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1.
Three types of fluid inclusions have been identified in olivine porphyroclasts in the spinel harzburgite and lherzolite xenoliths from Tenerife: pure CO2 (Type A); carbonate-rich CO2–SO2 mixtures (Type B); and polyphase inclusions dominated by silicate glass±fluid±sp±silicate±sulfide±carbonate (Type C). Type A inclusions commonly exhibit a “coating” (a few microns thick) consisting of an aggregate of a platy, hydrous Mg–Fe–Si phase, most likely talc, together with very small amounts of halite, dolomite and other phases. Larger crystals (e.g. (Na,K)Cl, dolomite, spinel, sulfide and phlogopite) may be found on either side of the “coating”, towards the wall of the host mineral or towards the inclusion center. These different fluids were formed through the immiscible separations and fluid–wall-rock reactions from a common, volatile-rich, siliceous, alkaline carbonatite melt infiltrating the upper mantle beneath the Tenerife. First, the original siliceous carbonatite melt is separated from a mixed CO2–H2O–NaCl fluid and a silicate/silicocarbonatite melt (preserved in Type A inclusions). The reaction of the carbonaceous silicate melt with the wall-rock minerals gave rise to large poikilitic orthopyroxene and clinopyroxene grains, and smaller neoblasts. During the metasomatic processes, the consumption of the silicate part of the melt produced carbonate-enriched Type B CO2–SO2 fluids which were trapped in exsolved orthopyroxene porphyroclasts. At the later stages, the interstitial silicate/silicocarbonatite fluids were trapped as Type C inclusions. At a temperature above 650 °C, the mixed CO2–H2O–NaCl fluid inside the Type A inclusions were separated into CO2-rich fluid and H2O–NaCl brine. At T<650 °C, the residual silicate melt reacted with the host olivine, forming a reaction rim or “coating” along the inclusion walls consisting of talc (or possibly serpentine) together with minute crystals of NaCl, KCl, carbonates and sulfides, leaving a residual CO2 fluid. The homogenization temperatures of +2 to +25 °C obtained from the Type A CO2 inclusions reflect the densities of the residual CO2 after its reactions with the olivine host, and are unrelated to the initial fluid density or the external pressure at the time of trapping. The latter are restricted by the estimated crystallization temperatures of 1000–1200 °C, and the spinel lherzolite phase assemblage of the xenolith, which is 0.7–1.7 GPa.  相似文献   

2.
A rare composite xenolith and abundant cumulative pyroxenites obtained from the Mesozoic Fangcheng basalts on the eastern North China Craton record a complex history of melt percolation and circulation in the subcontinental lithospheric mantle. The composite xenolith has a dunite core and an olivine clinopyroxenite rim. The dunite is of cumulative origin and has a granular recrystallized texture and extremely low Mg# [100 Mg/(Mg + Fe) = 81–82] contents in olivines. The olivine clinopyroxenite contains larger clinopyroxene and/or orthopyroxene with a few fine-grained olivine and tiny phlogopite, feldspar, and/or carbonate minerals interstitial to clinopyroxene. The clinopyroxene has low Mg# (83–85). Compositional similarity between dunitic olivine and pyroxenitic one indicates a sequential crystallization of dunite and pyroxenite from a precursor melt. Pyroxenite xenoliths include olivine websterites and clinopyroxenites, both are of cumulative origin. Estimation of the melt from major oxides in olivines and REE concentrations in clinopyroxenes in these composite and pyroxenite xenoliths suggests a derivation from subducted crustal materials, consistent with the highly enriched EMII-like Sr and Nd isotopic ratios observed in the pyroxenites. Occurrence of phlogopite, feldspar and carbonate minerals in some xenoliths requires the melt rich in alkalis (K, Na), silica and volatiles (water and CO2) at the latest stage as well, similar to highly silicic and potassic melts. Thus, the occurrence of these composite and pyroxenite xenoliths provides an evidence for voluminous injection of recycled crustal melts into the lithosphere beneath the southeastern North China Craton at the Late Mesozoic, a reason for the rapid lithospheric enrichment in both elemental and isotopic compositions.  相似文献   

3.
Spinel lherzolite xenoliths from Tertiary basaltic host magmas at Allyn River, eastern Australia reveal two distinct petrographic and geochemical types. One group is distinguished by xenoliths with undeformed, equilibrated microstructures and interstitial melt patches; The second group shows deformation and contains abundant fluid inclusions but no melt patches. Trace-element signatures of clinopyroxene in these xenoliths provide evidence for metasomatism by a silicate agent with hydrous component and by a carbonate-rich agent respectively.

