Temperature History of Sheared Mantle Xenoliths from the West Eifel, West Germany: Evidence for Mantle Diapirism Beneath the Rhenish Massif

Mantle xenoliths from the West Eifel, West Germany revealingdistinct disequilibrium textures were formed by strong sheardeformation from coarse grained, high temperature spinel peridotites.Foliated structures are caused by the roughly parallel alignmentof elongated orthopyToxenc porphyroclasts up to 8 ? 2 mm insize and streched patches of clinopyroxene and spinel in a matrixof recrystallized olivine, orthopyroxene, clinopyroxene, andspinel. Bulk chemical disequilibrium finds its expression in a highdegree of chemical heterogeneity which is most evident in orthopyroxene.In orthopyroxene porphyroclasts, unmixed lamellae of clinopyroxeneand chromium-aluminium spinel are confined to the grain cores,because concentration gradients of Al, Cr, and Ca existed atthe time of their exsolution. Orthopyroxene neoblasts also revealdiffusion controlled concentration gradients of Al, Cr, andCa, which decrease from core to rim. The temperature historydetermining the orthopyroxene chemistries was derived from Al-solubilitiesin orthopyroxene using an empirical geothermometer. From thetextural relationships, in conjunction with the temperaturehistory, it is inferred that the shear process causing the deformationof the porphyroclastic xenoliths was associated with a temperaturedecrease from at least 1100 to about 800?C. The observed linkage of deformation and cooling in the xenolithsis related to the diapiric uplift of hot mantle material intoa cooler uppermost mantle beneath the West Eifel. It rules outa deformation due to secular mantle flow or movements along‘cold’ shear zones.     

Peridotite xenoliths in silica-rich,potassic latite from the transition zone of the Colorado Plateau in north-central Arizona

Potassic latite in the transition zone of the Colorado Plateau near Chino Valley, Arizona, contains abundant eclogite and amphibolite xenoliths and minor websterite and pyroxenite xenoliths. One unit contains peridotite xenoliths; analyzed samples have mg-ratios of 68 and 71, 58 and 63 wt% SiO₂, and are enriched in potassium and other large ion lithophile (LIL) elements. Rare earth element (REE) patterns are light REE enriched with La greater than 100 times chondritic abundance. The peridotite xenoliths are partly to totally altered, but contain remnant olivine, orthopyroxene, and clinopyroxene; one harzburgite nodule also contains spinel. Mineral compositions from the xenoliths are relatively refractory and similar to those in other spinel peridotite xenoliths from the Colorado Plateau. Geothermometry on olivine-spinel and two-pyroxene pairs indicates equilibration temperatures of less than 800° C for the peridotite nodules. The relatively low temperatures calculated from mineral equilibria are consistent with temperature estimates for other mantle nodules from under the Colorado Plateau.Peridotite xenoliths, mg-ratios, and Ni contents are evidence that the latite magma was derived from mantle peridotite. The potassic nature of the magma probably accounts for its silica-rich composition. The potassic, silica-rich nature of the latite and its enrichment in LREE and other LIL elements are consistent with a source which was metasomatically enriched in these elements either before or during partial melting. The source could have been either spinel or garnet peridotite.     

Petrogenesis of the amphibole-rich veins from the Lherz orogenic lherzolite massif (Eastern Pyrenees, France): a case study for the origin of orthopyroxene-bearing amphibole pyroxenites in the lithospheric mantle

