Earth and Planetary Science Letters 331-332 (2012) 8–20

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Earth and Planetary Science Letters

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The role of CO2-rich fluids in trace element transport and metasomatism in thelithospheric mantle beneath the Central Pannonian Basin, Hungary, based on fluidinclusions in mantle xenoliths

Márta Berkesi a, Tibor Guzmics a, Csaba Szabó a,⁎, Jean Dubessy b, Robert J. Bodnar c,Károly Hidas d, Kitti Ratter e

a Lithosphere Fluid Research Lab, Institute of Geography and Earth Sciences, Eötvös University Budapest (ELTE), Pázmány P. stny. 1/C, H-1117 Budapest, Hungaryb G2R, Université de Lorraine, CNRS, CREGU, Boulevard des Aiguillettes, B.P.239, F-54506 Vandoeuvre lès Nancy, Francec Fluids Research Laboratory, Virginia Tech (VT), VA 24061 Blacksburg, United Statesd Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC & UGR, Avenida de las Palmeras 4 18100 Armilla, Spaine Department of Materials Physics, Institute of Physics, Eötvös University Budapest (ELTE), Pázmány P. stny. 1/A, H-1117 Budapest, Hungary

⁎ Corresponding author at: H-1117 Pázmány PéterTel.: +36 1 3722500x8338; fax: +36 1 3812212.

E-mail address: cszabo@elte.hu (C. Szabó).

0012-821X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.epsl.2012.03.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 October 2011Received in revised form 24 February 2012Accepted 3 March 2012Available online xxxx

Editor: B. Marty

Keywords:metasomatismupper mantle xenolithsglassdissolved silicate componentcarbonationfluid inclusions

Upper mantle peridotite xenoliths from the Tihany Maar Volcanic Complex, Bakony–Balaton HighlandVolcanic Field (Central Pannonian Basin, Hungary) contain abundant pyroxene-hosted negative crystalshaped CO2-rich fluid inclusions. The good correlation between enrichment of the clinopyroxenes in Al2O3,TiO2, Na2O, MREE and Zr, and the presence of fluid inclusions in the xenoliths provide strong evidence forfluid-related cryptic metasomatism of the studied xenoliths. The FIB-SEM (focused ion beam-scanning elec-tron microscopy) exposure technique revealed a thin glass film, covering the wall of the fluid inclusions,which provides direct evidence that the silicate components were formerly dissolved in the CO2-rich fluidphase. This means that at upper mantle conditions CO2-rich fluids are capable of transporting trace andmajor elements, and are the agents responsible for cryptic metasomatism of the peridotite wall rock.Several daughter phases, including magnesite, quartz and sulfide, were identified in the fluid inclusions.Magnesite and quartz are the products of a post entrapment carbonation reaction, whereby the reactantsare the CO2-rich fluid and the host orthopyroxene. It is likely that the thin glass film prevented or arrestedfurther growth of the magnesite and quartz by isolating the fluid from the host orthopyroxene, resulting inthe preservation of residual CO2 in the fluid inclusions.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

One of the most poorly understood processes associated with theformation and evolution of the deep lithosphere is related to themechanisms and role of fluid transport through the convectingmantle (e.g., Hidas et al., 2010; Izraeli et al., 2001; Malaspina et al.,2006; Rosenbaum et al., 1996). Supercritical aqueous fluids havebeen studied experimentally at upper mantle conditions (e.g.,Keppler, 1996; Kessel et al., 2005; Scambelluri and Philippot, 2001;Spandler et al., 2007); however, the great majority of mantle fluidsare not H2O-rich but, rather, are dominated by CO2, based on studiesof fluid inclusions in mantle xenoliths (e.g., Andersen and Neumann,2001; Frezzotti and Peccerillo, 2007; Roedder, 1983; Szabó andBodnar, 1996). In such fluid inclusions, minor fluid components,

stny 1/c, Budapest, Hungary.

l rights reserved.

