Origin of intergranular aggregates in mantle xenoliths from Krzeniów basanite
Abstract and Figures
Th e upper mantle harzburgite and dunite xenoliths occurring in the Miocene basanite from Krzeniów (Lower Silesia, SW Poland) contain abundant (up to 16 vol. %) fi ne-grained mineral aggregates. Th e xenoliths consist of olivine I, ortho-and clinopyroxene I and spinel I. Th e aggregates are formed of clinopyroxene II, olivine II ± spinel II ± glass ± feldspar ± sulfi des (Ni-pyrrhotite, pentlandite and chalcopyrite). Th e aggregates occur as intergranular "patches" or envelop grains of orthopyroxene I. Th ey originated due to the reaction of primary harzburgite/dunite phases with S-bearing alkaline silicate melt. Th e reaction led to changes of chemical composition of clinopyroxene I olivine I and spinel I or to crystallization of new phases (feldspar, sulfi des). Apart from aggregates, the xenoliths are cut by scarce veinlets fi lled mostly with feldspar and apatite. Th ose veinlets were formed due to infi ltration of Fsp-bearing P-rich melt aft er xenoliths entrainment into host magma. Lack of well-de-veloped host basanite/xenolith reaction zones suggest short but legible contact between them. Streszczenie Harzburgitowe i dunitowe ksenolity skał górnego płaszcza występujące w bazanicie z Krzeniowa (Dolny Śląsk, SW Polska) zawierają liczne (do 16 % obj.) drobnoziarniste agregaty międzyziar-nowe. Ksenolity zbudowane są z oliwinu I, orto-i klinopiroksenu I oraz spinelu I. Agregaty zbu-dowane są z klinopiroksenu II, oliwinu II ± spinelu II ± szkliwa ± skalenia ± siarczków (Ni-pirotyn, pentlandyt, chalkopiryt). Agregaty występują jako "kieszenie" międzyziarnowe lub tworzą otoczki dookoła ziaren ortopiroksenu I. Agregaty powstały w wyniku reakcji pierwotnych faz harzburgitu/ dunitu z alkalicznym stopem krzemianowym bogatym w S. Reakcja doprowadziła do zmian składu chemicznego klinopiroksenu I, oliwinu I i spinelu I, a także do krystalizacji nowych faz (skaleń, siarczki). Wydarzenie to miało miejsce krótko przed wprowadzeniem ksenolitów do wznoszącego Geo_1-księga.indb 25 Geo_1-księga.indb 25 Geoscience Notes 1, 2013 © for this edition by CNS 26 Magdalena Matusiak-Małek, Jacek Puziewicz, Theodoros Ntafl os się bazanitu. Oprócz agregatów w ksenolitach z Krzeniowa występują nieliczne żyłki zbudowane ze skalenia i apatytu. Żyłki te powstały w wyniku infi ltracji przez skalenionośny krzemianowy stop bogaty w P. Brak dobrze rozwiniętych stref kontaktowych pomiędzy ksenolitami a bazanitem z Krzeniowa sugeruje ich krótką, ale wyraźną interakcję.
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... (ii) Type II sulphides form anhedral or oval Po + Pn grains (50-400 μm) within interstitial melt patches (Fig. 5b, c) composed of secondary clinopyroxene, olivine, spinel and glass + feldspar. These sulphides consist of cores formed of vermicular intergrowths of Ni-pyrrhotite and pentlandite surrounded by a pentlandite rim (Matusiak-Małek et al., 2013;Bukała et al., 2015). This assemblage was likely formed at mantle depths by infiltration of alkaline silicate melt similar to that of host basanite (Matusiak-Małek et al., 2014). ...
... Prior studies on mantle xenoliths from the Bohemian Massif documented a chemical heterogeneity of the sampled portions of the upper mantle, which is a consequence of different degrees of partial melting and metasomatism by melts of variable compositions (Ackerman et al., 2015(Ackerman et al., , 2013b(Ackerman et al., , 2007Kukuła et al., 2015;Matusiak-Małek et al., 2013Medaris et al., 2015aMedaris et al., , 2015bPuziewicz et al., 2015Puziewicz et al., , 2011Špaček et al., 2013). The mantle xenoliths in this study are mostly harzburgitic and their silicate minerals show generally highly magnesian composition (Table 1). ...
