Glasses In Mantle Xenoliths Research Articles

Mafic and silica-rich glasses in mantle xenoliths from Wau-en-Namus, Libya: Textural and geochemical evidence for peridotite–melt reactions

Anhydrous spinel peridotite xenoliths in Quaternary nepheline-basanite and melilite- or sodalite-bearing lavas of the Wau-en-Namus volcano in S Libya range from lherzolites to harzburgites recording melt extraction in a shallow setting (≤ 2 GPa). Primary clinopyroxenes have distinct trace element characteristics documenting LILE (large ion lithophile element) depletion or enrichment events predating the formation of glass pockets and veins in the xenoliths. These glasses are aluminous and alkali-rich, range in composition from ultrabasic to silicic (43–67 wt.% SiO 2) and may contain empty vugs and micro-phenocrysts of olivine, clinopyroxene, spinel, plagioclase, sodalite, apatite that are similar in composition to phenocrysts in the host lavas. Reactions of infiltrating melt and xenolith minerals are documented by diffuse Fe–Ca-rich rims of olivine in contact with glass, and by spongy-textured reaction domains caused by incongruent dissolution of primary pyroxenes and spinel. Some glasses have trace element characteristics similar to that of the host Ne-basanite, suggesting they were derived from the same source during entrainment and transport to the surface. Incompatible element enrichment and Sr–Nd isotopic compositions of the analyzed host lava are similar to HIMU (high μ; μ = 238Pb/ 204Pb)-type magmas, but the Pb isotopic composition is less radiogenic compared to other intra-plate Neogene magmatic rocks from N Africa.

Redox potential of mantle magmatic systems

A new variant of the olivine-clinopyroxene-spinel oxygen barometer was developed on the basis of the equilibrium 3CaMgSi2O6(Cpx) + 2Fe3O4(Spl) = 3CaMgSiO4(Ol) + 3Fe2SiO4(Ol) + O2. Oxygen fugacity was estimated for the mineral assemblages of meymechites, olivine-bearing rocks of the Guli intrusion, and olivine and clinopyroxene microphenocrysts from interstitial glasses in mantle xenoliths containing metal alloys from Sal Island, Cape Verde Archipelago. It was shown that oxygen fugacity may vary in mantlederived magmatic systems by 7–8 orders of magnitude. Thermodynamic analysis showed that the low water activity in the lower part of the subcratonic lithosphere prevents the formation of hydrocarbons even at the presence of elemental carbon and low oxygen fugacity. The most probable mechanism of diamond formation is the reduction of carbonate components in the composition of near-solidus melts coming into the lithosphere from ascending mantle plumes.

A new version of the spinel-olivine-pyroxene oxybarometer and extreme redox differentiation in magmatic systems of mantle sources

A new version of an oxybarometer for olivine + clinopyroxene + spinel phase association based on consideration of the equilibrium (2) has been developed. Assessments of the oxygen fugasity for mineral associations of meimechites, Gulinskaya intrusion olivine-containing rocks, and microphenocrysts of olivine and clinopyroxene from intergranular glasses in mantle xenoliths from Sal Island, the Cape Verde archipelago, which contain grains of metallic alloys, were made. The oxygen fugasity values that characterize magmatic systems of mantle sources are shown to be capable of being varied by 7–8 orders of magnitude.

About this title ‐ Metasomatism in Oceanic and Continental Lithospheric Mantle

Twenty years have passed since Menzies & Hawkesworth extended the concept of metasomatism to mantle processes. The aim of this book is to gather together progress made on this topic since then. Most of the 14 papers reported in the volume rely on in situ major and trace element analyses of minerals and glasses in mantle xenoliths, and deal with different kinds of metasomatic agents at variable fluid/rock ratios in tectonic settings as different as intra-plate, mid-ocean ridge (ophiolites) and supra-subduction. The book contributes to the wide debate on the nature of the fluids migrating into the mantle wedge, as well as on the different residential times of the subduction signature. In addition papers on intra-plate settings deal with the problem of relating various metasomatic signatures to one single metasomatic event through an infiltration-reaction process.

