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1.
A comparison of mantle xenolith suites along the northern Canadian Cordillera reveals that the xenoliths from three suites exhibit bimodal populations whereas the xenoliths from the other four suites display unimodal populations. The bimodal suites contain both fertile lherzolite and refractory harzburgite, while the unimodal suites are dominated by fertile lherzolite xenoliths. The location of the three bimodal xenolith suites correlates with a newly discovered P-wave slowness anomaly in the upper mantle that is 200 km in width and extends to depths of 400–500 km (Frederiksen AW, Bostock MG, Van Decar JC, Cassidy J, submitted to Tectonophysics). This correlation suggests that the bimodal xenolith suites may either contain fragments of the anomalously hot asthenospheric mantle or that the lithospheric upper mantle has been affected by the anomalously hot mantle. The lherzolite xenoliths in the bimodal suites display similar major element compositions and trace element patterns to the lherzolite xenoliths in the unimodal suites, suggesting that the lherzolites represent the regional lithospheric upper mantle. In contrast, the harzburgite xenoliths are highly depleted in terms of major element composition, but their clinopyroxenes [Cpx] have much higher incompatible trace element contents than those in the lherzolite xenoliths. The major element and mildly incompatible trace element systematics of the harzburgite and lherzolite xenoliths indicate that they could be related by a partial melting process. The lack of textural and geochemical evidence for the former existence of garnet argues against the harzburgite xenoliths representing actual fragments of the deeper anomalous asthenospheric mantle. Furthermore, the calculated P-wave velocity difference between harzburgite and lherzolite end-members is only 0.8%, with the harzburgites having higher P-wave velocities. Therefore the 3% P-wave velocity difference detected teleseismically cannot be produced by the compositional difference between the lherzolite and harzburgite xenoliths. If temperature is responsible for the observed 3% P-wave velocity perturbation, the anomalous mantle is likely to be at least 200 °C higher than the surrounding mantle. Taken together these data indicate that the refractory harzburgite xenoliths represent the residue of 20–25% partial melting of a lherzolite lithospheric mantle. The incompatible trace element enrichment of the harzburgites suggests that this melting was accompanied by the ingress of fluids. The association of the bimodal xenolith suites with the mantle anomaly detected teleseismically suggests that anomalously hot asthenospheric mantle provided both the heat and volatiles responsible for the localized melting and enrichment of the lithospheric mantle. Received: 16 May 1997 / Accepted: 25 October 1997  相似文献   

2.
Current methods of geothermometry and geobarometry applicable to garnet lherzolite are reviewed with reference to recent experimental studies of the equilibration of natural garnet lherzolite and it is concluded that the Wells and Mori-Green formulations of the two pyroxene solvus provide the most reasonable temperature estimates. Pressures are best estimated by using these temperatures with Wood's formulation of the orthopyroxene-garnet geobarometer without chromium corrections. Pipe 200 garnet lherzolites are considered to have equilibrated at 907°–950° C at 30.0–34.5 kb.It is shown that the transport times of xenoliths from the mantle are sufficiently long (0.5–24 h) to allow thermal equilibration with kimberlite but are too short to allow chemical re-equilibration to occur. Xenolith suites therefore retain information regarding the pressure/temperature history of the upper mantle despite being heated to the temperature of the kimberlite magma during transport.The Pipe 200 xenolith suite indicates that the upper mantle beneath Lesotho has been perturbed to temperatures slightly above those defined by steady state geotherms. The Pipe 200 suite is derived from a narrow depth range (90–110 km) and derivation of chromite and garnet lherzolites from similar depths implies that the mantle is heterogeneous over short vertical distances. No simple stratigraphy, in which chromite lherzolites overlie garnet lherzolites is evident. Comparison with other suites of Lesotho garnet lherzolites shows that it is not possible to construct an upper mantle stratigraphy except in the most general terms because of the prevailing lateral and vertical heterogeneity and apparent limited depth range represented by the xenolith suites.  相似文献   

