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1.
Garnet and spinel peridotite xenoliths associated with the Phanerozoic Lambert-Amery Rift in eastern Antarctica contain evidence for several stages in the development of the mantle beneath the rift. Despite the fact that equilibria were only partly attained, a combination of petrography, whole-rock geochemistry, mineral chemistry and thermobarometry can be used to decipher four stages prior to entrainment of the xenoliths in the host magma during the initial stages of the breakup of Antarctica, India and Madagascar. The first chronological stage is represented by harzburgitic protoliths represented by rare occurrences of low-Ca olivines and orthopyroxenes in spinel lherzolites: these yield the lowest temperatures of 830-850 °C, and are also characterized by distinct trace element contents; lower Ti, Cr, V and Zn in olivine and orthopyroxene, and additionally lower Cu, Ni, Ga and Li in orthopyroxene. Some garnets are subcalcic, indicating that the spinel-garnet lherzolites also formed from harzburgitic protoliths. The second stage is the formation of garnet due to a pressure increase probably related to collision at 1.1 Ga. The third stage is marked by the growth of clinopyroxene, demonstrably in cpx-poor spinel lherzolites but probably in all xenolith groups: equilibrium of clinopyroxene with olivine and orthopyroxene was not attained in all samples, so that the non-judicious use of thermobarometers can produce bewildering results. The fourth stage is an enrichment episode that affected all spinel-garnet peridotites and about half of the spinel peridotites. During this stage, reaction rims were produced on the clinopyroxenes that formed during stage 3, the modal content of olivine and Mg/(Mg + Fe) in the rocks was reduced, CaO, Al2O3 and trace elements were enriched, and garnets were almost completely transformed to kelyphites. A later stage is documented by interstitial glasses and films around spinels related to infiltration of melt from the host magma. These post-date, and are more enriched in alkalies than, partially melted rims on clinopyroxenes, demonstrating that all the three earlier episodes were pre-entrainment events. Pressures indicated by the spinel + garnet lherzolites are restricted to 20-24 kbar at 1040-1180 °C. Early harzburgitic assemblages are interpreted to represent an earlier, cooler geotherm, whereas the kelyphite assemblages indicate temperatures 180-200 °C hotter than the main xenolith geotherm. This event also caused recrystallization of the clinopyroxene rims and is attributed to heating during rifting, but not due to the host magma itself. The preservation of evidence for three progressively hotter geotherms can be related to the upward movement of isotherms during the development of the sub-rift mantle.  相似文献   

2.
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.  相似文献   

3.
Li behaviour and distribution in the mantle were investigated by ion microprobe in situ measurements on co-existing olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and amphibole (amp) in xenoliths from the French Massif Central. The fertile spinel lherzolites of this study record increasing degrees of mantle metasomatism, from unmetasomatised anhydrous samples through cryptically metasomatised samples to highly metasomatised amphibole-rich samples. In anhydrous lherzolites, Li is preferentially incorporated into olivine (1.1-1.4 ppm, average values) compared to pyroxenes (0.2-0.9 ppm). The hydrous samples clearly show enrichment of Li in ol (1.5-5.0 ppm), opx (1.1-2.4 ppm) and cpx (2.4-5.4 ppm), while amphibole incorporates less Li than the co-existing phases (0.8-1.3 ppm). Average δ7Li values range from +7.6 to +14.5‰ in ol, from 5.1 to +13.7‰ in opx and from 8.8 to +10.3‰ in cpx from the anhydrous lherzolites. A layered peridotite sample (Sdi) shows higher Li content in all phases, with lighter isotopic composition in opx and cpx (−0.6 and −2‰ average δ7Li values, respectively). In the hydrous lherzolites average δ7Li values both overlap and extend beyond these ranges in ol (up to 17.5 ‰) and in opx (up to 22.9‰), and vary widely in cpx (−2.7 to +9.7‰). Low δ7Li values are observed in some opx (−10.4‰) and cpx (−13‰) from sample Sdi, and in cpx from three hydrous samples (from −9.7 to −5.3‰). The different anhydrous phases from the hydrous samples show large intra-grain variations in Li isotopic ratios (e.g., up to 18‰) compared to the same phases from the anhydrous samples (mostly less than 6‰), excepting sample Sdi which has up to 20.4‰ variation in cpx. Similar to the anhydrous silicates, amphiboles show a wide variation of δ7Li values on the intra-grain scale (2-27‰). These variations are interpreted to result from fractionation processes during metasomatism by a silicate melt undergoing compositional changes as it percolates through and reacts with the peridotite phases. Thus Li abundances and isotopic in situ measurements are useful for tracing metasomatic processes but the heterogeneities observed in the samples preclude any identification of a specific mantle source by its Li signature.  相似文献   

