Spinel peridotite xenoliths from the Atsagin-Dush volcano, Dariganga lava plateau, Mongolia: a record of partial melting and cryptic metasomatism in the upper mantle

Spinel peridotite xenoliths from the Atsagin-Dush volcanic centre, SE Mongolia range from fertile lherzolites to clinopyroxene(cpx)-bearing harzburgites. The cpx-poor peridotites typically contain interstitial fine-grained material and silicate glass and abundant fluid inclusions in minerals, some have large vesicular melt pockets that apparently formed after primary clinopyroxene and spinel. No volatile-bearing minerals (amphibole, phlogopite, apatite, carbonate) have been found in any of the xenoliths. Fifteen peridotite xenoliths have been analysed for major and trace elements; whole-rock Sr isotope compositions and O isotope composition of all minerals were determined for 13 xenoliths. Trace element composition and Sr-Nd isotope compositions were also determined in 11 clinopyroxene and melt pocket separates. Regular variations of major and moderately incompatible trace elements (e.g. heavy-rare-earth elements) in the peridotite series are consistent with its formation as a result of variable degrees of melt extraction from a fertile lherzolite protolith. The Nd isotope compositions of LREE (light-rare-earth elements)-depleted clinopyroxenes indicate an old (≥ 1 billion years) depletion event. Clinopyroxene-rich lherzolites are commonly depleted in LREE and other incompatible trace elements whereas cpx-poor peridotites show metasomatic enrichment that can be related to the abundance of fine-grained interstitial material, glass and fluid inclusions in minerals. The absence of hydrous minerals, ubiquitous CO₂-rich microinclusions in the enriched samples and negative anomalies of Nb, Hf, Zr, and Ti in primitive mantle-normalized trace element patterns of whole rocks and clinopyroxenes indicate that carbonate melts may have been responsible for the metasomatic enrichment. Low Cu and S contents and high δ³⁴S values in whole-rock peridotites could be explained by interaction with oxidized fluids that may have been derived from subducted oceanic crust. The Sr-Nd isotope compositions of LREE-depleted clinopyroxenes plot either in the MORB (mid-ocean-ridge basalt) field or to the right of the mantle array, the latter may be due to enrichment in radiogenic Sr. The LREE-enriched clinopyroxenes and melt pockets plot in the ocean island-basalt field and have Sr-Nd isotope signatures consistent with derivation from a mixture of the DMM (depleted MORB mantle) and EM (enriched mantle) II sources. Received: 18 January 1996 / Accepted: 23 August 1996     

Geochemistry and petrogenesis of spinel lherzolite xenoliths from Boeun,Korea

Geochemical characteristics of spinel lherzolite xenoliths, enclosed in Miocene alkali basalt from Boeun, Korea, provide important clues for understanding the lithosphere composition, equilibrium temperature and pressure conditions, and depletion and enrichment processes of subcontinental lithospheric mantle beneath Boeun. The spinel lherzolite xenoliths with protogranular to porpyroclastic textures were accidentally trapped by the ascending alkali basalt magma. The spinel lherzolite xenoliths originated at depths between 50 and 63 km with equilibrium temperatures ranging from 847 to 1030 °C. These xenoliths may have undergone small degrees (1–2%) of partial melting and cryptic metasomatism by an alkali basaltic melt. Based on Sr and Nd isotope compositions, the subcontinental lithospheric mantle beneath Boeun was heterogeneous and similar to that beneath East China and Central Mongolia rather than the Japanese Island Arc.     