3.
Low-Ca garnet harzburgite xenoliths contain garnets that are deficient in Ca relative to those that have equilibrated with diopside in the iherzolite assemblage. Minor proportions of these harzburgites are of wide-spread occurrence in xenolith suites from the Kaapvaal craton and are of particular interest because of their relation to diamond host rocks. The harzburgite xenoliths are predominantly coarse but one specimen from Jagersfontein and another from Premier have deformed textures similar to those of high-temperature peridotites. Analyses for many elements in the harzburgites and associated iherzolites form concordant overlapping trends. On the average, however, the harzburgites are deficient in Si, Ca, Al and Fe but enriched in Mg and Ni relative to the lherzolites. Both the harzburgites and lherzolites are enstatite-rich with mg numbers [100.Mg/(Mg+Fetotal)] greater than 92 and in these respects differ markedly from residues generated by extraction of MORB. Equilibration temperatures and depths calculated for the harzburgites have the ranges 600–1,400°C and 50–200 km. Those of deepest origin overlap the interval between low-and high-temperature lherzolites that commonly is observed in temperature-depth plots for the Kaapvaal craton, suggesting that some harzburgites may be concentrated relative to lherzolites at the base of the lithosphere. The low-Ca harzburgites and lherzolite xenoliths have overlapping depths of origin, gradational bulk chemical characteristics and similar textures, and therefore both are believed to have formed as residues of Archaen melting events. The harzburgites differ from the lherzolites only in that they are more depleted. Garnets and associated minerals in harzburgite xenoliths differ from minerals of the same assemblage that are included in diamonds in that the latter are more Cr-rich, Mg-rich and Ca-poor. Coarse crystals of low-Ca garnet with the compositional characteristics of diamond inclusions commonly occur as disaggregated grains in diamondiferous kimberlites. Their host rocks are presumed to have been harzburgites and dunites. The differences in composition between the disaggregated grains that are similar to diamond inclusions and those comprising xenoliths imply some differences in origin. Possibly the disaggregated harzburgites with diamond-inclusion mineralogy have undergone repeated partial melting and depletion near the base of the lithosphere subsequent to their primary depletion and aggregation in the craton. Equilibration with magnesite may have reduced the Ca contents of their garnets and decomposition of the magnesite during eruption may have caused their disaggregation.  相似文献   

4.
Two picrite flows from the SW rift zone of Mauna Loa containxenoliths of dunite, harzburgite, lherzolite, plagioclase-bearinglherzolite and harzburgite, troctolite, gabbro, olivine gabbro,and gabbronorite. Textures and olivine compositions precludea mantle source for the xenoliths, and rare earth element concentrationsof xenoliths and clinopyroxene indicate that the xenolith sourceis not old oceanic crust, but rather a Hawaiian, tholeiitic-stagemagma. Pyroxene compositions, phase assemblages and texturalrelationships in xenoliths indicate at least two different crystallizationsequences. Calculations using the pMELTS algorithm show thatthe two sequences result from crystallization of primitive MaunaLoa magmas at 6 kbar and 2 kbar. Independent calculations ofolivine Ni–Fo compositional variability in the plagioclase-bearingxenoliths over these crystallization sequences are consistentwith observed olivine compositional variability. Two parentsof similar bulk composition, but which vary in Ni content, arenecessary to explain the olivine compositional variability inthe dunite and plagioclase-free peridotitic xenoliths. Xenolithsprobably crystallized in a small magma storage area beneaththe rift zone, rather than the large sub-caldera magma reservoir.Primitive, picritic magmas are introduced to isolated rift zonestorage areas during periods of high magma flux. Subsequenteruptions reoccupy these areas, and entrain and transport xenolithsto the surface. KEY WORDS: xenolith; Hawaii; volcano plumbing; mineral composition; picrite  相似文献   

5.
Numerous ultramafic xenoliths occur within the A??n–Temouchent volcanic complex (Northwestern Oranie, Algeria). Most of them are type I mantle tectonites (lherzolites and harzburgites) and composite xenoliths (harzburgite/clinopyroxenite) are rare. Only a few samples of spinel lherzolites display relatively fertile compositions when the major part of type I xenoliths have refractory major element compositions but enriched LREE contents showing that they have been affected by mantle metasomatism. The composite xenoliths are witnesses of reactions of alkaline magmas with the upper mantle. An asthenospheric rising, in relation with the large strike slip fault affecting the North African plate margin at Trias time is proposed as a possible geodynamical setting. To cite this article: M. Zerka et al., C. R. Geoscience 334 (2002) 387–394.  相似文献   

