Reactions between mantle xenoliths and host magma beneath La Palma (Canary Islands): constraints on magma ascent rates and crustal reservoirs

Spinel-bearing peridotitic mantle xenoliths from the 1949 eruption on La Palma were modified mineralogically and chemically during prolonged reaction with their host magma. The magmatism that brought the peridotites to the surface caused two distinct generations of xenolith fractures: (1) Old fractures are characterized by crystalline selvages with cumulus textures towards the host magma, or by polymineralic veins. They are accompanied by 0.9–2 mm wide diffusion zones where peridotite olivine became less forsteritic through diffusive exchange with the host magma. Old fractures represent most of each xenolith's surface. (2) Young fractures show no selvages and only narrow diffusion zones of <0.02 mm width. Calculations based on a model of Fe-Mg interdiffusion give an age of 6 to 83 years and <4 days for old and young fractures, respectively. A combination of these data with fluid inclusion barometry indicates that selvages and veins formed during xenolith transport rather than representing wall-rock reactions or mantle metasomatism. The results provide ample evidence for prolonged storage of the xenoliths in the crust, constraining a multi-stage magma ascent: Years to decades prior to eruption, ascending magma ruptured peridotitic wall-rock possibly through hydraulic fracturing and stoping around magma reservoirs. Magma batches transported the peridotite xenoliths to the crust at ascent rates exceeding 0.2 ms⁻¹. The xenoliths and their host magma stagnated during at least 6 years in possibly sill-like reservoirs at 7–11 km depth. The xenoliths became deposited and subsequently embedded in a mush of settled phenocrysts, while selvages and veins crystallized until the eruption commenced. At the end of the eruption, the xenoliths were finally transported to the surface within hours to days. Decompression during the rapid ascent induced internal stresses and caused renewed fragmentation of the xenoliths, producing the young fractures. Received: 25 August 1997 / Accepted: 25 November 1997     

Multi-stage magma ascent beneath the Canary Islands: evidence from fluid inclusions

Gabbroic and ultramafic xenoliths and olivine and clinopyroxene phenocrysts in basaltic rocks from Gran Canaria, La Palma, El Hierro, Lanzarote and La Gomera (Canary Islands) contain abundant CO₂-dominated fluid inclusions. Inclusion densities are strikingly similar on a regional scale. Histogram maxima correspond to one or more of the following pressures: (1) minimum 0.55 to 1.0 GPa (within the upper mantle); (2) between 0.2 and 0.4 GPa (the Moho or the lower crust); (3) at about 0.1 GPa (upper crust). Fluid inclusions in several rocks show a bimodal density distribution, the lower-density maximum comprising both texturally early and late inclusions. This is taken as evidence for an incomplete resetting of inclusion densities, and simultaneous formation of young inclusions, at well-defined magma stagnation levels. For Gran Canaria, pressure estimates for early inclusions in harzburgite and dunite xenoliths and olivine phenocrysts in the host basanites overlap at 0.9 to 1.0 GPa, indicating that such magma reservoir depths coincide with levels of xenolith entrainment into the magmas. Magma chamber pressures within the mantle, inferred to represent levels of mantle xenolith entrainment, are 0.65–0.95 GPa for El Hierro, 0.60–0.68 GPa for La Palma, and 0.55–0.75 GPa for Lanzarote. The highest-density fluid inclusions in many Canary Island mantle xenoliths have probably survived in-situ near-isobaric heating at the depth of xenolith entrainment. Inclusion data from all islands indicate ponding of basaltic magmas at Moho or lower crustal depths, and possibly at an additional higher level, strongly suggestive of two main crustal accumulation levels beneath each island. We emphasize that repeated magmatic underplating of primitive magmas, and therefore intrusive accretion, are important growth mechanisms for the Canary Islands, and by analogy, for other ocean islands. Comparable fluid inclusion data from primitive rocks in other tectonic settings, including Iceland, Etna and continental rift systems (Hungary, South Norway), indicate that magma accumulation close to Moho depths shortly before eruption is not, however, restricted to oceanic intraplate volcanoes. Lower crustal ponding and crystallization prior to eruption may be the rule rather than the exception, independent of the tectonic setting. Received: 30 May 1997 / Accepted: 6 February 1998     