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

5.
Mantle peridotites entrained as xenoliths in the lavas of Ngao Bilta in the eastern branch of the continental Cameroon Line were examined to constrain mantle processes and the origin and nature of melts that have modified the upper mantle beneath the Cameroon Line.The xenoliths consist mainly of lherzolite with subordinate harzburgite and dunite.They commonly contain olivine,orthopyroxene,clinopyroxene and spinel although the dunite is spinel-free.Amphibole is an essential constituent in the lherzolites.Mineral chemistry differs between the three types of peridotite:olivines have usual mantle-like Mg#of around 90 in lherzolites,but follow a trend of decreasing Mg#(to 82)and NiO(to 0.06 wt.%)that is continuous in the dunites.Lherzolites also contain orthopyroxenes and/or clinopyroxenes with low-Mg#,indicating a reaction that removes Opx and introduces Cpx,olivine,amphibole and spinel.This is attributed to reaction with a silica-undersaturated silicate melt such as nephelinite or basanite,which originated as a low-degree melt from a depleted source as indicated by low Al2O3 and Na2O in Cpx and high Na2O/K2O in amphibole.Thermobarometric estimates place the xenoliths at pressures of 11–15 kbar(35–50 km)and temperatures of 863–957C,along a dynamic rift geotherm and shallower than the region where carbonate melts may occur.The melt/rock reactions exhibited by the Ngao Bilta xenoliths are consistent with their peripheral position in the eastern branch of the Cameroon Volcanic Line in an area of thinned crust and lithosphere beneath the Adamawa Uplift.  相似文献   

6.
A suite of spinel lherzolite and wehrlite xenoliths from a Devonian kimberlite dyke near Kandalaksha, Kola Peninsula, Russia, has been studied to determine the nature of the lithospheric mantle beneath the northern Baltic Shield. Olivine modal estimates and Fo content in the spinel lherzolite xenoliths reveal that the lithosphere beneath the Archaean–Proterozoic crust has some similarities to Phanerozoic lithospheric mantle elsewhere. Modal metasomatism is indicated by the presence of Ti-rich and Ti-poor phlogopite, pargasite, apatite and picroilmenite in the xenoliths. Wehrlite xenoliths are considered to represent localised high-pressure cumulates from mafic–ultramafic melts trapped within the mantle as veins or lenses. Equilibration temperatures range from 775 to 969 °C for the spinel lherzolite xenoliths and from 817 to 904 °C for the wehrlites.

