Fluidized CO2-sulphide-silicate media as agents of mantle metasomatism and megacrysts formation: evidence from a large druse in a spinel-lherzolite xenolith

doi:10.1016/0031-9201(87)90017-3

Physics of the Earth and Planetary Interiors

Volume 45, Issue 3, April 1987, Pages 280-293

https://doi.org/10.1016/0031-9201(87)90017-3 Get rights and content

Abstract

Two unique mantle-derived fragments demonstrating intensive fluid action have been found in 1.5 Ma-old basanitic breccia of Shavaryn-Tzaram maar, Khangai highland, northern Central Mongolia, known for abundant mantle xenoliths (dominant spinel therzolites and various spinel and garnet spinel peridotites and pyroxenites). One is a composite nodule in which common spinel lherzolite grades into black websterite with a druse of large cuhedrally terminated garnet and clinopyroxene crystals; the latter mineral contains macroscopic vugs. The Ti and Fe content of minerals increases from the lherzolite to the druse in which accessory phlogopite contains as much as 11.4 wt.% TiO₂. Minerals in the druse are practically identical in composition to those found as megacrysts in the same pipe. The other sample is a vesicular clinopyroxene megacryst with tubular vugs up to 5 mm across lined with fine-grained glassy aggregate, stretching parallel to the cleavage plane and making up at least $14$ of the sample. ‘Normal’ clinopyroxene megacrysts in this occurrence also frequently contain some subspherical macroscopic vugs. Clinopyroxene of both the druse and megacrysts contains microinclusions grouped into 4 types: (1) fluid, (2) sulphide-fluid, (3) silicate-sulphide-fluid, and (4) silicate-fluid, the first three types being primary. Composition of minerals in the inclusions is similar for the druse and megacrysts. They are inferred to have been formed from a multicomponent fluid system represented by an emulsion of silicate-sulphide melt drops (∼30%) in high-density CO₂ (∼ 70%). By combining the measured composition of major phases with estimates of their proportions in inclusions the chemical composition of the initial fluid system has been estimated.

These observations suggest that, in areas of alkali basaltic magmatism, subcontinental upper mantle may trap small reservoirs of fluidized CO₂-silicate-sulphide mixtures capable of producing profound metasomatic transformations of the enclosing mantle peridotites leading to the formation of veins and pockets of very coarse-grained iron-rich pyroxenites and eclogite-like rocks. Disintegration of the latter during their entrainment in and subsequent transportation by rapidly ascending alkali basalt magmas will lead to the formation of mineral fragments (discrete nodules) indistinguishable from garnet, clinopyroxene and mica megacrysts, that is, at least some of the megacrysts are of metasomatic origin.

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Cited by (19)