The Lherz orogenic lherzolite massif (Eastern French Pyrenees) displays one of the best exposures of subcontinental lithospheric mantle containing veins of amphibole pyroxenites and hornblendites. A reappraisal of the petrogenesis of these rocks has been attempted from a comprehensive study of their mutual structural relationships, their petrography and their mineral compositions. Amphibole pyroxenites comprise clinopyroxene, orthopyroxene and spinel as early cumulus phases, with garnet and late-magmatic K₂O-poor pargasite replacing clinopyroxene, and subsolidus exsolution products (olivine, spinel II, garnet II, plagioclase). The original magmatic mineralogy and rock compositions were partly obscured by late-intrusive hornblendites and over a few centimetres by vein–wallrock exchange reactions which continued down to subsolidus temperatures for Mg–Fe. Thermobarometric data and liquidus parageneses indicate that amphibole pyroxenites started to crystallize at P ≥ 13 kbar and recrystallized at P < 12 kbar. The high Al^VI/Al^IV ratio (>1) of clinopyroxenes, the early precipitation of orthopyroxene and the late-magmatic amphibole are arguments for parental melts richer in silica but poorer in water than alkali basalts. Their modelled major element compositions are similar to transitional alkali basalt with about 1–3 wt% H₂O. In contrast to amphibole pyroxenites, hornblendites only show kaersutite as liquidus phase, and phlogopite as intercumulus phase. They are interpreted as crystalline segregates from primary basanitic magmas (mg=0.6; 4–6 wt% H₂O). These latter cannot be related to the parental liquids of amphibole pyroxenites by a fractional crystallization process. Rather, basanitic liquids mostly reused pre-existing pyroxenite vein conduits at a higher structural level (P ≤ 10 kbar). A continuous process of redox melting and/or alkali melt/peridotite interaction in a veined lithospheric mantle is proposed to account for the origin of the Lherz hydrous veins. The transitional basalt composition is interpreted in terms of extensive dissolution of olivine and orthopyroxene from wallrock peridotite by alkaline melts produced at the mechanical boundary layer/thermal boundary layer transition (about 45–50 km deep). Continuous fluid ingress allowed remelting of the deeper veined mantle to produce the basanitic, strongly volatiles enriched, melts that precipitated hornblendites. A similar model could be valid for the few orthopyroxene-rich hydrous pyroxenites described in basalt-hosted mantle xenoliths. Received: 15 September 1999 / Accepted: 31 January 2000     

Petrology of ultramafic xenoliths from Kishyuku Lava,Fukue-jima,Southwest Japan

 Ultramafic xenoliths are found in Kishyuku Lava, Fukue-jima, Southwest Japan. These include spinel lherzolite, harzburgite and dunite, as well as pyroxenite. The compositions of the constituent minerals of the peridotite xenoliths are in the range of upper mantle peridotites. Variable Cr/(Cr+Al) ratios (0.1–0.5) of spinel, together with a limited range in olivine composition (Fo₉₀–Fo₉₂), indicate that the xenoliths are derived from slightly to highly depleted residual mantle. The combination of previously published clinopyroxene-olivine geothermobarometry and clinopyroxene-orthopyroxene geothermometry applied to the xenoliths yields a high geotherm of 1070° C at 1.0 GPa up to 1200° C at 2.2 GPa. Existence of such depleted upper mantle is compatible with the existing model of asthenospheric injection during the rifting of the Northeast China and the Japan Sea. The high geotherm is caused by thermal perturbation due to the injection of the hot asthenosphere and/or post-rifting uprise of mantle diapirs since 11 Ma. Received: 15 May 1995 / Accepted: 3 January 1996     

Silicic melts produced by reaction between peridotite and infiltrating basaltic melts: ion probe data on glasses and minerals in veined xenoliths from La Palma, Canary Islands

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

The upper mantle under La Palma,Canary Islands: formation of Si−K−Na-rich melt and its importance as a metasomatic agent

 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     

Subcalcic clinopyroxenites and associated ultramafic xenoliths in alkali basalt near Glen Innes, northeastern New South Wales, Australia

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

Mantle Xenoliths from the Southeastern Slave Craton: Evidence for Chemical Zonation in a Thick, Cold Lithosphere