Laser ablation ICP-MS data for incompatible trace elements in primary clinopyroxenes and metasomatic amphiboles from the spinel lherzolites show moderate levels of LREE enrichment. Replacement clinopyroxenes in the wehrlites are less enriched in LREE but richer in TiO2. Fractional melt modelling for Y and Yb concentrations in clinopyroxenes from the spinel lherzolites indicates 7–8% partial melting of a primitive source. Such a volume of partial melt could be related to the 2.4–2.5 Ga intrusion of basaltic magmas (now metamorphosed to garnet granulites) in the lower crust of the northern Baltic Shield. The lithosphere beneath the Kola Peninsula has undergone several episodes of metasomatism. Both the spinel lherzolites and wehrlites were subjected to an incomplete carbonatitic metasomatic event, probably related to an early carbonatitic phase associated with the 360–380 Ma Devonian alkaline magmatism. This resulted in crystallisation of secondary clinopyroxene rims at the expense of primary orthopyroxenes, with development of secondary forsteritic olivine and apatite. Two separate metasomatic events resulted in the crystallisation of the Ti–Fe-rich amphibole, phlogopite and ilmenite in the wehrlites and the low Ti–Fe amphibole and phlogopite in the spinel lherzolites. Alternatively, a single metasomatic event with a chemically evolving melt may have produced the significant compositional differences seen in the amphibole and phlogopite between the spinel lherzolites and wehrlites. The calculated REE pattern of a melt in equilibrium with clinopyroxenes from a cpx-rich pocket is identical to that of the kimberlite host, indicating a close petrological relationship.  相似文献   


7.
Numerous lenticular bodies of ultramafic rocks occur withinthe upper amphibolite- to granulitefacies metamorphic terraneof the Austrides between the Non and Ultimo valleys (Nonsbergregion), northern Italy. The ultramafic rocks are divided intotwo textural types: (a) coarse-type; and (b) finetype. The coarse-typerocks have the protogranular texture and are predominantly spinellherzolite. Some coarse-type spinel lherzolites have partlytransformed to garnet lherzolite. The fine-types are consideredto be metamorphic derivatives of the former, and the observedmineral assemblages are: (1) olivine + orthopyroxene + clinopyroxene+ garnet + amphibole ? spinel, (2) olivine + orthopyroxene +garnet + amphibole + spinel; (3) olivine + orthopyroxene + amphibole+ spinel; and (4) olivine+ orthopyroxene + amphibole + chlorite.Based on the microprobe analyses of constituent minerals fromten representative peridotite samples, physical conditions ofthe metamorphism, particularly that of the spinel to garnetlherzolite transformation, are estimated. Applications of pyroxenegeothermometry yield temperature estimates of 1100–1300?Cfor the formation of the primary spinel lherzolite, and 700–800?Cfor that of the fine-type peridotites. A pressure range of 16–28kb is obtained for the garnet lherzolite crystallization dependingon the choice of geobarometers. Two alternative P-T paths, i.e.(1) isobaric cooling or (2) pressure-increase and temperaturedecrease are considered and their geodynamic implications discussed.  相似文献   

8.
The course of crystallization of basalt forming clinopyroxenes in the olivine-basalt-hawaiite-mugearite-trachyte-and basanitoid (= basanite)-series from the Hocheifel area (Western Germany) has been presented by the author in two earlier papers (Hucken-holz, 1965 a, 1965 b). The present paper deals with the evolution of clinopyroxenes from the ankaramites in the same area. The ankaramites are richer in olivines and clinopyroxenes than the normal olivine basalts and may be a product of accumulation of olivines and clinopyroxenes in an olivine basalt magma. The first clinopyroxene formed in the ankaramites is a greenish chromian salite Ca47.1 Mg40.9 Fe12.0 which is preserved in the cores of the phenocrysts. Strongly zoned brown titansalite Ca45 Mg42Fe13 surrounds the chromian salite (HF 5) or appears in independent microphenocrysts (HF53). The groundmass clinopyroxene is titansalite Ca45Mg37Fe18 with a small optic axial angle. Phenocrysts of nickel-rich olivine Fe12–14 and chrome spinell were formed together with chromium clinopyroxene. The same minerals together with orthopyroxenes have been observed in the olivine basalts, hawaiites and basanitoids (= basanites). This paragenesis indicates high pressure and high temperature in the alkali basalts of the Hocheifel during an initial stage of magmatic evolution. With the decrease of pressure, mineral reaction occurred with the alkali basalt melt. The high pressure clinopyroxenes of the ankaramites and basanitoids (= basanites) were changed to a larger extent than the clinopyroxenes of the olivine basalts and hawaiites or were completely destroyed (HF 53) because they are in contact with the hot magma for a longer period of time. This favours the reaction between the solids and the melt, and the high pressure garnet and/or enstatite components in the clinopyroxenes are substituted by Ca-Tschermak’s and titanaugite molecules.   相似文献   