Sc,Cr, Co and Ni partitioning between minerals from spinel peridotite xenoliths

The partitioning of divalent (Co, Ni) and trivalent (Sc, Cr) trace elements between olivine, ortho- and clinopyroxene and spinel from spinel peridotite xenoliths has been investigated. These peridotites cover a wide range in modal composition from dunite to primitive lherzolites and have equilibrated in the upper mantle between >900° C and <1,200° C.The distribution of Co and Ni shows only minor variation through the whole sequence. In contrast, Sc partitioning between ortho- and clinopyroxene and olivine and clinopyroxene as well as Cr partitioning between olivine and clinopyroxene or olivine and orthopyroxene display high but systematic variations which can be assigned to dependences upon equilibration temperatues. Empirical temperature calibrations are given for Sc-orthopyroxene/clinopyroxene, Sc-olivine/clinopyroxene and Cr-olivine/clinopyroxene which, in principle, may permit to estimate equilibration temperatures not only for lherzolites or harzburgites but for orthopyroxene-free peridotites, too.Sc and Ni partition coefficients between spinel and mantle silicate minerals are primarily dependent upon the major element composition of spinel (e.g. Cr and Al) although a temperature dependence can still be identified. Probably such compositional effects are not observed for trace element partitioning between pyroxenes and olivine or ortho- and clinopyroxene only for the reason that in normal spinel peridotites these minerals show much less variation in major element composition than their coexisting spinels.     

Rare earth element partitioning between minerals from anhydrous spinel peridotite xenoliths

REE abundances in minerals from spinel peridotite xenoliths from West Germany, the south-western U.S. and Mongolia decrease in the order clinopyroxene > orthopyroxene > olivine > spinel. While clinopyroxenes are similar in absolute chondrite-normalized concentrations to those known from other studies, orthopyroxenes and olivines are significantly lower in LREE although comparable in HREE. Spinels are much lower in all REE than any previously reported values and are completely negligible for the REE budget of peridotites.Partition coefficients for most orthopyroxene/clinopyroxene pairs increase systematically from La to Lu. Olivine/clinopyroxene and spinel/clinopyroxene partition coefficients increase from the intermediate rare earth elements to Lu and normally are higher for La compared to Sm.The application of Nagasawa's (1966) elastic lattice model suggests that all heavy but only minor amounts of the light REE substitute into structural positions of orthopyroxene and olivine.Significant differences between orthopyroxene/clinopyroxene partition coefficients for various xenoliths may be assigned to dependences upon equilibration temperature and bulk chemistry.Apart from grain surface contaminations, fluid inclusions which are practically always present in mantle minerals, can highly concentrate light rare earth elements and thus may be responsible for unexpectedly high concentrations of incompatible elements frequently reported for mantle olivines or orthopyroxenes.     

Petrology of spinel harzburgite xenoliths from the Kishb Plateau,Saudi Arabia

Two spinel harzburgite xenoliths from a Pleistocene alkali basalt unit erupted at the northwestern corner of the Tertiary Kishb Plateau (Saudi Arabia) are characterized by an incipient transition from protogranular to porphyroclastic texture. Vermicular and interstitial spinels are closely associated with neoblasts of olivine, enstatite, and diopside. Sparse exsolution lamellae of high-Ca pyroxene occur in all the enstatite porphyroblasts. Olivine neoblasts are, in many cases, in contact with one another, with the triple grain junctions rarely approaching 120°. Chemical zoning is undetectable by microprobe in spinel and olivine, whereas zoning of Al in enstatite and diopside indicates that chemical equilibrium was not attained. Clear, palegreen glasses occur as veinlets about 10 microns or less in width along grain boundaries and cracks. Consistent counting rates for Na in these glasses were obtained only at 5 kV with a sample current of about 6 namps and counting time of less than 7 s. These glasses are chemically homogeneous and are characterized by relatively high contents of SiO₂ (55.8–58.7 wt%), Na₂O (6.4–7.6 wt%), and Al₂O₃ (20.0–21.6 wt%), with inferred volatile contents of less than 1 wt%. The glass is suggested to be of upper mantle origin rather than having developed from the host basalt or by decompressional melting upon ascent.Geothermometry and geobarometry indicate that the lithospheric upper mantle beneath the Arabian Shield had been locally heated to higher than 1,050° C during Miocene/ Pliocene, resulting in some degree of partial melting. Spinel was formed by reaction between aluminous pyroxenes and olivine during subsequent cooling, and intercrystalline Mg-Fe exchange reached a steady state at about 800° C. The geotherm beneath the Arabian Shield since Miocene is estimated to be somewhat lower than that representing the present oceanic upper mantle. The thermal history established is consistent with the tectonic history of the Red Sea area and indicates a two-stage magmatism in the Arabian Shield since Miocene.     

