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1.
 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   

2.
Laser ablation microprobe data are presented for olivine, orthopyroxeneand clinopyroxene in spinel harzburgite and lherzolite xenolithsfrom La Palma, Hierro, and Lanzarote, and new whole-rock trace-elementdata for xenoliths from Hierro and Lanzarote. The xenolithsshow evidence of strong major, trace element and Sr isotopedepletion (87Sr/86Sr 0·7027 in clinopyroxene in themost refractory harzburgites) overprinted by metasomatism. Thelow Sr isotope ratios are not compatible with the former suggestionof a mantle plume in the area during opening of the AtlanticOcean. Estimates suggest that the composition of the originaloceanic lithospheric mantle beneath the Canary Islands correspondsto the residues after 25–30% fractional melting of primordialmantle material; it is thus significantly more refractory than‘normal’ mid-ocean ridge basalt (MORB) mantle. Thetrace element compositions and Sr isotopic ratios of the mineralsleast affected by metasomatization indicate that the upper mantlebeneath the Canary Islands originally formed as highly refractoryoceanic lithosphere during the opening of the Atlantic Oceanin the area. During the Canarian intraplate event the uppermantle was metasomatized; the metasomatic processes includecryptic metasomatism, resetting of the Sr–Nd isotopicratios to values within the range of Canary Islands basalts,formation of minor amounts of phlogopite, and melt–wall-rockreactions. The upper mantle beneath Tenerife and La Palma isstrongly metasomatized by carbonatitic or carbonaceous meltshighly enriched in light rare earth elements (REE) relativeto heavy REE, and depleted in Zr–Hf and Ti relative toREE. In the lithospheric mantle beneath Hierro and Lanzarote,metasomatism has been relatively weak, and appears to be causedby high-Si melts producing concave-upwards trace element patternsin clinopyroxene with weak negative Zr and Ti anomalies. Ti–Al–Fe-richharzburgites/lherzolites, dunites, wehrlites and clinopyroxenitesformed from mildly alkaline basaltic melts (similar to thosethat dominate the exposed parts of the islands), and appearto be mainly restricted to magma conduits; the alkali basaltmelts have caused only local metasomatism in the mantle wall-rocksof such conduits. The various metasomatic fluids formed as theresults of immiscible separations, melt–wall-rock reactionsand chromatographic fractionation either from a CO2-rich basalticprimary melt, or, alternatively, from a basaltic and a siliceouscarbonatite or carbonaceous silicate melt. KEY WORDS: mantle xenoliths; mantle minerals; trace elements; depletion; carbonatite metasomatism  相似文献   

3.
Carbonate xenoliths containing olivine and rimmed by kaersutitic amphibole were collected in basaltic rocks of the Basal Complex of La Palma. The mineralogical composition and microscopic appearance may suggest a relationship with carbonatites in general, thus a major element, trace element and stable isotope study was conducted to investigate the origin of the carbonate formation. Based on electron microprobe analyses, the carbonate is calcite with up to 6.3 wt% MgO and 7.2 wt% SiO2. The elevated SiO2 content may suggest a melt origin for the carbonate. However, the C and O isotope compositions of the carbonate xenoliths (δ13C and δ18O around −1‰ and 13‰, respectively) are similar to those of calcite veins and amygdales in basaltic rocks of the Basal Complexes of La Palma and Fuerteventura and are interpreted as produced by fluid degassing and metasomatism by CO2-H2O fluid derived from mobilization of sedimentary material. Trace element contents determined by laser-ablation ICP-MS analyses support the assumed origin, thus, the relationship with carbonatitic melts can be excluded. Based on trace element compositions, the amphibole surrounding the xenoliths is not related genetically to the carbonate. The elevated SiO2 content of the calcite can be attributed to submicron relics of pyroxene, thus, the use of this feature as an evidence for melt origin is questionable.  相似文献   