9.
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.  相似文献   

10.
The melting reaction at the solidus of mantle peridotite is commonly peritectic in nature, with liquid and one or more solid phases produced upon melting. In some situations, one of the phases participating on the reactant side of the reaction is present in low abundance. This article explores the possible effects of the low abundance of a reactant phase on the melting behavior of mantle peridotite.For example, spinel lherzolite begins to melt via the peritectic reaction, clinopyroxene + orthopyroxene + spinel = olivine + liquid in the ∼1- to 2-GPa pressure range. In natural spinel lherzolites, spinel is a modally minor mineral and may be infrequently in contact with both clinopyroxene and orthopyroxene. If these mutual contacts are insufficient to generate an interconnected melt, then significant melting may not occur until a combination of minerals that are modally abundant and in contact begin to melt. This scenario could have implications for the physical process of melting and for the timing of formation of an interconnected melt network and separation of the melt from the residue.To begin to investigate this possibility, the spatial relationships between the constituent minerals in two fertile spinel lherzolites were determined by elemental mapping with the electron microprobe. Olivine, orthopyroxene, and clinopyroxene are of similar size, whereas the spinel was smaller and interstitial. Spinel and clinopyroxene are frequently in contact, but mutual contacts of spinel, clinopyroxene, and orthopyroxene are rare. Because of the changes in modal mineralogy anticipated for these lherzolites with increasing temperature, these mutual contacts will be even less common at the solidus. Therefore, an interconnected, potentially extractable, melt may not occur by the solidus spinel + orthopyroxene + clinopyroxene melting reaction.  相似文献   

11.
Peridotite xenoliths from Grenada,Lesser Antilles Island Arc   总被引：2，自引：2，他引：0  
Ultramafic xenoliths comprising harzburgite, lherzolite (reacted harzburgite) and spinel-rich dunite, occur in alkali olivine basalts (M series) of Grenada in the Lesser Antilles island arc. Textures are protogranular, porphyroclastic and granular; the latter are restricted to dunites and areas of the harzburgites/lherzolites where interaction with host magma has occurred. Primary mineralogy comprises olivine, orthopyroxene, clinopyroxene, and spinel. Harzburgites are residual from a fractional partial melting event totaling ~22%. Infiltration of harzburgite by (and reaction with) basalt has produced: a wehrlite, with partial dissolution of primary spinel, an increase in the oxygen fugacity (ƒO2) from primary values 1–2 log ƒO2 units above the fayalite-magnetite-quartz (FMQ) buffer, to 2–2.5 log units above the buffer; reaction of orthopyroxene to form patches of intergrown olivine and clinopyroxene, and bronzite andesite glass (60 wt%, SiO2 18–20 wt% Al2O3 and 3–4 wt% Na2O) with flat to light rare earth element-depleted, chondrite-normalized abundances. Refertilisation of the mantle by reacting melts, producing a clinopyroxene-rich lithology, may form a source of ankaramitic (high-Ca) arc basalts.Editorial responsibility: T.L. Grove  相似文献   

12.
Three methods of geothermometry, currently used for spinel lherzolites, are refined based on new experiments on subsolidus phase equilibria of olivine, pyroxenes and spinel in CaO-MgO-Al2O3-SiO2 and natural rock systems at 16 kb and 1200 °C. Although quasi-thermodynamic modelling is employed, the methods are essentially based on the pyroxene solvus, alumina contents in clinopyroxene and orthopyroxene. Increasing alumina contents in pyroxenes reduce enstatite and diopside components in clinopyroxene and orthopyroxene, respectively. Thus, neglect of alumina in pyroxenes causes underestimates of temperatures by the solvus method.The three geothermometers were tested by applying them to homogeneous spinel lherzolites which were especially selected for this purpose. Coincidence of the three temperatures thus estimated gives confidence in the effectiveness of the geothermometers.They were also applied to spinel lherzolite nodules in basalts and intrusive lherzolites described in the literature. It was found that equilibration temperature of the nodules varies from 1000 °C to 1300 °C, i.e., temperatures somewhat higher than have been generally thought. In contrast to the nodules, the intrusive spinel lherzolites show extensive disequilibrium, which is probably due to retrogressive metamorphism suffered by the intrusives.  相似文献   