such as H2O (Berkesi et al., 2009), CO ((Bergman and Dubessy,1984), H2S (Berkesi et al., 2009; Frezzotti and Peccerillo, 2007;Hidas et al., 2010), N2 (Andersen et al., 1995) and SO2 (Frezzotti etal., 2002) have also been reported. The existence of various solidphases within the fluid inclusions, including graphite (Bergman andDubessy, 1984), halite (Frezzotti et al., 2002), OH-bearing solids(Frezzotti et al., 2010) and carbonates (Frezzotti and Peccerillo,2007) that have all been interpreted to have been precipitated fromthe trapped fluids, demonstrate the complexity of mantle fluids.These solids provide valuable information concerning mantle fluiddiversity, however, little information is available on the volume pro-portions of the solid daughter phases, precluding a detailed estima-tion of the compositions of fluids in the inclusions.

This study demonstrates the applicability of the stepwise expo-sure technique involving focused ion beam-scanning electron micros-copy (FIB-SEM) to identify submicron-sized daughter phases in fluidinclusions. The presence of daughter phases and their compositionsleads to a better understanding of the role of CO2-rich fluids in mantlemetasomatism and post entrapment processes involving trappedfluids and host minerals.

9M. Berkesi et al. / Earth and Planetary Science Letters 331-332 (2012) 8–20

2. Geological framework and sampling

The studiedmantle xenoliths were collected from the TihanyMaarComplex (Németh et al., 2001), Bakony–Balaton Highland VolcanicField (BBHVF), Central Pannonian Basin, western Hungary. The de-tailed geological background of the BBHVF has already been described(e.g., Hidas et al., 2010 and references therein). The Tihany MaarVolcanic Complex represents the oldest (7.96±0.03 Ma, Wijbrans etal., 2007) volcanic products of post-extensional alkaline basaltic vol-canism in the BBHVF. The collected peridotite xenoliths have an aver-age size of 4 cm and irregular to rounded shape. Nine representativexenoliths (Table 1) that show coarse-grained poikilitic texture(Embey-Isztin et al., 1989; Xu et al., 1998) have been selected for de-tailed geochemical study (Fig. 1A–C). These are orthopyroxene-richspinel lherzolites and harzburgites, composed of olivine, orthopyrox-ene, clinopyroxene and spinel (Fig. 1A–C, Table 1). The fabric is dom-inated by 1–5 mm orthopyroxenes and 2–4 mm olivines (Fig. 1A,B).In contrast, clinopyroxenes and spinels are smaller, ranging from40–300 μm and 20–100 μm, respectively (Fig. 1C). The coarse grainedorthopyroxenes enclose euhedral to subhedral 50–500 μm olivine in-clusions (Fig. 1C). Subhedral olivines are also rarely enclosed in clin-opyroxenes. Both silicate-hosted and interstitial spinel are observed.

Previous microthermometric and Raman analyses on the fluid in-clusions in these samples (Berkesi et al., 2009) revealed high-density (1.01–1.12 g/cm3), CO2-rich fluids, with small amounts ofH2O and lesser H2S, that are typical of mantle fluids (e.g., Frezzottiet al., 2002; Hidas et al., 2010; Roedder, 1965). During microthermo-metry it was observed that the smallest inclusions preserved thehighest density CO2 and, as a result, they are thought to be the fluidsthat are most representative of the fluids present at the conditions ofentrapment.

3. Fluid inclusion petrography

Fluid inclusions are present in orthopyroxenes and less commonlyin clinopyroxene. The fluid inclusions are negative crystal shapedwith sizes varying between 3 and 70 μm (Fig. 1D–F). At room temper-ature the fluid inclusions contain one visible liquid phase (Fig. 1D–F).However, fluid inclusions that are partially decrepitated contain botha liquid and a vapor phase at room temperature. Using the definitionof Roedder (1984), the inclusions are classified as either primary(single fluid inclusions), or pseudosecondary (occurring along healedfractures without reaching the edges of the crystal) and secondary(occurring along healed fractures that crosscut the entire crystal)

Table 1Modal composition, rock type (after Streckeisen, 1976), texture, equilibrium temperature baperidotites and the CO2-densitya of the studied fluid inclusions (for details, see text).