We present the first data on the petrology of the mantle lithosphereof the Southeastern (SE) Slave craton, Canada. These are basedon petrographic, mineralogical and geochemical studies of mantlexenoliths in Pipe 5034 of the Cambrian Gahcho Kué kimberlitecluster. Major types of mantle xenoliths include altered eclogite,coarse garnet or spinel peridotite, and deformed garnet peridotite.The peridotites belong to the low-temperature suite and formedat T=600–1300°C and P= 25–80 kbar in a thick(at least 220–250 km), cool lithosphere. The SE Slavemantle is cooler than the mantle of other Archaean cratons andthat below other terranes of the Slave craton. The thick lithosphereand the relatively cool thermal regime provide favourable conditionsfor formation and preservation of diamonds beneath the SE Slaveterrane. Similar to average Archaean mantle worldwide, the SESlave peridotite is depleted in magmaphile major elements andcontains olivine with forsterite content of 91–93·5.With respect to olivine composition and mode, all terranes ofthe Slave mantle show broadly similar compositions and are relativelyorthopyroxene-poor compared with those of the Kaapvaal and Siberiancratons. The SE Slave spinel peridotite is poorer in Al, Caand Fe, and richer in Mg than deeper garnet peridotite. Thegreater chemical depletion of the shallow upper mantle is typicalof all terranes of the Slave craton and may be common for thesubcontinental lithospheric peridotitic mantle in general. Peridotiticxenoliths of the SE Slave craton were impregnated by kimberliticfluids that caused late-stage recrystallization of primary clinopyroxene,spinel, olivine and spinel-facies orthopyroxene, and formationof interstitial clinopyroxene. This kimberlite-related recrystallizationdepleted primary pyroxenes and spinel in Al. The kimberliticfluid was oxidizing, Ti-, Fe- and K-rich, and Na-poor, and introducedserpentine, chlorite, phlogopite and spinel into peridotitesat P < 35 kbar. KEY WORDS: kimberlite xenolith; lithosphere; mantle terrane; chemical zoning; thermobarometry; Slave craton     

The Mineral Chemistry of Ultramafic Xenoliths of Eastern China: Implications for Upper Mantle Composition and the Paleogeotherms

Ultramafic xenoliths of garnet lherzolite (?rare spinel), spinellherzolites, spinel harzburgites, clinopyroxenites, and clinopyroxenemegacrysts were collected from Cenozoic basalts in all partsof eastern China. From their modal composition and mineral chemistryall the xenoliths may be placed into three types representing:a fertile or more primitive mantle (garnet lherzolite and spinellherzolite), a refractory or more depleted mantle (spinel harzburgiteand dunite), and inclusions cognate with the host alkali basaltsat mantle pressures (pyroxenite and megacrysts). There are systematicdifferences between the mineral compositions of each type. Spinelshows a wide compositional range and the spinel cr-number [100Cr/(Cr + Al)] is a significant indicator of the xenolithtype. Spinel cr-number and Al₂O₃ of coexisting minerals (spinel,clinopyroxene, and orthopyroxene) are useful as refractory indicatorsfor spinel peridotite in that the cr-number increases and thepercentage of Al₂O₃ decreases with increasing degrees of melting.In garnet peridotite, however, the same functions vary withpressure, not degree of melting. According to P–T estimates,the various xenoliths were derived from a large range of depthsin the upper mantle: spinel peridotite from approximately 11to 22 kb (37–66 km), spinel/garnet lherzolite from 19to 24 kb (62–80 km), and garnet lherzolite from 24 to25 kb (79–83 km). We conclude that the uppermost mantlebeneath eastern China is heterogeneous, with a north-northeastzone of more depleted mantle lying beneath the continental marginand a more primitive mantle occurring towards the continentalinterior.     

Geochemical syntheses among the cratonic,off-cratonic and orogenic garnet peridotites and their tectonic implications