... This implies Re addition from percolating melts and/or from host basalts without apparently disturbed present-day 187 Os/ 188 Os systematics (Fig. 8a). The latter is more probable because plagioclase-bearing melt pockets clearly associated with syn-volcanic alkaline melt infiltration at mantle depths are commonly present in samples from Krzeniów (Matusiak-Małek et al., 2013). The Krzeniów and Lutynia samples have very low Os contents and they also contain two generations of sulphides with one generation clearly associated with a syn-volcanic melt infiltration (Matusiak-Małek et al., 2014). ...
Peridotite xenoliths brought to the surface by basaltic lavas attest to a variety of mantle processes, including partial melting, melt percolation or refertilization. The whole rock Re–Os concentrations and Os isotopic compositions were determined for 30 xenoliths collected from 11 localities across the northern Bohemian Massif in order to evaluate the Os model ages and attempt to relate the results to major crustal tectonic events during the history. Most samples were affected by variable extent of metasomatic overprint, which is commonly paralleled by very low Os concentrations (b 1 ppb). Rhenium concentrations in the whole suite are below the primitive mantle value. A subset of samples shows evidence for Re addition from host basaltic rocks, consistent with the presence of abundant melt pockets with secondary sulphides. The 187 Os/ 188 Os ratios range from 0.1162 to 0.1330 and cannot be directly related to individual mantle domains, implying the inability of more recent tec-tonic events to reset the original Os isotopic systematics. The calculated mantle extraction ages (T MA) range from b 0.1 to 2.1 Ga, whereas future ages obtained for nine samples indicate metasomatic overprints. The Re depletion ages (T RD) vary between b 0.1 and ~1.6 Ga. However, the T RD is not well suited for direct comparison with crustal ages because it represents a minimum age limit rather than specific age estimate. Therefore, a modified model age (T RDII) was calculated assuming a non-zero Re content during the pre-metasomatic stage and using a composition of the most depleted sample in our suite. A prominent peak in the calculated T RDII ages ranges between 0.5 and 0.6 Ga which corresponds to the Cadomian orogenic cycle.
... (ii) Type II sulphides form anhedral or oval Po + Pn grains (50-400 μm) within interstitial melt patches (Fig. 5b, c) composed of secondary clinopyroxene, olivine, spinel and glass + feldspar. These sulphides consist of cores formed of vermicular intergrowths of Ni-pyrrhotite and pentlandite surrounded by a pentlandite rim (Matusiak-Małek et al., 2013;Bukała et al., 2015). This assemblage was likely formed at mantle depths by infiltration of alkaline silicate melt similar to that of host basanite (Matusiak-Małek et al., 2014). ...
... Prior studies on mantle xenoliths from the Bohemian Massif documented a chemical heterogeneity of the sampled portions of the upper mantle, which is a consequence of different degrees of partial melting and metasomatism by melts of variable compositions (Ackerman et al., 2015(Ackerman et al., , 2013b(Ackerman et al., , 2007Kukuła et al., 2015;Matusiak-Małek et al., 2013Medaris et al., 2015aMedaris et al., , 2015bPuziewicz et al., 2015Puziewicz et al., , 2011Špaček et al., 2013). The mantle xenoliths in this study are mostly harzburgitic and their silicate minerals show generally highly magnesian composition (Table 1). ...
... This implies Re addition from percolating melts and/or from host basalts without apparently disturbed present-day 187 Os/ 188 Os systematics (Fig. 8a). The latter is more probable because plagioclase-bearing melt pockets clearly associated with syn-volcanic alkaline melt infiltration at mantle depths are commonly present in samples from Krzeniów (Matusiak-Małek et al., 2013). The Krzeniów and Lutynia samples have very low Os contents and they also contain two generations of sulphides with one generation clearly associated with a syn-volcanic melt infiltration (Matusiak-Małek et al., 2014). ...