Melt patches in the undeformed xenoliths contain secondary minerals including clinopyroxene, olivine, feldspar, Mg- and Ca-rich carbonate, apatite, ilmenite and spinel. They are interpreted to represent volatile-rich melt captured shortly prior to entrainment in the host basalt. Sulfide globules, now recrystallised to discrete sulfide phases but inferred to be molten at lithospheric mantle T and P, are closely associated with the melt patches. The close association between sulfide and highly mobile, volatile-bearing fluid has important implications for the mobility of Re and Os, the use of their isotopes in dating mantle events, and the possible effect of volatile-bearing metasomatic agents on their composition.  相似文献   


4.
A corundum-bearing mafic rock in the Horoman Peridotite Complex, Japan, was derived from upper mantle conditions to lower crustal conditions with surrounding peridotites. The amphiboles found in the rock are classified into 3 types: (1) as interstitial and/or poikilitic grains (Green amphibole), (2) as a constituent mineral of symplectitic mineral aggregates with aluminous spinel at grain boundary between olivine and plagioclase (Symplectite amphibole) and (3) as film-shaped thin grains, usually less than 10 μm in width, at grain boundary between olivine and clinopyroxene (Film-shaped amphibole). The Film-shaped amphibole is rarely associated with orthopyroxene extremely low in Al2O3, Cr2O3 and CaO (Low-Al OPX). These minerals were formed by infiltration of SiO2- and volatile-rich fluids along grain boundaries after the rock was recrystallized at olivine-plagioclase stability conditions, i.e. the late stage of the exhumation of the Horoman Complex.

Chondrite-normalized rare earth element patterns and primitive mantle-normalized trace-element patterns of the Green amphibole and clinopyroxene are characterized by LREE-depleted patterns with Eu positive and negative anomalies of Zr and Hf. These geochemical characteristics of the constituent minerals were inherited from original whole-rock compositions through a reaction involving both pre-existing clinopyroxene and plagioclase. We propose that the fluids were originally rich in a SiO2 component but depleted in trace-elements. Dehydration of the surrounding metamorphic rocks in the Hidaka metamorphic belt, probably related to intrusion of hot peridotite body into the Hidaka crust, is a plausible origin for the fluids.  相似文献   


5.
The paper discusses the results of mineralogical and petrographic studies of spinel lherzolite xenoliths and clinopyroxene megacrysts in basalt from the Jixia region related to the central zone of Cenozoic basaltic magmatism of southeastern China. Spinel lherzolite is predominantly composed of olivine (Fo89.6–90.4), orthopyroxene (Mg# = 90.6–92.7), clinopyroxene (Mg# = 90.3–91.9), and chrome spinel (Cr# = 6.59–14.0). According to the geochemical characteristics, basalt of the Jixia region is similar to OIB with asthenospheric material as a source. The following equilibrium temperatures and pressures were obtained for spinel peridotite: 890–1269°C and 10.4–14.8 kbar. Mg# of olivine and Cr# of chrome spinel are close to the values in rocks of the enriched mantle. It is evident from analysis of the textural peculiarities of spinel lherzolite that basaltic melt interacted with mantle rocks at the xenolith capture stage. Based on an analysis of the P–T conditions of the formation of spinel peridotite and clinopyroxene megacrysts, we show that mantle xenoliths were captured in the course of basaltic magma intrusion at a significantly lower depth than the area of partial melting. However, capture of mantle xenoliths was preceded by low-degree partial melting at an earlier stage.  相似文献   