Garnet-bearing mantle peridotites, occurring as either xenoliths in volcanic rocks or lenses/massifs in high-pressure and ultrahigh-pressure terrenes within orogens, preserve a record of deep lithospheric mantle processes. The garnet peridotite xenoliths record chemical equilibrium conditions of garnet-bearing mineral assemblage at temperatures (T) ranging from ~700 to 1,400°C and pressures (P) > 1.6–8.9 GPa, corresponding to depths of ~52–270 km. A characteristic mineral paragenesis includes Cr-bearing pyropic garnet (64–86 mol% pyrope; 0–10 wt% Cr₂O₃), Cr-rich diopside (0.5–3.5 wt% Cr₂O₃), Al-poor orthopyroxene (0–5 wt% Al₂O₃), high-Cr spinel (Cr/(Cr + Al) × 100 atomic ratio = 2–86) and olivine (88–94 mol% forsterite). In some cases, partial melting, re-equilibration involving garnet-breakdown, deformation, and mantle metasomatism by kimberlitic and/or carbonatitic melt percolations are documented. Isotope model ages of Archean and Proterozoic are ubiquitous, but Phanerozoic model ages are less common. In contrast, the orogenic peridotites were subjected to ultrahigh-pressure (UHP) metamorphism at temperature ranging from ~700 to 950°C and pressure >3.5–5.0 GPa, corresponding to depths of >110–150 km. The petrologic comparisons between 231 garnet peridotite xenoliths and 198 orogenic garnet peridotites revealed that (1) bulk-rock REE (rare earth element) concentrations in xenoliths are relatively high, (2) clinopyroxene and garnet in orogenic garnet peridotites show a highly fractionated REE pattern and Ce-negative anomaly, respectively, (3) Fo contents of olivines for off-cratonic xenolith are in turn lower than those of orogenic garnet and cratonic xenolith but mg-number of garnet for orogenic is less than that of off-cratonic and on-cratonic xenolith, (4) Al₂O₃, Cr₂O₃, CaO and Cr# of pyroxenes and chemical compositions of whole rocks are very different between these garnet peridotites, (5) orogenic garnet peridotites are characterized by low T and high P, off-cratonic by high T and low P, and cratonic by medium T and high P and (6) garnet peridotite xenoliths are of Archean or Proterozoic origin, whereas most of orogenic garnet peridotites are of Phanerozoic origin. Taking account of tectonic settings, a new orogenic garnet peridotite exhumation model, crust-mantle material mixing process, is proposed. The composition of lithospheric mantle is additionally constrained by comparisons and compiling of the off-cratonic, on-cratonic and orogenic garnet peridotite.     

Clinopyroxene of spinel-peridotite mantle xenoliths from Lake Nji (Cameroon Volcanic Line, W Africa): crystal chemistry and petrological implications

The crystal chemistry of six clinopyroxenes enclosed in protogranular spinel-peridotite mantle xenoliths from Lake Nji (Cameroon, W Africa) was studied by means of single-crystal X-ray diffraction and electron microprobe. These spinel-peridotite xenoliths are characterised by clinopyroxene contents distinctly higher than those reported by Lee et al. (1996) for spinel-peridotite xenoliths of the same region (19–11% and 15–8% respectively) and by high clinopyroxene/orthopyroxene ratios: 0.7–2.7 (present study) and about 0.4 (Lee et al. 1996). The clinopyroxene crystal chemistry indicates that the xenoliths show the compositional features of a mantle source contaminated by small-volume melts, responsible for the high clinopyroxene content. The protogranular texture of the Cameroon xenoliths (upper portion of lithospheric mantle) testifies that the spinel peridotite and the associated small-volume melts completely re-equilibrated at the spinel-peridotite facies. This is also supported by the petrological and geochemical data of Lee et al. (1996) on the Etinde–Biu Plateau spinel-peridotite xenoliths, which underwent Late Proterozoic–Early Paleozoic enrichment in incompatible trace elements. Received: 6 October 1999 / Accepted: 27 March 2000     

Ultrahigh-pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern Alps

New evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation.     

Ultra-depleted, shallow cratonic mantle beneath West Greenland: dunitic xenoliths from Ubekendt Ejland

Dunitic xenoliths from late Palaeogene, alkaline basalt flows on Ubekendt Ejland, West Greenland contain olivine with 100 × Mg/(Mg + Fe), or Mg#, between 92.0 and 93.7. Orthopyroxene has very low Al₂O₃ and CaO contents (0.024–1.639 and 0.062–0.275 wt%, respectively). Spinel has 100 × Cr/(Cr + Al), or Cr#, between 46.98 and 95.67. Clinopyroxene is absent. The osmium isotopic composition of olivine and spinel mineral separates shows a considerable span of ¹⁸⁷Os/¹⁸⁸Os values. The most unradiogenic ¹⁸⁷Os/¹⁸⁸Os value of 0.1046 corresponds to a Re-depletion age of ca. 3.3 Gy, while the most radiogenic value of 0.1336 is higher than present-day chondrite. The Os isotopic composition of the xenoliths is consistent with their origin as restites from a melt extraction event in the Archaean, followed by one or more subsequent metasomatic event(s). The high Cr# in spinel and low modal pyroxene of the Ubekendt Ejland xenoliths are similar to values of some highly depleted mantle peridotites from arc settings. However, highly depleted, arc-related peridotites have higher Cr# in spinel for a given proportion of modal olivine, compared to cratonic xenolith suites from Greenland, which instead form coherent trends with abyssal peridotites, dredged from modern mid-ocean ridges. This suggests that depleted cratonic harzburgites and dunites from shallow lithospheric mantle represent the residue from dry melting in the Archaean.     

