Significance of silicate melt pockets in upper mantle xenoliths from the Bakony Balaton Highland Volcanic Field, Western Hungary

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1 Lithos 61 (2002) Significance of silicate melt pockets in upper mantle xenoliths from the Bakony Balaton Highland Volcanic Field, Western Hungary Enikon Bali a, Csaba Szabó a, *, Orlando Vaselli b,kálmán Török c a Lithosphere Fluid Research Lab, Department of Petrology and Geochemistry, Eötvös University, Pázmány Péter sétány 1/c, H-1117 Budapest, Hungary b Department of Earth Sciences, University of Florence, Via G. La Pira 4, I-50121, Italy c Department of Geophysics, Eötvös University, Pázmány Péter sétány 1/c, H-1117 Budapest, Hungary Received 16 October 2000; accepted 9 November 2001 Abstract Silicate melt pockets with or without carbonate occur in 10% of upper mantle xenoliths from the alkali basalts of the Bakony Balaton Highland Volcanic Field (BBHVF), Western Hungary. Based on the estimated bulk composition of the melt pockets, both the carbonate-free and the carbonate-bearing ones are considered to be the result of the reaction between primary mantle clinopyroxene and/or amphibole and external CaO, Al 2 O 3, alkali-rich and MgO-poor fluids/melts, as metasomatic agents, migrating in the upper mantle. The metasomatic melt that produced the carbonate-bearing melt pockets was extremely rich in volatiles, whereas metasomatic melt that contributed to the formation of the carbonate-free melt pockets was particularly rich in silica and relatively poor in volatiles. These metasomatizing melts could have originated from the melting of the previously metasomatized upper mantle due to Middle Miocene mantle diapirism. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Pannonian Basin; Upper mantle xenolith; Melt pockets; Mantle metasomatism; Carbonatite; Silicate melt 1. Introduction Silicate melt accumulations in ultramafic xenoliths can be divided into two main types, silicate melt inclusions, enclosed into the mantle minerals, and interstitial silicate melt veins and pockets, providing significant information on processes within the shallow subcontinental lithospheric mantle (e.g. Ionov et al., 1993, 1996; Szabó et al., 1996; Schiano and Bourdon, 1999; Schiano et al., 2000). Silicate melt inclusions in upper mantle xenoliths are considered to represent mantle melts that migrated and were trapped in high * Corresponding author. Tel.: ext. 8338; fax: ext address: cszabo@iris.geobio.elte.hu (C. Szabó). pressure and temperature in equilibrium with the peridotitic assemblages. However, melts forming the interstitial silicate melt pockets and veins are considered to be open systems, which reacted continuously with their environment preserving only the last equilibrium state of the melt (Schiano and Bourdon, 1999). Silicate melt pockets in ultramafic xenoliths can be produced by decompression melting of H 2 O-bearing phases (mostly pargasite and phlogopite) or nominally dry mantle minerals (mostly clinopyroxene and orthopyroxene) of xenoliths after trapping in the host basaltic magma (e.g. Stosch and Seck, 1980; Tracy, 1980; Francis, 1987). Furthermore, migrating volatile-rich upper mantle melt or fluid can react with the primary mantle minerals causing metasomatic alteration of the primary assemblage (Dawson, 1984; Ionov et al., 1993) /02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S (01)

2 80 E. Bali et al. / Lithos 61 (2002) Fig. 1. Schematic geological map of the Carpathian Pannonian Region, based on the Geological Map of Hungary (Jugovics, 1967). Black filled circles show the ultramafic xenolithbearing Plio-Pleistocene alkai basalts. (1) Stryrian Basin, (2) Little Hungarian Plain, (3) Bakony Balaton Highland, (4) Nógrád Gömör Region, (5) Persany Mts.

3 E. Bali et al. / Lithos 61 (2002) Silicate melt pockets in upper mantle xenoliths of the Bakony Balaton Highland Volcanic Field (BBHVF) (Hungary) have been mentioned by Embey-Isztin et al. (1989), however, detailed study has not yet been done. The main aim of this study is to infer about the different processes that have affected the upper mantle beneath the central part of the Pannonian Basin based on the study of the melt pockets in ultramafic xenoliths of the BBHVF. 2. Geological background Thrusting and folding of the Carpathians were accompanied by the formation of the Miocene Pliocene subduction-related calc-alkaline volcanic complex of the Inner Carpathian Volcanic arc (Royden and Burchfiel, 1989; Szabó et al., 1992). Extension and subsidence of the adjacent Pannonian Basin were also accompanied by sporadic Pliocene Pleistocene alkali basalts (Embey-Isztin et al., 1993) (Fig. 1). This volcanism appeared when the upwelled asthenospheric mantle dome started to cool down (Szabó et al., 1992). The lithospheric mantle beneath the BBHVF, which is located in the center of the Carpathian Pannonian Region (Fig. 1), is more deformed than that located towards the edges of the Carpathian Pannonian Region (Downes et al., 1992; Szabó et al., 1995). 3. Petrography of the ultramafic xenoliths More than 150 ultramafic xenoliths from localities of the BBHVF (Bondoró-hegy, Szentbékkálla and Szigliget) were collected. Silicate melt pockets were recognized in 10% of the xenoliths. After microscopic observations, six ultramafic xenoliths were selected for further study from Szentbékkálla (Fig. 1) on which interaction between the xenolith and the host alkaline basalt is not recognizable. Samples selected are mostly spinel peridotites but one olivine clinopyroxenite xenolith was also included (Table 1). The peridotite xenoliths show equigranular and protogranular textures (based on the classification of Mercier and Nicolas, 1975). The olivine clinopyroxenite is a coarse-medium-grained igneous-textured xenolith (Table 1). Beside the most common primary mantle minerals [olivine (ol-i), orthopyroxene (opx-i), clinopyroxene (cpx-i) and spinel (sp-i)], amphibole (amp- I) also occurs in some of the xenoliths (Table 1). 4. Petrography of the melt pockets The melt pockets represent from 12 to 25 vol% of the studied samples. The size of the silicate melt pockets is variable (from mm in diameter). Individual melt pockets are usually connected to each other by thin ( mm) silicate melt veins (Fig. 2A). Modal compositions of the silicate melt pockets were determined by image analysis of photomicrographs (for the average bulk melt compositions, five or more melt pockets per sample were determined; Table 2). Carbonate-bearing and carbonate-free melt pockets can be distinguished. (1) The borders of the carbonate-bearing melt pockets (up to 4.5 mm in diameter) are mostly rectilinear or occasionally curvilinear (Fig. 2B). The melt Table 1 Textural types, modal proportions and calculated equilibrium temperature (Brey and Köhler, 1990), pressure (Mercier, 1980) and oxygen fugacity values (Ballhaus et al., 1991) of studied ultramafic xenoliths from the Bakony Balaton Highland Volcanic Field Sample Rock type Texture Melt pocket Modal composition Equilibrium type Olivine Orthopyroxenpyroxene Clino- Amphibole Melt T pocket (jc) P DlogfO 2 (MPa) Szb04 harzburgite equigranular cb-free Szb21 wehrlite equigranular cb-bearing Szb50 olivin coarse-grained cb-bearing clinopyroxenite igneous Szb55 lherzolite equigranular cb-bearing Szb59 harzburgite protogranular cb-bearing Szb52 lherzolite protogranular cb-free