6.
Don Francis   《Lithos》2003,71(2-4):135-152
The Earth's continents are cored by Archean cratons underlain by seismically fast mantle roots descending to depths of 200+ km that appear to be both more refractory and colder than the surrounding asthenospheric mantle. Low-temperature mantle xenoliths from kimberlite pipes indicate that the shallow parts of these cratonic mantle roots are dominated by refractory harzburgites that are very old (3+ Ga). A fundamental mass balance problem arises, however, when attempts are made to relate Archean high-Mg lavas to a refractory restite equivalent to the refractory lithospheric mantle roots beneath Archean cratons. The majority of high-Mg Archean magmas are too low in Al and high in Si to leave behind a refractory residue with the composition of the harzburgite xenoliths that constitute the Archean mantle roots beneath continental cratons, if a Pyrolitic primitive mantle source is assumed. The problem is particularly acute for 3+ Ga Al-depleted komatiites and the Si-rich harzburgites of the Kaapvaal and Slave cratons, but remains for cratonic harzburgites that are not anomalously rich in orthopyroxene and many Al-undepleted komatiites. This problem would disappear if fertile Archean mantle was richer in Fe and Si, more similar in composition to chondritic meteorites than the present Pyrolitic upper mantle of the Earth. Accepting the possibility that the Earth's convecting upper mantle has become poorer in Fe and Si over geologic time not only provides a simpler way of relating Archean high-Mg lavas to the lithospheric mantle roots that underlie Archean cratons, but could lead to new models for the nature Archean magmatism and the lower mantle sources of modern hot-spot volcanism.  相似文献   

7.
Phlogopite has been recognized for the first time in ultramaficxenoliths from the Canadian Cordillera. The phlogopite-bearingxenoliths are hosted in post-glacial basanitoid flows and ejectaof the Kostal Lake volcanic center, British Columbia. The xenolithassemblage consists of 60% cumulate-textured wehrlites, and40% coarse-textured lherzolites, harzburgites, dunites, andolivine websterites. The phlogopite occurs: (1) as sub-euhedral grains along grainboundaries in dunite and lherzolite xenoliths; or (2) alongorthopyroxene lamellae exsolved from intercumulus clinopyroxenein the wehrlite xenoliths; or (3) as grains hosted in 10–100pm diameter fluid inclusions in clinopyroxene of all xenoliths.The phlogopites do not show any reaction relationships withother phases in any of the xenoliths studied. Phlogopites ina given xenolith have Mg/Mg + Fe2+ similar to that of coexistingolivine, clinopyroxene, and orthopyroxene. The partitioningof Fe and Mg between phlogopite and coexisting olivine and clinopyroxeneis similar to that observed in other phlogopite-bearing mantlexenoliths, and in high-pressure melting experiments on rockswith similar bulk compositions. This indicates that the phlogopitesin xenoliths from Kostal Lake have equilibrated with these coexistingphases. The occurrence of phlogopites in fluid inclusions containingNa, K, Cl, P, and S, suggests that incompatible element-enrichedhydrous fluids/melts fluxed this part of the upper mantle beneatheastern British Columbia. Metasomatism of the upper mantle beneathKostal Lake probably occurred prior to Quaternary alkaline magmatism(7550–400 B.P.) and after the initial volcanism whichformed the wehrlite cumulates (3–5 Ma). Metasomatism causedoverall oxidation of the upper mantle beneath this area butwas not responsible for the anomalously Fe-rich nature of somexenoliths from the Kostal Lake eruptive center.  相似文献   

8.
A suite of mainly spinel peridotite and subordinate pyroxenite xenoliths and megacrysts were studied in detail, enabling us to characterize upper mantle conditions and processes beneath the modern North American–Eurasian continental plate boundary. The samples were collected from 37-Ma-old basanites cropping out in the Main Collision Belt of the Chersky Range, Yakutia Republic (Russian Far East). The spinel lherzolites reflect a mantle sequence, equilibrated at temperatures of 890–1,025 °C at pressures of 1.1–2 GPa, with melt extraction estimated to be around 2–6 %. The spinel harzburgites are characterized by lower P–T equilibration conditions and estimated melt extraction up to 12 %. Minor cryptic metasomatic processes are recorded in the clinopyroxene trace elements, revealing that percolating hydrous fluid-rich melts and basaltic melts affected the peridotites. One of the lherzolites preserves a unique melt droplet with primary dolomite in perfect phase contact with Na-rich aluminosilicate glass and sodalite. On the basis of the well-constrained P–T frame of the xenolith suite, as well as the rigorously documented melt extraction and metasomatic history of this upper mantle section, we discuss how a carbonated silicate melt infiltrated the lherzolite at depth and differentiated into an immiscible carbonate and silicate liquid shortly before the xenolith was transported to the surface by the host basalt. Decreasing temperatures triggered crystallization of primary dolomite from the carbonate melt fraction and sodalite as well as quenched glass from the Na-rich aluminosilicate melt fraction. Rapid entrainment and transport to the Earth’s surface prevented decarbonatization processes as well as reaction phenomena with the host lherzolite, preserving this exceptional snapshot of upper mantle carbonatization and liquid immiscibility.  相似文献   