The influence of water on melting of mantle peridotite

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     

Ultrahigh-pressure garnet peridotites from the devolatilization of sea-floor hydrated ultramafic rocks

Garnet peridotites occur in quartzofeldspathic gneisses in the Northern Qaidam Mountains, western China. They are rich in Mg and Cr, with mineral compositions similar to those in mantle peridotites found in other orogenic belts and as xenoliths in kimberlite. Garnet‐bearing lherzolites interlayered with dunite display oriented ilmenite and chromite lamellae in olivine and pyroxene lamellae in garnet that have been interpreted to indicate pressures in excess of 6 GPa. However, some garnet porphyroblasts include hornblende, chlorite and spinel + orthopyroxene symplectite after garnet; some clinopyroxene porphyroblasts include abundant actinolite/edenite, calcite and lizardite in the lherzolite; some olivine porphyroblasts (Fo₉₂) include an earlier generation Mg‐rich olivine (Fo_95–99), F‐rich clinohumite, pyroxene, chromite, anthophyllite/cummingtonite, Cl‐rich lizardite, carbonates and a new type of brittle mica, here termed ‘Ca‐phlogopite’, in the associated dunite. The pyrope content of garnet increases from core to rim, reaching the pyrope content (72 mol.%) of garnet typically found in the xenoliths in kimberlite. The simplest interpretation of these observations is that the rock association was formerly mantle peridotite emplaced into the oceanic crust that was subjected to serpentinization by seawater‐derived fluids near the sea floor. Dehydration during subduction to 3.0–3.5 GPa and 700 °C transformed these serpentinites into garnet lherzolite and dunite, depending on their Al and Ca contents. Pseudosection modelling using thermocalc shows that dehydration of the serpentinites is progressive, and involved three stages for Al‐rich and two stages for Al‐poor serpentinites, corresponding to the breakdown of the key hydrous minerals. Static burial and exhumation make olivine a pressure vessel for the pre‐subduction mineral inclusions during ultrahigh‐pressure (UHP) metamorphism. The time span of the UHP event is constrained by the clear interface between the two generations of olivine to be very short, implying rapid subduction and exhumation.     

揭示地质现象的本质与核心

青藏高原的形成与演化是举世瞩目的现代地学前缘 ,岩石圈结构及其演变对于探讨高原隆升及其资源环境效应具有重要意义。多年来 ,对于青藏高原岩石圈结构的认识主要基于地球物理探测结果和地质研究 ,尚未发现深达地幔的物质。我们在西昆仑地区康西瓦新生代火山岩中首次发现了以含金云母方辉橄榄岩为主的深源包体 ,其主要组成矿物为橄榄石和斜方辉石 ,含有少量单斜辉石、尖晶石和金云母。橄榄岩具有平衡粒状变晶结构 ,部分橄榄石颗粒仍保留有扭折带。电子探针分析结果表明 ,尖晶石的x(Cr) /x(Cr +Al)变化为 2 5%～ 8% ,属于尖晶石铬铁矿系列。根据CarrollWebb和Wood(1986 )提出的温压图解 ,橄榄岩形成压力大致为16 0 0MPa ,约相当于深度 56km。据此 ,认为康西瓦新生代玄武岩中产出的橄榄岩包体系岩浆活动过程中从岩石圈地幔携带上来的幔源物质 ,对于探讨青藏高原北部的岩石圈结构及其演化具有重要意义。     

Phlogopite-biotite parageneses from the K-mafic-carbonatite effusive magmatic association of Katwe-Kikorongo, SW Uganda