Peridotite xenoliths brought to the surface by basaltic lavas attest to a variety of mantle processes, including partial melting, melt percolation or refertilization. The whole rock Re–Os concentrations and Os isotopic compositions were determined for 30 xenoliths collected from 11 localities across the northern Bohemian Massif in order to evaluate the Os model ages and attempt to relate the results to major crustal tectonic events during the history. Most samples were affected by variable extent of metasomatic overprint, which is commonly paralleled by very low Os concentrations (< 1 ppb). Rhenium concentrations in the whole suite are below the primitive mantle value. A subset of samples shows evidence for Re addition from host basaltic rocks, consistent with the presence of abundant melt pockets with secondary sulphides. The 187Os/188Os ratios range from 0.1162 to 0.1330 and cannot be directly related to individual mantle domains, implying the inability of more recent tectonic events to reset the original Os isotopic systematics. The calculated mantle extraction ages (TMA) range from < 0.1 to 2.1 Ga, whereas future ages obtained for nine samples indicate metasomatic overprints. The Re depletion ages (TRD) vary between < 0.1 and ~ 1.6 Ga. However, the TRD is not well suited for direct comparison with crustal ages because it represents a minimum age limit rather than specific age estimate. Therefore, a modified model age (TRDII) was calculated assuming a non-zero Re content during the pre-metasomatic stage and using a composition of the most depleted sample in our suite. A prominent peak in the calculated TRDII ages ranges between 0.5 and 0.6 Ga which corresponds to the Cadomian orogenic cycle.
... Orthopyroxene lamellae-free recrystallized rims are scarce and may poikilitically enclose olivine I grains. Some of the orthopyroxene I crystals are partly or completely surrounded by a thin (up to 10 mm) zone of vermicular clinopyroxene III; this zone always occurs if orthopyroxene I is in contact with an intergranular fine-grained aggregate (Matusiak-Malek et al., 2013). Scarce orthopyroxene II forms lamellae 10^30 mm long and $3 mm across the host clinopyroxene. ...
... % of the Krzenio¤ w xenoliths. The aggregates have been described in detail elsewhere (Matusiak-Malek et al., 2013), but as they represent a significant volume of the xenoliths a brief summary of their petrography, chemical composition and origin is presented here. The aggregates are oval to ellipsoidal in shape; their longer axes may be up to 1cm. ...
... The major element compositions of the minerals forming the intergranular aggregates led Matusiak-Malek et al. (2013) to the conclusion that they originated by infiltration of a Si-undersaturated, S-bearing silicate melt. The infiltration must have taken place shortly prior to eruption as the compositions of some of the aggregate-forming minerals mimic those of the primary, peridotite-forming minerals. ...
Miocene basanite rz'eniow (SW Poland, eastern part of CenKoic Central European Volcanic Province) contains scarce, small (usually <4 cm in diameter) spinel harzburgite and dunite mantle xenoliths. Woo groups are &fined based on the forsterite (Pb) content in olivine: group A (F0,90.4- 91.7) and group B bo88.0 89.8). Both group A and B rocks are either clinopyroxenefree or clinopyroxene-poor. The group A o)thoproxene (mg# 0-91 0 92) is Al-poor and strongly light To-6 earth element (TREE) depleted The mg# of group A clinop.yroxene varies from 0.92 to 0.94 and is negatively correlated with its Al content. The trace element compositions of clinopyroxene define two patterns: Al, U-shaped 1(La/Lu), almost linear LREE enriched [La/Lu)N=11.91-14.001. The group B orthopyroxene is also Al poor and LREE depleted /La/La) N = 0-030-047 but its mg# is lower than that in group A orthopyrovene (0-900-92). 'The mg# of group B clinopyrovene (0-900-927 is lower than that in group A, whereas the Al content is similar. The REE patterns of group 13 clinopyroxene mimic those of subgroup, N=5.64-11.5'07. The,youp A harzhargites and dunites represent spina:facies lilltospheric mantle that underwent metasumatism by CO,-bearing silicate melts subjected to chromalogn ic fractionation. Locally (arbor tile silicale immiscible melts were generated. The peridotites in the distal parts of the chromatographic system We're little affected by metasomatism and preserve much of their pre-metasomatic, depleted characteristics. The group B rocks are similar to those of group A except for the lower mg# of orthopyroxene and olivine. They were age cted by percolating alkaline silicate melt unders-aturated in clinopyroxene which lowered the mg# of the olivine and orthopyroxene (Fe metasomatism'). The A and B peridotites are representative of the Miocene lithospheric mantle close to the malgins of the neighbouring Eger Rift.
... Orthopyroxene lamellae-free recrystallized rims are scarce and may poikilitically enclose olivine I grains. Some of the orthopyroxene I crystals are partly or completely surrounded by a thin (up to 10 mm) zone of vermicular clinopyroxene III; this zone always occurs if orthopyroxene I is in contact with an intergranular fine-grained aggregate (Matusiak-Malek et al., 2013). Scarce orthopyroxene II forms lamellae 10^30 mm long and $3 mm across the host clinopyroxene. ...