6.
In the Kakkaponnu area within the Achankovil Shear Zone (ACSZ), southern India, an undeformed ultramafic body occurs within intensely deformed granulite facies metamorphic rocks of Pan-African age. The Kakkaponnu ultramafic body is composed of spinel-dunite, phlogopite-dunite, glimmerite, graphite-spinel-glimmerite, and phlogopite-graphite-spinellite. The spinel-dunite is a fine- to medium-grained rock composed mainly of olivine and aluminous spinel and is characterized by relatively high MgO (50.39–50.90 wt.%), (Mg/ (Mg+Fe) = 0.95), Al2O3 (7.8–8.98 wt.%), and low Ni (10–14 ppm). The phlogopite-dunite comprises serpentinized olivine, phlogopite and subordinate amounts of dolomite and is high in MgO (36.5 wt.%), Mg# [(Mg/(Mg+Fe) = 0.97], and K2O (%%5.5 wt.%). Olivine in the spinel-dunite is marked by unusually high MgO (Mg# = 0.96) and extremely low NiO (<0.14 wt.%). Spinels in all rock variants are highly aluminous with low Cr# [Cr/(Al+Cr)] ratio (<0.01). Magnesian ilmenite [Mg# = 59], rutile, zirconolite and baddeleyite are main accessory phases. No significant compositional variation is noted between large grains and small inclusions for all minerals. Abundant graphite, magnesite, melt and ubiquitous CO2 fluid inclusions are identified in the olivine and spinel grains. The data imply that the Kakkaponnu ultramafic body was formed by progressive crystallization of highly potassic CO2-rich melts injected into lower crustal levels. K-Ar ages of 470.5±9.3 and 464.5±9.2 Ma are obtained for phlogopite separates from glimmerite and phlogopite-dunite respectively. These ages are comparable to the phlogopite K-Ar ages reported from lithospheric shear zones in southern Madagascar, which was once conjugated to the Southern Peninsular India prior to the Gondwana breakup. This implies widespread highly potassic CO2-rich fluid/melt influx along shear zones in this part of East Gondwana continent.  相似文献   

7.
The Uintjiesberg kimberlite diatreme occurs within the Proterozoic Namaqua–Natal Belt, South Africa, approximately 60 km to the southwest of the Kaapvaal craton boundary. It is a group I, calcite kimberlite that has an emplacement age of 100 Ma. Major and trace element data, in combination with petrography, are used to evaluate its petrogenesis and the nature of its source region. Macrocryst phases are predominantly olivine with lesser phlogopite, with very rare garnet and Cr-rich clinopyroxene. Geochemical variation amongst the macrocrystic samples (Mg# 0.85–0.87, SiO2=27.0–29.3%, MgO=26.1–30.5%, CaO=10.9–13.5%) is shown to result from 10% to 40% entrainment and partial assimilation of peridotite xenoliths, whereas that shown by the aphanitic samples (Mg# 0.80–0.83, SiO2=19.1–23.0%, MgO=17.9–23.9%, CaO=16.5–23.7%) is consistent with 7–25% crystal fractionation of olivine and minor phlogopite. Changing trajectories on chemical variation diagrams allow postulation of a primary magma composition with 25% SiO2, 26% MgO, 2.3% Al2O3, 5%H2O, 8.6% CO2 and Mg#=0.85.

Forward melting models, assuming 0.5% melting, indicate derivation of the primary Uintjiesberg kimberlite magma from a source enriched in light rare earth elements (LREE) by 10× chondrite and heavy REE (HREE) by 0.8–2× chondrite, the latter being dependent on the proportion of residual garnet. Significant negative Rb, K, Sr, Hf and Ti anomalies present in the inferred primary magma composition are superimposed on otherwise generally smooth primitive mantle-normalized trace element patterns, and are inferred to be a characteristic of the primary magma composition. The further requirement for a source with chondritic or lower HREE abundances, residual olivine with high Fo content (Fo94) suggests derivation from a mantle previously depleted in mafic melt but subsequently enriched in highly incompatible elements prior to kimberlite genesis. These requirements are interpreted in the context of melting of continental lithospheric mantle previously enriched by metasomatic fluids derived from a sublithospheric (plume?) source.  相似文献   