Petrology and geochemistry of spinel peridotite xenoliths from Hannuoba and Qixia, North China craton

We report mineralogical and chemical compositions of spinel peridotite xenoliths from two Tertiary alkali basalt localities on the Archean North China craton (Hannuoba, located in the central orogenic block, and Qixia, in the eastern block). The two peridotite suites have major element compositions that are indistinguishable from each other and reflect variable degrees (0–25%) of melt extraction from a primitive mantle source. Their compositions are markedly different from typical cratonic lithosphere, consistent with previous suggestions for removal of the Archean mantle lithosphere beneath this craton. Our previously published Os isotopic results for these samples [Earth Planet. Sci. Lett. 198 (2002) 307] show that lithosphere replacement occurred in the Paleoproterozoic beneath Hannuoba, but in the Phanerozoic beneath Qixia. Thus, we see no evidence for a compositional distinction between Proterozoic and Phanerozoic continental lithospheric mantle. The Hannuoba xenoliths equilibrated over a more extensive temperature (hence depth) interval than the Qixia xenoliths. Neither suite shows a correlation between equilibration temperature and major element composition, indicating that the lithosphere is not chemically stratified in either area. Trace element and Sr and Nd isotopic compositions of the Hannuoba xenoliths reflect recent metasomatic overprinting that is not related to the Tertiary magmatism in this area.     

Geochemistry and mineralogy of two spinel peridotite suites from Dreiser Weiher,West Germany

Among the spinel peridotite nodules from Dreiser Weiher. West Germany which represent fragments of the earth's upper mantle two series may be distinguished. One group (Ib) is anhydrous while the second one (Ia) is characterized by the presence of amphibole and/or breakdown-products of amphibole. Both suites display a wide range in modal composition. Pyroxene geothermometry yields equilibration temperatures of ~ 1150°C for group Ib and ~950°C for group Ia.Rare earth element (REE) patterns reveal marked differences between both groups: nearly unfractionated, light REE depleted or slightly light REE enriched chondrite normalized patterns in nodules from group Ib, but a high relative light REE enrichment in the amphibole containing suite Ia. An inverse correlation between the clinopyroxene content (or CaO) and relative light REE enrichment is observed in group Ia only. Two sources of light REE enrichment of the nodules can be distinguished: The first is a contaminant on mineral surfaces and can be removed by acid leaching. The second is an integral part of the constituent minerals of the nodules. Trace element abundances and modelling show that nodules of group Ib cannot be derived from upper mantle pyrolite by a one-stage partial melting process. Nodules of type Ia are interpreted as being the result of a reaction between Ib-type mantle and a fluid or liquid which provides H₂O and incompatible elements. At Dreiser Weiher this ‘open system’ upper mantle metasomatism may be related to the young uplift of the Rhenish Shield in a similar way as previously proposed by Lloyd and Bailey (1975).A model for the lithospheric mantle below the Westeifel is different from the San Carlos model proposed by Frey and Prinz (1978) and more complex.     