4.
Gabbro xenoliths reported in this paper were collected in northern Fuerteventura, the Canary Island located closest to the coast of Africa. The xenoliths are very fresh and consist of Ti–Al-poor clinopyroxene + plagioclase (An87–67) + olivine (Fo72–86) ± orthopyroxene. Clinopyroxene and orthopyroxene are constantly and markedly depleted in light rare earth elements (LREE) relative to heavy REE (HREE), as expected for cumulus minerals formed from highly refractory N-MORB-type melts. In contrast, whole-rock Primordial Mantle-normalized trace element patterns range from mildly S-shaped (mildly depleted in Pr–Sm relative to both the strongly incompatible elements Rb–La and the HREE) to enriched. Estimates show that the trace element compositions of the rocks and their minerals are compatible with formation as N-MORB gabbro cumulates, which have been infiltrated at various extents (≤1% to >5%) by enriched alkali basaltic melts. The enriched material is mainly concentrated along grain boundaries and cracks through mineral grains, suggesting that the infiltration is relatively recent, and is thus associated with the Canary Islands magmatism. Our data contradict the hypothesis that a mantle plume was present in this area during the opening of the Atlantic Ocean. No evidence of continental material that might reflect attenuated continental crust in the area has been found. Gabbro xenoliths with REE and trace element compositions similar to those exhibited by the Fuerteventura gabbros are also found among gabbro xenoliths from the islands of La Palma (western Canary Islands) and Lanzarote. The compositions of the most depleted samples from these islands are closely similar, implying that there was no significant change in chemistry during the early stages of formation of the Atlantic oceanic crust in this area. Strongly depleted gabbros similar to those collected in Fuerteventura have also been retrieved in the MARK area along the central Mid-Atlantic Ridge. The presence of N-MORB oceanic crust beneath Fuerteventura implies that the continent–ocean transition in the Canary Islands area must be relatively sharp, in contrast to the situation both further north along the coast of Morocco, and along the Iberian peninsula.  相似文献   

5.
 Two types of melt pockets occur in Hawaiian mantle xenoliths: amphibole-bearing (AMP) and spinel-bearing (SMP). AMPs contain amphibole (kaersutite), olivine (Fo92), clinopyroxene (with 7–11% Al2O3), vesicles and glass. SMPs contain olivine, clinopyroxene, spinel, glass, and vesicles. The glasses in SMPs (SiO2=44–45%, 11–12% alkalis, La=90–110 ppm) and AMPs (SiO2=49–54%, 6–8.5% alkalis, La=8–14 ppm) are distinct in color and composition. Both glasses are generally characterized by LREE-enriched (chondrite-normalized) patterns. Amphibole and clinopyroxene have gently convex upward-to-moderately LREE-enriched patterns. Mineral/glass trace element abundance ratio plots show a strong negative Ti anomaly and a gentle negative Zr anomaly for clinopyroxene/glass; whereas amphibole/glass patterns show a distinctive positive Ti spike. The amphibole/glass trace element ratios are similar to published megacryst/lava values. An earlier study showed that the Hawaiian spinel lherzolites (lithosphere) have largely been metasomatized during post-erosional Honolulu magmatic activity. REE abundances of SMP glasses (melts) overlap the REE abundances calculated for such metasomes. The occurrence of hydrous, alkaline, mafic melt pockets in Hawaiian upper mantle xenoliths implies that (1) such hydrous liquids are generated in the upper mantle, and (2) water plays a role in magmatic activity associated with the Hawaiian plume. Although we are uncertain about the source (plume, lithosphere, or asthenosphere) of this water, we speculate that such melts and other alkalic lavas erupted on Oahu and on the sea-floor over the Hawaiian arch were generated from a broad „wet“ rim of a radially layered Hawaiian plume, whose hot and „dry“ core supplied the shield-forming magmas. Received: 6 February 1995 / Accepted: 28 August 1995  相似文献   