Compositional characteristics of the MORB mantle and bulk silicate earth based on spinel peridotites from the Tariat Region, Mongolia
2019, Geochimica et Cosmochimica Acta
Citation Excerpt :
Another argument for the recent metasomatism are Tariat xenoliths containing veins and patches of phlogopite, amphibole and apatite (Ionov et al., 1997). Furthermore, the Shava eruption breccia contains very large (>10 cm) “megacrysts” of cpx, garnet, and sanidine that are interpreted as fragments of coarse-grained veins in the mantle produced by crystallization of pockets of residual metasomatic fluid-rich melts (Kovalenko et al., 1987). The abundance of such megacrysts in the breccia shows the availability of the metasomatic fluids in the mantle shortly before the eruption.
A new collection of spinel-peridotite xenoliths from Quaternary basaltic centers in the Tariat region of Mongolia provide a sample of the shallow lithospheric mantle beneath this area. Ninety-seven of these xenoliths were analyzed for whole rock and mineral major element composition. Both orthopyroxenes and clinopyroxenes separated from the samples were analyzed for trace element content. For a selection of samples, whole rocks were analyzed for trace element contents and clinopyroxenes for the isotopic composition of Sr, Nd, Hf, and Pb. A number of the whole rocks also were analyzed for their Re-Os systematics.
Mineral thermometry provides equilibration temperatures in the range of 858–1073 °C that places the origin of these samples in the shallow upper mantle just below the crust of this region. Over half of the samples have fertile major element compositions with Al₂O₃ concentrations between 3.5 and 4.5 wt%. About a third of the samples are more refractory, yet show enrichment in highly incompatible elements suggestive of recent metasomatism that is hosted both by major minerals and interstitial materials. About 10% of the samples show the compositional consequences of having experienced addition of cumulates from mafic melts in the mantle, including composite samples with pyroxenite and peridotitic portions.
The fertile peridotite xenoliths from the Tariat region are samples of mantle that is compositionally and isotopically similar to that which supplies modern mid-ocean ridge basalt (MORB) volcanism. While the major element compositions of the majority of these samples approach the composition estimated for the Bulk-Silicate-Earth (BSE), the peridotites with fertile major element chemistry all show a slight degree of depletion in the more highly incompatible elements and have radiogenic isotopic compositions (⁸⁷Sr/⁸⁶Sr mostly from 0.7021 to 0.7035, εNd from +5.6 to +11.0 and εHf from +12.0 to +18.5) consistent with small volume melt removal in the mid-Proterozoic. The composition of these xenoliths is well-matched by models that suggest the MORB-source mantle is produced in a dynamic Earth by the many melt extraction events that have built up the continental crust and created a portion of the mantle depleted in the elements that are enriched in continental crust. For this reason, the most obvious petrogenetic explanation for this section of continental lithospheric mantle is that it is essentially undifferentiated MORB-source mantle that was accreted beneath the Paleozoic crust of this region during the ocean-closing events that formed the Central Asian Orogenic Belt. The composition of these xenoliths provides an opportunity to examine details of estimates for the composition of both the depleted mantle sources of MORB and the BSE. The Tariat xenoliths clearly support the observation that the MORB mantle is unexpectedly depleted in Nb, Ta, and possibly Ti compared to models that create the MORB mantle by extraction of continental crust. Given these characteristics, the Tariat xenoliths, though sampled in the middle of Earth’s largest continent, may provide the least altered look at the composition of the upper mantle that provides volcanism along the world’s ocean ridges.
Pyroxenites and megacrysts from Vitim picrite-basalts (Russia): Polybaric fractionation of rising melts in the mantle?
2011, Journal of Asian Earth Sciences
Citation Excerpt :
Megacrystalline pyroxenites were probably formed during the ascent of the host magma to the surface (Lorand, 1989) and their crystallization may be a result of polybaric fractionation (Irving, 1984; Eggler and Mccallum, 1976; Neal, 1995) accompanied by AFC (Neal and Davidson, 1989). Garnet peridotites and pyroxenite xenoliths and pyrope megacrysts are not frequent in alkali basalt (Beeson and Jackson, 1970; Chapman, 1976; Kepezhinskas, 1979; Nixon and Boyd, 1979; Delaney et al., 1979; Kovalenko et al., 1987; Mukhopadyay and Manton, 1994; Ashchepkov et al., 1996; Stern et al., 1999; Upton et al., 2003; Bjerg et al., 2005). The picrite-basalt tuffs from Vitim (Russia) and closely located volcanic centers (Ashchepkov et al., 2003) contain a nearly complete set of megacrysts, peridotites and pyroxenites as described in the literature.
Picrite basalt tuffs and lavas from the Miocene basalt plateau of Vitim (Trans Baikal, Russia) contain abundant megacrysts and varied pyroxenite and mantle lherzolite xenoliths (spinel facies and upper part of the garnet-facies) and crustal cumulates. Black pyroxenites and megacrysts show decreasing temperatures from 1350 to 900 °C, and range from high-T dark green websterites and clinopyroxenites, to low-T black megacrystalline garnet clinopyroxenites and phlogopite–ilmenite-bearing varieties. Garnet-bearing Cr-diopside veins and zoned veins with mica and rare amphiboles cross-cut peridotite xenoliths. Veins consisting of almost pure amphibole are more common in spinel lherzolite xenoliths. P–T calculations for pyroxenites yield pressure intervals at 3.3–2.3, 2.2–2.0, 1.9–1.5 and 1.3–1.0 GPa, probably corresponding to the locations of dense magmatic vein networks in mantle.
Major and trace elements for clinopyroxenes from the black megacrystalline series can be modeled by fractional crystallization of a picrite-basalt melt. In contrast, green high-temperature pyroxenites and black giant-grained garnet pyroxenites with lower LREE-enrichment and variable LILE and HFSE concentrations probably result from AFC processes and mixing with partial melts derived from older pyroxenites and peridotites. Gray low-Cr garnet clinopyroxenites with highly fractionated and inflected trace element patterns may have been formed by remelting of metasomatic veins within peridotites. Multistage melting of metasomatic assemblages with selective removal of clinopyroxene in vein contacts produce the REE patterns with low MREE concentrations and usually with elevated HFSE contents. Cr-diopside veins were most probably formed by partial melting of phlogopite- and/or amphibole-bearing lherzolites. The trace element and Sr–Nd isotopic features of the megacrystalline pyroxenites suggest that they crystallized from magma volumes that evolved in separate systems during formation of pre-eruption vein networks and magma chambers, which together formed the feeding system for the host basalts.
Mantle Volatiles-Distribution and Consequences
2003, Treatise on Geochemistry
Experimental petrology of upper mantle materials, processes and products
1995, Journal of Geodynamics