8.
南美洲南部的Pali Aike火山岩区第四纪碱性玄武岩中普遍发育含石榴石的斜方辉石岩包体。这种斜方辉石岩既作为独立的捕掳体存在又以细脉的形式穿插于橄榄岩捕掳体中。斜方辉石岩普遍含富Ti矿物,并且次生斜方辉石含橄榄石和单斜辉石残晶。与含石榴石橄榄岩中的斜方辉石相比,这种次生的斜方辉石以高TiO2、中等含量的Al2O3以及低Mg#为特征,表明它是在一种高度分异演化的富Ti熔体交代作用下通过消耗橄榄石和单斜辉石方式形成的。斜方辉石岩全岩的Co、Ni略低,Cr和铂族元素(PGE)含量与地幔橄榄岩相当,表明这些元素在交代作用过程中相对稳定,而交代介质带入的组分以碱质(K2O+Na2O)、Ti、Si、Al和S为主。交代的斜方辉石在现代活动岛弧和古克拉通的地幔橄榄岩捕掳体中多有报道。与这些环境中地幔样品的斜方辉石相比,PaliAike地区的次生斜方辉石含有相对高的Ti和Al,以及相对低的Mg。高Ti低Mg属性反映了交代介质可能来源于下伏的软流圈地幔并且经历了高度的分异和演化过程。Pali Aike地区所见到的这种交代斜方辉石和斜方辉石岩在其他被上涌软流圈影响的陆下岩石圈地幔中可能普遍存在。这些研究对了解中国华北-东北中生代以来的岩石圈地幔减薄机制有着重要的借鉴意义。  相似文献   

9.
A detailed petrographic, major and trace element and isotope (Re–Os) study is presented on 18 xenoliths from Northern Lesotho kimberlites. The samples represent typical coarse, low-temperature garnet and spinel peridotites and span a P–T range from 60 to 150 km depth. With the exception of one sample (that belongs to the ilmenite–rutile–phlogopite–sulphide suite (IRPS) suite first described by [B. Harte, P.A. Winterburn, J.J. Gurney, Metasomatic and enrichment phenomena in garnet peridotite facies mantle xenoliths from the Matsoku kimberlite pipe, Lesotho. In: Menzies, M. (Ed.), Mantle metsasomatism. Academic Press, London 1987, 145–220.]), all samples considered here have high Mg# and show strong depletion in CaO and Al2O3. They have bulk rock Re depletion ages (TRD) >2.5 Ga and are therefore interpreted as residua from large volume melting in the Archaean. A characteristic of Kaapvaal xenoliths, however, is their high SiO2 concentrations, and hence, modal orthopyroxene contents that are inconsistent with a simple residual origin of these samples. Moreover, trace element signatures show strong overall incompatible element enrichment and REE disequilibrium between garnet and clinopyroxene. Textural and subtle major element disequilibria were also observed. We therefore conclude that garnet and clinopyroxene are not co-genetic and suggest that (most) clinopyroxene in the Archaean Kaapvaal peridotite xenoliths is of metasomatic origin and crystallized relatively recently, possibly from a melt precursory to the kimberlite.

Possible explanations for the origin of garnet are exsolution from a high-temperature, Al- and Ca-rich orthopyroxene (indicating primary melt extraction at shallow levels) or a majorite phase (primary melting at >6 GPa). Mass balance calculations, however, show that not all garnet observed in the samples today is of a simple exsolution origin. The extreme LREE enrichment (sigmoidal REE pattern in all garnet cores) is also inconsistent with exsolution from a residual orthopyroxene. Therefore, extensive metasomatism and probably re-crystallization of the lithosphere after melt-depletion and garnet exsolution is required to obtain the present textural and compositional features of the xenoliths. The metasomatic agent that modified or perhaps even precipitated garnet was a highly fractionated melt or fluid that might have been derived from the asthenosphere or from recycled oceanic crust. Since, to date, partitioning of trace elements between orthopyroxene and garnet/clinopyroxene is poorly constrained, it was impossible to assess if orthopyroxene is in chemical equilibrium with garnet or clinopyroxene. Therefore, further trace element and isotopic studies are required to constrain the timing of garnet introduction/modification and its possible link with the SiO2 enrichment of the Kaapvaal lithosphere.  相似文献   


