs of the Okete Volcanics have compositions which include basanites, alkali olivine basalts, olivine tholeiites, and hawaiites. Most rocks have Mg numbers >66, >250 p.p.m. Ni, >500 p.p.m. Cr, and often contain ultramafic xenoliths, which indicate that they are very close to being primary magmas. The Okete Volcanics show geochemical trends, from basanite to hawaiite, of progressive depletion of both compatible and incompatible trace elements, progressive increase in Al2O3, and heavy REE and Y enrichment with crossingover REE patterns in the hawaiites. These geochemical trends can be accounted for by varying degrees of partial melting of a light REE enriched garnet peridotite with subsequent modification of the melts near source or during ascent by fractional crystallization of olivine and minor clinopyroxene. Mass balance calculations cannot quantitatively constrain the degree of partial melting or fractional crystallization, but nevertheless indicate that the Okete alkali olivine basalts, olivine tholeiites, and hawaiites have been derived by successively larger degrees of partial melting relative to basanites, and have also been progressively more modified by fractional crystallization than have the basanites. Sources of the alkalic melts lay at depths corresponding to >20 kb, and most of the ultramafic xenoliths, apart from some which may be cognate cumulates, are unrelated to the magmas that brought them to the surface. Magmas have changed in composition with time from older smaller-volume volcanoes of basanite or alkali olivine basalt compositions, to younger and more voluminous volcanoes which contain hawaiites. The geochemical trends shown by the Okete Volcanics and their spatial association with voluminous tholeiitic volcanism, are features which are different from those observed elsewhere in the Pliocene to Recent basaltic fields of northern North Island, and may be related to their unique tectonic setting, situated in a distinct structural domain.  相似文献   

19.
We present a comprehensive mineral chemical dataset (～400 analyses) on subalkaline meimechitic (Mg-number?=?74–80) and ferropicritic (Mg-number?=?67–69) dike samples from the Antarctic extension of the Karoo large igneous province (LIP) in Vestfjella, western Dronning Maud Land. Some of the meimechites, previously considered to be cumulates from ferropicritic magmas, are characterized by forsteritic olivine (with core composition up to Fo92) that is in, or close to Fe-Mg equilibrium with the host rock. The olivines are subhedral to euhedral, contain Ti-rich (volcanic) spinel inclusions, have a high CaO content (≥0.19 wt. %), and are thus unlikely to represent xenocrysts from mantle peridotite. Igneous amphibole is found in olivine-hosted, crystallized melt inclusions, indicating that the parental magmas had a H2O content of 1–2 wt. %. The olivine data suggests generation of extremely MgO-rich (up to 25 wt. %) melts during the Karoo magmatism. Based on our petrogenetic modeling, such melts are likely to have originated from the partial melting of garnet peridotite at high pressures (5–6 GPa) and mantle potential temperatures (>1,600°C) that are compatible with the involvement of a mantle plume in the generation of the Karoo LIP. A geochemical comparison of the Vestfjella meimechites with meimechites from the Siberian Traps LIP and the assumed komatiitic parental melts of the Horingbai picrites (Paraná-Etendeka LIP) reveals key similarities, suggesting that all these suites were generated from broadly similar sources and/or by similar melting processes in anomalously hot subcontinental mantle.  相似文献   

20.
The Ronda high temperature peridotite: Geochemistry and petrogenesis   总被引：2，自引：0，他引：2  
The Ronda peridotite in southern Spain is a large (~300 km2) exposure of upper mantle which provides direct information about mantle processes on a scale much larger than that provided by mantle xenoliths in basalt. Ronda peridotites range from harzburgite to lherzolite, and vary considerably in major element content, e.g., Al2O3 from 0.9 to 4.8%, and trace element abundances, e.g., Sr, Zr and La abundances vary by factors of 20 to 40. These compositional variations are systematic and correlate with (pyroxene + garnet)/olivine ratios and olivine compositions. The data are consistent with formation of residual peridotites by variable degrees of melting (~0 to 30%) of a compositionally homogeneous peridotite. None of the peridotites have geochemical characteristics of residues formed by extensive (?5%) fractional melting and the data can be explained by equilibrium (batch) melting, possibly with incomplete melt segregation in some samples. Based on compositional differences between Ronda peridotites, the segregated melts were picritic (12–22% MgO) with relative rare earth element abundances similar to mid-ocean ridge basalt (MORB). Prior to the melting event the Ronda peridotite body was a suitable source for MORB. The compositional characteristics of Ronda peridotites are consistent with diapiric rise of a fertile mantle peridotite with relatively small degrees of melting near the diapir-wall rock interface yielding residues of garnet iherzolite, and larger degrees of melting in the diapir interior yielding residues of garnet-free peridotite. Subsequently these residual rocks were recrystallized at sub-solidus conditions (Obata, 1980), and emplaced in the crust by thrusting (Lundeen, 1978).  相似文献   

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