13.
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.  相似文献   

14.
Spinel, which constitutes from 0.7% to 3% of lherzolite inclusions, occurs as primary anhedral grains (chrome-rich variety) and as a secondary phase as breakdown products of garnet (alumina-rich variety). Although individual primary spinel grains are chemically homogeneous, spinels are characterized by a wide range of Cr/Al ratios and a relatively narrow range of Mg/Fe″ ratios, even in a single lherzolite sample. The chemical variations of spinels are considered to have the following origin: When garnet lherzolite enters the stability field of the spinel peridotite facies as a consequence of slow upward transport, both orthopyroxenes and clinopyroxenes are recrystallized with loss of jadeite and some Tschermak's component to reach equilibrium. A part of the Tschermak's component reacts with olivine to form pyroxene and spinel. This secondary spinel component is alloted to the primary chromian spinel. However, these reactions did not always reach equilibrium with the major constituent minerals in the lherzolites.  相似文献   

15.
Patches of glass with a second generation of small crystals of olivine, clinopyroxene, and spinel are abundant in hydrous peridotite mantle xenoliths with tabular equigranular textures from two maar-type volcanoes, Meerfelder Maar and Dreiser Weiher (West Eifel, Germany). The patches are similar in size to the main phases of the hosting peridotite. Their central part is often occupied by relics of pargasitic amphibole. Mass-balance calculations show that the patches were formed by surface controlled incongruent thermal breakdown of amphibole according to the reaction: amphibole olivine + clinopyroxene + spinel + melt. Simultaneously with the decomposition of amphibole, small crystals of olivine, clinopyroxene, and spinel grew radially from the patch/peridotite interface toward the centre of the patch. Apart from sector zoning of clinopyroxene, the crystals are virtually homogeneous and are separated from the amphibole by a seam of melt (glass). Secondary olivines reveal higher Mg-numbers, secondary clinopyroxenes higher Cr2O3 concentrations than olivines and clinopyroxenes, respectively, of the host peridotite. The silica contents of melts produced by the above breakdown reaction range from 48 to 52% SiO2 as a function of the composition of the parent amphiboles. Patches surrounded by primary olivines only reveal no reaction with the host peridotite. The variation of SiO2, MgO and CaO in melts from these patches is the result of minor precipitation of olivine and clinopyroxene during fast cooling. If patches are in contact with primary olivine and orthopyroxene, melts are additionally modified by the reaction liquid 1 + orthopyroxene liquid 2 + olivine + clinopyroxene resulting in more silica-rich compositions between 54 and 58%. For the rare glasses richer in silica, a more complex formation is required. Veinlets along grain boundaries are filled with glasses which are chemically identical to those from nearby patches. This suggests that the veinlets were filled by melts formed by amphibole breakdown during entrainment of the xenoliths to the host magmas.  相似文献   

