Some implications of xenolith glasses for the mantle sources of alkaline mafic magmas

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

Amphibole pyroxenite xenoliths: Cumulate or replacement phenomena from the upper mantle,Nunivak Island,Alaska

Interstitial to poikilitic amphibole found in garnet pyroxenite xenoliths has been interpreted, in the past, to represent a critically silica undersaturated, residual intercumulus melt trapped by its cumulate assemblage of anhydrous phases. The textural features of such amphibole in pyroxenite xenoliths from Nunivak Island, Alaska, however, are more compatible with an origin by replacement of the anhydrous phases of the pyroxenite, following a period of cooling and sub-solidus recrystallization in the upper mantle. The reaction of amphibole and olivine to give orthopyroxene, observed in two specimens, requires that the associated fluid phase was not critically silica undersaturated. The amphibole is therefore thought to reflect the interaction of an alkali-bearing, migratory, aqueous fluid and an upper mantle consisting of spinel lherzolite cut by veins of spinel and garnet pyroxenite.     

Na Metasomatism in the Island-Arc Mantle by Slab Melt--Peridotite Interaction: Evidence from Mantle Xenoliths in the North Kamchatka Arc

The Pliocene (7 Ma) Nb-enriched arc basalts of the ValovayamVolcanic Field (VVF) in the northern segment of the Kamchatkaarc, Russia, host abundant mantle xenoliths, including spinelIherzolites. Textural and microstructural evidence for high-temperature,multi-stage, creep-related deformations in spinel Iherzolitessupports a sub-arc mantle derivation. Pyroxene chemistry indicatesthe existence of two compositional suites: (1) a Cr-diopsidesuite with low-Tt, moderate-Al clinopyroxene compositions, and(2) an Al-augite suite with high Al and Tt, and low Cr concentrationsin clinopyroxene. Some spinel lherzolite xenoliths contain metasomaticAl-augite-type clinopyroxene, Al-Tt spinel, and felsic veinssimilar to trondhjemite melt. The Al-augite series xenolithstypically contain high-Na plagioclase, Cr-poor, Al-Fe-Mg andAl-Tt-Fe spinels, with occasional almandine-grossularite garnetand high-Al and -Na pargasitic amphibole. Pyroxene and spinel compositional trends suggest that the Crdiopsideseries xenoliths from the VVF Nb-enriched arc basalts representan island-arc mantle affected by a metasomatic event. Occurrenceof high-Na plagioclase and trondhjemitic veins favors the additionof a metasomatic component with high Na, Al and Si to the northernKamchatka arc mantle. Trondhjemitic veins, representing siliceousslab melts, compositionally exemplify the metasomatic component.Na metasomatism by peridotite-slab melt interaction is an importantmantle hybridization process responsible for arc-related alkalinemagma generation from a veined sub-arc mantle. KEY WORDS: metasomatism; island arc; mantle xenoliths; Kamchatka; mantle     

An Example of Consequent Mantle Metasomatism in Peridotite Inclusions from Nunivak Island, Alaska

Many of the coarse-grained peridotite inclusions in basanitesfrom Nunivak Island, Alaska, contain amphibole and a smallerfraction also contain phlogopite and apatite. All of these peridotiteshave light REE/heavy REE abundance ratios greater than chondritesand many have abundances of K, Rb, Sr, Ba and light REE whichexceed estimates for primitive mantle. On the basis of mineraltextures and compositions we infer that the clinopyroxene, amphibole,phlogopite and apatite equilibrated with a metasomatic fluid.Isotopic (Sr and Nd) ratios and parent-daughter abundance datafor the coarse-grained peridotites constrain the age of themetasomatism to be less than 200 million years. Associated amphibole pyroxenite inclusions are not metasomatized;these inclusions probably formed as crystal segregates froman alkalic magma. Both pyroxenites and coarse-grained peridotitesare isotopically similar to basalts from Nunivak Island. Usingthese data, we propose a model in which the metasomatized peridotiteswere wallrocks located adjacent to the pyroxenites, and thatmetasomatism of these peridotites was caused by the infiltrationof a residual silicate melt or volatile-rich fluid derived fromthe parental magma of the pyroxenites; i.e. the metasomatismwas a consequence of basaltic magmatism. Furthermore, the parentalmagma of the pyroxenites was probably petrogenetically relatedto the Nunivak volcanism. REE modelling of fluids in equilibriumwith clinopyroxenes from the coarse-grained peridotites is consistentwith this model.     