10.
首次报道了来自东北地区岩石圈地幔水含量的数据。通过对吉林龙岗和汪清新生代玄武岩中的橄榄岩包体矿物进行电子探针(EMP)和激光熔蚀等离子体质谱(LA-ICPMS)的分析,得到了矿物的主量元素和微量元素的数据,结果显示这些橄榄岩是原始地幔经历了不同程度部分熔融的残余,大部分样品的熔融程度可能<10%。橄榄岩样品在后期还经历了地幔交代作用,大部分样品受到硅酸岩熔体的交代,少部分样品受到碳酸岩熔体的交代。显微傅里叶变换红外光谱(FTIR)的分析结果显示,橄榄岩样品中的单斜辉石、斜方辉石均含有以结构羟基形式存在的水,而橄榄石中没有明显的羟基吸收峰。龙岗样品中单斜辉石的水含量为(48~464)×10-6(H2O, 质量分数),斜方辉石水含量为(28~104)×10-6;汪清样品中单斜辉石的水含量为(34~403)×10-6,斜方辉石的水含量为(13~89)×10-6;所有样品全岩水含量为(8~92)×10-6。样品的水含量可以代表龙岗和汪清地区岩石圈地幔的水含量信息,并且水含量变化范围较大,造成这种变化的原因可能是由于地幔源区初始水含量的不均一,以及部分熔融和地幔交代作用叠加的结果。  相似文献   

11.
Reaction zones around minerals in mantle xenoliths have been reported from many localities worldwide. Interpretations of the origins of these textures fall into two groups: mantle metasomatic reaction or reaction during transport of the xenoliths to the surface. A suite of harzburgitic mantle xenoliths from Sal, Cape Verde show clear evidence of reaction during transport. The reactions resulted in the formation of olivine–clinopyroxene and Si- and alkali-rich glass reaction zones around orthopyroxene and sieve-textured clinopyroxene and sieve textured spinel, both of which are associated with a Si- and alkali-rich glass similar to that in the orthopyroxene reaction zones. Reaction occurred at pressures less than the mantle equilibration pressure and at temperatures close to the liquidus temperature of the host magma. In addition, there is a clear spatial relation of reaction with the host lava: reaction is most intense near the lava/xenolith contact. The residence time of the xenoliths in the host magma, determined from Fe–Mg interdiffusion profiles in olivine, was approximately 4 years. Our results cannot be reconciled with a recent model for the evolution of the mantle below the Cape Verde Archipelago involving mantle metasomatism by kimberlitic melt. We contend that alkali-rich glasses in the Sal xenoliths are not remnants of a kimberlitic melt, but rather they are the result of reaction between the host lava or a similar magma and xenolith minerals, in particular orthopyroxene. The formation of a Si- and alkali-rich glass by host magma–orthopyroxene reaction appears to be a necessary precursor to formation of sieve textured spinel and clinopyroxene.  相似文献   

12.
Summary Mantle-derived xenoliths from Baarley in the Quaternary West Eifel volcanic field contain six distinct varieties of glass in veins, selvages and pools. 1) Silica-undersaturated glass rich in zoned clinopyroxene microlites that forms jackets around and veins within the xenoliths. This glass is compositionally similar to groundmass glass in the host basanite. 2) Silica-undersaturated alkaline glass that contains microlites of Cr-diopside, olivine and spinel associated with amphibole in peridotites. This glass locally contains corroded primary spinel and phlogopite. 3) Silica-undersaturated glass associated with diopside, spinel ± olivine and rh?nite microlites in partly to completely broken down amphibole grains in clinopyroxenites. 4) Silica-undersaturated to silica-saturated, potassic glass in microlite-rich fringes around phlogopite grains in peridotite. 5) Silica-undersaturated potassic glass in glimmerite xenoliths. 6) Silica-rich glass around partly dissolved orthopyroxene crystals in peridotites. Geothermometry of orthopyroxene–clinopyroxene pairs (P = 1.5 GPa) gives temperatures of ∼ 850 °C for unveined xenoliths to 950–1020 °C for veined xenoliths. Clinopyroxene – melt thermobarometry shows that Cr-diopside – type 2 glass pairs in harzburgite formed at 1.4 to 1.1 GPa and ∼ 1250 °C whereas Cr-diopside – type 2 glass pairs in wehrlite formed at 0.9 to 0.7 GPa and 1120–1200 °C. This bimodal distribution in pressure and temperature suggests that harzburgite xenoliths may have been entrained at greater depth than wehrlite xenoliths. Glass in the Baarley xenoliths has three different origins: infiltration of an early host melt different in composition from the erupted host basanite; partial melting of amphibole; reaction of either of these melts with xenolith minerals. The composition of type 1 glass suggests that jackets are accumulations of relatively evolved host magma. Mass balance modelling of the type 2 glass and its microlites indicates that it results from breakdown of disseminated amphibole and reaction of the melt with the surrounding xenolith minerals. Type 3 glass in clinopyroxenite xenoliths is the result of breakdown of amphibole at low pressure. Type 4 and 5 glass formed by reaction between phlogopite and type 2 melt or jacket melt. Type 6 glass associated with orthopyroxene is due to the incongruent dissolution of orthopyroxene by any of the above mentioned melts. Compositional gradients in xenolith olivine adjacent to type 2 glass pools and jacket glass can be modelled as Fe–Mg interdiffusion profiles that indicate melt – olivine contact times between 0.5 and 58 days. Together with the clinopyroxene – melt thermobarometry calculations these data suggest that the glass (melt) formed over a short time due to decompression melting of amphibole and infiltration of evolved host melt. None of the glass in these xenoliths can be directly related to metasomatism or any other process that occurred insitu in the mantle. Received November 23, 1999; revised version accepted September 5, 2001  相似文献   