16.
The kinetics of lherzolite dissolution in an alkali basalt and a basaltic andesite was examined experimentally at 1,300°C and 1 GPa using the dissolution couple method. Dissolution of lherzolite in basaltic liquids produces either the melt-bearing dunite–harzburgite–lherzolite (DHL) sequence or the melt-bearing harzburgite–lherzolite sequence depending on whether the reacting melt is or close to olivine saturation (alkali basalt) or olivine + orthopyroxene saturation (basaltic andesite). The dunite in the DHL sequence is pyroxene-free and the harzburgites in both sequences are clinopyroxene-free. The melt fraction and olivine grain size in the dunite are larger than those in the harzburgite. The olivine grain size in the dunite and harzburgite in the DHL sequence also increases as a function experimental run time. Across the sharp dunite–harzburgite and harzburgite–lherzolite interfaces, systematic compositional variations are observed in the reacting melt, interstitial melt, olivine, and to a lesser extent, pyroxenes as functions of distance and time. The systematic variations in lithology, grain size, mineral chemistry, and melt compositions are broadly similar to those observed in the mantle sections of ophiolites. The processes of lherzolite dissolution in basaltic liquids involve dissolution, precipitation, reprecipitation, and diffusive transport in the interstitial melts and surrounding minerals. Preferential dissolution of olivine and clinopyroxene and precipitation of orthopyroxene in the basaltic andesite produces the melt-bearing harzburgite–lherzolite sequence. Preferential dissolution of clinopyroxene and orthopyroxene and precipitation of olivine results in the melt-bearing DHL sequence. Preferential mineral dissolution can also affect the composition of the through-going melt in a dunite channel or harzburgite matrix. Systematic variations in melt fraction and mineral grain size in the peridotite sequences are likely to play an important role in the development of channelized or diffuse porous melt flow in the mantle.An erratum to this article can be found at  相似文献   

17.
Spinel lherzolite found in Damaping, northern Zhangjiakou, Hebei Province occurs as xenoliths in the Hannuoba basalts that consist of alkali basalt and tholeiite. Spinel lherzolites contain 50%–70% olivine (Fo: 90%), 10%–20% clinopyroxene (predominantly Di), 10%–30% orthopyroxene (predominantly En), and less than 5% spinel.3He/4He and40Ar/38Ar ratios in the olivine are 7.56×10−7 and 299.1, respectively.3He/4He and40Ar/38Ar ratios in the orthopyroxene (enstatite) are 9.1×10−7 and 307, respectively. Olivine grains are fractured irregularly, and pyroxene grains characterized by well developed cleavages, which would have resulted from explosion during the rapid eruption of lava from the deep interior to the surface. The lower isotope ratios of helium and argon may indicate that the spinel lherzolite xenoliths were derived from the strongly degassed and depleted upper mantle, and that the mantle is inhomogeneous.3He losses to some extent might affect the helium isotope ratios. The project was financially supported by the National Natural Science Foundation of China (No. 49273185).  相似文献   