大同新生代火山岩浆的快速喷发——来自橄榄石地幔捕掳晶的证据

量化研究幔源岩浆从源区运移至喷发或者侵位的时间尺度，对理解基性岩浆作用具有重要意义。然而，对于岩浆的喷发和就位的时间尺度研究仍缺乏有效的约束，位于我国华北克拉通北部山西大同新生代火山岩群是理想的研究地区。本文以该火山群～0.2 Ma喷发的神泉寺碱性玄武岩为研究对象，重点研究其中携带的地幔橄榄石捕掳晶来约束喷发前的时间尺度。通过对其开展详细的矿物化学研究，发现地幔捕掳晶核部的Fo值高达97.7,为极富镁橄榄石，结合其极低的Ca、Mn和Ni含量特征，认为它们捕获自被交代的地幔橄榄岩。另外，地幔橄榄石捕掳晶发育明显的CaO成分环带，表明其在地壳岩浆系统内经历了复杂的岩浆演化过程。地幔橄榄石捕掳晶反应边宽度变化很大，说明它们在源区被捕获时及在运移过程中经过了多次破碎过程。橄榄石捕掳晶最边缘的Fo值为70左右，平衡计算表明它们在边部已与主岩浆(碱性玄武岩)达到平衡。Fe-Mg元素扩散计时结果显示，橄榄石地幔捕掳晶仅在岩浆中滞留了几个月的时间。对于40～70 km的岩石圈地幔厚度来说，岩浆平均上升速率最快可能超过500 m/d。     

Fast kimberlite ascent rates estimated from hydrogen diffusion profiles in xenolithic mantle olivines from southern Africa

Fourier transform infrared spectrometry (FTIR) analyses of olivines from peridotite xenoliths found in southern African kimberlites indicate 0 to 80 ppm H₂O concentrations. OH absorbance profiles across olivine grains show homogeneous H contents from core to edge for most samples. In one sample the olivines are H-free, while another has olivines characterized by lower H contents at the grain edges compared to the cores, indicating H loss during transport of the xenolith to the surface. Flat or near-flat H profiles place severe constraints on the duration of H loss from olivine grains, with implications for kimberlite magma ascent rates. Diffusion equations were used to estimate times of H loss of about 4 h for the sample with heterogeneous olivine H contents. Resulting kimberlite ascent rates are calculated to be 5-37 m s⁻¹ minimum, although these estimates are highly dependent on volatile contents and degassing behavior of the host kimberlite magma. Xenolithic olivines from alkali basalts generally have lower H contents and more pronounced H diffusion profiles than do those from kimberlites. This difference is likely caused by higher magma temperatures and lower ascent rates of alkali basalts compared to kimberlites.     

The origin of reaction textures in mantle peridotite xenoliths from Sal Island, Cape Verde: the case for “metasomatism” by the host lava

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.     

Primitive andesites from the Taupo Volcanic Zone formed by magma mixing

Andesites with Mg# >45 erupted at subduction zones form either by partial melting of metasomatized mantle or by mixing and assimilation processes during melt ascent. Primitive whole rock basaltic andesites from the Pukeonake vent in the Tongariro Volcanic Centre in New Zealand’s Taupo Volcanic Zone contain olivine, clino- and orthopyroxene, and plagioclase xeno- and antecrysts in a partly glassy matrix. Glass pools interstitial between minerals and glass inclusions in clinopyroxene, orthopyroxene and plagioclase as well as matrix glasses are rhyolitic to dacitic indicating that the melts were more evolved than their andesitic bulk host rock analyses indicate. Olivine xenocrysts have high Fo contents up to 94%, δ¹⁸O_(SMOW) of +5.1‰, and contain Cr-spinel inclusions, all of which imply an origin in equilibrium with primitive mantle-derived melts. Mineral zoning in olivine, clinopyroxene and plagioclase suggest that fractional crystallization occurred. Elevated O isotope ratios in clinopyroxene and glass indicate that the lavas assimilated sedimentary rocks during stagnation in the crust. Thus, the Pukeonake andesites formed by a combination of fractional crystallization, assimilation of crustal rocks, and mixing of dacite liquid with mantle-derived minerals in a complex crustal magma system. The disequilibrium textures and O isotope compositions of the minerals indicate mixing processes on timescales of less than a year prior to eruption. Similar processes may occur in other subduction zones and require careful study of the lavas to determine the origin of andesite magmas in arc volcanoes situated on continental crust.     