13.
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   

14.
Peridotite xenoliths found in Cenozoic alkali basalts of northern Victoria Land, Antarctica, vary from fertile spinel-lherzolite to harzburgite. They often contain glass-bearing pockets formed after primary pyroxenes and spinel. Few samples are composite and consist of depleted spinel lherzolite crosscut by amphibole veins and/or lherzolite in contact with poikilitic wehrlite. Peridotite xenoliths are characterized by negative Al2O3–Mg# and TiO2–Mg# covariations of clino- and orthopyroxenes, low to intermediate HREE concentrations in clinopyroxene, negative Cr–Al trend in spinel, suggesting variable degrees of partial melting. Metasomatic overprint is evidenced by trace element enrichment in clinopyroxene and sporadic increase of Ti–Fetot. Preferential Nb, Zr, Sr enrichments in clinopyroxene associated with high Ti–Fetot contents constrain the metasomatic agent to be an alkaline basic melt. In composite xenoliths, clinopyroxene REE contents increase next to the veins suggesting metasomatic diffusion of incompatible element. Oxygen isotope data indicate disequilibrium conditions among clinopyroxene, olivine and orthopyroxene. The highest δ18O values are observed in minerals of the amphibole-bearing xenolith. The δ18Ocpx correlations with clinopyroxene modal abundance and geochemical parameters (e.g. Mg# and Cr#) suggest a possible influence of partial melting on oxygen isotope composition. Thermobarometric estimates define a geotherm of 80°C/GPa for the refractory lithosphere of NVL, in a pressure range between 1 and 2.5 GPa. Clinopyroxene microlites of melt pockets provide P–T data close to the anhydrous peridotite solidus and confirm that they originated from heating and decompression during transport in the host magma. All these geothermometric data constrain the mantle potential temperature to values of 1250–1350°C, consistent with the occurrence of mantle decompressional melting in a transtensive tectonic regime for the Ross Sea region.  相似文献   