4 82 E. Bali et al. / Lithos 61 (2002) pockets are composed of newly formed clinopyroxene (cpx-ii), olivine (ol-ii), spinel (sp-ii) (Table 2), all between 0.05 and 0.5 mm in diameter, and interstitial colorless glass (gl). The melt pockets are characterized by the presence of mosaic textured carbonate (cb) globules (Fig. 2B). Resorbed amphiboles (amp-i) also occur in the carbonate-bearing melt pockets in lherzolite Szb55 (Fig. 2C). (2) The shape of the carbonate-free melt pockets (up to 5 mm in diameter) is always irregular (Fig. 2A, D). Modal compositions of these melt pockets are similar to the carbonate-bearing melt pockets (Table 2). In most cases, resorbed amphibole (amp-i) and empty bubbles (bub) can be also observed (Fig. 2D). 5. Geochemistry 5.1. Analytical method Electron microprobe analysis of the primary mantle minerals and the phases in the silicate melt pockets Fig. 2. Photomicrographs of silicate melt pockets in ultramafic xenoliths of the Bakony Balaton Highland Volcanic Field. (A) Carbonate-free silicate melt pockets connected to each other by silicate melt vein (Szb04 equigranular harzburgite, plane-polarized light). (B) Carbonatebearing silicate melt pocket showing straight border lines (Szb50 medium-grained igneous-textured olivine clinopyroxenite, reflected light). (C) Carbonate-bearing silicate melt pocket containing resorbed primary amphibole (amp-i) (Szb55 equigranular lherzolite, plane-polirized light). (D) Carbonate-free silicate melt pocket containing resorbed primary amphibole (amp-i) and empty bubbles (bub) (Szb52 protogranular spinel lherzolite, plane-polirized light). Legend: ol-i primary mantle olivine, opx-i primary mantle orthopyroxene, cpx-i primary mantle clinopyroxene, ol-ii olivine crystallized in the melt pocket, cpx-ii clinopyroxene crystallized in the melt pocket, sp-ii spinel crystallized in the melt pocket, cb carbonate crystallized in the melt pocket, bub empty bubble occurring in the melt pocket, gl silicate glass interstitial to the mineral phases in the melt pocket.

5 E. Bali et al. / Lithos 61 (2002) Table 2 Modal compositions (m.c.) and standard deviations (s.d.) of silicate melt pockets in ultramafic xenoliths from the Bakony Balaton Highland Volcanic Field Phases\samples Szb52 (n = 8) Szb04 (n = 9) Szb55 (n = 6) Szb50 (n = 8) Szb21 (n = 5) Szb59 (n =5) m.c. s.d. m.c. s.d. m.c. s.d. m.c. s.d. m.c. s.d. m.c. s.d. Olivine-II II Clinopyroxene-II Glass Carbonate Primary mantle amphiboles (amp-i), occurring as residual phases in the melt pockets, and empty bubbles (bub) (Fig. 2a, c and d), have been ignored from the modal composition. Letter n indicates number of melt pockets studied; olivine-ii, spinel-ii, clinopyroxene-ii, glass, carbonate indicate phases forming the melt pockets. was carried out on a JEOL Superprobe JXA-8600 WDS at the Department of Earth Sciences, University of Florence. The accelerating voltage was 15 kv with 10 na sample current. Analysis of mineral phases was performed using a beam diameter of 5 Am, except for the analysis of H 2 O-bearing minerals and silicate glass Table 3 Average chemical compositions of primary mantle minerals from upper mantle xenoliths of the Bakony Balaton Highland Volcanic Field Sample Szb50 Szb59 Szb21 Szb52 Szb55 Szb04 Rock type Olivine clinopyroxenite harzburgite Wehrlite lherzolite Lherzolite harzburgite Texture Igneous Protogranular Equigranular Protogranular Equigranular Equigranular n Fo in olivine mg# in spinel cr# in spinel mg# in orthopyroxene Szb52 Szb55 Clinopyroxene Amphibole n SiO TiO Al 2 O Cr 2 O FeO MnO n.d MgO CaO Na 2 O K 2 O n.a. n.a. n.a. n.a. n.a. n.a Total mg# The table shows the Fo-content of olivines, the mg# and cr# of spinels and mg# of orthopyroxenes, clinopyroxenes and amphiboles. n = number of analysis, n.a. = not analyzed, n.d. = not detected. mg# in spinel = 100Mg/(Mg + Fe 2+ ). cr# in spinel = 100Cr/(Cr + Al + Fe 3+ ). mg# in clinopyroxene = 100Mg/(Mg + Fe t ). mg# in orthopyroxene = 100Mg/(Mg + Fe t ). mg# in amphibole = 100Mg/(Mg + Fe t ).