9.
 We have investigated new samples from the Gees mantle xenolith suite (West Eifel), for which metasomatism by carbonatite melt has been suggested. The major metasomatic change is transformation of harzburgites into phlogopite-rich wehrlites. Silicate glasses are associated with all stages of transformation, and can be resolved into two major groups: a strongly undersaturated alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, variably alkaline glass present exclusively within the xenoliths. Si-Al-rich glasses (up to 72 wt% SiO2 when associated with orthopyroxene (Opx) are usually interpreted in mantle xenoliths as products of decompressional breakdown of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to phlogopite breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those related to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Both phases crystallize as microlites from the glass. The chemical composition of the Si-Al-enriched glass shows that it cannot be derived from decompressional melting of the Gees xenoliths, but must have been present prior to their entrainment in the host magma. Simple mass-balance calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which glass + microlite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline andesite (55–60 wt% SiO2), characterized by high Al2O3 (ca. 18 wt%). The obtained composition is very similar to high-alumina, calc-alkaline melts that should form by AFC-type reactions between basalt and harzburgite wall rock according to the model of Kelemen (1990). Thus, we suggest that the Si-Al-enriched glasses of Gees, and possibly of other suites as well, are remnants of upper mantle hybrid melts, and that the Gees suite was metasomatized by silicate and not carbonatite melts. High-Mg, high-Ca composition of metasomatic olivine and clinopyroxene in mantle xenoliths have been explained by carbonatite metasomatism. As these features are also present in the Gees suite, we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show that they are a simple function of silica activity. High-Ca compositions are attained at low a SiO2 and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespective of whether it is a carbonatite or a silicate melt. Such low a SiO2 is recorded by the microlites in the Gees Si-Al-rich glasses. Our results imply that xenolith suites cannot confidently be related to carbonatite metasomatism if the significance of silicate glasses, when present, is not investigated. Received: 2 March 1995 / Accepted: 12 June 1995  相似文献   

10.
The composition, structure and thermal state of the lithosphere beneath the Slave craton have been studied by analysing over 300 peridotitic mantle xenoliths or multiphase xenocrysts entrained within kimberlites in the Lac de Gras area. These xenoliths are derived from seven kimberlites located on the Ekati Diamond Mine™ property and define a detailed stratigraphic profile through the central Slave lithosphere from less than 120 km down to 200 km. Two dominant peridotite types are present, namely garnet-bearing harzburgite and lherzolite with rare occurrences of chromite-facies peridotite, websterite and wehrlite. The pressures and temperatures (P–T's) defined by the entire data-set range from 28 to 62 kbar and 650 to 1250 °C, respectively, and approximately intersect the diamond stability field at 900 °C and 42 kbar. There is no apparent change in the geotherm with depth that is discernable beyond the resolution of the various thermobarometers. The peridotites can be divided into two compositional zones—a shallow layer dominated by garnet harzburgite that straddles the diamond–graphite boundary and a deeper layer that is strongly dominated by garnet lherzolite. Compositionally, the harzburgites (and to a lesser extent, the shallow lherzolites) are ultra-depleted relative to the more fertile deeper layer, irrespective of whether they reside within the graphite or diamond stability field. This ultra-depleted layer beneath Ekati continues to 150 km.  相似文献   