15.
Mantle petrology and mineralogy of the Thetford Mines Ophiolite Complex   总被引：4，自引：0，他引：4  
The Ordovician Thetford Mines ophiolite complex (TMOC) formed by boninite-fed seafloor-spreading, probably in a fore-arc environment. The mantle section is dominated by foliated harzburgite (≤ 5–6% clinopyroxene), cut by dunitic (± chromitite cores) and orthopyroxenitic veins and dykes. Contrasting structures, textures and mineral compositions allow us to subdivide the mantle. The granular-textured rocks of the Duck Lake Block (DLB) have two steeply-dipping foliations. The older foliation strikes NW, is sub-perpendicular to the Moho, and is interpreted to have resulted from upflow of the asthenosphere beneath the spreading ridge. This fabric is overprinted by a 2nd ductile foliation striking ENE, oriented sub-parallel to the Moho, which we interpreted as having formed by crust–mantle shear as the lithosphere migrated away from the spreading ridge. The DLB mantle has a limited range of spinel Cr# (100Cr / (Cr + Al) = 51–71). Comparison with experimentally determined residual spinel compositions (equilibrium melting) implies a maximum loss of 27–38% melt if the protolith had a fertile MORB mantle composition. However, interstitial-textured clinopyroxene may have high TiO2 (< 0.04wt.%) and Na2O (< 0.27wt.%), and some interstitial spinel has higher TiO2 (< 0.09wt.%), suggesting interaction with (or crystallization from) an “impregnating” melt. Interstitial tremolitic amphibole also indicates the passage of late hydrous fluids. The harzburgite in the Caribou Mountain Block (CMB) has a porphyroclastic texture, with a strong, locally mylonitic foliation striking roughly N–S, parallel to the orientation of seafloor-spreading related paleo-normal faults in the crust. These fabrics and textures imply a colder, lithospheric deformation, possibly related to tectonic denudation (oceanic core complex). This would explain problematic lava/mantle contacts, favour infiltration of seawater, serpentinization, and reduced fO2 conditions. The CMB mantle shows a wider range of mineral compositions than the DLB, with spinel Cr# (28–86) implying ≤ 15–45% of equilibrium melting. Locally higher TiO2 in spinel (< 0.05wt.%) and clinopyroxene (< 0.11wt.%), a local rimward decrease in spinel Cr#, clinopyroxene Cr#, and olivine Fo-content, and traces of interstitial amphibole, are attributed to the circulation of an evolved hydrous melt during peridotite deformation. This suggests that the lower limit to the extent of melting inferred for the CMB (15%), established on the basis of Al-rich spinel rims and neoblasts, is probably too low. On the other hand, the higher inferred degree of depletion of the CMB is probably unaffected by the metasomatic overprint and is a more robust conclusion.  相似文献   

16.
Troctolitic gabbros from Valle Fértil and La Huerta Ranges, San Juan Province, NW‐Argentina exhibit multi‐layer corona textures between cumulus olivine and plagioclase. The corona mineral sequence, which varies in the total thickness from 0.5 to 1 mm, comprises either an anhydrous corona type I with olivine|orthopyroxene|clinopyroxene+spinel symplectite|plagioclase or a hydrous corona type II with olivine|orthopyroxene|amphibole|amphibole+spinel symplectite|plagioclase. The anhydrous corona type I formed by metamorphic replacement of primary olivine and plagioclase, in the absence of any fluid/melt phase at <840 °C. Diffusion controlled metamorphic solid‐state replacement is mainly governed by the chemical potential gradients at the interface of reactant olivine and plagioclase and orthopyroxene and plagioclase. Thus, the thermodynamic incompatibility of the reactant minerals at the gabbro–granulite transition and the phase equilibria of the coronitic assemblage during subsequent cooling were modelled using quantitative μMgO–μCaO phase diagrams. Mineral reaction textures of the anhydrous corona type I indicate an inward migration of orthopyroxene on the expense of olivine, while clinopyroxene+spinel symplectite grows outward to replace plagioclase. Mineral textures of the hydrous corona type II indicate the presence of an interstitial liquid trapped between cumulus olivine and plagioclase that reacts with olivine to produce a rim of peritectic orthopyroxene around olivine. Two amphibole types are distinguished: an inclusion free, brownish amphibole I is enriched in trace elements and REEs relative to green amphibole II. Amphibole I evolves from an intercumulus liquid between peritectic orthopyroxene and plagioclase. Discrete layers of green amphibole II occur as inclusion‐free rims and amphibole II+spinel symplectites. Mineral textures and geochemical patterns indicate a metamorphic origin for amphibole II, where orthopyroxene was replaced to form an inner inclusion‐free amphibole II layer, while clinopyroxene and plagioclase were replaced to form an outer amphibole+spinel symplectite layer, at <770 °C. Calculation of the possible net reactions by considering NCKFMASH components indicates that the layer bulk composition cannot be modelled as a ‘closed’ system although in all cases the gain and loss of elements within the multi‐layer coronas (except H2O, Na2O) is very small and the main uncertainties may arise from slight chemical zoning of the respective minerals. Local oxidizing conditions led to the formation of orthopyroxene+magnetite symplectite enveloping and/or replacing olivine. The sequence of corona reaction textures indicates a counter clockwise P–T path at the gabbro–granulite transition at 5–6.5 kbar and temperatures below 900 °C.  相似文献   