Immiscibility of calcium fluoride and aluminosilicate melts in ongonite from the Ary-Bulak intrusion, Eastern Transbaikal region

The addition of fluorides of Na, K, Ca, and other elements into silicate melts lowers the degree of their homogeneity owing to the formation of sybotaxic groups enriched in fluorine and cations-modifiers [1]. Many fluoride–silicate systems are characterized by a microheteregoneous structure in the hyperliquidus region related to immiscibility [2]. Under certain conditions, the evolution of natural magmatic systems gives rise to the formation of fluoride melts. Such examples are not numerous and mostly pertain to the effects of immiscibility in sodic fluoride melts (melts– brines) associated with peralkaline granitic magmas. The existence of calcium fluoride melts is confirmed by inclusions of fluoritic glass in mantle xenoliths captured by alkali basalt in New Zealand [3]. As was suggested previously, the residual glasses in ongonite from the Ary-Bulak intrusion were formed as a result of quenching of a calcium alumino-silicofluoride melt consisting of Ca, F, Al, and Si with an admixture of Na and K [4]. In this communication, we present new data on the active participation of calcium fluoride melts in crystallization of this rock.

Glasses in mantle xenoliths as geochemical indicators of metasomatic agents

An extensive comparison between world-wide occurrences of mantle glasses in both continental and oceanic settings is presented here. Several localities were chosen on the basis of the available major (and trace) element data, coupled with a clear identification of the nature of the metasomatizing agents. Xenoliths bearing amphibole and/or phlogopite, which can strongly affect the glass chemistry, were not considered. Despite their considerable geochemical variability world-wide, glasses are rather homogeneous within single xenoliths and their main geochemical features are broadly maintained in the same area. Orthopyroxene always plays a major role in glass genesis, as suggested by the relationship between its presence in the primary assemblage and the silica saturation degree, as well as by the FeO vs. SiO 2 decreasing trends. At the same time, the absence of any correlation between mg# and SiO 2 rules out the possibility of glass chemistry resulting from (or being affected by) secondary mineral fractionation. Thus, the geochemical features of mantle glasses are a powerful tool for investigating the nature of the metasomatic melts. According to available data, glasses in mantle xenoliths from Gran Comore, Samoa, Spitsbergen, Mongolia and Hoggar have been related to carbonatite metasomatism, whereas glasses in mantle xenoliths from Mt. Lessini, Hawaii, Cape Verde and Yitong have been related to metasomatism by alkali silicate basic melts. Moreover, the distinctly different alkali contents in Mt. Lessini and Hawaii glasses as compared with those of Cape Verde and Yitong allow a sodic signature to be attributed to the alkali silicate metasomatic melt of the former with respect to a potassic characteristic of the latter. Glasses related to carbonatite metasomatism are characterized by high CaO, Na 2O, and low SiO 2 and K 2O contents, with Na 2O/K 2O ratios usually greater than 2; they tend to have the highest Nb and LREE and the lowest Zr and Ti contents. On the other hand, glasses related to K-alkali silicate metasomatism are mainly characterized by high SiO 2 and K 2O and low CaO and NaO contents, with Na 2O/K 2O ratios less than 1; they generally present higher Rb and lower Nb and REE contents. Glasses related to Na-alkali silicate metasomatism are characterized by SiO 2 contents comparable to those related to K-alkali silicate metasomatism, but present higher Na 2O contents (in any case lower than those of carbonatite metasomatism), with Na 2O/K 2O ratios usually ranging between 1 and 2. On these bases, diagrams for discriminating between carbonatite- and Na-alkali or K-alkali silicate-related metasomatism are presented.

In situ origin for glass in mantle xenoliths from southeastern Australia: insights from trace element compositions of glasses and metasomatic phases

Siliceous, aluminous and alkali-rich glasses, commonly found in patches and veins in spinel peridotite xenoliths, have been attributed to a number of different origins. These include low-degree primary melts of the mantle, exotic metasomatic melts influxing into the lithosphere, or breakdown of amphibole, and other phases during high-temperature transport of the xenoliths to the surface in their host magmas. We present new laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of trace element abundances in glasses, and in metasomatically introduced phases (clinopyrox-ene, amphibole, phlogopite, apatite) from a suite of spinel wehrlite, lherzolite and harzburgite xenoliths from southeastern Australia. The majority of glass compositions are best explained by melting of amphibole (usually complete, as amphibole is now absent from most samples) with varying but significant contributions from partial melting of clinopyroxene. However, some glasses require additional components derived from partial or complete modal melting of phlogopite, or apatite. The data confirm our earlier model, that the glass present in patches in these samples derives from high-temperature, transport-related breakdown of a metasomatic phase assemblage (amphibole + clinopyroxene ± phlogopite =b apatite) present in the xenoliths prior to their entrainment in the host magmas.