11.
Lithospheric thinning beneath the eastern North China Craton is widely recognized, but the mechanism and timing of the thinning are contentious. New data on peridotitic xenoliths from the Cretaceous (∼100 Ma) Fuxin basalts at the northern edge of the craton have been integrated with data from other localities across the craton, to provide an overview of the processes involved. The Fuxin peridotite xenoliths can be subdivided into three types, which can also be recognized in other xenolith suites across the craton. The dominant Type 1, lherzolites with olivine Mg# ∼90, represents fertile mantle (5-12% partial-melt extraction) that makes up much of the Late Mesozoic-Cenozoic lithosphere beneath the craton. Type 2 consists of magnesian (olivine Mg# >92) harzburgites, interpreted as shallow relics of the Archean cratonic mantle. Type 3, minor lherzolite xenoliths with olivine Mg# ∼86 reflect the interaction of the lithosphere with magmas similar to the host basalts. In-situ Re-Os data on sulfides in xenoliths from Hebi (4 Ma, interior of the craton) and Hannuoba (22 Ma, northern edge of the Trans-North China Orogen within the craton) basalts give model ages of 3.1-3.0, 2.5, 2.2-2.1, 1.4 and 0.8 Ga, These correspond to the U-Pb ages of zircons from early Mesozoic (178 Ma) peridotitic xenoliths at the southern margin of the craton, and record events during which the Archean lithospheric mantle was modified. The dominance of fertile peridotite xenoliths in the 100 Ma Fuxin basalts indicates that the mantle replacement beneath the eastern North China Craton at least partly took place before that time. The regional synthesis suggests that Mesozoic-Cenozoic lithospheric thinning and mantle replacement was heterogeneously distributed across the North China Craton in space and time. Lateral spreading of the lithosphere, accompanied by asthenospheric upwelling and melt-peridotite interaction, is the most probable mechanism for the lithospheric thinning beneath the eastern part of the craton. Subsequent cooling of the upwelled asthenosphere caused some re-thickening of the lithosphere; this overall more fertile and hence denser lithosphere resulted in widespread basin formation.  相似文献   

12.
The assumption that mafic alkaline magmas are derived from mantle sources with a lherzolite mineralogy has become entrenched in the petrologic literature. Although it is commonly assumed that highly alkaline magmas require metasomatised mantle sources, there is little understanding of the spatial relation of such sources with respect to those of associated more Si-rich transitional magmas. Glasses developed in mantle xenoliths represent natural experiments which may provide some insight on this problem. Highly silica undersaturated glasses developed in the amphibole-garnet clinopyroxenite portion of a composite xenolith from Nunivak Island, Alaska, become quartz normative where they penetrate adjacent spinel lherzolite. A comparison of glass compositions in mantle pyroxenite and lherzolite xenoliths reveals that glasses developed in amphibole pyroxenite xenoliths are in general more silica undersaturated than those in lherzolite xenoliths. This suggests that some highly silica undersaturated magmas such as nephelinites may in fact be derived by the preferential melting of amphibole or amphibole-garnet pyroxenite veins and that the spectrum from nephelinite to transitional alkaline basalt that characterizes many individual alkaline volcanic suites is produced by mixing with melt derived from the host lherzolite as the degree of partial melting increases.  相似文献   

13.
New major- and trace-element data of bulk-rocks and constituent minerals, and whole-rock Re-Os isotopic compositions of samples from the Lherz Massif, French Pyrenees, reveal complex petrological relationships between the dominant lithologies of lherzolite ± olivine-websterite and harzburgite. The Lherz peridotite body contains elongate, foliation parallel, lithological strips of harzburgite, lherzolite, and olivine-websterite cross-cut by later veins of hornblende-bearing pyroxenites. Peridotite lithologies are markedly bimodal, with a clear compositional gap between harzburgites and lherzolites ± olivine-websterite. Bulk-rock and mineral major-element oxide (Mg-Fe-Si-Cr) compositions show that harzburgites are highly-depleted and result from ∼20-25 wt.% melt extraction at pressures <2 GPa. Incompatible and moderately-compatible trace-element abundances of hornblendite-free harzburgites are analogous to some mantle-wedge peridotites. In contrast, lherzolites ± olivine-websterite overlap estimates of primitive mantle composition, yet these materials are composite samples that represent physical mixtures of residual lherzolites and clinopyroxene dominated cumulates equilibrated with a LREE-enriched tholeiitic melt. Trace-element compositions of harzburgite, and some lherzolite bulk-rocks and pyroxenes have been modified by; (1) wide-spread interaction with a low-volume LREE-enriched melt +/− fluid that has disturbed highly-incompatible elements (e.g., LREEs, Zr) without enrichment of alkali- and Ti-contents; and (2) intrusion of relatively recent, small-volume, hornblendite-forming, basanitic melts linked to modal and cryptic metasomatism resulting in whole-rock and pyroxene Ti, Na and MREE enrichment.Rhenium-Os isotope systematics of Lherz samples are also compositionally bimodal; lherzolites ± olivine-websterite have chondritc to suprachondritic 187Os/188Os and 187Re/188Os values that overlap the range reported for Earth’s primitive upper mantle, whereas harzburgites have sub-chondritic 187Os/188Os and 187Re/188Os values. Various Os-model age calculations indicate that harzburgites, lherzolites, and olivine-websterites have been isolated from convective homogenisation since the Meso-Proterozoic and this broadly coincides with the time of melt extraction controlled by harzburgite Os-isotope compositions. The association between harzburgites resulting from melting in mantle-wedge environments and Os-rich trace-phases (laurite-erlichmanite sulphides and Pt-Os-Ir-alloys) suggests that a significant portion of persistent refractory anomalies in the present-day convecting mantle of Earth may be linked to ancient large-scale melting events related to wide-spread subduction-zone processing.  相似文献   