17.
Dunite, wehrlite and websterite are rare members of the mantle xenolith suite in the Kimberley kimberlites of the Kaapvaal Craton in southern Africa. All three types were originally residues of extensive melt extraction and experienced varying amounts and types of melt re-enrichment. The melt depletion event, dated by Re-Os isotope systematics at 2.9 Ga or older, is evidenced by the high Mg# (Mg/(Mg + Fe)) of silicate minerals (olivine (0.89-0.93); pyroxene (0.88-0.93); garnet (0.72-0.85)), high Cr# (Cr/(Cr + Al)) of spinel (0.53-0.84) and mostly low whole-rock SiO2, CaO and Al2O3 contents. Shortly after melt depletion, websterites were formed by reaction between depleted peridotites and silica-rich melt (>60 wt% SiO2) derived by partial melting of eclogite before or during cratonization. The melt-peridotite interaction converted olivine into orthopyroxene.All three xenolith types have secondary metasomatic clinopyroxene and garnet, which occur along olivine grain boundaries and have an amoeboid texture. As indicated by the preservation of oxygen isotope disequilibrium in the minerals and trace-element concentrations in clinopyroxene and garnet, this metasomatic event is probably of Mesozoic age and was caused by percolating alkaline basaltic melts. This melt metasomatism enriched the xenoliths in CaO, Al2O3, FeO and high-field-strength-elements, and might correspond to the Karoo magmatism at 200 Ma. The websterite xenoliths experienced both the orthoyproxene-enrichment and clinopyroxene-garnet metasomatic events, whereas dunite and wehrlite xenoliths only saw the later basaltic melt event, and may have been situated further away from the source of melt migration channels.  相似文献   

18.
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   

19.
Sieve-textured clinopyroxene and spinel are common in mantle xenoliths and have been interpreted to be the result of partial melting, mantle metasomatism and host magma–xenolith reaction during transport. In this paper, we test the latter hypothesis with a series of reduced and oxidized experiments at 1,200 and 1,156°C at one atmosphere using a synthetic leucitite melt and discs of natural peridotite. Our results show that sieve texture development on clinopyroxene and spinel in mantle xenoliths is the result of a multistage reaction process. In the first step, orthopyroxene undergoes incongruent dissolution to produce a silica and alkali-rich melt together with olivine. As this melt migrates along grain boundaries it causes incongruent dissolution of clinopyroxene and spinel. The incongruent dissolution mechanism involves complete dissolution of the clinopyroxene or spinel followed by nucleation and growth of a secondary clinopyroxene or spinel once the reacting melt is saturated. The reaction of orthopyroxene, clinopyroxene and spinel with infiltrated host magma results in a range of melt compositions that are very similar to those interpreted to be due to very small degrees of partial melting. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

20.
Alkaline magmas from the late-Cenozoic Marie Byrd Land Volcanic Province, West Antarctica, have entrained lithospheric xenoliths which vary from spinel lherzolites to supracrustal rocks. Lower crustal xenoliths have been collected from the Executive Committee Range (Mounts Hampton in the north and Mount Sidley in the south) in central Marie Byrd Land, and their petrological characteristics together with preliminary geochemical data are discussed here. Granulite xenoliths include metaigneous gabbros and norites with varying proportions of clinopyroxene, spinel and either olivine or orthopyroxene. Pyroxenites occur together with granulites, which on the basis of their similar mineral assemblage, texture and composition are considered to be related to the granulites. The composition of xenoliths from Mounts Hampton and Sidley differ (e.g. Mount Sidley xenoliths have Mg# 32–80, are relatively LREE enriched and have 87Sr/86Sr of 0.70286–0.70376 and 143Nd/144Nd of 0.512864–0.512870, whereas Mount Hampton xenoliths have Mg# 68–78, are LREE depleted and have 87Sr/86Sr of 0.70420–0.70458 and 143Nd/144Nd of 0.512771–0.512819), defining a major lateral lower crustal discontinuity beneath the Executive Committee Range. Relict igneous textures and low abundances of incompatible elements indicate that the xenoliths initially formed as cumulates rather than as trapped melts. The xenolith suite differs in composition to the host rocks (Mount Sidley VOLCANICS = 87Sr/86Sr of 0.70300–0.70312 and 143Nd/144Nd of 0.512814–0.512907) and cannot be co-genetic with them. They are interpreted here to represent the cumulates of mantle melts that evolved by crystal fractionation at lower crustal depths.  相似文献   

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