6.
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   

7.
Spinel-bearing peridotitic mantle xenoliths from the 1949 eruption on La Palma were modified mineralogically and chemically during prolonged reaction with their host magma. The magmatism that brought the peridotites to the surface caused two distinct generations of xenolith fractures: (1) Old fractures are characterized by crystalline selvages with cumulus textures towards the host magma, or by polymineralic veins. They are accompanied by 0.9–2 mm wide diffusion zones where peridotite olivine became less forsteritic through diffusive exchange with the host magma. Old fractures represent most of each xenolith's surface. (2) Young fractures show no selvages and only narrow diffusion zones of <0.02 mm width. Calculations based on a model of Fe-Mg interdiffusion give an age of 6 to 83 years and <4 days for old and young fractures, respectively. A combination of these data with fluid inclusion barometry indicates that selvages and veins formed during xenolith transport rather than representing wall-rock reactions or mantle metasomatism. The results provide ample evidence for prolonged storage of the xenoliths in the crust, constraining a multi-stage magma ascent: Years to decades prior to eruption, ascending magma ruptured peridotitic wall-rock possibly through hydraulic fracturing and stoping around magma reservoirs. Magma batches transported the peridotite xenoliths to the crust at ascent rates exceeding 0.2 ms−1. The xenoliths and their host magma stagnated during at least 6 years in possibly sill-like reservoirs at 7–11 km depth. The xenoliths became deposited and subsequently embedded in a mush of settled phenocrysts, while selvages and veins crystallized until the eruption commenced. At the end of the eruption, the xenoliths were finally transported to the surface within hours to days. Decompression during the rapid ascent induced internal stresses and caused renewed fragmentation of the xenoliths, producing the young fractures. Received: 25 August 1997 / Accepted: 25 November 1997  相似文献   

8.
Gabbroic and ultramafic xenoliths and olivine and clinopyroxene phenocrysts in basaltic rocks from Gran Canaria, La Palma, El Hierro, Lanzarote and La Gomera (Canary Islands) contain abundant CO2-dominated fluid inclusions. Inclusion densities are strikingly similar on a regional scale. Histogram maxima correspond to one or more of the following pressures: (1) minimum 0.55 to 1.0 GPa (within the upper mantle); (2) between 0.2 and 0.4 GPa (the Moho or the lower crust); (3) at about 0.1 GPa (upper crust). Fluid inclusions in several rocks show a bimodal density distribution, the lower-density maximum comprising both texturally early and late inclusions. This is taken as evidence for an incomplete resetting of inclusion densities, and simultaneous formation of young inclusions, at well-defined magma stagnation levels. For Gran Canaria, pressure estimates for early inclusions in harzburgite and dunite xenoliths and olivine phenocrysts in the host basanites overlap at 0.9 to 1.0 GPa, indicating that such magma reservoir depths coincide with levels of xenolith entrainment into the magmas. Magma chamber pressures within the mantle, inferred to represent levels of mantle xenolith entrainment, are 0.65–0.95 GPa for El Hierro, 0.60–0.68 GPa for La Palma, and 0.55–0.75 GPa for Lanzarote. The highest-density fluid inclusions in many Canary Island mantle xenoliths have probably survived in-situ near-isobaric heating at the depth of xenolith entrainment. Inclusion data from all islands indicate ponding of basaltic magmas at Moho or lower crustal depths, and possibly at an additional higher level, strongly suggestive of two main crustal accumulation levels beneath each island. We emphasize that repeated magmatic underplating of primitive magmas, and therefore intrusive accretion, are important growth mechanisms for the Canary Islands, and by analogy, for other ocean islands. Comparable fluid inclusion data from primitive rocks in other tectonic settings, including Iceland, Etna and continental rift systems (Hungary, South Norway), indicate that magma accumulation close to Moho depths shortly before eruption is not, however, restricted to oceanic intraplate volcanoes. Lower crustal ponding and crystallization prior to eruption may be the rule rather than the exception, independent of the tectonic setting. Received: 30 May 1997 / Accepted: 6 February 1998  相似文献   

9.
The influence of water on melting of mantle peridotite   总被引：47，自引：8，他引：39  
This experimental study examines the effects of variable concentrations of dissolved H2O on the compositions of silicate melts and their coexisting mineral assemblage of olivine + orthopyroxene ± clinopyroxene ± spinel ± garnet. Experiments were performed at pressures of 1.2 to 2.0 GPa and temperatures of 1100 to 1345 °C, with up to ∼12 wt% H2O dissolved in the liquid. The effects of increasing the concentration of dissolved H2O on the major element compositions of melts in equilibrium with a spinel lherzolite mineral assemblage are to decrease the concentrations of SiO2, FeO, MgO, and CaO. The concentration of Al2O3 is unaffected. The lower SiO2 contents of the hydrous melts result from an increase in the activity coefficient for SiO2 with increasing dissolved H2O. The lower concentrations of FeO and MgO result from the lower temperatures at which H2O-bearing melts coexist with mantle minerals as compared to anhydrous melts. These compositional changes produce an elevated SiO2/(MgO + FeO) ratio in hydrous peridotite partial melts, making them relatively SiO2 rich when compared to anhydrous melts on a volatile-free basis. Hydrous peridotite melting reactions are affected primarily by the lowered mantle solidus. Temperature-induced compositional variations in coexisting pyroxenes lower the proportion of clinopyroxene entering the melt relative to orthopyroxene. Isobaric batch melting calculations indicate that fluid-undersaturated peridotite melting is characterized by significantly lower melt productivity than anhydrous peridotite melting, and that the peridotite melting process in subduction zones is strongly influenced by the composition of the H2O-rich component introduced into the mantle wedge from the subducted slab. Received: 7 April 1997 / Accepted: 9 January 1998  相似文献   