14.
Poikiloblastic harzburgite xenoliths (P-type) from Borée, France are characterised by large (>1 cm), essentially unstrained olivines and high equilibrium temperatures (>1200 °C). Mineralogical data, trace element abundances and Sr-Nd-O isotopes of the constituent minerals are consistent with formation as a result of melt percolation-reactions in a lherzolite precursor during lithospheric erosion by an upwelling plume. This petrogenetic model contrasts with previous models involving isochemical recrystallisation from a granular lherzolite precursor (G-type) or derivation as metacumulates from tholeiitic magmas. Numerical simulation of percolation reactions at the lithosphere-plume boundary using the plate model of Vernières et al. (1997) indicates that the different textured xenoliths may represent mantle from different levels in a percolation-reaction column. If correct then the P-type harzburgites resulted from pyroxene-dissolving and olivine-producing reactions at increasing melt fraction (>3%) at the lower part of column (base of the lithosphere), whereas the G-type lherzolites were located within the low-porosity domain (<0.1%) above a permeability barrier, and are formed through a melt-rock reaction at decreasing melt mass. Given the very low melt fraction, the REE fractionation in this zone is controlled by chromatographic effects coupled with source effects of reaction. The variations in porosity, melt/rock ratio and melt-rock reaction mechanism are believed to be responsible for the diversity of REE patterns and striking correlation between REE abundance and texture in Borée xenoliths. Received: 15 June 1997 / Accepted: 7 January 1998  相似文献   

15.
《Chemical Geology》1999,153(1-4):11-35
Anhydrous mantle peridotite xenoliths from a single volcanic vent in the French Massif Central are compositionally varied, ranging from relatively fertile lherzolites to refractory harzburgites. Fertile lherzolites closely resemble previous estimates of undepleted mantle compositions but the average of the Ray Pic xenoliths is much less enriched in LILE and LREE than McDonough's (1990) average mantle [McDonough, W.F., 1990. Constraints on the composition of the continental lithospheric mantle. Earth Planet. Sci. Lett., 101, 1–18]. The wide geochemical variation in the bulk rocks reflects significant heterogeneities that can be attributed to two major processes within the shallow lithospheric mantle. The first process is depletion, related to variable degrees of partial melting and melt extraction from an originally near-chondritic mantle. This process has largely controlled the major elements and much of the trace element variation between fertile lherzolites and refractory peridotites. LREE-depleted compositions are also produced by this process. During partial melting, HREE behaved coherently with the major oxides and the moderately incompatible trace elements (Y, V and Sc). A subsequent process of enrichment is indicated by high concentrations of incompatible trace elements in many of the xenoliths. Sr, Ba, K, Th, U, Nb and LREE abundance are independent of major oxide variations and reflect enrichment related to infiltration by alkaline silicate melts/fluids. Both fertile and refractory mantle were enriched but harzburgites were particularly affected. Modal metasomatism occurred only rarely and is indicated by Cr-diopside-rich veins and patches in a few samples. Their chemistry suggests that they were also formed by migration of similar magmas/fluids from the asthenospheric mantle, although the presence of wehrlitic patches may indicate interaction with carbonate melts. In both depleted and enriched xenoliths, trace element patterns for separated clinopyroxenes closely reflect those of the bulk rock, except for Rb, Ba and Nb, which are probably hosted by other phases.  相似文献   