6 84 Table 4 Average chemical compositions of olivines, spinels and clinopyroxenes from melt pockets occurring in upper mantle xenoliths of the Bakony Balaton Highland Volcanic Field Olivine Clinopyroxene Sample Szb59 Szb50 Szb21 Szb52 Szb04 Szb55 Szb59 Szb50 Szb21 Szb52 Szb55 Szb04 Szb59 Szb50 Szb21 Szb52 Szb04 Szb55 Rock type hzb Olivine cpx-ite Wehrlite lhz hzb Lhz hzb Olivine cpx-ite Wehrlite Texture Proto Igneous Equi Proto Equi Equi Proto Igneous Equi Proto Equi Equi Proto Igneous Equi Proto Equi Equi n SiO TiO 2 n.a. n.a. n.a. n.a. n.a. n.a Al 2 O 3 n.a. n.a. n.a. n.a. n.a. n.a Cr 2 O 3 n.a. n.a. n.a. n.a. n.a. n.a FeO MnO n.d NiO n.a. n.a. n.a. n.a. n.a. n.a. MgO CaO Na 2 O n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Total mg# cr# n = number of analyzed melt pocket, n.a. = not analyzed element. mg# in spinel = 100Mg/(Mg + Fe 2+ ). cr# in spinel = 100Cr/(Cr + Al + Fe 3+ ). mg# in olivine = 100Mg/(Mg + Fe t ). mg# in clinopyroxene = 100Mg/(Mg + Fe t ). lhz Lhz hzb hzb Olivine cpx-ite Wehrlite lhz hzb Lhz E. Bali et al. / Lithos 61 (2002)

7 E. Bali et al. / Lithos 61 (2002) for which defocused beam in size of 10 Am was used. Counting times were 20 s for all the elements. Natural standards were employed for the analysis, and the method of Bence and Albee (1968) was applied for correction calculations Chemistry of the primary mantle minerals Olivines in the peridotites have Fo content between 88.7 and 90.8, whereas those from the olivine clinopyroxenite show a considerable low value (83.3) (Table 3). Mg-numbers of orthopyroxenes are slightly higher than those of the coexisting olivines, ranging from 85.2 to 91.6 (Table 3). Clinopyroxenes show variable Al 2 O 3 ( wt.%), Cr 2 O 3 ( wt.%) and TiO 2 ( wt.%) contents. Their mg-number varies between 87.5 and 93.2, the lowest value being the characteristic for the olivine clinopyroxenite (Table 3). The amphiboles are pargasites (Leake, 1978) with low TiO 2 and relatively high Cr 2 O 3 contents ( , wt.%, respectively) (Table 3). compositions are highly variable with cr-number (Cr/Cr + Al Fe 3+ ) and mgnumber (Mg/Mg + Fe 2+ ) ranging between 9.1 and 39.4, and 69.3 and 77.8, respectively (Table 3). The lowest values are characteristic of the olivine clinopyroxenite xenolith. Similar mineral compositions of BBHVF xenoliths have been reported by Embey- Isztin et al. (1989) and Downes et al. (1992) Phase chemistry of the melt pockets Mg-number of olivines in the silicate melt pockets is between 90.1 and 94.4, i.e. higher than those in the coexisting primary mantle olivines (Table 4). Clinopyroxene phenocrysts show variable compositions particularly in TiO 2 ( wt.%), Cr 2 O 3 ( wt.%) and Al 2 O 3 ( wt.%) content. These values are generally higher than those in the primary mantle clinopyroxenes (Tables 3 and 4). The mg-number of clinopyroxenes is between 86.3 (Szb50 olivine clinopyroxenite) and 91.2 (Szb55 lherzolite) (Table 4). Mg-numbers and cr-numbers of the spinels are between and wt.%, respectively (Table 4). The TiO 2 contents in spinel phenocrysts vary between 0.25 to 0.62 wt.% (Table 4). Table 5 Chemical compositions of glasses in silicate melt pockets of ultramafic xenoliths from the Bakony Balaton Highland Volcanic Field Sample Szb59 Szb50 Szb04 Szb21 Szb52 Szb55 Rock type harzburgite Olivine clinopyroxenite harzburgite Wehrlite lherzolite Lherzolite Texture Protogranular Igneous Equigranular Equigranular Protogranular Equigranular n SiO TiO Al 2 O Cr 2 O FeO MnO MgO CaO Na 2 O K 2 O P 2 O Cl Total Na 2 O/K 2 O mg# First columns show average compositions, second columns show compositional ranges of glasses in samples. n = number of analyzed melt pockets. n.d. = not detected element. mg# = 100Mg/(Mg + Fe t ).