Metasomatism in mantle xenoliths from Gees,West Eifel,Germany: evidence for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment

 We have investigated new samples from the Gees mantle xenolith suite (West Eifel), for which metasomatism by carbonatite melt has been suggested. The major metasomatic change is transformation of harzburgites into phlogopite-rich wehrlites. Silicate glasses are associated with all stages of transformation, and can be resolved into two major groups: a strongly undersaturated alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, variably alkaline glass present exclusively within the xenoliths. Si-Al-rich glasses (up to 72 wt% SiO₂ when associated with orthopyroxene (Opx) are usually interpreted in mantle xenoliths as products of decompressional breakdown of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to phlogopite breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those related to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Both phases crystallize as microlites from the glass. The chemical composition of the Si-Al-enriched glass shows that it cannot be derived from decompressional melting of the Gees xenoliths, but must have been present prior to their entrainment in the host magma. Simple mass-balance calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which glass + microlite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline andesite (55–60 wt% SiO₂), characterized by high Al₂O₃ (ca. 18 wt%). The obtained composition is very similar to high-alumina, calc-alkaline melts that should form by AFC-type reactions between basalt and harzburgite wall rock according to the model of Kelemen (1990). Thus, we suggest that the Si-Al-enriched glasses of Gees, and possibly of other suites as well, are remnants of upper mantle hybrid melts, and that the Gees suite was metasomatized by silicate and not carbonatite melts. High-Mg, high-Ca composition of metasomatic olivine and clinopyroxene in mantle xenoliths have been explained by carbonatite metasomatism. As these features are also present in the Gees suite, we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show that they are a simple function of silica activity. High-Ca compositions are attained at low a SiO₂ and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespective of whether it is a carbonatite or a silicate melt. Such low a SiO₂ is recorded by the microlites in the Gees Si-Al-rich glasses. Our results imply that xenolith suites cannot confidently be related to carbonatite metasomatism if the significance of silicate glasses, when present, is not investigated. Received: 2 March 1995 / Accepted: 12 June 1995     

矿物结构特征对岩浆混合过程的指示： 以新西兰Ruapehu全新世安山岩中的辉石为例

根据矿物结构可以有效的识别开放性体系内的各种岩浆过程,在岩相学研究基础上,对全新世喷发的Ruapehu火山 Whakapapa组安山岩内的辉石斑晶进行了系统的矿物学分析。所研究样品中辉石斑晶以普通辉石及顽火辉石为主,具多样 结构与形态,根据其结构和成分特征可分为3类：（1）骸晶状的辉石（SP型）,具有破碎的、筛孔状外形,核部具有异常高 的FeOT含量,可能为壳源捕掳晶;（2）具均一结构及成分的辉石（HP型）,其内部未显示明显的熔融结构,形成于岩浆房 的内部,基本未受基性岩浆混合的影响;（3）具核-幔-边结构的 “绿核”辉石（GCP型）,其幔部相比核部及边部具有更 高的En、Mg#、CaO、MgO和Cr2O3含量以及更低的FeOT、TiO2含量。且GCP辉石边部相比核部更加富CaO、MgO而贫 FeOT、TiO2。GCP型辉石幔部（边部）与核部形成明显的反环带结构,两者界线清晰,成分上存在突变,是偏基性岩浆注 入岩浆房边界层时,岩浆发生大规模扩散、混合之前形成于脉状岩浆“通道”内;而GCP辉石边部则结晶于混合后的岩浆 之中。辉石的结构及岩相学特征暗示,安山岩在喷发前经历了岩浆混合及地壳物质的混染,混合过程主要发生于地壳浅部 岩浆房的边界层及粥状层内,且这种混合过程可能是诱发火山喷发的重要机制之一。     

Primary fluids entrapped at blueschist to eclogite transition: evidence from the Tianshan meta-subduction complex in northwestern China