18.
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19.
Six crystalline mixtures, picrite, olivine-rich tholeiite, nepheline basanite, alkali picrite, olivine-rich basanite, and olivine-rich alkali basalt were recrystallized at pressures to 40 kb, and the phase equilibria and sequences of phases in natural basaltic and peridotitic rocks were investigated.The picrite was recrystallized along the solidus to the assemblages (1) olivine+orthopyroxene+ clinopyroxene +plagioclase+spinel below 13 kb, (2) olivine+orthopyroxene+clinopyroxene+spinel between 13 kb and 18 kb, (3) olivine+orthopyroxene+clinopyroxene+ garnet+spinel between 18 kb and 26 kb, and (4) olivine+clinopyroxene+garnet above 26 kb. The solidus temperature at 1 atm is slightly below 1,100° and rises to 1,320° at 20 kb and 1,570° at 40 kb. Olivine is the primary phase crystallizing from the melt at all pressures to 40 kb.The olivine-rich tholeiite was recrystallized along the solidus into the assemblages (1) olivine+ clinopyroxene+plagioclase+spinel below 13 kb, (2) clinopyroxene+orthopyroxene+ spinel between 13 kb and 18 kb, (3) clinopyroxene+garnet+spinel above 18 kb. The solidus temperature is slightly below 1,100° at 1 atm, 1,370° at 20 kb, and 1,590° at 40 kb. The primary phase is olivine below 20 kb but is orthopyroxene at 40 kb.In the nepheline basanite, olivine is the primary phase below 14 kb, but clinopyroxene is the first phase to appear above 14 kb. In the alkali-picrite the primary phase is olivine to 40 kb. In the olivine-rich basanite, olivine is the primary phase below 35 kb and garnet is the primary phase above 35 kb. In the olivine-rich alkali basalt the primary phase is olivine below 20 kb and is garnet at 40 kb.Mineral assemblages in a granite-basalt-peridotite join are summarized according to reported experimental data on natural rocks. The solidus of mafic rock is approximately given by T=12.5 P Kb+1,050°. With increasing pressure along the solidus, olivine disappears by reaction with plagioclase at 9 kb in mafic rocks and plagioclase disappears by reaction with olivine at 13 kb in ultramafic rocks. Plagioclase disappears at around 22 kb in mafic rocks, but it persists to higher pressure in acidic rocks. Garnet appears at somewhat above 18 kb in acidic rocks, at 17 kb in mafic rocks, and at 22 kb in ultramafic rocks.The subsolidus equilibrium curves of the reactions are extrapolated according to equilibrium curves of related reactions in simple systems. The pyroxene-hornfels and sanidinite facies is the lowest pressure mineral facies. The pyroxene-granulite facies is an intermediate low pressure mineral facies in which olivine and plagioclase are incompatible and garnet is absent in mafic rocks. The low pressure boundary is at 7.5 kb at 750° C and at 9.5 kb at 1,150° C. The high pressure boundary is 8.0 kb at 750° C and 15.0 kb at 1,150° C. The garnet-granulite facies is an intermediate high pressure facies and is characterized by coexisting garnet and plagioclase in mafic rocks. The upper boundary is at 10.3 kb at 750° C and 18.0 kb at 1,150° C. The eclogite facies is the highest pressure mineral facies, in which jadeite-rich clinopyroxene is stable.Compositions of minerals in natural rocks of the granulite facies and the eclogite facies are considered. Clinopyroxenes in the granulite-facies rocks have smaller jadeite-Tschermak's molecule ratios and higher amounts of Tschermak's molecule than clinopyroxenes in the eclogite-facies rocks. The distribution coefficients of Mg between orthopyroxene and clinopyroxene are normally in the range of 0.5–0.6 in metamorphic rocks in the granulite facies. The distribution coefficients of Mg between garnet and clinopyroxene suggest increasing crystallization temperature of the rocks in the following order: eclogite in glaucophane schist, eclogite and granulite in gneissic terrain, garnet peridotite, and peridotite nodules in kimberlite.Temperatures near the bottom of the crust in orogenic zones characterized by kyanitesillimanite metamorpbism are estimated from the mineral assemblages of metamorphic rocks in Precambrian shields to be about 700° C at 7 kb and 800° C at 9 kb, although heat-flow data suggest that the bottom of Precambrian shield areas is about 400° C and the eclogite facies is stable.The composition of liquid which is in equilibrium with peridotite is estimated to be close to tholeiite basalt at the surface pressure and to be picrite at around 30 kb. The liquid composition becomes poorer in normative olivine with decreasing pressure and temperature.During crystallization at high pressure, olivine and orthopyroxene react with liquid to form clinopyroxene, and a discontinuous reaction series, olivine orthopyroxene clinopyroxene is suggested. By fractional crystallization of pyroxenes the liquid will become poorer in SiO2. Therefore, if liquid formed by partial melting of peridotite in the mantle slowly rises maintaining equilibrium with the surrounding peridotite, the liquid will become poorer in MgO by crystallization of olivine, and tholeiite basalt magma will arrive at the surface. On the other hand, if the liquid undergoes fractional crystallization in the mantle, the liquid may change in composition to alkali-basalt magma and alkali-basalt volcanism may be seen at a late stage of volcanic activity.Publication No. 681, Institute of Geophysics and Planetary Physics, University of California, Los Angeles.  相似文献   

20.
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.  相似文献   

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