16.
This experimental study simulates the interaction of hotter, deeper hydrous mantle melts with shallower, cooler depleted mantle, a process that is expected to occur in the upper part of the mantle wedge. Hydrous reaction experiments (~6 wt% H2O in the melt) were conducted on three different ratios of a 1.6 GPa mantle melt and an overlying 1.2 GPa harzburgite from 1060 to 1260 °C. Reaction coefficients were calculated for each experiment to determine the effect of temperature and starting bulk composition on final melt compositions and crystallizing assemblages. The experiments used to construct the melt–wall rock model closely approached equilibrium and experienced <5% Fe loss or gain. Experiments that experienced higher extents of Fe loss were used to critically evaluate the practice of “correcting” for Fe loss by adding iron. At low ratios of melt/mantle (20:80 and 5:95), the crystallizing assemblages are dunites, harzburgites, and lherzolites (as a function of temperature). When the ratio of deeper melt to overlying mantle is 70:30, the crystallizing assemblage is a wehrlite. This shows that wehrlites, which are observed in ophiolites and mantle xenoliths, can be formed by large amounts of deeper melt fluxing though the mantle wedge during ascent. In all cases, orthopyroxene dissolves in the melt, and olivine crystallizes along with pyroxenes and spinel. The amount of reaction between deeper melts and overlying mantle, simulated here by the three starting compositions, imposes a strong influence on final melt compositions, particularly in terms of depletion. At the lowest melt/mantle ratios, the resulting melt is an extremely depleted Al-poor, high-Si andesite. As the fraction of melt to mantle increases, final melts resemble primitive basaltic andesites found in arcs globally. An important element ratio in mantle lherzolite composition, the Ca/Al ratio, can be significantly elevated through shallow mantle melt–wall rock reaction. Wall rock temperature is a key variable; over a span of <80 °C, reaction with deeper melt creates the entire range of mantle lithologies from a depleted dunite to a harzburgite to a refertilized lherzolite. Together, the experimental phase equilibria, melt compositions, and reaction coefficients provide a framework for understanding how melt–wall rock reaction occurs in the natural system during melt ascent in the mantle wedge.  相似文献   

17.
Jurassic basanite necks occurring at the junction of two major fault zones in Scania contain ultramafic (peridotites, pyroxenites) and mafic xenoliths, which together indicate a diversity of upper mantle and lower crustal assemblages beneath this region. The peridotites can be subdivided into lherzolites, dunites and harzburgites. Most lherzolites are porphyroclastic, containing orthopyroxene and olivine porphyroclasts. They consist of Mg-rich silicates (Mg# = Mg/(Mg + Fetot) × 100; 88–94) and vermicular spinel. Calculated equilibration temperatures are lower in porphyroclastic lherzolites (975–1,007°C) than in equigranular lherzolite (1,079°C), indicating an origin from different parts of the upper mantle. According to the spinel composition the lherzolites represent residues of 8–13% fractional melting. They are similar in texture, mineralogy and major element composition to mantle xenoliths from Cenozoic Central European volcanic fields. Dunitic and harzburgitic peridotites are equigranular and only slightly deformed. Silicate minerals have lower to similar Mg# (83–92) as lherzolites and lack primary spinel. Resorbed patches in dunite and harzburgite xenoliths might be the remnants of metasomatic processes that changed the upper mantle composition. Pyroxenites are coarse, undeformed and have silicate minerals with partly lower Mg# than peridotites (70–91). Pyroxenitic oxides are pleonaste spinels. According to two-pyroxene thermometry pyroxenites show a large range of equilibration temperatures (919–1,280°C). In contrast, mafic xenoliths, which are mostly layered gabbronorites with pyroxene- and plagioclase-rich layers, have a narrow range of equilibration temperatures (828–890°C). These temperature ranges, together with geochemical evidence, indicate that pyroxenites and gabbroic xenoliths represent mafic intrusions within the Fennoscandian crust.  相似文献   

18.
Spinel lherzolite and harzburgite xenoliths from the Basin and Range and Colorado Plateau Provinces in the southwestern United States (SWUS) show a broad range in incompatible element distributions from depleted to enriched, both within and between sites. The most fertile xenoliths occur in the Basin and Range Province and the most refractory in the Colorado Plateau or Transition Zone. Mass balance calculations indicate that up to 80% of the high field strength (Nb, Ta, Zr, Hf, Th, Ti) and rare earth elements (50% for Yb) in these xenoliths occur in phases along grain boundaries (or in microfractures). In lower crustal and eclogite xenoliths, up to 90% of these elements occur in grain-boundary phases. Nb/Ta, Zr/Hf, La/Sm and Nb/Th ratios and Nb-Ta anomalies in both types of xenoliths are also controlled by grain-boundary phases. To interpret these ratios in whole-rock analyses, it is critical to understand the timing and origin of the grain-boundary components in the xenoliths. Most of the enriched mantle xenoliths appear to have been enriched by metasomatic fluids related to silicate magmas and not to carbonatite magmas.Because of a complex multi-event history that affected the composition of the xenoliths, there is no simple geochemical relationship between magmatic or metamorphic history of the lower crust and upper mantle in the SWUS as reflected by the xenolith populations.  相似文献   