Sample Modal percentages Rock type Tex

Ol Opx Cpx Spl

Tih0304 72.6 22.6 3.8 0.9 Harzburgite cg-pTih0305 81.7 11.1 5.9 1.3 Lherzolite cg-pTih0310 85.6 10.1 2.4 1.9 Harzburgite cg-pTih0501 63.3 29.8 5.5 1.4 Lherzolite cg-pTih0503 68.1 25.8 5.3 0.9 Lherzolite cg-pTih0504 72.4 24.4 2.3 1 Harzburgite cg-pTih0506 72.4 24.5 2.6 0.6 Harzburgite cg-pTih0507 55.1 41.8 2.5 0.6 Harzburgite cg-pTih0509 58.5 26.3 12.4 2.8 Lherzolite cg-p

NOFLUID — no fluid inclusions found in the rock.FLUIDPOOR — fluid inclusion present in the rock tough with a small amount.FLUIDRICH — fluid inclusion present in the rock in large amount.Abbreviations: Ol — olivine, Opx — orthopyroxene, Cpx — clinopyroxene, Spl — spinel.FI — fluid inclusion.cg — coarse grained, av — average, T — calculated equilibrium temperature.

a Modeling pure CO2.

fluid inclusions. Infrequently, 4–10 μm rhombohedral-shaped crystalsare observed in the fluid inclusions (Fig. 1F), however, because of thepoor visibility their size cannot be precisely determined. It is note-worthy to mention that no silicate melt inclusions are associatedwith the fluid inclusions.

Based on the abundance of fluid inclusions, the studied peridotitexenoliths are divided into three groups: (1) fluid inclusion-absent,referred to as NOFLUID xenoliths (Tih 0304, Tih 0305, Tih 0501, Tih0503); (2) fluid inclusion-poor, referred to as FLUIDPOOR xenoliths(Tih 0504, Tih 0506); and (3) fluid inclusion-rich, referred to asFLUIDRICH (Tih 0310, Tih 0507, Tih 0509) xenoliths (Table 1).

Note that in many mantle peridotite xenoliths worldwide (de Vivoet al., 1988; Frezzotti and Peccerillo, 2007; Szabó and Bodnar, 1996;Török and de Vivo, 1995) small (up to 15 μm), rounded to worm-like fluid inclusions occur along intergranular microfractures. On thebasis of their petrographic features, these are late-stage inclusionsthat represent a different environment and later time of formationcompared to the negative crystal shaped fluid inclusions studiedhere. Therefore, they are not considered in this study.

4. Analytical techniques

From the nine selected xenoliths, 100–130 μm doubly-polishedthick sections were prepared. The rock forming minerals were ana-lyzed for major elements by CAMECA SX-100 electron microprobe(EMPA) at the University of Vienna (Austria) using a 5 to 10 μmbeam. Accelerating voltage and beam current were 20 kV and 10 nA,respectively. The counting time was 40 s for each element. Naturalstandards were used for the analyses and ZAF correction was applied.

The fluid inclusions were analyzed by Raman spectroscopy in twodifferent laboratories using different instruments and analytical set-tings. At Budapest University of Technology and Economics (Hungary),a Jobin Yvon confocal Labram Raman instrument using a frequency-doubled Nd-YAG laser with an excitation wavelength 532 nm, and50mW and 20 mW laser energy at the laser source and at the samplesurface, respectively. A 50× microscope objective was used to focusthe laser onto the sample and to collect the Raman signal. At G2RLaboratory, Nancy, France a Dilor® Labram-type Raman spectrometerwith edge filter, 514 nm Ar+ laser, 200 mW laser power at the lasersource and ~80 mW at the sample surface and 80× objective wereapplied. The analytical settings for both instruments included a200–500 μm confocal hole, 200 μm spectral slit, 600 or 1800 g/mmspectrograph gratings, 2–10× accumulations and 2–150 s acquisitiontime (all depending on themaximum intensity). The spectral resolution

sed on two-pyroxene thermometer (Brey and Köhler, 1990), classification of the Tihany

ture Av. grain size(mm)