内蒙古集宁新生代玄武岩中橄榄岩包体和巨晶的发现及意义

内蒙古集宁新生代玄武岩产于华北克拉通北缘，属于广义的“汉诺坝玄武岩”。首次在三义堂附近的碱性玄武岩中发现了大量的橄榄岩包体、辉石和长石以及少量的含钛磁铁矿巨晶。包体主要是尖晶石二辉橄榄岩，造岩矿物为橄榄石＋斜方辉石＋单斜辉石＋尖晶石，辉石巨晶主要为透辉石和普通辉石，长石巨晶主要是歪长石和少量斜长石。1．5GPa条件下，尖晶石二辉橄榄岩样品所记录的平衡温度与汉诺坝相近，集中于950℃左右，大体上反映了华北克拉通北缘大陆岩石圈地幔尖晶石相部分的温度状态。单斜辉石巨晶的结晶温压大于幔源包体的平衡温压，表明巨晶的来源深度可能大于包体。包体的结构及矿物成分分析表明，这些包体是玄武质岩浆上升过程中偶然捕获岩石圈地幔的岩石碎块。二辉橄榄岩包体中橄榄石高Mg#值（89．5～91．7）和较高的NiO含量（0．29％～0．55％），暗示集宁玄武岩中的橄榄岩包体来自较高熔融程度的岩石圈地幔。     

内蒙古阿巴嘎地幔岩捕掳体与岩石圈地幔性质探讨

阿巴嘎地区新生代地幔捕掳体岩性主要为尖晶石二辉橄榄岩,矿物组成有橄榄石(Fo89-91)、单斜辉石(Wo43-49En44-49Fs3-11)、斜方辉石(Wo0.7-2.2En88-91Fs8-11)和铬尖晶石(Cr6#-20)。橄榄岩结构以碎斑结构为主,除此还可见海绵边结构、重结晶结构、粗粒结构和变晶结构,部分橄榄石发育膝折带和伊丁石化现象,这些特征暗示该区地幔经历了较强烈的熔体-岩石圈相互作用和复杂的演化过程。大地构造位置上,阿巴嘎与大兴安岭哈拉哈河-绰尔河及五大连池-科洛同处在兴蒙造山带上,但阿巴嘎表现出与华北克拉通西部北缘相似的热状态及饱满型-过渡型的岩石圈地幔性质,区别于大兴安岭中部具有古老残留的岩石圈地幔,暗示着兴蒙造山带之下的岩石圈地幔存在时空上的不均一性。而这种大尺度岩石圈地幔的不均一性可能与软流圈-岩石圈相互作用程度、上地幔复杂的地质演化及兴蒙造山带内部不同块体的活动强度、时间等性质有关。     

Rare earth abundances and distribution in some spinel peridotite xenoliths from Assab (Ethiopia)

REE partition data between solid phases in nine spinel peridotite xenoliths from Assab (Ethiopia) are presented together with bulk-rock REE composition. REE partitioning between clinopyroxene and other coexisting solid phases varies over a relatively wide range. The largest ranges are for LREE in clinopyroxene/orthopyroxene and clinopyroxene/olivine pairs, while clinopyroxene/spinel partitioning of REE is more restricted. The range of REE partition values between coexisting phases is due to compositional dependency effects and is correlated with systematic variations in major element composition of the bulk rocks. The measured REE concentration in the Assab mantle harzburgites do not match with the compositions calculated by mass balance from the modal proportions and REE analyses in individual phases. Inconsistencies for HREE may be due to variable HREE amounts in the clinopyroxene, orthopyroxene phases within a single specimen, while the high LREE contents in the whole rocks are due to contamination during transport to the surface. A geochemical model based on theoretical treatment of the REE partition data suggests that the Assab harzburgites acquired their residual character during a batch melting episode in the upper mantle under the Afar region.     