... % of the Krzenio¤ w xenoliths. The aggregates have been described in detail elsewhere (Matusiak-Malek et al., 2013), but as they represent a significant volume of the xenoliths a brief summary of their petrography, chemical composition and origin is presented here. The aggregates are oval to ellipsoidal in shape; their longer axes may be up to 1cm. ...
... The major element compositions of the minerals forming the intergranular aggregates led Matusiak-Malek et al. (2013) to the conclusion that they originated by infiltration of a Si-undersaturated, S-bearing silicate melt. The infiltration must have taken place shortly prior to eruption as the compositions of some of the aggregate-forming minerals mimic those of the primary, peridotite-forming minerals. ...
... Fine-grained intergranular patches are formed of nepheline, fedspar, secondary clinopyroxene, and carbonate (sample FK28 V, F312 and AS29); they are interpreted as an effect of host magma infiltration shortly before eruption or infiltration/percolation of the silica-undersaturated alkali silicate melts (e.g. Matusiak-Malek et al., 2013). Megacrysts of clinopyroxene and olivine are common in the lavas of Jbel Saghro. ...
... Fine-grained intergranular patches are formed of nepheline, fedspar, secondary clinopyroxene, and carbonate (sample FK28 V, F312 and AS29); they are interpreted as an effect of host magma infiltration shortly before eruption or infiltration/percolation of the silica-undersaturated alkali silicate melts (e.g. Matusiak-Malek et al., 2013). Megacrysts of clinopyroxene and olivine are common in the lavas of Jbel Saghro. ...
... Fine-grained intergranular patches are formed of nepheline, fedspar, secondary clinopyroxene, and carbonate (sample FK28 V, F312 and AS29); they are interpreted as an effect of host magma infiltration shortly before eruption or infiltration/percolation of the silica-undersaturated alkali silicate melts (e.g. Matusiak-Malek et al., 2013). Megacrysts of clinopyroxene and olivine are common in the lavas of Jbel Saghro. ...
A suite of mafic pyroxenite xenoliths and clinopyroxene megacrysts was brought to the surface by Cenozoic nephelinites of the Jbel Saghro Volcanic Field (Anti-Atlas, Morocco). The large population of samples was subdivided into five groups: (i) clinopyroxenites sensu stricto; (ii) olivine clinopyroxenites; (iii) mica-bearing clinopyroxenites; (iv) kaersutite-bearing clinopyroxenites; (v) clinopyroxene megacrysts. These xenoliths display a cumulate texture (adcumulate, heteradcumulate with poikilitic clinopyroxene including olivine). The clinopyroxenes have the composition of augite and show an appreciable variation of MgO (7.02-14.80 wt.%), TiO2 (0.58-5.76 wt.%) and Al2O3 (2.81-12.38 wt.%) contents in grains. The clinopyroxenes are characterized by convex upward chondrite-normalized REE patterns, they display very similar trace element compositions with low contents of incompatible elements such as Rb (0-0.9 ppm), Ba (0.1-8.3 ppm), Th (0.1-0.3 ppm), U (0.01-0.04 ppm) and Nb (1.3-3.2 ppm). REE contents of the calculated melts in equilibrium with the clinopyroxene megacrysts and clinopyroxene from pyroxenite xenoliths are similar to those of the nephelinites exposed in Jbel Saghro. Crystallization temperatures of pyroxenite xenoliths and clinopyroxene megacrysts range from 950 °C to 1150 °C. Clinopyroxene barometry yielded pressure of crystallization ranging from 0.4 to 0.8 GPa for pyroxenite xenoliths and 0.3 to 0.7 GPa for clinopyroxene megacrysts. This pressure range is in agreement with pyroxenite xenoliths and clinopyroxene megacrysts being crystallized from their parental melts at the lower and upper crust.
... It is recorded as "fine-grained aggregates", sometimes glass-bearing, which occur commonly in mantle peridotite xenoliths (e. g. Lu et al. 2015;Matusiak-Małek et al. 2013;Bali et al. 2002, Shaw andKlügel 2002). Minerals occurring in these aggregates (typically olivine and clinopyroxene ± opaques) have compositions typical of mantle, indicating that melt The effects of reactive melt infiltration which precedes xenolith entrainment and host lava eruption are easy to recognize because of their textural appearance, described above. ...