8 86 E. Bali et al. / Lithos 61 (2002) Glass compositions show high silica, aluminum and alkali contents. The SiO 2 content varies between 49.2 and 56.7 wt.%, Al 2 O 3 ranges from 18.6 to 27.6 wt.%, the total alkali content shows wide variation from 2.88 to 10.1 wt.% (Table 5). Glasses in those carbonate-bearing melt pockets, which do not contain resorbed amphibole (samples Szb21, Szb50 and Szb59), show increasing TiO 2, alkalis and decreasing Al 2 O 3 content with an increase of SiO 2 (Fig. 3). The glass compositions of the carbonate-bearing sample Szb55 do not follow the observed trends (Fig. 3) but show higher TiO 2 and lower Al 2 O 3 and alkali contents compared to the previously mentioned amphibole-free samples. The compositions of the glasses found in carbonate-free melt pockets are very variable showing somewhat lower TiO 2 content variable Al 2 O 3 and higher alkali contents compared to the glasses of the carbonate-bearing melt pockets (Table 5). Carbonates in the melt pockets are Mg-calcites with low FeO (up to 0.54 wt.%), MgO ( wt.%), MnO (up to 1.51 wt.%) and SrO (up to 0.12 wt.%) contents (Table 6). 6. Bulk composition of the melt pockets The average bulk compositions of the silicate melt pockets have been estimated by mass balance calculation (Table 7). The results show relatively high MgO contents ( wt.%), whereas the FeO t content Fig. 3. Harker diagrams for glasses in silicate melt pocket of ultramafic xenoliths in the BBHVF.

9 E. Bali et al. / Lithos 61 (2002) Table 6 Chemical composition of carbonates in silicate melt pockets of ultramafic xenoliths from the Bakony Balaton Highland Volcanic Field Sample Szb50 Szb21 Szb59 Szb55 Rock type Olivine clinopyroxenite Wehrlite harzburgite Lherzolite Texture Igneous Equigranular Protogranular Equigranular n FeO MnO n.d. n.d MgO CaO SrO BaO n.d. n.d n.d. n.d. n.d. n.d. Total mg# First columns show average compositions, second columns show compositional ranges of carbonates in samples. n = number of analyzed melt pockets. n.d. = not detected element. mg# = 100Mg/(Mg + Fe t ). is relatively low ( wt.%). The CaO content shows a wide range ( wt.%). There is no significant difference between the bulk composition of the two melt pocket types. Only the SiO 2 and Cr 2 O 3 contents of the carbonate-free melt pockets show elevated values compared to the carbonate-bearing ones (Table 7). A significant difference between the carbonate-bearing and carbonate-free melt pockets presents in their volatile content. The estimated volatile content (CO 2 calculated from carbonates + H 2 O {on 100% total basis}) of the melt pockets varies from 0.35 to 11.3 wt.% with higher values in the carbonate-bearing melt pockets. Bulk compositions of the melt pockets considerably differ from those of the basanitic melt, which carried the xenoliths to the surface (Embey-Isztin et al., 1993). The K 2 O, FeO t contents of the silicate melt pockets are much lower, whereas the CaO and MgO contents are much higher than those in the host basanitic melt (Table 7). 7. Discussion 7.1. Presence of carbonate minerals in the upper mantle Primary carbonates have been observed in upper mantle xenoliths or peridotite massifs (e.g. Amundsen, 1987; Ionov et al., 1993, 1996; Varela et al., 1998; Charlot-Prat and Arnold, 1999; Zanetti et al., 1999; Laurora et al., 2001). In most cases, carbonates coexisting with silicate glass show an immiscible-like texture in silicate melt pockets and veins in mantle xenoliths (e.g. Ionov et al., 1996; Laurora et al., 2001). Carbonates is generally considered to have been associated with carbonatitic metasomatism in the upper mantle (e.g. Amundsen, 1987; Ionov et al., 1993, 1996; Varela et al., 1998; Charlot-Prat and Arnold, 1999). High pressure and temperature experimental data also confirm that carbonates can be stable phases in peridotitic assemblages (e.g. Olafsson and Eggler, 1983; Woermann and Rosenhauer, 1985; Wallace and Green, 1988; Dalton and Wood, 1993a,b; Lee and Wyllie, 2000; Lee et al., 2000). The estimated equilibrium temperature and pressure range of ultramafic xenoliths in this study [T = jc using geothermometers of Brey and Köhler, 1990 and approximately P = MPa using the geobarometer of Mercier, 1980] basically fall in the experimental stability field of Mg-calcite [T = jc, P = MPa (Dalton and Wood, 1993a)] (Fig. 4). The appearance of Mg-calcites in the ultramafic xenoliths studied (Fig. 2B, Table 6) and their estimated pressure range are in agreement with the conclusion of Embey-Isztin et al. (1989) and Downes et al. (1992) that BBHVF ultramafic xenoliths could not be derived from greater depth than 45

10 88 Table 7 Calculated average bulk compositions of the silicate melt pockets occurring in the Bakony Balaton Highland Volcanic Field ultramafic xenoliths (first columns); the standard deviations (s.d.) are also shown (second columns) Sample Szb52 s.d. Szb04 s.d. Szb21 s.d. Szb50 s.d. Szb55 s.d. Szb59 s.d. Composition of basanites Rock type lherzolite harzburgite Wehrlite Olivine clinopyroxenite Lherzolite harzburgite from the BBHVF (Embey-Isztin et al., 1993) Texture Protogranular Equigranular Equigranular Igneous Equigranular Protogranular average s.d. Melt pocket cb-free cb-free cb-bearing cb-bearing cb-bearing cb-bearing type SiO TiO Al 2 O Cr 2 O FeO(t) MnO MgO CaO Na 2 O K 2 O P 2 O CO H 2 O Total mg# For comparison, the average composition and standard deviation of the basanites from the BBHVF (Embey-Isztin et al., 1993) are also shown. H 2 O* = 100 total. CO 2 = determined from carbonate content. mg# = 100Mg/(Mg + Fe t ). E. Bali et al. / Lithos 61 (2002)