Structure of the Ethiopian lithosphere: Xenolith evidence in the Main Ethiopian Rift

The lithospheric and sublithospheric processes associated with the transition from continental to oceanic magmatism during continental rifting are poorly understood, but may be investigated in the central Main Ethiopian Rift (MER) using Quaternary xenolith-bearing basalts. Explosive eruptions in the Debre Zeyit (Bishoftu) and Butajira regions, offset 20 km to the west of the contemporaneous main rift axis, host Al-augite, norite and lherzolite xenoliths, xenocrysts and megacrysts. Al-augite xenoliths and megacrysts derived from pressures up to 10 kb are the dominant inclusion in these recent basalts, which were generated as small degree partial melts of fertile peridotite between 15 and 25 kb. Neither the xenoliths nor the host basalts exhibit signs of carbonatitic or hydrous (amphibole + phlogopite) metasomatism, suggesting that infiltration of silicate melts resulting in pervasive Al-augite dyking/veining dominates the regional lithospheric mantle. Recent geophysical evidence has indicated that such veining/dyking is pervasive and segmented, supporting the connection of these Al-augite dykes/veins to the formation of a proto ridge axis. Al-augite xenoliths and megacrysts have been reported in other continental rift settings, suggesting that silicate melt metasomatism resulting in Al-augite dykes/veins is a fundamental processes attendant to continental rift development.     

辽西中生代粗面玄武岩中地幔和下地壳捕虏体的发现及其地质意义

辽西中生代粗面玄武岩的KAr同位素年龄为84.76±1.67Ma,其 中 含有丰富的超镁 铁质岩和深部壳源捕虏体,主要有角闪尖晶二辉橄榄岩、尖晶石二辉橄榄岩、二辉橄榄岩、 含斜长石的角闪二辉石岩、二辉麻粒岩和辉石斜长片麻岩等。包体的组构及矿物化学研究表 明超镁质岩捕虏体来源于上地幔,麻粒岩和片麻岩则为深部壳源捕虏体。该类包体的发现对 中国东部中生代上地幔和下地壳的研究具有重要意义。     

Petrology of high pressure clinopyroxenite series xenoliths,Mount Carmel,Israel

The Mount Carmel xenolith suite is composed of a series of garnet granulites of probable lower crustal origin, and a high pressure clinopyroxenite series. The clinopyroxenite series is petrologically diverse with the most common lithologies being garnet-clinopyroxenite, clinopyroxenite, garnet-amphibole-clinopyroxenite, amphibole-clinopyroxenite, amphibole-mica-clinopyroxenite plus megacrystalline nodules of clinopyroxene, garnet, amphibole and mica. Orthopyroxene is extremely rare (1 sample) and olivine is absent in the clinopyroxenite series xenoliths. The clinopyroxenite series is divided into three rock associations based on textures, mineralogy, mineral chemistry and equilibration temperatures: the metaclinopyroxenite, the magmatic garnet-clinopyroxenite and the amphibole-mica-clinopyroxenite associations. Many of the xenoliths contain late phases, largely amphibole, as microphenocrysts in glass and altered glass that was intruded into the xenoliths. Each of the three associations plus the late phases represents the crystallization products of one or more magma batchs. Garnet-clinopyroxene geothermometry and phase relations for alkaline basalts allow estimates of theP/T conditions of equilibration to be made for some of the xenoliths. The metaclinopyroxenites were equilibrated at 1.5–3 GPa and 990–1,115° and the magmatic garnet-clinopyroxenites were equilibrated at 2–3 GPa and 1,160–1,190°. The Mt. Carmel xenoliths are samples from the depth range 50–95 km and fall in the sampling gap between xenoliths typical of alkali basalts (d<60 km) and those typical of kimberlites (d> 90 km).     

Mantle upwelling within the Pannonian Basin: evidence from xenolith lithology and mineral chemistry

Five spinel lherzolite xenoliths hosted in Neogene alkali basalts from the marginal parts of the Pannonian Basin (Styrian Basin in Austria and Persani Mts. in the Eastern Transylvanian Basin, Romania) contain orthopyroxene–clinopyroxene–spinel clusters, which are believed to represent former garnet in lherzolitic mantle material. 'Palaeo' equilibrium pressure of this former garnet lherzolite was estimated to be equivalent to depths of 90–120 km using calculated garnet compositions and measured orthopyroxene compositions from the clusters. 'Neo' equilibrium pressures of the xenoliths indicate depths of 55–65 km, suggesting c.  50–60 km uprise of the mantle section represented by these xenoliths. This petrological result confirms the observations from previous geophysical studies that significant mantle uplift has occurred beneath the Pannonian Basin.     