Petrographic observations as well as new and published mineral major and trace element compositions, in part complemented by electron backscatter diffraction, evidence that representative mantle xenolith suites from several Cenozoic basalt locations in the European Variscan orogen can be grouped into three main types. Each type is derived from parts of the sub-continental lithospheric mantle which have experienced a different geological history. The oldest type, referred to as orogenic mantle, is dominated by strongly depleted harzburgites, which represent a fossil Variscan mantle wedge with slices of mantle of continental and oceanic plates attached to it during collision. This mantle lithosphere has been overprinted by Cenozoic carbonated alkali basalt melts, and clinopyroxene, if present, is LREE-enriched and re-introduced. This type of mantle is exemplified by xenoliths from Lower Silesia (Poland) and the northern Massif Central (France). The second type of lithospheric mantle lies beneath parts of the Variscan orogen which experienced Cenozoic rifting. This mantle is dominated by harzburgites and lherzolites formed by multiple episodes of reactive percolation of basaltic melts generated at various stages of continental rifting. The clinopyroxene REE patterns range from flat to LREE-enriched. The xenoliths from Vogelsberg (Germany) are an example of such a lithospheric mantle. The third mantle type consists of lherzolites which originated by refertilization of a harzburgitic protolith by melts derived from upwelled asthenosphere. Lherzolites contain primary clinopyroxene characterized by LREE-depleted-patterns. The xenoliths from south Massif Central (France) are an example of that third type.
The xenoliths in Cenozoic basalts considered so far show that the mantle root of the Variscan orogen in Central Europe consists of various domains which in part conserve their characteristics from the time of the Variscan collision, and in part are overprinted by metasomatism caused by late-orogenic asthenosphere upwelling or by Cenozoic rifting. The metasomatically affected domains are decoupled from the Variscan structure of the orogen.
... Spinel is typically sparse and is commonly associated with fine-grained aggregates representing infiltrated silicate melt (Matusiak-Małek et al. 2013). We suspect that spinels in this kind of aggregates have been chemically affected by the melt (e.g. ...
Mantle xenoliths in Oligocene–Miocene alkaline lavas in Lower Silesia (SW Poland) and adjacent part of Upper Lusatia (SE Germany) are samples of the subcontinental lithospheric mantle at the time of culmination of rifting in the Eger Rift (Bohemian Massif, Central Europe). The xenoliths come from the spinel mantle facies and show that two major lithologies occur in the area: A—highly magnesian (olivine Fo 90.5–92.0) harzburgites, and B—less magnesian (olivine Fo 84.0–90.0) harzburgites. The protolith of group A was clinopyroxene-free harzburgite being the residue after extensive melting. It was affected by chromatographic carbonatite/silicate melt metasomatism, with the carbonatite metasomatism only recorded in distal parts of the chromatographic systems. The B harzburgites were penetratively metasomatised by percolating alkaline silicate melts at the time of volcanism. That metasomatism was mostly anhydrous and typically cryptic; it lowered the Mg/(Mg + Fe) ratio of olivine and orthopyroxene in the peridotites subjected to melt percolation and led in places to dissolution of clinopyroxene. The mostly harzburgitic subcontinental mantle lithospheric domain beneath Lower Silesia and Upper Lusatia differs from the lherzolitic/harzburgitic ones located to W and SW beneath other parts of European Variscan orogen.
Mantle xenoliths in the 20 Ma Wilcza Góra basanite (Lower Silesia, NE Bohemian Massif) are mostly harzburgites, some with amphibole which is exceptional in the region. Forsterite content in olivine defines two Groups of peridotites: Group A (Fo89.1–91.5) and Group B (Fo84.2–89.2). Hornblende-clinopyroxenite, websterite and one composite xenolith consisting of dunite, olivine-hornblendite and pyroxene-hornblende-peridotite contain olivine with Fo77.3–82.5 and are classified as Group C. Group A xenoliths contain Al-poor orthopyroxene and some contain LREE-enriched clinopyroxene with negative Ti, Zr-Hf and Nb-Ta anomalies. Spinel (Cr# 0.57–0.68) is scarce in Group A, and Cr-rich pargasite occurs in only two xenoliths. Group B xenoliths contain less magnesian orthopyroxene and clinopyroxene. The REE patterns of Group B clinopyroxene are convex downward, less enriched in LREE and have smaller negative Ti, Zr-Hf and Nb-Ta anomalies than those in Group A. The Cr# in Group B spinel is 0.26–0.56, while pargasite is Ti-rich and Cr-poor. Clinopyroxene from Group C is low magnesian, slightly enriched in LREE and has no negative Ti, Zr-Hf and Nb-Ta anomalies. Group C pargasite is rich in Ti and poor in Cr. Equilibration temperatures recorded in all groups vary within the range 905–970°C.