11 E. Bali et al. / Lithos 61 (2002) Fig. 4. Stability fields of carbonates in equilibrium with ultramafic rocks [where olivines are at least Fo = 90 (Dalton and Wood, 1993a,b)]. BBHVF xenoliths studied showing their texture types fall mostly in the field of Mg-calcite lherzolite. For a comparison, equilibrium P T range of carbonate-bearing ultramafic xenoliths from Spitsbergen is also shown. (1) Reaction of 2CaCO 3 + 2MgSiO 3 = CaMg(CO 3 ) 2 + CaMgSi 2 O 6, (Dalton and Wood, 1993a,b), (2) reaction of CaCO 3 + MgCO 3 + 2MgSiO 3 = CaMgSi 2 O 6 +CO 2 +Mg 2 SiO 4 (Dalton and Wood, 1993a,b). km due to the mantle diapirism beneath the area (e.g. Royden et al., 1983; Royden and Dövényi, 1988; Horváth et al., 1988). The estimated equilibrium temperature and pressure ranges of Spitsbergen carbonate-bearing ultramafic xenoliths, in a geodynamic environment similar to that of the BBHVF, are jc and MPa (Fig. 4). The two suites have similar pressure and temperature range, however, the equilibrium temperature range of the BBHVF xenoliths is wider and the equilibrium pressure range might be higher than the Spitsbergen xenoliths. The result of a stable isotopic study of Demény and Embey-Isztin (1997) undertaken on carbonates in ultramafic xenoliths from the BBHVF led the authors to conclude that carbonates could be the results of weathering and formed by the alteration of mantle minerals. However, a recent stable isotopic

12 90 E. Bali et al. / Lithos 61 (2002) study of Demény et al. (2000) rather indicated that the formation of carbonates in melt pockets might have formed in association with subduction environments Origin of the silicate melt pockets Based on microscopic observations, melting of the primary mantle minerals, basically clinopyroxenes, could play a significant role in the formation of the silicate melt pockets. However, the presence of resorbed amphiboles (Fig. 2C, D) in two carbonatefree (xenoliths of Szb04 and Szb52) and one carbonate-bearing (xenolith of Szb55) samples also suggests that amphibole could have been the major melting phase in these xenoliths. Significant incorporation of melted orthopyroxene in the studied melt pockets can be ruled out as silicate glasses related to orthopyroxene break-down would have to show much higher silica ( > 70.0 wt.%) and lower Al 2 O 3 and CaO content ( < 17.0 wt.%, < 2.0 wt.%, respectively) (e.g. Zinngrebe and Foley, 1995) as compared to the compositions of the glasses in this study (Table 5). The calculated bulk composition of the carbonatebearing melt pockets (Table 7), which contain no resorbed amphibole and proposed to be pseudomorphs after primary mantle clinopyroxene (Fig. 2B), differs considerably from the compositions of the primary clinopyroxenes (Table 3) in the studied xenoliths (Fig. 2). The Al 2 O 3 and alkali contents are higher (Fig. 5a, d, Tables 3 and 7), whereas SiO 2, MgO and CaO contents are lower (Fig. 5b, c and Tables 3 and 7) than those of primary mantle clinopyroxenes. Furthermore, the mg-numbers ( ) differ from those of the primary mantle minerals ( ) and are much higher than the host basanite reported by Embey-Isztin et al. (1993) (Table 7). Carbonate-bearing melt pockets, containing resorbed primary amphibole (lherzolite Szb55), show low MgO and high CaO content as compared to the coexisting primary mantle amphibole (Fig. 4b, c). However, in lherzolite Szb55, the average mg-number of the silicate melt pockets is 84.2, similar to the primary amphiboles (Tables 3 and 7). The mg-numbers of the carbonate-free silicate melt pockets vary between 85.3 and 89.4 (Table 7). These values are similar to those of the coexisting primary amphiboles, however, the calculated bulk compositions show higher SiO 2, CaO, and lower Al 2 O 3 and MgO and alkali contents than the primary amphiboles (Fig. 4a, b, d and Tables 3 and 7). Based on the calculated bulk compositions of the silicate melt pockets and the compositions of primary mantle clinopyroxenes and amphiboles, it is clear that neither the carbonate-bearing nor the carbonate-free melt pockets could have originated solely from simple in situ melting of these mantle phases. This compositional discrepancy and the presence of carbonate-bearing melt pockets in some xenoliths (Fig. 2B, C) indicate that volatile-rich external melt(s) could have been added to the in situ melt formed after melting of the primary mantle minerals (basically amphibole and clinopyroxene). The question is raised, what could be the compositions and source of these external melts? Carbonate-free silicate melt pockets The petrographic features indicate that the carbonate-free melt pockets originate from in situ melting of amphibole and the surrounding phases (clinopyroxene and minor olivine, spinel, orthopyroxene) (Fig. 2A, D). Assuming only in situ melting, different proportions of the melting phases can be estimated by model calculation knowing the calculated bulk composition of the melt pockets and the potential melting phases. For this model calculation, different percentages of the dry mantle minerals (clinopyroxene, olivine, orthopyroxene, spinel) were added to the amphibole composition, as potential melted phases, iteratively until the best-fit composition to the melt pockets was obtained. The results of these model calculations confirm that the major melting phases were indeed clinopyroxene and amphibole. The calculated model melt pocket compositions show always lower Si, Al, alkali and higher Mg contents than the bulk compositions of the melt pockets estimated by mass balance calculation (see Appendix A). This suggests the incorporation of external melt, which was enriched in Si, Al and alkalis and depleted in Mg. The incorporation of external melts is also supported by the high volatile content of the silicate melt pockets. The carbonate-free silicate melt pockets contain high amount of empty bubbles (Fig. 2A, D). The bubbles take the 3 7 vol% of the whole volume of the melt pockets. Assuming that these empty