Experiments on rock materials and volatile components provide an array of phase equilibrium boundaries, giving the depth-temperature framework for the phase transitions experienced by rock masses as they move up or down within the Earth in response to the dynamic processes of mantle convection and plate tectonics. Sub-solidus phase transitions in mantle materials correlate well with upper mantle structure determined from seismic studies, but debate continues about whether there is a change in composition at some seismic boundaries. The interface at 650 km correlates with the transformation of most minerals into the perovskite structure, which may have significant effects on mantle dynamics. Recent research on mantle xenoliths has been concerned with extension of standard thermometers and barometers to higher pressures and their practical assessment, along with the development and refinement of new geothermobarometers. The experimental partial melting of mantle peridotite has been elucidated by detailed studies of model systems and new experimental techniques. Parameterization of the data makes prediction possible. The origin of MORBs involves a complex process of fractional fusion. Evidence from static olivine flotation experiments and shock wave compression experiments on molten komatiite indicates that the densities of mantle melts may exceed that of the residual rock at depths greater than 400 km, which would prohibit ascent of the magma. Recent investigations have identified many dense hydrous magnesian silicates (DHMS), stable through the upper mantle between ~ 300 and 650 km, and reaching the peridotite-volatile solidus curve through part of this interval. The near-solidus liquid composition in peridotite-CO₂-H₂O above ~ 2 GPa is carbonatitic (calcic dolomite), potentially a powerful agent for metasomatism. Experimental studies of trace element distributions between mantle minerals and carbonatitic liquids are beginning to permit quantification. Determination of the effect of reduced oxygen fugacity has introduced mantle scenarios with melting induced by redox changes There is indirect experimental evidence that the solidus may terminate at a critical end-point at depths of a few hundred kilometers. Recent experiments related to subduction of oceanic crust include measurements of liquid composition from the melting of H₂O-undersaturated peridotite, the vapour-absent melting of amphibolite, and partial melting of pelagic clays. Much H₂O is expelled during subduction, but the current estimates of low temperatures support the deep subduction and longterm storage in the mantles of both H₂O and CO₂.
Content and isotopic composition of sulphur in ultramafic xenoliths from central Asia
1992, Earth and Planetary Science Letters
Sulphur contents and isotopic compositions have been determined in 90 fresh mantle-derived garnet and spinel peridotite and pyroxenite xenoliths from six regions of Cenozoic alkali basaltic volcanism in southern Siberia and Mongolia. Sulphur contents in most of the peridotite nodules fall in the range 6–25 ppm, with largely positive δ³⁴S values clustering between −1 and +7‰ (average +2‰). By contrast, the pyroxenites are richer in sulphur (25–140 ppm), and their δ³⁴S values are close to 0‰ (−1.5 to +1.4‰). In the peridotite nodules, sulphur content correlates negatively with MgO, while their δ³⁴S values correlate positively with MgO. Most of the fertile lherzolites (MgO= 37–39%, CaO= 3–4%) have δ³⁴S values close to 0‰ (−1 to +2‰), similar to meteorite values, while in the harzburgites these values reach +3 to +5‰. These features apparently reflect sulphur depletion during partial melting of peridotite mantle accompanied by sulphur isotopic fractionation between residual peridotites and generated basaltic melts. Clinopyroxene-poor xenoliths from southern Mongolia yielded highest δ³⁴S values of +5 to +7‰ accompanied by high Ba and K contents and high ⁸⁷Sr/⁸⁶Sr ratios; these may be a result of interaction with fluids derived from subducted oceanic crust which followed the partial melting and magmatic fractionation episodes.
Low average sulphur concentrations ( < 50 ppm) and largely positive δ³⁴S values may predominate in the continental lithospheric mantle worldwide. Sulphur isotope compositions typical of tholeiitic basalts and MORB (0 to +1‰ [1,2]) may be produced by melting of moderately depleted lherzolites. Primary melts with positive δ³⁴S values may be generated from mantle peridotites with larger degrees of depletion and/or from rocks metasomatised by subduction-related fluids.
Fluid inclusion evidence for immiscibility in magmatic differentiation
1992, Geochimica et Cosmochimica Acta
After a brief review of recent work on melt inclusions in diamond and in normal magmatic environments, the nature of the fluid-inclusion evidence for various stages of magmatic immiscibility is summarized.
During magmatic differentiation by crystal fractionation, from originally low-volatile content silicate melts (yielding normal plutonic rocks), through hydrous silicate melts (yielding pegmatites), to late-stage water-rich fluids (yielding quartz veins and ore deposits), the composition of the residual liquid changes drastically. In some geologic environments, this change in composition of the residual liquid may be continuous, with no phase change in the fluid part of the system. Under most natural conditions, however, it is far more likely to have one or more stages in the evolution during which globules of a separate, new immiscible fluid phase exsolve. The term immiscibility is used here in its more general sense to refer to the existence, at equilibrium, of two or more non-crystalline polycomponent solutions (fluids), differing in properties and generally in composition. I believe that the great bulk of the magmatic inclusions that are being studied today have come about through the intervention of one or more such stages of immiscible fluid separation, i.e., magmatic differentiation by fluid immiscibility. Examples include silicate melt/dense CO₂ fluid, silicate melt/sulfide melt, silicate melt/silicate melt, silicate melt/low density vapor (i.e., vesiculation), and, as presented in more detail in this paper, various stages of silicate melt/hydrous saline melt/aqueous fluid/CO₂ fluid immiscibility.
Generally, only parts of these stages are recorded by the trapping of fluid inclusions, and the interpretation of the inclusion record is commonly ambiguous and difficult, particularly that stage between hydrous silicate melts and the hydrous saline melts (and aqueous solutions) responsible for many ore deposits. The ambiguity results mainly from problems of inclusion origin and nonrepresentative trapping within any given individual inclusion. Thus the probability of trapping of only one, vs. various amounts of two, immiscible fluids, or of trapping solid phases along with the fluid, present severe difficulties. In addition, there is the almost universal superposition of various stages of trapping of fluids within any given sample. These problems can only be approached by very careful petrography and analysis of optimum material, combined with field data and experimental P-V-T-X studies of pertinent systems. Completely unequivocal statements of the course of evolution of the fluids in such natural systems may never be possible.