The evolution of the European Cenozoic Rift System (ECRIS) and the Alpine orogen is discussed on the base of a set of palaeotectonic maps and two retro-deformed lithospheric transects which extend across the Western and Central Alps and the Massif Central and the Rhenish Massif, respectively.
During the Paleocene, compressional stresses exerted on continental Europe by the evolving Alps and Pyrenees caused lithospheric buckling and basin inversion up to 1700 km to the north of the Alpine and Pyrenean deformation fronts. This deformation was accompanied by the injection of melilite dykes, reflecting a plume-related increase in the temperature of the asthenosphere beneath the European foreland. At the Paleocene–Eocene transition, compressional stresses relaxed in the Alpine foreland, whereas collisional interaction of the Pyrenees with their foreland persisted. In the Alps, major Eocene north-directed lithospheric shortening was followed by mid-Eocene slab- and thrust-loaded subsidence of the Dauphinois and Helvetic shelves. During the late Eocene, north-directed compressional intraplate stresses originating in the Alpine and Pyrenean collision zones built up and activated ECRIS.
At the Eocene–Oligocene transition, the subducted Central Alpine slab was detached, whereas the West-Alpine slab remained attached to the lithosphere. Subsequently, the Alpine orogenic wedge converged northwestward with its foreland. The Oligocene main rifting phase of ECRIS was controlled by north-directed compressional stresses originating in the Pyrenean and Alpine collision zones.
Following early Miocene termination of crustal shortening in the Pyrenees and opening of the oceanic Provençal Basin, the evolution of ECRIS was exclusively controlled by west- and northwest-directed compressional stresses emanating from the Alps during imbrication of their external massifs. Whereas the grabens of the Massif Central and the Rhône Valley became inactive during the early Miocene, the Rhine Rift System remained active until the present. Lithospheric folding controlled mid-Miocene and Pliocene uplift of the Vosges-Black Forest Arch. Progressive uplift of the Rhenish Massif and Massif Central is mainly attributed to plume-related thermal thinning of the mantle-lithosphere.
ECRIS evolved by passive rifting in response to the build-up of Pyrenean and Alpine collision-related compressional intraplate stresses. Mantle-plume-type upwelling of the asthenosphere caused thermal weakening of the foreland lithosphere, rendering it prone to deformation.
Peridotite mantle xenoliths collected north of Gobernador Gregores, Patagonia, affected by cryptic and modal metasomatism bear melt pockets of unusually large size. Melt pockets consist of second generation olivine (ol2), clinopyroxene (cpx2) and spinel (sp2) ± relict amphibole (amph) immersed in a yellowish vesicular glass matrix. Amphibole breakdown was responsible for melt pocket generation as suggested by textural evidence and proved by consistent mass-balance calculations: amph → cpx2 + ol2 + sp2 + melt. Composition of calculated amphibole in amphibole-free melt pockets is very similar to that measured in amphibole-bearing melt pockets from the same xenolith, i.e. amphibole was consumed in the melt pocket generation process. In melt pockets devoid of relict amphibole, mass-balance calculations show remarkable differences between the calculated amphibole and the measured amphibole compositions in melt pockets from the same xenolith. The participation of minor proportions of a consumed reactant phase could be a reasonable explanation. In some samples the calculated phase proportion of glass is in excess compared to modal estimations based on backscattered electron images, probably because a portion of the generated melt was able to migrate out of the melt pockets. Compositional inhomogeneity of cpx2 and variable Ti Kd in cpx2 vs. glass in the same melt pocket reflect fast nucleation and growth and disequilibrium crystallisation, respectively. This and the difference between forsterite content in calculated equilibrium olivine and second generation olivine, suggest that mineral equilibrium was inhibited by rapid quenching of melt pockets.