Orthopyroxene and olivine exposed along the rim of a harzburgite xenolith from La Palma (Canary Islands) show polycrystalline selvages and diffusion zones that result from contact with mafic, alkaline, silica-undersaturated melts during at least 10-100 years before eruption. The zoned selvages consist of a fine-grained reaction rim towards the xenolith and a coarser grained, cumulate-like layer towards the melt contact. The diffusion zones are characterized by decreasing magnesium number from about 89-91 in the xenolith interior to 79-85 at the rims, and clearly result from Fe-Mg exchange with surrounding mafic melt. The width of the diffusion zones is 80-200 µm in orthopyroxene and 1,020-1,730 µm in olivine. Orthopyroxene also shows decreasing Al₂O₃ and Cr₂O₃ and increasing MnO and TiO₂ towards the reaction rims. Textural relations and comparisons with dissolution experiments suggest that orthopyroxene dissolution by silica-undersaturated melt essentially ceased after days to weeks of melt contact, possibly because of decreasing temperature and formation of the reaction rims. The short dissolution phase was followed by prolonged growth of diffusion zones through cation exchange between xenolith minerals and melt across the reaction rims, and by the growth of cumulus crystals. The observations indicate that orthopyroxene xenocrysts and harzburgite xenoliths can survive in mafic, silica-undersaturated, subliquidus magmas at 1,050-1,200 °C and 200-800 MPa for tens of years. Modeling and comparison of the diffusion zones indicate that the average Fe-Mg interdiffusion coefficient D_FeMg in orthopyroxene is 2 log units lower than that in olivine; at 1,130 °C and QFM-buffered oxygen fugacity, D_FeMg^opx = 3 ×10 ^{- 19}  m²  s^{- 1}D_{FeMg}^{opx} = 3 \times 10^{ - 19} \,{\rm m}^2 \,{\rm s}^{{\rm - 1}} . The new data overlap well with recently published data for D_FeMg in diopside, and indicate that D_FeMg ^opxD_{FeMg\,}^{opx} (as predicted by previous authors) may be extrapolated to higher temperatures and oxygen fugacities. It is suggested that D_FeMg ^opx D_{FeMg\,}^{opx} and D_FeMg in Mn-poor ferromagnesian garnet are similar within 0.5 log units at temperatures between 1,050 and 1,200 °C.     

Magma ascent rate and initial water concentration inferred from diffusive water loss from olivine-hosted melt inclusions

As the water concentration in magma decreases during magma ascent, olivine-hosted melt inclusions will reequilibrate with the host magma through hydrogen diffusion in olivine. Previous models showed that for a single spherical melt inclusion in the center of a spherical olivine, the rate of diffusive reequilibration depends on the partition coefficient and diffusivity of hydrogen in olivine, the radius of the melt inclusion, and the radius of the olivine. This process occurs within a few hours and must be considered when interpreting water concentration in olivine-hosted melt inclusions. A correlation is expected between water concentration and melt inclusion radius, because small melt inclusions are more rapidly reequilibrated than large ones when the other conditions are the same. This study investigates the effect of diffusive water loss in natural samples by exploring such a correlation between water concentration and melt inclusion radius, and shows that the correlation can be used to infer the initial water concentration and magma ascent rate. Raman and Fourier transform infrared spectroscopy measurements show that 31 melt inclusions (3.6–63.9 μm in radius) in six olivines from la Sommata, Vulcano Island, Aeolian Islands, have 0.93–5.28 wt% water, and the host glass has 0.17 wt% water. The water concentration in the melt inclusions shows larger variation than the data in previous studies (1.8–4.52 wt%). It correlates positively with the melt inclusion radius, but does not correlate with the major element concentrations in the melt inclusions, which is consistent with the hypothesis that the water concentration has been affected by diffusive water loss. In a simplified hypothetical scenario of magma ascent, the initial water concentration and magma ascent rate are inferred by numerical modeling of the diffusive water loss process. The melt inclusions in each olivine are assumed to have the same initial water concentration and magma ascent rate. The melt inclusions are assumed to be quenched after eruption (i.e., the diffusive water loss after eruption is not considered). The model results show that the melt inclusions initially had 3.9–5.9 wt% water and ascended at 0.002–0.021 MPa/s before eruption. The overall range of ascent rate is close to the lower limit of previous estimates on the ascent rate of basalts.     