T(°C)

Classification CO2-densityof FIs

oikilitic 1.2–1.5 955 NOFLUIDoikilitic ~1.0 989 NOFLUIDoikilitic 1.5–2.5 1070 FLUIDRICH 0.81–1.06oikilitic 1.0–1.5 1015 NOFLUIDoikilitic 0.4–0.6 1005 NOFLUIDoikilitic 2.0–3.0 925 FLUIDPOOR 0.66–1.02oikilitic 2.5–3.0 1015 FLUIDPOOR 0.88–1.01oikilitic 2.0–2.5 1150 FLUIDRICH 0.90–1.05oikilitic 2.0–2.5 1165 FLUIDRICH 0.92–1.12

Opx

Ol

Cpx

Spl

Ol

Opx

Cpx

Ol

Spl

A

4 mm

Opx

OlB

Spl

FOpx

Mgs

L

Opx

E

L

C

0.6 mm

Spl

Ol

OlCpx

Opx

Spl

Ol

D

120 μm

40 μm20 μm

Opx

Fluid inclusion trail

1.5 mm

B

Fig. 1. Photomicrographs showing the petrographic characteristics of the Tihany peridotite xenoliths and their contained fluid inclusions. A) and B) Coarse-grained orthopyroxene-rich xenolith showing that the orthopyroxenes occur mainly in clusters together with lesser amounts of clinopyroxene. Xenolith: Tih 0504 harzburgite, stereomicroscopic view.C) Typical petrographic features observed in Tihany peridotites. The dominant orthopyroxene occurs together with less abundant clinopyroxene. Spinel is found mainly as crystalinclusions, euhedral and subhedral olivine crystals are found in both ortho- and clinopyroxene. Xenolith: Tih 0506 harzburgite, stereomicroscopic view. D) Trail of orthopyroxene-hosted, negative crystal-shaped fluid inclusions Xenolith: Tih 0509 lherzolite. Picture taken using plane polarized light, 1 N. E) Cluster of partially decrepitated, negative crystal-shaped fluid inclusions containing mainly one phase liquid at room temperature hosted by orthopyroxene. Xenolith: Tih 0310 harzburgite. Picture taken using plane polarizedlight, 1 N. F) Partially decrepitated negative crystal-shaped fluid inclusions in orthopyroxene. In addition to the liquid phase, a magnesite daughter phase (confirmed by Ramanmicrospectroscopy) is present. Xenolith: Tih 0310 harzburgite. Images taken using plane polarized light, 1 N. Abbreviations: Ol — olivine, Opx — orthopyroxene, Cpx —

clinopyroxene, Spl — spinel, L — liquid phase, Mgs — magnesite.

10 M. Berkesi et al. / Earth and Planetary Science Letters 331-332 (2012) 8–20

of themeasurements variedwithwavenumber butwas≤2 cm−1 usingthe grating with 1800 grooves per mm. The method of Berkesi et al.(2009) was used to test for small amounts of water in the inclusions.To characterize the bands in the Raman spectra and identify thephases present, the online Raman database of the French Society ofMineralogy and Crystallography (http://wwwobs.univ-bpclermont.fr/

sfmc/ramandb2/index.html) and that of Bonelli and Frezzotti (2003,http://www.dst.unisi.it/geofluids/raman) were referred to. Raw datawere processed using LabSpec v5.25.15 software designed forJobin-Yvon Horiba LabRam instruments.