滇东南建水地区高镁火山岩包体的成因和构造背景

在滇东南建水地区发现产于峨眉山玄武岩中的高镁火山岩包体,这对于地幔柱的形成演化具有重要研究意义.对这些包体进行了锆石U-Pb年代学、地球化学和岩矿分析.高镁火山岩包体具斑状结构,致密块状构造,斑晶主要为贵橄榄石和透辉石.13颗锆石U-Pb LA-ICP-MS加权平均年龄为259±2Ma(MSWD=1.9),显示与寄主岩石同期形成.包体岩石具有高镁(Mg~#=68～75)、低硅(SiO_2=45.11%～45.93%)特征,轻稀土元素(LREE)、高场强元素(HFSE)富集而重稀土元素(HREE)亏损,属于亚碱性、拉斑玄武岩系列,具有板内玄武岩(IPB)特征.火山岩包体的原始岩浆起源于石榴子石二辉橄榄岩低程度部分熔融的产物,岩浆演化过程中发生了橄榄石和单斜辉石的分离结晶作用,在侵位上升过程中未受明显的地壳混染作用.该高镁火山岩的存在,显示地幔柱除了垂直上升运动外,在地球深部不同的边界还有多次侧向扩展移动,表明滇东南晚二叠世存在峨眉山地幔柱的一个分支-地幔枝活动.     

中国东部深源包体和新生代辉石地温的研究

中国东部新生代宏玄武岩广泛发育,它们在时间和空间上表现为明显的分带。在新生代玄武岩中赋存着丰富的地幔碎裂－二辉橄榄岩包体和高压巨晶,为研究上地幔组成和岩浆演化提供大量信息。人选必其他研究者的资料,从岩石学,矿物学,地球化学及热力学的观点,对中国东部新生代玄武岩中赋存的深源包体和新生代辉石地温进行探讨。     

Xenoliths in the Honolulu Volcanic Series, Hawaii

The Pleistocene to Holocene Honolulu Volcanic Series was eruptedfrom about 37 vents scattered over the older Koolau tholeiiteshield. The rocks of this series are compositionally zoned withrespect to the shield; near the Koolau caldera the predominantrocks are melilitenepheline basalts, but these give way outwardto nepheline basalts, and ultimately, at the apron of the shield,to alkalic olivine basalts. The xenoliths in these are likewisezoned: most of those in the caldera area consist of dunite,most of those at intermediate distances of lherzolite, and someof those in the apron of the shield consist of garnet pyroxeniteand peridotite. The zoning of the xenoliths, however, does notcoincide with that of the enclosing rocks. We believe that copiouseruption of Koolau tholeiite produced a lateral and verticalheterogeneity in the mantle beneath Oahu, and that the zoningin both Honolulu lavas and their xenoliths is caused by thatheterogeneity. The textures of the xenoliths indicate that thebasalts were mainly produced by fractional melting rather thanfractional crystallization. There is some evidence that thedunite xenoliths are mantle residua produced during the generationof the tholeiite, and that the Honolulu magmas were generatedat greater depths than the Koolau magmas, probably as a resultof elastic unloading.     

Carbonate-bearing mantle peridotite xenoliths from Spitsbergen: phase relationships,mineral compositions and trace-element residence