Mechanisms of orthopyroxene dissolution in silica-undersaturated melts at 1 atmosphere and implications for the origin of silica-rich glass in mantle xenoliths

Experiments dissolving orthopyroxene (En93) in a variety of Si-undersaturated alkaline melts at 1 atmosphere and variable f O2 demonstrate that orthopyroxene dissolves to form olivine, Si-rich melt and clinopyroxene. These phases form a texturally and chemically distinct boundary layer around the partly dissolved orthopyroxene crystals. The occurrence of clinopyroxene in the boundary layer is due to inward diffusion of Ca from the solvent melt to the boundary layer causing clinopyroxene saturation. Compositional profiles through the solvent and the boundary layer for a number of experiments demonstrate rapid diffusion of cations across the boundary layer – solvent interface. SiO2 diffuses outward from the boundary layer whereas CaO and Al2O3 diffuse toward the Si-enriched boundary layer melt. The rate of Al diffusion is slower under reducing conditions compared to the rates in experiments performed in air. Concentrations of FeO and MgO in the boundary layer and solvent are approximately equal indicating rapid diffusion and attainment of equilibrium despite ongoing crystallisation of clinopyroxene within the boundary layer. The behaviour of Na2O and K2O is strongly affected by f O2. Under reducing conditions Na2O and K2O concentrations are approximately equal in the boundary layer and solvent indicating normal diffusion down the concentration gradient and attainment of equilibrium. Under oxidising conditions, K2O and to a lesser extent Na2O, have compositional profiles indicative of uphill diffusion likely due to their preference for more polymerised Si- and Al-rich melts. Under reduced conditions Al-enrichment in the boundary layer melt is not as extreme and uphill diffusion did not occur. The composition of the solvent melt after the experiments indicates that it was contaminated by the boundary layer by convective mixing due to the onset of hydrodynamic instabilities brought on by density and viscosity contrasts between the two melts. Despite using a wide variety of solvent melt compositions we find that the boundary layer melts converge toward a common composition at high SiO2 contents. The composition of glass generated by orthopyroxene dissolution at 1 atmosphere is similar in many respects to Si-rich glass found in many orthopyroxene-rich mantle xenoliths that have been attributed to high pressure in situ processes including mantle metasomatism. The results of this study suggest that at least some Si-rich melts are likely to have formed by dissolution of xenolith orthopyroxene at low pressure possibly by their Si-undersaturated host magmas.

The Origin of Highly Silicic Glass in Mantle Xenoliths from the Canary Islands

The Origin of Highly Silicic Glass in Mantle Xenoliths from the Canary Islands

The Origin of Highly Silicic Glass in Mantle Xenoliths from the Canary Islands

Spinel harzburgite, lherzolite, dunite and wehrlite mantle xenoliths from the Canary Islands (La Palma, Hierro, Tenerife and Lanzarote) contain a spectrum of silicate glasses as inclusions in minerals, along grain boundaries, and in interstitial glass pockets. These glasses show a range in composition form basaltic (∼44 wt % SiO2), to highly silicic, TiO2–FeO–MgO–CaO–P2O5-poor types (up to 71 wt % SiO2). Glasses in spinel harzburgites and lherzolites are generally silica oversaturated, whereas those in spinel dunites and wehrlites have somewhat lower SiO2 contents and are generally silica undersaturated. Glasses in xenoliths from La Palma and Tenerife are rich in K2O compared with those from Hierrro and Lanzarote. Daughter minerals coexisting with highly silicic glass in polyphase inclusions are similar in composition to the main phases in the host xenoliths (Fo>90, Cr-diopside, chromite), whereas those in less silicic glasses are richer in Al2O3, TiO2 and FeO, and poorer in MgO. The systematic relations found to exist between glass composition, mineralogy of the host xenolith and locality (island) cannot reflect random variations in the geochemistry of ‘exotic’ melts infiltrating the mantle lithosphere, but instead suggest a cogenetic relationship between the melts and their mantle host xenoliths. The silicic glasses are interpreted as the products of reactions at 8–12 kbar between infiltrating alkali basaltic magmas and peridotitic wall-rocks which, in orthopyroxene-bearing rock-types, involves formation of silicic melt+olivine at the expense of orthopyroxene. In xenoliths from La Palma and Tenerife, where interstitial phlogopite is commonly present, phlogopite has been partly or totally consumed by the reactions between relatively mafic melts and peridotite, giving rise to silicic glasses with high K2O contents and K2O/Na2O ratios. The low K2O concentrations and K2O/Na2O ratios in glasses in anhydrous xenoliths suites from Hierro and Lanzarote are believed to result for reactions between infiltrating melts and anhydrous and/or amphibole-bearing mantle wall-rocks. The silicic melts appear to have been mobile over distances exceeding the diameter of a xenolith, that is, at least 20–30 cm.