Fragmentation processes and xenolith transport in a Proterozoic minette dyke,Grenville Province,Québec

 The Mesoproterozoic Rivard minette dyke provides a case example of dyke-parallel fracturing induced by propagation of low-viscosity melts at deep crustal levels. The dyke constitutes a xenolith-choked intrusion breccia and provides samples of an extensive section of the lithosphere underlying the southwestern Grenville Province of Québec at 1.08 Ga. The xenoliths occupy approximately 50% of the volume of the breccia. They are extremely diverse and include pyroxenites and mafic granulites from the upper mantle and lower crust, and Grenvillian gabbroids, gneisses and quartzites. Wall-rock fragments are detached to various extents along dyke-parallel fractures and apophyses. The host lamprophyre magma has a calculated viscosity of ca. 40 Pa·s. Rapid ascent of magma and turbulence are suggested by the presence of large dense fragments and the brittle-to-ductile deformation at the margins of some xenoliths. A minimum ascent velocity of ca. 50 cm·s^–1 is estimated from the settling velocity of a 40 cm long pyroxenite xenolith. The progressive addition of xenoliths increased the effective viscosity of the liquid-solid mixture to 10³ Pa·s during emplacement. This favoured Bingham behaviour and laminar flow of the magma, and xenolith suspension. Field evidence suggests that xenoliths were formed through (1) early fracturing of wall rock by inelastic deformation during dyke propagation, and (2) continuous delamination of wall rock by intrusion of magma along the dyke-parallel fractures. This led to removal of chilled margins, and to fluid infiltration, partial melting and microbrecciation in the wall rock. Pre-existing discontinuities played a minor role in the fracturing process. Xenoliths may thus be abundant in alkaline magmas not only because these magmas ascend rapidly and can transport xenoliths, but possibly also, because their low viscosities promote intense fracturing of wall rock. Received: 10 June 1995 / Accepted: 10 February 1996     

Magmatism-related localized deformation in the mantle: a case study

A deformed composite peridotite-pyroxenite xenolith in Pliocene alkali basalts from the Pannonian Basin (Szentbékkálla, Bakony—Balaton Highland Volcanic Field) has been studied in detail. A narrow shear zone of intense deformation marked by porphyroclast elongation and recrystallization runs along the peridotite-pyroxenite contact. The xenolith contains a large volume of euhedral olivine neoblasts and tablet grains of olivine away from the shear zone interpreted as products of annealing and recrystallization in the presence of grain boundary fluid. Estimates of the time required for growth of recrystallized olivine grains suggest that the annealing took place in situ in the mantle and not during transport of the xenolith in the basalt magma. The grain boundary fluid present during recrystallization was a vapor rich silicate-melt different from the host basaltic melt that entrained the xenolith. This study demonstrates that high-stress deformation zones and associated fluid-assisted recrystallization, which are common features in kimberlite mantle xenoliths, also occur in some mantle xenoliths from alkali basalts. The suggested high-stress deformation zones in the mantle beneath the Pannonian Basin may be produced by paleoseismic events in the lithosphere associated with faulting related to the ascent of basalt magma.Editorial responsibility: J. Hoefs

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Evidence for xenolith–host basalt interaction from chemical patterns in Fe–Ti-oxides from mafic granulite xenoliths of the Bakony–Balaton Volcanic field (W-Hungary)

We investigate the effects of xenolith–host basalt interaction on lower crustal mafic granulite xenoliths from the Central Pannonian Basin. The xenoliths are devoid of any signs of melting, nevertheless various phenomena are identified, which indicate that the original mineralogy and chemistry of the xenoliths was modified during interaction with the host basalt. The rock-forming silicates are only slightly affected by alteration, but the Fe–Ti-oxides are overprinted to a significant extent. Complex chemical zoning patterns are detected using high-resolution element mapping in ilmenites and in lamellar titanomagnetite–ilmenite intergrowths. The chemical alteration of the Fe–Ti-oxides was diffusion-controlled and, hence, time and temperature dependent. On the basis of diffusion profiles in titanomagnetite we estimate the duration of xenolith–host basalt interaction to be at least 9–20 h. This is comparable to the time necessary for the ascent of the host basalt to the surface. It is too short to reflect alteration during granulite facies metamorphism in the deep crust. The chemical alteration of the titanomagnetite thus reflects the total duration of the xenolith–host basalt interaction.     