A large body of recent work has linked the origin of Si-Al-rich alkaline glass inclusions to metasomatic processes in the
upper mantle. This study examines one possible origin for these glass inclusions, i.e., the dissolution of orthopyroxene in
Si-poor alkaline (basanitic) melt. Equilibrium dissolution experiments between 0.4 and 2 GPa show that secondary glass compositions
are only slightly Si enriched and are alkali poor relative to natural glass inclusions. However, disequilibrium experiments
designed to examine dissolution of orthopyroxene by a basanitic melt under anhydrous, hydrous and CO2-bearing conditions show complex reaction zones consisting of olivine, ± clinopyroxene and Si-rich alkaline glass similar
in composition to that seen in mantle xenoliths. Dissolution rates are rapid and dependent on volatile content. Experiments
using an anhydrous solvent show time dependent dissolution rates that are related to variable diffusion rates caused by the
saturation of clinopyroxene in experiments longer than 10 minutes. The reaction zone glass shows a close compositional correspondence
with natural Si-rich alkaline glass in mantle-derived xenoliths. The most Si-and alkali-rich melts are restricted to pressures
of 1 GPa and below under anhydrous and CO2-bearing conditions. At 2 GPa glass in hydrous experiments is still Si-␣and alkali-rich whereas glass in the anhydrous and
CO2-bearing experiments is only slightly enriched in SiO2 and alkalis compared with the original solvent. In the low pressure region, anhydrous and hydrous solvent melts yield glass
of similar composition whereas the glass from CO2-bearing experiments is less SiO2 rich. The mechanism of dissolution of orthopyroxene is complex involving rapid incongruent breakdown of the orthopyroxene,
combined with olivine saturation in the reaction zone forming up to 60% olivine. Inward diffusion of CaO causes clinopyroxene
saturation and uphill diffusion of Na and K give the glasses their strongly alkaline characteristics. Addition of Na and K
also causes minor SiO2 enrichment of the reaction glass by increasing the phase volume of olivine. Olivine and clinopyroxene are transiently stable
phases within the reaction zone. Clinopyroxene is precipitated from the reaction zone melt near the orthopyroxene crystal
and redissolved in the outer part of the reaction zone. Olivine defines the thickness of the reaction zone and is progressively
dissolved in the solvent as the orthopyroxene continues to dissolve. Although there are compelling reasons for supporting
the hypothesis that Si-rich alkaline melts are produced in the mantle by orthopyroxene – melt reaction in the mantle, there
are several complications particularly regarding quenching in of disequilibrium reaction zone compositions and the mobility
of highly polymerized melts in the upper mantle. It is considered likely that formation of veins and pools of Si-rich alkaline
glass by orthopyroxene – melt reaction is a common process during the ascent of xenoliths. However, reaction in situ within
the mantle will lead to equilibration and therefore secondary melts will be only moderately siliceous and alkali poor.
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.
Two types of melt pockets, closed melt pocket (CMP) and open melt pocket (OMP), are recognized from the peridotite xenoliths
entrained in the Cenozoic kamafugites in western Qinling (秦岭), Central China. The Haoti (好梯) CMPs have a mineral assemblage
of olivine+ clinopyroxene+amphibole+K-feldspar, whereas the Baiguan (白关) CMPs are composed of olivine+clinopyroxene+ilmenite+carbonate.
The components of the OMPs are more complicated. In the Haoti OMPs, there are olivine, clinopyroxene, glass, low modal abundances
of amphibole, K-feldspar (Kfs), ilmenite, sulfide, chlorite, perovskite, chromite and phlogopite. The Baiguan OMPs contain
olivine, clinopyroxene, glass, chlorite and chromite. Compositionally, olivines in the CMPs and OMPs are both apparently depleted
in Ni, and those in the OMPs are also depleted in Fe and Mg, and enriched in Ca compared to the primary ones. Clinopyroxenes
display large and systematical compositional variations between the CMPs and OMPs, particularly in Al, Cr, Na, Ca and Ti.
Glasses are generally depleted in Si compared to the worldwide glasses in melt pockets, although they still have large variations.
Amphiboles and K-feldspars have relatively restricted compositional variations. The petrographical observations and mineral
chemistry suggest that the Haoti and Baiguan CMPs were generated by the in-situ decompression melting of orthopyroxenes, olivines
and clinopyroxenes, and by the addition of minor external K-rich and Ca-rich melt/fluids. The OMPs formed during the latest
metasomatic event in the lithospheric mantle beneath the western Qinling.