 Carbonates of mantle origin have been found in xenoliths from Quaternary basaltic volcanoes in NW Spitsbergen. The carbonates range from dolomite to Mg-bearing calcite and have high Mg-numbers [Mg/(Mg+Fe)=(0.92–0.99)]. In some samples they occur interstitially, e.g. at triple junctions of silicate minerals and appear to be in textural and chemical equilibrium with host lherzolite. Most commonly, however, the carbonates make up fine-grained aggregates together with (Ca,Mg)-rich olivine and (Al,Cr,Ti)-rich clinopyroxene that typically replace spinel, amphibole, and orthopyroxene as well as primary clinopyroxene and olivine. Some lherzolites contain amphibole and apatite that appear to have formed before precipitation of the carbonates. In situ analyses by proton microprobe show very high contents of Sr in the clinopyroxene, carbonates and apatite; the apatite is also very rich in LREE, U, Th, Cl, Br. Disseminated amphibole in carbonate-bearing rocks is very poor in Nb and Zr, in contrast to vein amphibole and mica from carbonate-free rocks that are rich in Nb and Zr. Overall, the Spitsbergen xenoliths provide evidence both for the occurrence of primary carbonate in apparent equilibrium with the spinel lherzolites (regardless of the nature of events that emplaced them) and for the formation of carbonate-bearing pockets consistent with metasomatism by carbonate melts. Calcite and amorphous carbonate-rich materials occur in com- posite carbonate-fluid inclusions, veins and partial melting zones that appear to be related to fluid action in the mantle, heating of the xenoliths during their entrainment in basaltic magma, and to decompression melting of the carbonates. Magnesite is a product of secondary, post-eruption alteration of the xenoliths. Received: 6 October 1995/Accepted: 17 June 1996     

The geochemistry of carbon in mantle peridotites

Carbon abundances have been determined in mantle xenoliths from alkalic basalts and kimberlites and interpreted in terms of the nature and distribution of the C-rich phases. Anhydrous Cr-diopside Group I spinel lherzolites from basalts typically contain 15–50 ppm C, and amphibole-bearing ones have only marginally higher concentrations (40–100 ppm). Carbon abundances in Al-augite Group II pyroxenites are not significantly different from those of the Group I rocks. Although most LREE-depleted lherzolite xenoliths contain less C than enriched samples, there is no clear relationship between abundances of C and the incompatible trace elements.In the suite of deformed cumulate peridotite and dunite xenoliths of the 1801 Kaupulehu flow of the Hualalai volcano, Hawaii, C abundances are clearly related to texture, modal composition, and style of deformation. The most C-rich rocks are wehrlites in which the clinopyroxenes deformed more brittly and thus possess higher fluid inclusion and crack densities than the surrounding olivines.Regardless of their lithology, all xenoliths from kimberlites (including both peridotites and eclogites) are C-rich compared to those from basalts. Most of the C in these xenoliths exists as calcite or carbonaceous matter associated with serpentine veins and was thus probably contributed by the kimberlite host. Primary carbonates are extremely rare in all xenoliths, although occasionally they have been observed as daughter products in fluid inclusions.Although most C exists as inclusions of CO₂-rich vapor, condensed carbonaceous matter also appears to occur in all rocks as discrete platy grains and as a film on natural surfaces such as grain boundaries and cracks.     

The mineral chemistry and origin of xenoliths from the lavas of Anjouan,Comores Archipelago,Western Indian Ocean

Mineralogical data for xenoliths occurring as inclusions in the fissure erupted alkali basalts and the basanitic tuffs of Anjouan reveal three xenolith suites: 1) the lherzolites, 2) the dunites and wehrlites, 3) the gabbros and syenites. The dunite-wehrlite suite and the gabbro suite are shown to represent high-level cumulate sequences resulting from ankaramitic fractionation of the hy-normative shield-building lavas and cotecictic fractionation of the alkali basalt lavas respectively, whilst the syenitic xenoliths represent evolved high-level intrusions. Mineralogical and rare earth element (REE) data indicate that the most likely origin for the spinel lherzolite xenoliths is by extraction of a basaltic phase from spinel peridotite, leaving a light REE-poor spinel lherzolite residuum. REE models, constructed using model peridotite assemblages, imply that the hy-normative basalt lavas may be derived by partial melting of spinel peridotite at pressures of <20–25 kb leaving a residual lherzolite, and that the alkali basalt and basanite melts are formed by small degrees of melting of a garnet-peridotite source at pressures of >20–25 kb. The spinel lherzolite source for the hy-normative basalts has been accidentally sampled during explosive eruption of the alkali basalt and basanite magmas.     