The pressure and temperature conditions and timing of glass formation in mantle-derived xenoliths from Baarley, West Eifel, Germany: the case for amphibole breakdown, lava infiltration and mineral – melt reaction

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     

The magma plumbing system for the 1971 Teneguía eruption on La Palma,Canary Islands

The 1971 Teneguía eruption is the most recent volcanic event of the Cumbre Vieja rift zone on La Palma. The eruption produced basanite lavas that host xenoliths, which we investigate to provide insight into the processes of differentiation, assimilation and magma storage beneath La Palma. We compare our results to the older volcano magmatic systems of the island with the aim to reconstruct the temporal development of the magma plumbing system beneath La Palma. The 1971 lavas are clinopyroxene-olivine-phyric basanites that contain augite, sodic-augite and aluminium augite. Kaersutite cumulate xenoliths host olivine, clinopyroxene including sodic-diopside, and calcic-amphibole, whereas an analysed leucogabbro xenolith hosts plagioclase, sodic-augite-diopside, calcic-amphibole and hauyne. Mineral thermobarometry and mineral-melt thermobarometry indicate that clinopyroxene and plagioclase in the 1971 Teneguía lavas crystallised at 20–45 km depth, coinciding with clinopyroxene and calcic-amphibole crystallisation in the kaersutite cumulate xenoliths at 25–45 km and clinopyroxene, calcic-amphibole and plagioclase crystallisation in the leucogabbro xenolith at 30–50 km. Combined mineral chemistry and thermobarometry suggest that the magmas had already crystallised, differentiated and formed multiple crystal populations in the oceanic lithospheric mantle. Notably, the magmas that supplied the 1949 and 1971 events appear to have crystallised deeper than the earlier Cumbre Vieja magmas, which suggests progressive underplating beneath the Cumbre Vieja rift zone. In addition, the lavas and xenoliths of the 1971 event crystallised at a common depth, indicating a reused plumbing system and progressive recycling of Ocean Island plutonic complexes during subsequent magmatic activity.     

Contamination of mafic magma by partial melting of dolomitic xenoliths

Mg-skarns enclosed in dunite cumulates of the Neo-Proterozoic Ioko-Dovyren intrusion (northern Baikal region, Russia) can be traced to silica-poor dolomitic host rock layers. The dominant minerals of the skarns are brucite (pseudomorph after periclase), forsterite and Cr-poor spinel. Rapid heating of quartz-poor dolomitic xenoliths led to the formation of minor olivine, followed by the breakdown of dolomite to calcite and periclase. Xenoliths were partially melted upon further heating resulting in a calcite melt. This low-density melt was quantitatively squeezed out, mixed with the surrounding mafic magma and left behind periclase and olivine. This caused the crystallization of new olivine with elevated CaO contents in zones above skarn-bearing horizons. Mixing of calcite melt with the surrounding mafic magma also resulted in the crystallization of Cats-rich clinopyroxene instead of plagioclase. The mineralogy of contaminated dunite cumulates is consistent with assimilation of approximately 4wt% CaO by the Ioko-Dovyren mafic magma.     

Chalcophile element constraints on magma differentiation of Quaternary volcanoes in Tengchong,SW China

The Tengchong volcanic field comprises numerous Quaternary volcanoes in SW China. The volcanic rocks are grouped into Units 1–4 from the oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, and Unit 2 consists of hornblende-bearing dacite. This rock assemblage resembles those of arc volcanic sequences related to oceanic slab subduction. Rocks of Units 1 and 3 contain olivine phenocrysts with Fo contents ranging from 65 to 85 mole%, indicating early fractionation of olivine and chromite prior to the eruption of magma. All the rocks from Units 1, 3 and 4 have very low PGE concentrations, with <0.05 ppb Ru and Rh, <0.2 ppb Pt and Pd, and Ir that is commonly close to, or slightly higher than detection limits (0.001 ppb). The small variations of Pt/Pd ratios (0.4–2.2) are explained by fractionation of silicate and oxide minerals. The 5-fold variations in Cu/Pd ratios (200,000–1,000,000) for the lavas at Tengchong, which do not vary systematically with fractionation, likely reflect retention of variable amounts of residual sulfide in the mantle source. In addition, all the rocks from Units 1, 3 and 4 have primitive mantle-normalized chalcophile element patterns depleted in PGE relative to Cu. Together with very low Cu/Zr ratios (0.06–0.24), these rocks are considered to have undergone variable degrees of sulfide-saturated differentiation in shallow crustal staging magma chambers. Large amounts of olivine and chromite crystallization probably triggered sulfide saturation of magma at depth for Units 1 and 3, whereas crustal contamination was responsible for sulfide saturation during ascent of magma for Unit 4.     