10.
Laboratory experiments on natural, hydrous basalts at 1–4 GPa constrain the composition of “unadulterated” partial melts of eclogitized oceanic crust within downgoing lithospheric slabs in subduction zones. We complement the “slab melting” experiments with another set of experiments in which these same “adakite” melts are allowed to infiltrate and react with an overlying layer of peridotite, simulating melt:rock reaction at the slab–mantle wedge interface. In subduction zones, the effects of reaction between slab-derived, adakite melts and peridotitic mantle conceivably range from hybridization of the melt, to modal or cryptic metasomatism of the sub-arc mantle, depending upon the “effective” melt:rock ratio. In experiments at 3.8 GPa, assimilation of either fertile or depleted peridotite by slab melts at a melt:rock ratio 2:1 produces Mg-rich, high-silica liquids in reactions which form pyrope-rich garnet and low-Mg# orthopyroxene, and fully consume olivine. Analysis of both the pristine and hybridized slab melts for a range of trace elements indicates that, although abundances of most trace elements in the melt increase during assimilation (because melt is consumed), trace element ratios remain relatively constant. In their compositional range, the experimental liquids closely resemble adakite lavas in island-arc and continental margin settings, and adakite veins and melt inclusions in metasomatized peridotite xenoliths from the sub-arc mantle. At slightly lower melt:rock ratios (1:1), slab melts are fully consumed, along with peridotitic olivine, in modal metasomatic reactions that form sodic amphibole and high-Mg# orthopyroxene.  相似文献   

11.
Summary Mesozoic melilite-bearing ultramafic lamprophyres are developed as sill, dyke and plug-like intrusive bodies in the East Antarctic Beaver Lake area. They consist of varying amounts of olivine, melilite, phlogopite, nepheline, titanomagnetite and perovskite as major phases, accompanied by minor amounts of apatite, carbonate, spinel, glass and, rarely, monticellite. The rocks are mineralogically and geochemically broadly similar to olivine melilitites, differing in higher CO2 and modal phlogopite and carbonate contents. The ultramafic lamprophyres are MgO-rich (13.4–20.5 wt%) and SiO2-poor (32.8–37.2 wt%), indicative of a near-primary nature. Major and trace element features are consistent with minor fractionation of olivine and Cr-spinel from melts originating at depths of 130–140 km. Primary melts originated by melting of upper mantle peridotite which had been veined by phlogopite + carbonate + clinopyroxene-bearing assemblages less than 200 Ma before eruption. The presence of the veins and their time of formation is required to explain high incompatible trace element contents and growth of 87Sr/86Sr, leaving 143Nd/144Nd unaffected. The major element, compatible trace element, and most radiogenic isotope characteristics are derived from melting of the wall-rock peridotite. The depth of about 130 km is indicated by the presence of phlogopite rather than amphibole in the veins, by control of the REE pattern by residual garnet, by the high MgO content of the rocks, and by the expected intersection of the rift-flank geotherm with the solidus at this depth. The higher CO2 contents than are characteristic for olivine melilitites favoured the crystallization of melilite at crustal pressures, and suppressed the crystallization of clinopyroxene. The Beaver Lake ultramafic lamprophyres are a distal effect of the breakup of Gondwanaland, too distal to show a geochemical signature of the Kerguelen plume. Upward and outward movement of the asthenosphere-lithosphere boundary beneath the Lambert-Amery rift led first to the production of phlogopite- and carbonate-rich veins, and later to the generation of the ultramafic lamprophyres themselves. Received March 31, 2000; revised version accepted September 3, 2001  相似文献   