The lithosphere beneath the Sangilen Plateau, Siberia: evidence from peridotite, pyroxenite and gabbro xenoliths from alkaline basalts

Summary Ultramafic and mafic xenoliths in Ordovician Agardag alkaline basalt dikes from the Sangilen Plateau, southeastern Siberia, provide samples from the upper mantle and crust beneath central Asia. Three major groups were distinguished among the xenoliths: Group I xenoliths are spinel lherzolites, Group II xenoliths are spinel-garnet clinopyroxenites, and Group III comprises gabbroic xenoliths with two subgroups: Group IIIa comprises garnet bearing gabbroids and Group IIIb is represented by garnet-free gabbroids. The spinel lherzolite xenoliths represent the uppermost lithospheric mantle beneath the Sangilen Plateau and have geochemical characteristics similar to those of primitive mantle. Spinel-garnet clinopyroxenite and gabbroic xenoliths are of igneous origin and represent fragments of intrusive bodies crystallized at depths close to the mantle-crust boundary, as well as in the lower and the upper crust. The gabbroic xenoliths are evidently the crystallization products of melts similar in major and trace element composition to parental magma of the Bashkymugur gabbronorite-monzodiorite intrusion. Gabbroic xenoliths from the Ordovician Agardag alkaline basalt dikes demonstrate the presence of intermediate magmatic chambers within the crust beneath the Sangilen Plateau during the Early Palaeozoic. The relatively high equilibration temperatures of the mantle and lower crust xenoliths in the Agardag alkaline basalt dikes are largely attributable to a plume occurring beneath the Sangilen Plateau during the Ordovician.     

Depleted and enriched mantle processes under the Rio Grande rift: spinel peridotite xenoliths

Upper mantle xenoliths from the southern Rio Grande rift axis (Potrillo and Elephant Butte) and flank (Adam’s Diggings) have been investigated to determine chemical depletion and enrichment processes. The variation of modal, whole rock, and mineral compositions reflect melt extraction. Fractional melting is the likely process. Fractional melting calculations show that most spinel peridotites from rift axis locations have undergone <5% melting versus 7–14% melting for xenoliths from the rift shoulder, although the total range of fractional melting overlaps at all three locations. In the rift axis, deformed (equigranular and porphyroclastic texture) spinel peridotites are generally characterized by significantly less fractional melting (2–5%) than undeformed (protogranular) xenoliths (up to 16%). This difference may reflect undeformed xenoliths being derived from greater depths and higher temperatures than deformed rocks. Spinel peridotites from the axis and shoulder of the Rio Grande rift have undergone mantle metasomatism subsequent to melt extraction. Under the rift shoulder spinel peridotites have undergone both cryptic and patent (modal) metasomatism, possibly during separate events, whereas the upper mantle under the rift axis has undergone only cryptic metasomatism by alkali basaltic magma.     

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.     

Hf isotope compositions and HREE variations in off-craton garnet and spinel peridotite xenoliths from central Asia

Garnet-facies continental mantle is poorly understood because the vast majority of mantle xenoliths in continental basalts are spinel peridotite. Peridotite xenoliths from Vitim (southern Siberia) and Mongolia provide some of the best samples of garnet and garnet-spinel facies off-craton lithospheric mantle. Garnets in those fertile to moderately depleted lherzolites show a surprisingly broad range of HREE abundances, which poorly correlate with modal and major oxide compositions. Some garnets are zoned and have Lu-rich cores. We argue that these features indicate HREE redistribution after the partial melting, possibly related to spinel-garnet phase transition on isobaric cooling. Most peridotites from Vitim have depleted to ultra-depleted Hf isotope compositions (calculated from mineral analyses: ε_Hf(0) = +17 to +45). HREE-rich garnets have the most radiogenic ε_Hf values and plot above the mantle Hf-Nd isotope array while xenoliths with normal HREE abundances usually fall within or near the depleted end of the MORB field. Model Hf isotope ages for the normal peridotites indicate an origin by ancient partial melt extraction from primitive mantle, most likely in the Proterozoic. By contrast, an HREE-rich peridotite yields a Phanerozoic model age, possibly reflecting overprinting of the ancient partial melting record with that related to a recent enrichment in Lu. Clinopyroxene-garnet Lu-Hf isochron ages (31-84 Ma) are higher than the likely eruption age of the host volcanic rocks (∼16 Ma). Garnet-controlled HREE migration during spinel-garnet and garnet-spinel phase transitions may be one explanation for extremely radiogenic ¹⁷⁶Hf/¹⁷⁷Hf reported for some mantle peridotites; it may also contribute to Hf isotope variations in sub-lithospheric source regions of mantle-derived magmas.     