Key Wordsmantle metasomatism-melt pocket-partial melting-peridotite xenolith-western Qinling
K-Ar dating was carried out on the Tertiary alkali basaltic rocks represented by melanephelinite and melabasanite lavas and plugs in the Opole region, Lower Silesia (Poland). The most probable geological age of the volcanic activity in this area is given by the mean K-Ar age of the melabasanites, calculated at 26.5 Ma: Late Oligocene (Chattian, middle part). Some melabasanite plugs yielded K-Ar dates significantly older than those from the lavas which - by good geological evidence - pre-dated them. This might be explained by the presence of excess argon in the plugs.
Based on phase geochemistry and Re-Os isotopic ratios, an exotic (in an oceanic setting) K-rich silicate melt, named kimberlite-type, has been claimed to be the metasomatizing agent interacting with subcontinental lithospheric mantle fragments beneath the Cape Verde Archipelago. On the basis of textural features and major-and trace-element chemistry, we constrain key geochemical indicators able to discriminate percolation at depth of this exotic melt from infi ltration of the host magma in Cape Verde mantle xenoliths. Cape Verde type A lherzolites and harzburgites show evidence of dissolution of the primary phases (mainly pyroxenes) and the presence of large patches of secondary mineral (and glass) assemblages, and they do not show textural evidence of host basalt infi ltration. Cape Verde type A mantle xenoliths frequently contain almost pure K-feldspar (An3.8-8.8, Ab6-24, Or72-89) in the secondary mineral assemblage. They have an anomalously high K content (up to 0.49 wt%), and K/Na ratios generally >1, with respect to Cape Verde peridotites clearly affected by host basalt infi ltration (type B samples). The dichotomy between Na and K observed in the two textural types suggests that the Na-alkaline host basalt (K/Na <1), which infi ltrated at low pressure, was able to modify the whole-rock Na content of the xenoliths (type B samples). In turn, a completely different K-rich alkaline melt, which interacted at depth with the peridotite, imposed its alkali ratio (K/Na >1) on the bulk composition and formed the type A xenoliths. The kimberlitetype metasomatic agent, which reacted with the Cape Verde peridotite assemblage (mainly orthopyroxenes) in those regions where the mantle xenoliths are entrapped in the host basalt (P = 17 kbar; T = 1092°C), reasonably tends toward SiO 2-saturated, K-rich basic magmas (lamproite-type?) with K-feldspars as the "liquidus" phase. Isotopic data on separate clinopyroxenes do not contribute to discrimination between metasomatism and infi ltration processes but certainly concur to reinforce the hypothesis that a fragment of subcontinental lithospheric mantle domain was preserved during the opening of the Atlantic Ocean, forming K-rich undersaturated silicate melts that percolated through the peridotite matrix. Whole-rock major-and trace-element and isotopic geochemistry alone would not contribute to the interpretation of the processes occurring in the mantle xenolith. The most reliable tool would be an in situ mineral (and glass) study, which would be valid for Cape Verde mantle xenoliths and others. Small-melting-degree undersaturated silicate melts percolating at depth are olivine-saturated and may form, by reaction and dissolution of pyroxene, type A olivine without substantially modifying the original Fe/Mg peridotite ratio. By contrast, under low-pressure (<1.5 GPa), high-temperature regimes, olivine silicate melts infi ltrating the mantle xenoliths form type B olivine, in which Fe/Mg ratios will be controlled by fractionation. Mantle diopsides interact (at depth) with undersaturated silicate melts, rearranging the most fusible elements into a new diopside composition: type A clinopyroxene. By contrast, diopsides that interact with melts at progressively lower pressure react and are locally rearranged in a new chemical structure that is able to accommodate the high diffusive elements (i.e., Fe and Ti): type B aegirineaugites. Fe3+ in spinel is a key element in the investigation of the processes acting on Cape Verde mantle xenoliths. As a metasomatic product, secondary chromian spinel tends toward a Fe 3+-buffered composition, mainly depending on pressure and chemistry of the magma. A decompression system is able to change the percolation regime from porous flow to open conduit. At this stage, the chromian spinel would be the low-pressure phase able to accommodate larger amounts of Fe3+. Type A glasses have exceptionally high K2O content, and, when associated with K-feldspars, they are buffered at ~9 K2O wt%, in a silica range of 55.7-66.8 wt%. By contrast, type B glasses follow a hypothetical major-element trend toward the host basanites. In conclusion, the compositional features (in particular major elements) of minerals and glasses in relation to their chemical behavior in mantle systems are the most effi cient tools to distinguish metasomatism-related (type A) from infi ltration-related (type B) samples and consequently to place the mantle xenoliths in a correct genetic framework.