A first insight into the constitution of the upper mantle under the hoggar area (Southern Algeria): The lherzolite xenoliths in the alkali-basalts

Quaternary alkali olivine basalts and nephelinites from the Hoggar area (Southern Algeria) contain numerous lherzolite inclusions. The investigated nodules have been classified into three textural categories: coarse, porphyroclastic and granuloblastic. Microprobe analyses have been carried out on the coexisting phases (olivine, clinopyroxene, orthopyroxene, spinel, pargasite, Al rich glass) from 10 selected samples, with a special attention to pyroxene porphyroclasts which are zoned in Al and Cr. Most of the porphyroclastic xenoliths contain secondary pargasite (or its breakdown products) which is thought to reflect a metasomatic event in mantle conditions. Equilibrium temperatures and pressures have been calculated for the three groups, using the single-pyroxene method: the coarse samples have been equilibrated at higher pressures (20–25 kb) and temperatures (1,000° C–1,100° C) than the granuloblastic samples (about 10 kb and 900° C); with regards to the porphyroclastic xenoliths, the estimated T and P have been related to two stages of crystallization (corresponding to porphyroclasts and neoblasts). Relationships between phase compositions, rock-textures and metasomatism are briefly discussed.     

吉林蛟河地幔橄榄岩捕虏体的Sr-Nd-Hf-Os同位素特征与岩石圈地幔时代

吉林省蛟河市境内大石河新生代玄武岩中含有丰富的地幔橄榄岩包体,详细的岩石学与矿物学研究显示,这些包体的主要岩石类型为尖晶石二辉橄榄岩-方辉橄榄岩,未发现石榴石橄榄岩。岩相学及地球化学资料显示它们都是经历过熔体抽取而形成的岩石圈地幔残留。矿物平衡温度计算发现,本区的这些地幔橄榄岩包体来自地下40～60km 深度,且下部以二辉橄榄岩为主,而上部以贫单斜辉石的二辉橄榄岩和方辉橄榄岩为主,显示明显的岩石圈地幔分层现象。Sr-Nd-Hf 同位素资料反映这些地幔包体均表现为亏损性质,而 Re-Os 同位素资料确定上述岩石圈地幔形成于中元古代,明显老于上覆地壳的新元古宙时代,反映壳幔年龄上的解耦。因此我们推测,该区曾经历过华北克拉通类似的早期岩石圈地幔的整体丢失事件,然后形成于其它地区的中元古宙岩石圈地幔在本区增生。     

Variation in mantle lithology and composition beneath the Ngao Bilta volcano,Adamawa Massif,Cameroon volcanic line,West-central Africa

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 863–957
C,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.     

Quantitative Untersuchungen mit der Elektronen-Mikrosonde an Pyroxenen aus Basalten und Peridotit-Einschlüssen

The chemical composition of the pyroxenes and olivines of 12 basaltic rocks and 5 lherzolite nodules was determined quantitatively by electron micro-probe analysis. The composition of the pyroxenes depends on the type of basalt in which they occur. Tholeiitic basalts with normative quartz contain three pyroxenes: orthorombic pyroxenes, pigeonites and augites. All pyroxene phases are zoned and do not show any exsolution. Their Ti and Al contents (Ca-Tschermaks and Ti-augite molecules) are small. All pyroxene phases were formed under disequilibrium with each other and with the melt because of rapid quenching. The sequence of crystallization: orthopyroxene—pigeonite—augite could be established by their Cr content.The alkali olivine basalts undersatured in SiO₂ and the olivine nephelinites are characterized by Ti and Al-rich clinopyroxenes. The distribution of Ti and Al in the pyroxenes of the alkali olivine basalts shows a differentiation trend from the cores of the phenocrysts to their outer zones and to the crystals of the ground mass. Thereby the Ca-Tschermaks molecule is being replaced more and more by the Ti-augite molecule. The Ti content of the pyroxenes of the olivine nephelinites decreases in the last stage of differentiation because simultaneously increasing amounts of titaniferous magnetite crystallize.The pyroxenes of lherzolite peridotite nodules are characterized by high Al and low Ti contents which differ according to the type of basalt (alkali olivine basalt or olivine nephelinite) in which the nodules occur. The homogeneous distribution of the elements within the single grains indicates crystallization under equlibrium conditions. The conditions of their formation are comparable to those of Al-pyroxene peridotites in the upper mantle. The composition of pyroxenes of early accumulates of alkali basaltic melts differ from those of peridotite nodules. Therefore lherzolite nodules can be taken as residues of deeper peridotite masses.     

Geochemical and O-isotope constraints on the evolution of lithospheric mantle in the Ross Sea rift area (Antarctica)

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–Fe_tot. Preferential Nb, Zr, Sr enrichments in clinopyroxene associated with high Ti–Fe_tot 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 δ¹⁸O_cpx 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.     

Xenolith evidence for lithospheric melting above anomalously hot mantle under the northern Canadian Cordillera

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