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Fluidized CO2-sulphide-silicate media as agents of mantle metasomatism and megacrysts formation: evidence from a large druse in a spinel-lherzolite xenolith

Abstract

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Phys. Chem. Earth

Deep-seated inclusions in alkalic basaltoids of the Shavaryn-Tzaram pipe, Mongolian People's Republic

Doklady Earth Sci. Sec.

The trapped fluid phase in upper mantle xenoliths from Victoria, Australia: implications for mantle metasomatism

Contrib. Mineral. Petrol.

CO2CO fluid inclusions in a composite peridotite xenolith: implications for upper mantle oxygen fugacity

Contrib. Mineral. Petrol.

Stable isotope, chemical and petrographic studies of high-pressure amphiboles and micas: evidence for metasomatism in the mantle source regions of alkali basalts and kimberlites

Am. J. Sci.

The effect of CO2 upon partial melting of peridotite in the system Na2OCaOAl2O3MgOSiO2CO2 to 35 kbar, with an analysis of melting in a peridotite-H2OCO2 system

Am. J. Sci.

Spinel peridotite xenoliths from the Shavaryn-Tsaram volcano, northern Mongolia: petrography, major element chemistry and mineralogy

Geologica Carpathica (Bratislava, CSSR)

Micas from mantle nodules in Mongolian alkaline basalts

Doklady AN SSSR

On the first occurrence of amphibole in mantle xenoliths from Mongolian alkaline basalts

Doklady AN SSSR

Petrology and geochemistry of composite ultramafic xenoliths in alkalic basalts and implications for magmatic processes within the mantle

Am. J. Sci.

Cenozoic Alkali Basaltoids of Mongolia and Related Deep Inclusions

Volatile components in magmatic processes

Geochem. Int.

Garnet-clinopyroxene druse: an example of fluid crystallization in the mantle

Doklady AN SSSR

Fluidized CO₂-sulphide-silicate media as agents of mantle metasomatism and megacrysts formation: evidence from a large druse in a spinel-lherzolite xenolith

CO₂CO fluid inclusions in a composite peridotite xenolith: implications for upper mantle oxygen fugacity

The effect of CO₂ upon partial melting of peridotite in the system Na₂OCaOAl₂O₃MgOSiO₂CO₂ to 35 kbar, with an analysis of melting in a peridotite-H₂OCO₂ system