Summary Ti-bearing phlogopite-biotite is dominant in Ugandan kamafugite-carbonatite effusives and their entrained alkali clinopyroxenite xenoliths. It occurs as xeno/phenocrysts, microphenocrysts and groundmass minerals and also as a major xenolith mineral. Xenocrystic micas in kamafugites and carbonatites are aluminous (> 12 wt% Al₂O₃), typically contain significant levels of Cr (up to 1.1 wt% Cr₂O₃), and are Ba-poor. Microphenocryst and groundmass micas in feldspathoidal rocks extend to Al-poor compositions, are depleted in Cr, and are generally enriched in Ba. In general, xenocrystic micas occupy the Al₂O₃ and TiO₂ compositional field of the xenolith mica, and on the basis of Mg#, and high P, T experimental evidence they probably crystallised at mantle pressures. Mica xenocryst Cr contents range from those in Cr-poor megacryst and MARID phlogopite to higher values found in primary and metasomatic phlogopites in kimberlite-hosted peridotite xenoliths. Such Cr contents in Ugandan mica xenocrysts are considered consistent with derivation from carbonate-bearing phlogopite wehrlite and phlogopite-clinopyroxenite mantle. Olivine melilitite xenocryst micas are distinguished by higher Mg# and Cr content than mica in clinopyroxenite xenoliths and mica in Katwe-Kikorongo mixed melilitite-carbonatite tephra. Higher Al₂O₃ distinguishes Fort Portal carbonatite xenocrysts and some contain high Cr. It is suggested that the genesis of Katwe-Kikorongo olivine melilitite and Fort Portal carbonatite involves a carbonate-bearing phlogopite wehrlite source while the source of the mixed carbonatite-melilitite rocks may be carbonate-bearing phlogopite clinopyroxenite. Received January 24, 2000; revised version accepted September 27, 2001     

Erratum to: Iron isotope variations in spinel peridotite xenoliths from North China Craton: implications for mantle metasomatism

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

Petrochemical Study of Mantle Xenoliths in Alkali Basalts from South Korea: P/T Regime of the Upper Mantle

Mantle xenoliths in alkali basalt at three locations in South Korea—Boun, the Gansung area, and Baegryung Island—are spinel lherzolites composed of olivine, orthopyroxene, clinopyroxene, and spinel. The xenoliths generally display triple junctions between grains, kink-banding in olivine and pyroxenes, and protogranular and equigranular textures, with no preferred crystal orientation. Anhedral brown spinels occur interstitially. Minerals in lherzolites from each of the three localities are compositionally homogeneous. Olivine compositions have Fo_89.0 to Fo_90.2, low CaO (.03 to 0.12 wt%), and NiO of 0.34 to 0.40 wt%; the orthopyroxene is enstatite with En_89.0 to En_90.0 Al₂O₃ of 4 to 5 wt%; the clinopyroxene is diopside with En_47.2 to En_49.1 and Al₂O₃ of 7.42 to 7.64 wt% from Boun and 4.70 to 4.91 wt% from Baegryung. Spinel chemistry shows a distinct negative trend, with increasing Al corresponding with decreasing Cr, and Mg# and Cr# of 75.1 to 81.9 and 8.5 to 12.6, respectively.

Temperatures and pressures of equilibration for these mantle xenoliths were estimated using various pyroxene geothermometers (Wood and Banno, 1973; Wells, 1977; Mercier, 1980; Sachtleben and Seck, 1981; Bertrand and Mercier, 1985; Brey and Köhler, 1990) and the Al-solubility geobarometer (Mercier, 1980; Lane and Ganguly, 1980). Temperature estimates from the recipes of Mercier (1980) and Sachtleben and Seck (1981) are compatible. The equilibrium temperatures of these xenoliths, taken as the average obtained from these two methods, lie between 970 and 1020° C, and equilibrium pressures derived from Mercier (1980) fall within the range of 12 to 19 kbar (i.e., 42 to 63 km). These temperatures and pressures are reinforced by considerations of the Al-isopleths in the MAS system (Lane and Ganguly, 1980), as adjusted for the Fe effect on Al solubility in orthopyroxene (Lee and Ganguly, 1988).

The equilibrium temperatures and pressures of xenoliths, as considered in P/T space, belong to the oceanic geotherm, based upon the various mantle geotherms presented by Mercier (1980). This geotherm is completely different from continental geotherms, e.g., from South Africa (Lesotho) and southern India. Mineral compositions of spinel-lherzolites in South Korea and eastern China are primitive; paleogeotherms of both are quite similar, but degrees of depletion of the upper mantle could vary locally. This is demonstrated by eastern China, which has various depleted xenoliths caused by different degrees of partial melting.     

Feldspar-bearing lherzolite xenoliths in alkali basalts from Hamar-Daban,southern Baikal region,Russia

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