新疆西克尔碧玄岩中的地幔橄榄岩包体

在新疆西克尔地区发现了尖晶石相橄榄岩包体.这些包体的寄主岩石为碧玄岩,其K-Ar同位素年龄为19.76～21.90 Ma.岩相学和矿物化学研究表明西克尔橄榄岩包体具有典型的岩石圈地幔橄榄岩包体的特征.利用矿物温压计对包体的平衡温压进行估算,发现西克尔地幔橄榄岩包体的平衡温压为736～1017℃和1.7～2.2GPa,与西南天山托云地区晚白垩纪火山岩中尖晶石二辉橄榄岩包体(平衡温压为818～1113℃和1.5～2.0GPa)相比,具有温度明显偏低,而压力明显偏高的特点.这说明西克尔地区的地幔橄榄岩包体没有受到地幔热异常事件的影响,因此可以代表塔里木板块岩石圈地幔的原始性质.这对于研究塔里木盆地岩石圈地幔的热结构和地球化学特征以及塔里木盆地内大量幔源岩浆的成因具有重要意义.     

Clinopyroxene-orthopyroxene major and rare earth elements partitioning in spinel peridotite xenoliths from Assab (Ethiopia)

Major element and rare earth element (REE) partitioning among coexisting clinopyroxene-orthopyroxene pairs from mantle xenoliths of the Assab Range (Ethiopia) are discussed in terms of crystal-chemistry.Major element partitioning indicates relatively uniform conditions of subsolidus equilibration over a narrow range of temperatures (mean value about 1100 C) in the spinel peridotite stability field. Major element distributions and correlations, moreover, seem to indicate that the mantle material studied underwent slightly different depletions prior to the metamorphic equilibration.In spite of the rather homogeneous major element compositions for both cpx and opx, clinopyroxenes show chondrite-normalized REE patterns which are widely variable both in shape and absolute values, whereas orthopyroxenes exhibit more restricted ranges and concordant profiles.REE activity ratios have been investigated by applying Iiyama's (Bull. Soc. fr. Minéral. Christallogr.97, 143–151) thermodynamic model: the estimated activity patterns exhibit a good coherence for the different pyroxene pairs, in spite of the contrasting features of their REE concentration ratios. The wide ranges in the measured partition values for the same rare earth element in different pyroxene pairs have been related to coupled substitutions involving A1 in the Z site and REE in the M₂ site of clinopyroxene.     

Trace element compositions of minerals in garnet and spinel peridotite xenoliths from the Vitim volcanic field, Transbaikalia, eastern Siberia

Peridotite xenoliths from the Bereya alkali picrite tuff in the Vitim volcanic province of Transbaikalia consist of garnet lherzolite, garnet–spinel lherzolite and spinel lherzolite varieties. The volcanism is related to the Cenozoic Baikal Rift. All peridotites come from pressures of 20–23 kbar close to the garnet to spinel peridotite transition depth, and the presence of garnet can be attributed to cooling of spinel peridotites, probably during formation of the lithosphere. The peridotites show petrographic and mineral chemical evidence for infiltration by an alkaline silicate melt shortly before their transport to the Earth's surface. The melt infiltration event is indicated petrographically by clinopyroxenes which mimic melt morphologies, and post-dates outer kelyphitic rims on garnets which are attributed to an isochemical heating event within the mantle before transport to the Earth's surface. Single-mineral thermometry gives reasonable temperature estimates of 1050±50°C, whereas two-mineral methods involving clinopyroxene are falsified by secondary components in clinopyroxene introduced during the melt infiltration event. Excimer Laser–ICP-MS analysis has been performed for an extensive palette of both incompatible and compatible trace elements, and manifests the most thorough dataset available for this rock type. Orthopyroxene and garnet show only partial equilibration of trace elements with the infiltrating melt, whereas clinopyroxene and amphibole are close to equilibration with the melt and with each other. The incompatible element composition of the infiltrating melt calculated from the clinopyroxene and amphibole analyses via experimental mineral/melt partition coefficients is similar to the host alkali picrite, and probably represents a low melt fraction from a similar source during rift propagation. The chemistry and chronology of the events recorded in the xenoliths delineates the series of events expected during the influence of an expanding rift region in the upper mantle, namely the progressive erosion of the lithosphere and the episodic upward and outward propagation of melts, resulting in the evolution of the Vitim volcanic field.     