13 E. Bali et al. / Lithos 61 (2002) Fig. 5. Harker diagrams for bulk compositions of carbonate-free silicate melt pockets in ultramafic xenoliths from the Bakony Balaton Highland Volcanic Field. Composition fields of primary mantle clinopyroxenes (cpx), amphiboles (amp), and calculated compositions of the external metasomatizing melts using results of the model calculations in Appendix B, and silicate melt compositions produced by high PT experiments of Yaxley and Green (1994, 1998) are also shown. The arrows show possible mixing trends between the mantle minerals and metasomatizing melt.

14 92 E. Bali et al. / Lithos 61 (2002) bubbles contained volatiles (e.g. H 2 O) and taking into account that the volatile content of the glasses in these melt pockets is around 0.8 wt.% (Table 5), it seems that the amphibole break-down (amphiboles contain max wt.% H 2 O) cannot be solely responsible for the high volatile content in the melt pockets. Consequently, we propose the incorporation of low amount of external silicate melt into the in situ melt of amphibole and clinopyroxene to the carbonate-free silicate melt pockets. To estimate the composition (and the amount) of this external melt, the method of Zinngrebe and Foley (1995) was used. Based on the K D (Fe/Mg) values (Zinngrebe and Foley, 1995) between the bulk composition of the melt pockets (Table 7) and the primary mantle olivines (Table 3), the silicate melt in the melt pockets is not in equilibrium with their environment. The mg-values of olivines in the samples Szb04 and Szb52 (Table 3) require silicate melt with mg# between 72 and 75 to be in equilibrium. Following the method of Zinngrebe and Foley (1995), we subtracted the mantle phases (basically amphibole and clinopyroxene) from the bulk composition of the melt pockets (Table 7) until the mg# s of the residual melt reached the required mg-values ( f 72 75) (details are seen in Appendix B). These calculated melts for both of the carbonatefree samples (Szb04 and Szb52) are basalt to basaltic andesite in composition showing high SiO 2,Al 2 O 3 and CaO and low MgO contents ( wt.%; wt.%; wt.%; wt.%, respectively) with variable alkali content ( wt.%). These calculated melts show composition surprisingly similar to that of subduction-related silicate melt inclusions found in Nógrád Gömör Region (North-Pannonian Basin) ultramafic peridotite suite, reported by Szabó et al. (1996). The calculated bulk compositions of the silicate melt pockets from the BBHVF fall among the fields of the compositions of mixture of amphibole and clinopyroxene and those of the calculated external melts for both samples studied (Fig. 5). It is interesting enough that compositions of the external melts for both of the carbonatefree xenoliths (Szb04 and Szb52) studied are similar to those of the subduction-related silica-rich melts produced by the experiment of Yaxley and Green (1994, 1998), except CaO and alkalis content. This suggests complex mixing processes among the in situ melts of amphibole and clinopyroxene and an external andesitic dacitic melts released from subducted slab (Fig. 5) Carbonate-bearing silicate melt pockets The bulk compositions of the carbonate-bearing melt pockets differ from the composition of the primary mantle minerals (basically clinopyroxenes and amphiboles), which are assumed to be the principal phases that produced the melt pockets (Tables 3 and 7). Furthermore, the carbonate contents of the melt pockets suggest that the external melts, which reacted with the primary mantle amphiboles and clinopyroxenes or with their in situ melts, were indeed carbonate-rich. To estimate the composition of the external melt, we used the same calculation procedure (Zinngrebe and Foley, 1995) as for the carbonate-free melt pockets (Appendix C). The external melts show variable SiO 2 ( wt.%), CaO ( wt.%) and CO 2 ( ), high Al 2 O 3 ( wt.%) and alkali ( wt.%), low MgO ( wt.%) contents. The CaO contents of the calculated external melts decrease, whereas the alkali content increases with increasing SiO 2 -content (Fig. 6c, d). The Al 2 O 3 and MgO content of the calculated external melts does not show variation with SiO 2 content (Fig. 6a, b). The composition of these external melts is similar to the bulk composition of the carbonate-bearing silicate melt inclusion from an upper mantle xenoliths in subduction settings at Papua New Guinea (McInnes and Cameron, 1994) (Fig. 6). The bulk composition of the silicate melt pockets shows transitional compositions between the primary mantle minerals (amphiboles and clinopyroxenes) and the calculated external melts (Fig. 6). The bulk composition of the silicate melt pockets in the amphibole-bearing Szb55 xenolith shows higher CaO and lower total alkali content as compared to the primary mantle amphiboles (Fig. 6c, d). These features can be explained by a significant clinopyroxene incorporation in these melt pockets (Appendix C). The calculated CO 2 contents of the external melts can reflect to their high carbonate content. Based on the average composition of the carbonates in the BBHVF xenolith suite and the CO 2 content of the calculated external melt, we estimated the carbonate content of the external melt. The average composition of the carbonates in the studied xenoliths (Table 6) was