12.
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   

13.
Mineral and whole-rock chemical data for peridotite xenolithsin basaltic lavas on Spitsbergen are examined to reassess mechanismsof melt–fluid interaction with peridotites and their relativerole versus melt composition in mantle metasomatism. The enrichmentpatterns in the xenoliths on primitive mantle-normalized diagramsrange from Th–La–Ce ‘inflections’ inweakly metasomatized samples (normally without amphibole) toa continuous increase in abundances from Ho to Ce typical foramphibole-bearing xenoliths. Numerical modelling of interactionbetween depleted peridotites and enriched melts indicates thatthese patterns do not result from simple mixing of the two end-membersbut can be explained by chromatographic fractionation duringreactive porous melt flow, which produces a variety of enrichmentpatterns in a single event. Many metasomatized xenoliths havenegative high field strength element and Pb anomalies and Srspikes relative to rare earth elements of similar compatibility,and highly fractionated Nb/Ta and Zr/Hf. Although amphiboleprecipitation can produce Nb–Ta anomalies, some of thesefeatures cannot be attributed to percolation-related fractionationalone and have to be a signature of the initial melt (possiblycarbonate rich). In general, chemical and mineralogical fingerprintsof a metasomatic medium are strongest near its source (e.g.a vein) whereas element patterns farther in the metasomatic‘column’ are increasingly controlled by fractionationmechanisms. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; trace elements; theoretical modelling  相似文献   

14.
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   

15.
Mantle xenoliths in alkaline lavas of the Kerguelen Islandsconsist of: (1) protogranular, Cr-diopside-bearing harzburgite;(2) poikilitic, Mg-augite-bearing harzburgite and cpx-poor lherzolite;(3) dunite that contains clinopyroxene, spinel phlogopite, andrarely amphibole. Trace element data for rocks and mineralsidentify distinctive signatures for the different rock typesand record upper-mantle processes. The harzburgites reflectan initial partial melting event followed by metasomatism bymafic alkaline to carbonatitic melts. The dunites were firstformed by reaction of a harzburgite protolith with tholeiiticto transitional basaltic melts, and subsequently developed metasomaticassemblages of clinopyroxene + phlogopite ± amphiboleby reaction with lamprophyric or carbonatitic melts. We measuredtwo-mineral partition coefficients and calculated mineral–meltpartition coefficients for 27 trace elements. In most samples,calculated budgets indicate that trace elements reside in theconstituent minerals. Clinopyroxene is the major host for REE,Sr, Y, Zr and Th; spinel is important for V and Ti; orthopyroxenefor Ti, Zr, HREE, Y, Sc and V; and olivine for Ni, Co and Sc. KEY WORDS: mantle xenoliths; mantle metasomatism; partition coefficients; Kerguelen Islands; trace elements  相似文献   

16.
Spinel peridotite xenoliths in alkali basalts at Tok, SE Siberian craton range from fertile lherzolites to harzburgites and wehrlites; olivine-rich (70-84%) rocks are dominant. REE patterns in the lherzolites range from nearly flat for fertile rocks (14-17% cpx) to LREE-enriched; the enrichments are positively correlated with modal olivine, consistent with high-permeability of olivine-rich rocks during melt percolation. Clinopyroxene in olivine-rich Tok peridotites typically has convex-upward trace element patterns (La/NdPM < 1 and Nd/YbPM ? 1), which we consider as evidence for equilibration with evolved silicate liquids (with higher REE and lower Ti contents than in host basalts). Whole-rock patterns of the olivine-rich xenoliths range from convex-upward to LREE-enriched (La/NdPM > 1); the LREE-enrichments are positively correlated with phosphorus abundances and are mainly hosted by accessory phosphates and P-rich cryptocrystalline materials. In addition to apatite, some Tok xenoliths contain whitlockite (an anhydrous, halogen-poor and Na-Mg-rich phosphate), which is common in meteorites and lunar rocks, but has not been reported from any terrestrial mantle samples. Some olivine-rich peridotites have generations of clinopyroxene with distinct abundances of Na, LREE, Sr and Zr. The mineralogical and trace element data indicate that the lithospheric mantle section represented by the xenoliths experienced a large-scale metasomatic event produced by upward migration of mafic silicate melts followed by percolation of low-T, alkali-rich melts and fluids. Chromatographic fractionation and fractional crystallisation of the melts close to the percolation front produced strong LREE-enrichments, which are most common in the uppermost mantle and are related to carbonate- and P2O5-rich derivatives of the initial melt. Reversal and gradual retreat of the percolation front during thermal relaxation to ambient geotherm (“retrograde” metasomatism) caused local migration and entrapment of small-volume residual fluids and precipitation of volatile-rich accessory minerals. A distinct metasomatic episode, which mainly produced “anhydrous” late-stage interstitial materials was concomitant with the alkali basaltic magmatism, which brought the xenoliths to the surface.  相似文献   