Glasses in mantle xenoliths from western Victoria, Australia, and their relevance to mantle processes

Mantle-derived xenoliths from western Victoria, Australia, contain glass in patches and veinlets, and as secondary inclusions within xenolith minerals. Glass patches are commonly associated with primary clinopyroxene, spinel and in some cases relict pargasitic amphibole, phlogopite, apatite or calcitic carbonate. The patches and veins are filled with secondary microphenocrysts of olivine, clinopyroxene and spinel. The siliceous, aluminous, alkali-rich glasses display ranges in major element compositions which are similar to those of glasses in mantle xenoliths worldwide. Textures and compositions of glass and associated phases in the Victorian xenolith suite argue strongly against origins involving equilibrium partial melting of spinel peridotite, or migration of exotic metasomatic melts. Metasomatism of the southeastern Australian lithosphere is expressed by formation of amphibole, phlogopite, and apatite, and increased abundances of clinopyroxene at the expense of orthopyroxene, in lherzolite or harzburgite. Partial or complete melting of this metasomatic assemblage occurred immediately prior to, or during, entrainment of the xenolith in the host magmas, and locally produced disequilibrium liquids, now preserved as glass. Melting in the xenoliths was caused by the thermal and decompressional effects of transport in the host magma, or by thermal and metasomatic effects in the lithosphere associated with adjacent intrusion of magmas. Large inter-xenolithic variation in glass compositions was caused by variations in the nature and proportions of the precursor metasomatic assemblage, by reaction of the melts with primary orthopyroxene in the lherzolites and harzburgites, and by rapid, disequilibrium crystallization from the melts of an assemblage of olivine, clinopyroxene and spinel in all samples.

Glasses in Mantle Xenoliths from Olmani, Tanzania

Nineteen mantle xenoliths from Olmani, Tanzania, have deformation textures and fluid inclusions like those in olivine-rich peridotites from Hawaii. Group A comprises 7 barren harzburgites (spinel, TiO2 <. 1 wt %) and group B (spinel, TiO2.8–2.6 wt %) contains 6 clinopyroxene-dunites, 2 lherzolites, 1 wehrlite, 1 two-pyroxene-dunite, and 1 olivine-orthopyroxenite. Both groups lack garnet and porphyroclastic textures and display fingerprint intergrowths of spinel with ortho-or clinopyroxene. Many secondary inclusions of several generations within olivine are rich in Co2. Alkali-rich glasses (Na2O.4–7.3 wt %, K2O 3.3–5.1, P2O5 ∼1) in B xenoliths are unrelated chemically to the olivine-augite-phyric ankaramite host. Aphyric inter-granular glass occurs along with larger patches of residual glasses associated with euhedral olivine, clinopyroxene and spinel. Clinopyroxenes of B xenoliths show decreasing Na2O (2.1–.3 wt %) and Cr2O3 from olivine-brown cores to clear margins of larger grains, and to finer grains associated with residual glass. These properties of the xenoliths are consistent with establishment of a coarse texture during prolonged metamorphism in the mantle (∼28 kb, 1100 K?), partial melting in the presence of a CO2-rich fluid (∼26 kb, 1400 K?), transport by ankaramite magma to the Earth's surface, and rapid crystallization of some of the melts at 1300–1200 K.