15 E. Bali et al. / Lithos 61 (2002) Fig. 6. Harker diagrams for bulk compositions of carbonate-bearing silicate melt pockets in ultramafic xenoliths from the Bakony Balaton Highland Volcanic Field. Beside the bulk composition fields of the melt pocket, composition fields of primary mantle clinopyroxenes (cpx), amphiboles (amp), and of the calculated compositions of the external metasomatizing carbonatitic melt ( carbonatite ) are also shown based on the model calculations in Appendix C. extracted from the composition of the suspected external melt until the total consummation of the CO 2. Based on our calculation, the external melt incorporated into the composition of the carbonate-bearing melt pockets could have contained as high carbonate content as wt.% (Appendix C). We assume that the compositions of these volatile-rich external melts could be similar to those found in veins of an

16 94 E. Bali et al. / Lithos 61 (2002) upper mantle xenolith from the BBHVF, and composed of carbonate and silicate glass (Bali, 1999) Origin of the external melts Carbonate-free external melt(s) Silicate melts with high silica, aluminum and alkali and low magnesium content are commonly found as silicate melt inclusions trapped in upper mantle materials (e.g. Schiano and Clocchiatti, 1994; Schiano et al., 1994, 1995; Szabó et al., 1996; Schiano and Bourdon, 1999). These silicate melt inclusions are thought to be the remnants of melts migrated in the upper mantle predating the host volcanism. The possible existence of silicate melts with this composition in the upper mantle is also supported by numerous experimental works (e.g. Draper and Green, 1997; Yaxley and Green, 1994, 1998). Draper and Green (1997) suggested that glasses with high silica, alkali and low MgO content can be formed by the partial melting of the upper mantle, and these melts can migrate in equilibrium with the upper mantle assemblage. The results of Yaxley and Green (1994, 1998) on high pressure ( P = 2500 MPa) and temperature (T = jc and T = jc) indicate that such a melt enriched in Si, Al, and alkali and depleted in Mg could have been originated from subducted carbonated oceanic crust. Presence of partial melts from a subducted carbonated oceanic crust beneath the BBHVF cannot be excluded based on the geological setting of the studied area. Detailed geophysical studies in the Western Carpathians (Hamilton, 1990; Tomek and Hall, 1993; Bezák et al., 1997; Bielik et al., 1998, and references therein) suggest southwards (probably beneath the BBHVF) subducted oceanic slab related to the Paleogene and Neogene evolution of the Carpathian Pannonian Region. Isotopic ratios 87/86 Sr/ 143/ 144 Nd and 206/204 Pb/ 207/204 Pb in clinopyroxenes from BBHVF peridotitic xenoliths, reported by Downes et al. (1992) and Rosenbaum et al. (1997), suggest that Sr, U, and Pb-enriched fluids and silicate melts, which are related probably to subduction, could have modified the composition of the mantle beneath the region Carbonate-bearing external melt(s) Experimental work of Wallace and Green (1988) (at P = kbar and T = jc) demonstrated that an ephemeral carbonatitic melt can be produced from amphibole-bearing peridotitic assemblage whenever CO 2 vapor acts in the mantle producing carbonates. The question raises whether the source of the CO 2 can also be a subducted slab. Recent studies (e.g. Peacock, 1990; Yaxley and Green, 1994; Zanetti et al., 1999; Coltorti et al., 1999; Molina and Poli, 2000) suggest that source of the CO 2 acting in peridotitic assemblage could be the sedimentary or basaltic part of a subducted slab. Yaxley and Green (1994) and Molina and Poli (2000) also concluded that at pressure below 2000 MPa, subducting oceanic crust can generate high amounts of CO 2. Furthermore, the role of subduction in the generation of carbonatitic melts is supported by the presence of upper mantle xenoliths metasomatized by carbonatitic melts in present subduction settings in Papua New Guinea (McInnes and Cameron, 1994) and Kamchatka (Kepezinskas and Defant, 1996). Based on the geological environment of the BBHVF, the carbonate-bearing external melt, that incorporated into the composition of the studied carbonate-bearing silicate melt pockets, could also have been subduction-related melt. This is supported by the preliminary study of Demény et al. (2000) which suggested that the stable isotope composition of some carbonate globules, found also in interstitial silicate melt accumulations in the ultramafic xenoliths of the BBHVF, reflects their relation to subduction environment Scenario for the metasomatic evolution of the lithospheric mantle beneath the bbhvf Subduction-related metasomatism (Early Miocene) Suspected subduction of a slab of the flysch ocean southward beneath the Western Carpathians (Tomek and Hall, 1993; Bezák et al., 1997; Bielik et al., 1998) could have been carbonated and thus generated CO 2 at pressure < 2000 MPa as is predicted by the experiments of Yaxley and Green (1994) and Molina and Poli (2000) (silica-rich melt can be produced at T >750 jc and P>1500 MPa). The volatile and silica-rich melts could have been released from the subducted slab and caused the observed metasomatism above the subducting slab recorded in upper mantle xenoliths of the Pannonian Basin (Downes et al., 1992; Szabó et