Compositional variations and heterogeneity in fertile lithospheric mantle: peridotite xenoliths in basalts from Tariat,Mongolia

Clinopyroxene-rich, poorly metasomatised spinel lherzolites are rare worldwide but predominate among xenoliths in five Quaternary basaltic eruption centres in Tariat, central Mongolia. High-precision analyses of the most fertile Tariat lherzolites are used to evaluate estimates of primitive mantle compositions; they indicate Mg#_PM = 0.890 while lower Mg# in the mantle are likely related to metasomatic enrichments in iron. Within a 10 × 20 km area, and between ~45 and ≥60 km depth, the sampled xenoliths suggest that the Tariat mantle does not show km-scale chemical heterogeneities and mainly consists of residues after low-degree melt extraction at 1–3 GPa. However, accessory (<1%) amphibole and phlogopite are unevenly distributed beneath the eruption centres. Ca abundances in olivine are controlled by temperature whereas Al and Cr abundances also depend on Cr/Al in coexisting spinel. Comparisons of conventional and high-precision analyses obtained for 30 xenoliths show that high-quality data, in particular for whole-rocks and olivines, are essential to constrain the origin of mantle peridotites. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Two diamond-bearing peridotite xenoliths from the finsch kimberlite,South Africa

Two diamond bearing xenoliths found at Finsch Mine are coarse garnet lherzolites, texturally and chemically similar to the dominant mantle xenoliths in that kimberlite. A total of 46 diamonds weighing 0.053 carats have been recovered from one and 53 diamonds weighing 0.332 carats from the other. The diamonds are less corroded than diamonds recovered from the kimberlite. Geothermobarometric calculations indicate that the xenoliths equilibrated at 1,130° C and pressures 50 kb which is within the diamond stability field; this corresponds to depths of 160 km and would place the rocks on a shield geotherm at slightly greater depths than most coarse garnet lherzolites from kimberlite. The primary minerals in the two rocks are very similar to each other but distinctly different to the majority of mineral inclusions in Finsch diamonds. This suggests a different origin for the diamonds in the kimberlite and the diamonds in the xenoliths although the equilibration conditions for both suites are approximately coincident and close to the wet peridotite solidus.     

Garnet-spinel-pyroxenite xenoliths from hyblean plateau (South-eastern Sicily, Italy)