17.
Seven alkali basalt centers in the southern Canadian Cordilleracontain mantle xenolith suites that comprise spinel Cr-diopsideperidotites, spinel augite-bearing wehrlites and orthopyroxene-poorlherzolites, and minor pyroxenites. The Cr-diopside peridotitesappear to be residues of the extraction of Mg-rich basalts byup to 15% partial melting (median 5–10%) of a pyrolite-likesource in the spinel stability field. The xenoliths are similarto other mantle xenolith suites derived from beneath convergentcontinental margins, but are less depleted, less oxidized, andhave lower spinel mg-number than peridotites found in fore-arcsettings. Their dominant high field strength element depletedcharacter, however, is typical of arc lavas, and may suggestthat fluids or melts circulating through the Canadian Cordilleralithosphere were subduction related. Modeling using MELTS isconsistent with the augite-bearing xenoliths being formed byinteraction between crystallizing alkaline melts and peridotite.Assimilation–fractional crystallization modeling suggeststhat the trace element patterns of liquids in equilibrium withthe augite xenoliths may represent the initial melts that reactedwith the peridotite. Moreover, the compositions of these meltsare similar to those of some glasses observed in the mantlexenoliths. Melt–rock interaction may thus be a viablemechanism for the formation of Si- and alkali-rich glass inperidotites. KEY WORDS: Canadian Cordillera; mantle xenolith; peridotite; wehrlite; melt–rock reaction  相似文献   

18.
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   

19.
Globules of iron-dominated (59–69 wt% FeOtot) and titanium-dominated (43.5 wt% TiO2) oxide melts have been detected in igneous xenoliths from Pliocene-to-Pleistocene alkali basalts of the Western Carpathians. Fluid inclusion and mineral composition data indicate immiscible separation of the high-iron-oxide melt (HIM) at magmatic temperatures. The HIM separation occurred during clinopyroxene (augite) accumulation in an alkali trachybasalt and continued during crystallization of amphibole (kaersutite) and K-feldspar (anorthoclase), the latter coexisting with trachyte and alkalic rhyolite residual melts. Some HIM was also expelled from sub-alkalic rhyolite (70–77% SiO2), coexisting with An27–45 plagioclase and quartz in granitic (tonalite-trondhjemite) xenoliths. Oxygen fugacities during HIM separation range from −1.4 to +0.6 log units around the QFM buffer. A close genetic relationship between HIM-hosted xenoliths and mantle-derived basaltic magma is documented by mineral 18O values ranging from 4.9 to 5.9‰ V-SMOW. δD values of gabbroic kaersutite between −61 and −86‰ V-SMOW are in agreement with a presumed primary magmatic water source. Most trace elements, except Li, Rb and Cs, have preferentially partitioned into the HIM. The HIM/Si-melt partition coefficients for transition elements (Sc, V, Cr, Co, Ni) and base metals (Zn, Cu, Mo) are between 2–160, resulting in extreme enrichment in the HIM. La and Ce also concentrate in the silicic melt, whereas Tb-Tm in the HIM. Hence, the immiscible separation causes REE fractionation and produces residual silicic melt enriched in LREE and depleted in HREE. The weak fractionation among Tb-Tm and Yb, Lu can be attributed to recurrent extraction of the HIM from the magmatic system, while flat HREE chondrite-normalized patterns are interpreted to indicate no or little loss of the HIM. Received: 30 September 1997 / Accepted: 23 March 1998  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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