17 E. Bali et al. / Lithos 61 (2002) al., 1996; Rosenbaum et al., 1997), as well as produced a carbonated wehrlite assemblage, as is indicated by the experiments of Lee and Wyllie (2000). Also, under these pressure and temperature conditions (i.e. T = jc and P = MPa), amphibole can be generated in the mantle (Olafsson and Eggler, 1983; Wallace and Green, 1988) (Fig. 7, stage #1) Mantle diapirism related partial melting of metasomatized mantle (Late Miocene) Following the Early Miocene subduction, the Pannonian Basin was formed by lithospheric extension during the Badenian Sarmatian (Horváth and Royden, 1981). The driving mechanism of extension is thought to be subduction roll-back of the European plate subducted southward (Royden, 1988; Csontos et al., 1992; Horváth, 1993). Subsidence and thermal history modeling of sediments revealed that the mantle part of the lithosphere was considerably more thinned than the crust (Sclater et al., 1980; Royden et al., 1983; Royden and Dövényi, 1988; Horváth et al., 1988) suggesting that a mantle diapir could have played a significant role in the development of the Pannonian Basin. According to Falus et al. (2000), the asthenospheric mantle beneath the edges of the Carpathian Pannonian Region upwelled by a minimum of km, which is in good agreement with the stretching rate of the Pannonian Basin indicated by geophysical studies (e.g. Sclater et al., 1980). Due to the diapirism, the metasomatized mantle (actually the carbonated wehrlite assemblage) could have been upwelled from 2000 MPa ( f 70 km) to 1500 MPa ( f 45 km). During upwelling, the carbonated wehrlite could have partially melted and produced Ca-rich carbonatitic melt as the experiments of Lee and Wyllie (2000) lead us to assume (Fig. 7, stage #2) Development of the silicate melt pockets during cooling of the mantle diapir (Late Miocene Pliocene) The carbonatite melt could have migrated in equilibrium with the wehrlite over wide temperature range during the mantle upwelling and the subsequent cooling (Fig. 7). On the further decrease of pressure and temperature, immiscible volatile-rich and silica-rich external melts could have formed, based on the experiments of Lee and Wyllie (1997), which mixed with the primary mantle minerals (basically clinopyroxene and amphibole) or their in situ melts during the cooling of the diapir to create the melt pockets. Due to the continuously decreasing temperature, the system could have crossed the diopside + liquid = calcite + - forsterite + diopside + vapor reaction line shown in Fig. 7 (stage #3), which provides a wide pressure/ temperature range to crystallize carbonates in upper mantle conditions. The CO 2 vapor forming during this reaction could have yielded the source of CO 2 fluid inclusions in the BBHVF ultramafic xenoliths (Török and De Vivo, 1995; Szabó et al., 1997) (Fig. 7). Fig. 7. Model of the metasomatic evolution of the lithospheric mantle beneath the BBHVF: (1) wehrlite + calcite (S) + calcite (L), after subduction-related metasomatism in the Early Miocene; (2) wehrlite + silicate (L) + calcite (L), after mantle diapirism in the Middle to Late Miocene; (3) lherzolite + calcite (S), after reaction between the carbonatitic melt and the lherzolitic mantle in the Late Miocene Pliocene during cooling of the mantle diapir; (4) P T path of the host basanitic magma in the Pleistocene. The stability fields of solid and liquid carbonates in equilibrium with ultramafic rocks after Lee and Wyllie (2000). 8. Conclusions The carbonate-bearing and carbonate-free silicate melt pockets in ultramafic xenoliths of the BBHVF are the results of upper mantle metasomatism predating the Plio Pleistocene volcanism. Due to mantle upwelling in the Middle Miocene, an earlier metasomatic (carbonated and amphibole bearing) wehrlite melted and

18 96 Appendix A Model calculation of the bulk composition of the carbonate-free silicate melt pockets by the entire melting of primary mantle minerals (amphibole, clinopyroxene, orthopyroxene and spinel) in case of hazburgite Szb04. First column shows the bulk composition of silicate melt pockets, 2nd 6th columns show the chemical compositions of the primary mantle minerals melted to the melt pockets; 7th, 9th and 11th columns show example model compositions of melt pocket calculated by adding different percentage of the primary mantle minerals into the melt pockets; 8th, 10th and 12th columns show the difference between the calculated average bulk composition of melt pockets and the model composition of melt pockets. The major differences are shown in bold. Szb04 Bulk composition of silicate melt pocket In situ melted mantle minerals Model melt pocket compositions calculated by in situ melting of the primary mantle minerals amp-i ol-i cpx-i opx-i sp-i 52% amp + 40% cpx +3% sp + 5% opx bulk composition model composition 41% amp + 52% cpx + 7% opx bulk composition model composition 59% amp + 33% cpx +8% sp bulk composition model composition (1st) (2nd) (3rd) (4th) (5th) (6th) (7th) (8th) (9th) (10th) (11th) (12th) SiO TiO Al 2 O Cr 2 O FeO MnO NiO MgO CaO Na 2 O K 2 O CO H 2 O total H 2 O=(100 total). CO 2 = determined from carbonate content. E. Bali et al. / Lithos 61 (2002)

19 Appendix B Calculation method of the external silicate melts incorporated into the composition of the carbonate-free silicate melt pockets. Bulk composition of silicate melt pocket, Szb52 In situ melted mantle minerals Bulk melt pocket 35% amp % cpx + 9.5% ol SiO TiO Al 2 O Cr 2 O FeO MnO NiO MgO CaO Na 2 O K 2 O P 2 O H 2 O* total mg# Fe/Mg Kd melt/ol Bulk composition of silicate melt pocket, Szb04 E. Bali et al. / Lithos 61 (2002) amp-i ol-i cpx-i opx-i sp-i In situ melted mantle minerals amp-i ol-i cpx-i opx-i sp-i Bulk melt pocket 20% amp + 27% cpx + 4% ol SiO TiO Al 2 O Cr 2 O FeO MnO NiO MgO CaO Na 2 O K 2 O P 2 O H 2 O* total mg# Fe/Mg Kd melt/ol H 2 O*=(100 total). Composition of melt incorporated to melt pockets in 39 wt.% Composition of melt incorporated to melt pockets in 49 wt.%