Environments of Crystallization and Compositional Diversity of Mauna Loa Xenoliths

Two picrite flows from the SW rift zone of Mauna Loa containxenoliths of dunite, harzburgite, lherzolite, plagioclase-bearinglherzolite and harzburgite, troctolite, gabbro, olivine gabbro,and gabbronorite. Textures and olivine compositions precludea mantle source for the xenoliths, and rare earth element concentrationsof xenoliths and clinopyroxene indicate that the xenolith sourceis not old oceanic crust, but rather a Hawaiian, tholeiitic-stagemagma. Pyroxene compositions, phase assemblages and texturalrelationships in xenoliths indicate at least two different crystallizationsequences. Calculations using the pMELTS algorithm show thatthe two sequences result from crystallization of primitive MaunaLoa magmas at 6 kbar and 2 kbar. Independent calculations ofolivine Ni–Fo compositional variability in the plagioclase-bearingxenoliths over these crystallization sequences are consistentwith observed olivine compositional variability. Two parentsof similar bulk composition, but which vary in Ni content, arenecessary to explain the olivine compositional variability inthe dunite and plagioclase-free peridotitic xenoliths. Xenolithsprobably crystallized in a small magma storage area beneaththe rift zone, rather than the large sub-caldera magma reservoir.Primitive, picritic magmas are introduced to isolated rift zonestorage areas during periods of high magma flux. Subsequenteruptions reoccupy these areas, and entrain and transport xenolithsto the surface. KEY WORDS: xenolith; Hawaii; volcano plumbing; mineral composition; picrite     

广西平南玄武岩的地球化学特征

平南碱性玄武岩产于 NE向的区域性断裂中 ,岩石类型有两类 ,一为碧玄岩 ,全岩 K- Ar年龄为 50 .8Ma,含各类包体 ,有以下特征 :1)含地幔岩包体 ,2 ) [Mg]=0 .78,3)在 Fe O/mol- Mgo/mol图解上液相线橄榄石的成分位于 Fo90～ Fo94之间 ,与橄榄岩包体中的橄榄石成分完全一致。4 ) Ni=0 .0 4 ( % ) <0 .0 5( % ) ,5)其熔浆密度为 2 .62 g/cm3,粘度为 2 .52 Pa· s,携带 4 cm直径包体上升的速度为 37.7cm/s,上升的速度较快 ,具原生碧玄岩浆的特点 ,岩浆液相线的温度为12 86℃ ,起源压力为 1.8GPa,玄武岩富集轻稀土可能与地幔性质有关。一为粗面玄武岩 ,不含包体 ,全岩 K- Ar年龄为 4 9.9Ma,岩浆液相线的温度为 1162℃ ,是进化岩浆。玄武岩的 Nd、Sr同位素显示为亏损地幔源区。它们都是大陆拉张环境下的产物 ,与中国东部新生代玄武岩的形成背景一致。     

Origin of xenoliths in the trachyte at Puu Waawaa,Hualalai Volcano,Hawaii

Rare dunite and 2-pyroxene gabbro xenoliths occur in banded trachyte at Puu Waawaa on Hualalai Volcano, Hawaii. Mineral compositions suggest that these xenoliths formed as cumulates of tholeiitic basalt at shallow depth in a subcaldera magma reservoir. Subsequently, the minerals in the xenoliths underwent subsolidus reequilibration that particularly affected chromite compositions by decreasing their Mg numbers. In addition, olivine lost CaO and plagioclase lost MgO and Fe₂O₃ during subsolidus reequilibration. The xenoliths also reacted with the host trachyte to form secondary mica, amphibole, and orthopyroxene, and to further modify the compositions of some olivine, clinopyroxene, and spinel grains. The reaction products indicate that the host trachyte melt was hydrous. Clinopyroxene in one dunite sample and olivine in most dunite samples have undergone partial melting, apparently in response to addition of water to the xenolith. These xenoliths do not contain CO₂ fluid inclusions, so common in xenoliths from other localities on Hualalai, which suggests that CO₂ was introduced from alkalic basalt magma between the time CO₂-inclusion-free xenoliths erupted at 106±6 ka and the time CO₂-inclusion-rich xenoliths erupted within the last 15 ka.