19.
The petrology and geochemistry of peridotite xenoliths in the Cenozoic basalts from Fanshi, the central North China Craton (NCC), provide constraints on the evolution of sub-continental lithospheric mantle. These peridotite xenoliths are mainly spinel-facies lherzolites with minor harzburgites. The lherzolites are characterized by low forsterite contents in olivines (Fo < 91) and light rare earth element (LREE) enrichments in clinopyroxenes. In contrast, the harzburgites are typified by high-Fo olivines (> 91), high-Cr# spinels and clinopyroxenes with low abundances of heavy REE (HREE). These features are similar to those from old refractory lithospheric mantle around the world, and thus interpreted to be relics of old lithospheric mantle. The old lithospheric mantle has been chemically modified by the influx of melts, as evidenced by the Sr–Nd isotopic compositions of clinopyroxenes and relatively lower Fo contents than typical Archean lithospheric mantle (Fo > 92.5). The Sr–Nd isotopic compositions of harzburgites are close to EM1-type mantle, and of the lherzolites are similar to bulk silicate earth. The latter could be the result of recent modification of old harzburgites by asthenospheric melt, which is strengthened by fertile compositions of minerals in the lherzolites. Therefore, the isotopic and chemical heterogeneities of the Fanshi peridotite xenoliths reflect the refertilization of ancient refractory lithospheric mantle by massive addition of asthenospheric melts. This may be an important mechanism for the lithospheric evolution beneath the Central NCC.  相似文献   

20.
D.A. Carswell 《Lithos》1980,13(2):121-138
Occurrences, petrography and major element chemistry of lherzolite nodules are reviewed. Spinel-garnet stability relationships in these nodules are discussed and are shown to be controlled by chemical as well as physical (P/T) considerations. On the basis of a survey of spinel compositions it is proposed that three classes of spinel bearing lherzolite nodules should be recognised: namely Al-spinel lherzolites [spinel 100 Cr/(Cr + Al)<25], Cr-spinel lherzolites [spinel 100 Cr/(Cr + Al) 25–65] and chromite lherzolites [spinel 100 Cr/(Cr + Al)>65].All lherzolite nodules yield sub-solidus P/T equilibration estimates and are interpreted as fragments of upper mantle wall rocks incorporated during the volatile charged eruptions. Depths of derivation increase from < 60 kms for most nodules in alkali basaltic magmas to > 150 kms for some nodules in kimberlites. The fact that Al-spinel lherzolites are the most common nodule type in magmas of the alkali basalt suite whilst garnet lherzolites are dominant in kimberlites is attributed to the combined effects of a typically steeper geotherm in the mantle beneath areas of alkali basaltic volcanism and a shallower depth of origin for this type of magmatism. Al-spinel lherzolites do, however, occur in the kimberlites of the western U.S.A. and south-west Greenland in contrast to their apparent absence in the kimberlites of southern Africa and Yakutia, U.S.S.R. This suggests that the uppermost mantle beneath these latter regions (as represented by nodules of chromite lherzolites and chromite or Cr-spinel harzburgites) has a more refractory residual type composition with a higher Cr/(Cr + Al) ratio—although the evidence indicates an overall decrease in the level of depletion in ‘basaltic’ magma yielding constituents with depth. Lherzolite nodules generally have chemical compositions which are depleted in such constituents relative to the pyrolite model compositions for primitive or pristine mantle; nevertheless their composition range is thought to encompass both highly ‘depleted’ and essentially ‘undepleted’ upper mantle compositions.The fact that consistent temperature estimates can be obtained from the various calibrated element exchange reaction geothermometers for some lherzolite nodules but not for others (notably those with strikingly porphyroclastic textures) may indicate that some lherzolite nodules comprise mantle derived rocks which failed to totally re-equilibrate following the diapiric movements which immediately preceded their incorporation and rapid transportation to the surface.  相似文献   

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