Summary Rare garnet-spinel pyroxenite xenoliths occur in some basaltic tuff-breccia levels of Miocene age from the Valle Guffari (Hyblean Plateau, Sicily), together with a number of spinel-bearing mantle xenoliths. The garnet-bearing pyroxenites may be divided into two groups (a and b) on textural and mineralogical bases. Garnet-bearing spinel websterites with a fully recrystallized texture represent the first group (a). Here the garnet (Py_54.5 A1m₃₂ Gr_13.5), with a diffuse kelyphitic alteration, forms a reaction corona between coarse spinel grains and the in contact pyroxenes. The transition from the spinel-pyroxenite to the garnet-pyroxenite field may depend on isobaric cooling from higher (magmatic?) temperatures. Garnet-pyroxene geothermometry indicates that the last equilibration most probably occurred at P = 1.0 GPa (ca.), T = 750 °C (ca).The second lithotype (b) is an orthopyroxene-bearing garnet-spinel clinopyroxenite, exhibiting a complex texture. It consists of zones of coarse clinopyroxene grains enclosing euhedral spinel passing to zones where tiny rounded crystals of the same pyroxene and spinel are enclosed in relatively large patches of extensively kelyphitisized garnet (Py_64.8 Alm_25.6 Gr_9.6). Garnet also occurs as inclusion-free grains up to 4 mm in diameter. P-T calculations give significantly higher values than for the former case (a). The origin of the b-type garnet may also depend on subsolidus reaction of spinel and pyroxenes after an isobaric cooling from still higher temperatures, but a primary magrnatic origin might also be possible, especially for the granular garnets.P-T estimates for both the pyroxenite types closely match a steady geotherm for 100 mW/m² surface heat flow. Such a relatively intense heat flow may suggest the occurrence of huge masses of hot magma intruding the Hyblean lithospheric mantle and lower crust at different levels.

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Granat-Spinell-Pyroxenit-Xenolithe aus dem Iblei-Plateau (Südost-Sizilien, Italien)

Zusammenfassung Seltene Granat-Spinell-Pyroxenit-Xenolithe kommen in einigen basaltischen Tuff-Breckzien Horizonten miozänen Alters aus dem Valle Guffari (lblei-Plateau, Sizilien) zusammen mit einer Anzahl von Spinell-führenden Mantel-Xenolithen vor. Aufgrund textureller und mineralogischer Kriterien können die Granat-führenden Pyroxenite in zwei Gruppen (a und b) unterteilt werden. Granat-führende Spinell-Websterite mit vollkommen rekristallisierter Textur repräsentieren die erste Gruppe (a). Hier bildet Granat (Py_54.5 Alm₃₂ Gr_13.5) mit einer diffusen kelyphitischen Umwandlung, einen Reaktionssaum zwischen grobkörnigem Spinell und Pyroxenen, mit denen er in Kontakt ist. Der übergang vom Spinell-Pyroxenit- zum Granat-Pyroxenit-Feld kann auf isobarische Abkühlung von höheren (magmatischen ?) Temperaturen zurückgehen. Granat-Pyroxen-Geothermometrie zeigt, dass die letzte Equilibrierung sehr wahrscheinlich bei P = 1.0 GPa (ca.), T = 750°C (ca.) erfolgte.Der zweite Typ von Granat-führenden Pyroxeniten ist ein (b) Orthopyroxenführender Granat-Spinell-Klinopyroxenit, der komplexe Texturen zeigt. Er besteht aus Zonen von grobkörnigem Klinopyroxen mit Einschlüssen von idiomorphem Spinell, der in Zonen übergeht, wo kleine gerundete Kristalle des gleichen Pyroxens und Spinells in relativ große Bereiche von extensiv kelyphitisiertem Granat (Py_64,8 Alm_25,6 Gr_9,6) eingeschlossen sind. Granat kommt auch als einschlußfreie Körner mit bis zu 4 mm Durchmesser vor. P-T Berechnungen geben wesentlich höhere Werte als für die Gesteine des Types (a). Die Entstehung der b-Typ-Granaten kann auch durch Subsolidus-Reaktion von Spinell und Pyroxen nach isobarischer Abkühlung von noch höheren Temperaturen beeinflußt sein; ein primärer magmatischer Ursprung könnte auch möglich sein, besonders für die körnigen Granate.P-T Abschätzungen für beide Pyroxenit-Typen sind gut einer Geotherme für 100 mW/m² Wärmefluß an der Oberfläche zuzuordnen. Ein solcher, relativ intensiver Wärmefluß könnte auf das Vorkommen von großen heißen Magmenkörpern hinweisen, die den lithosphärischen Mantel unter dem Iblei-Plateau und die untere Kruste in verschiedenen Niveaus intrudierten.

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With 4 Figures