The trace element composition of the rock formingminerals and thesemiquantitative composition of the fluid inclusions were determined

11M. Berkesi et al. / Earth and Planetary Science Letters 331-332 (2012) 8–20

by laser ablation inductively coupled plasma mass spectrometry (LA–ICP-MS) with an Agilent 7500ce octopol spectrometer (ORS) andGeoLas laser ablation system (Department of Geosciences, VirginiaTech, USA) using 193 nm ArF laser beam, 103 Pa vacuum, 1.2 l/minHe-flow with 10–50 μm spot size, 0.01 s dwell time, 5 Hz repetitionrate and 150 mV output energy. NIST reference glass standard SRM610 was used for external standardization. Data processing was carriedout by the AMS data analysis software (Mutchler et al., 2008) and bySILLS (Guillong et al., 2008). When examined under the microscope,no crystals and/or silicate melt inclusions were observed within theablation volume for the trace element analyses of the pyroxene hosts.For fluid inclusion analyses the beam size was always set to be greaterthan the diameter of the fluid inclusions (>10 μm) in order to samplethe entire inclusion plus some of the host immediately around the in-clusion. Semi-quantitative trace element ratios for the fluid inclusionswere calculated using the method of Hidas et al., 2010.

Ten orthopyroxene-hosted fluid inclusions (6 in the Tih 0310harzburgite and 4 in the Tih 0509 lherzolite), ranging in size from 3to 8 μm and located within a few microns of the host mineral surface,were selected for FIB-SEM analyses. The analyses were conductedusing an FEI QUANTA 3D FIB-SEM apparatus having both secondaryand backscattered electron detectors, together with a silicon driftenergy dispersive spectrometer (EDS), located in the Material Scienceand Biological Research Center of Eötvös University, Budapest, Hun-gary. Accelerating voltage and current of the electron beam were10–20 kV (depending on inclusion size and elements of interest)and 0.46–2 nA, respectively, which allowed major elements havingmasses from beryllium through oxygen to barium to be analyzed. Be-sides primary electrons (Supplementary Fig. 1B), Ga-ions (used in themilling process) were also used for secondary electron imaging (Sup-plementary Fig. 1A). After locating the pre-selected area to be ana-lyzed, an ultra-thin (200–300 nm) platinum layer was depositedwith the electron beam to identify the area of interest on the ion in-duced image. When the sample is tilted to an angle of 52°, the millingGa-ion beam is perpendicular to the sample surface (SupplementaryFig. 1A,C). At this tilted position a thin (1–2 μm) layer of platinumwas deposited onto the pre-selected area with the ion beam to pro-tect that part of the sample from abrasion by the Ga-ion beam(Wirth, 2004). At the beginning of the milling process three largetrenches are milled around three edges of the platinum strip (Supple-mentary Fig. 1A,B,D). A somewhat deeper trench is milled along theedge of the platinum from which the inclusion exposure starts (re-ferred to as ‘front trench’ on Supplementary Fig. 1A). Because thesetrenches were milled using a high ion current (15 to 45 nA), acolumn-like structure forms on the walls (Supplementary Fig. 1B).The use of lower ion current (3–5 nA) allowed us to mill a perfectlyflat surface (Supplementary Fig. 1D) and expose the inclusions insteps. Each step involves first cutting (milling) of an arbitrary amountof material from the exposed surface of the inclusion, using the ionbeam, then analyzing the exposed portion of the inclusion by SEMand, finally, analyzing the exposed daughter phases with EDS. Inorder to produce the most precise estimate for the volume propor-tions of the daughter phases, an equal amount of material was re-moved from the inclusion in each step.

Identification of daughter phases was based on their morphologyon the secondary electron images, their brightness on the backscat-tered electron images and examination of EDS spectra (Fig. 2) andpreviously obtained Raman spectra of the same inclusion. Becauseof the small size of the daughter phases in fluid inclusions (from100 nm to 2 μm) (Fig. 2), the signals detected by EDS are mixed sig-nals, as X-rays from the adjacent area (host phase and/or otherdaughter minerals) are also detected. To better distinguish thedaughter phase signal from the host mineral, EDS control spectra forthe host mineral were also taken in the proximity and from thesame depth as the solid phases after each daughter mineral spectrumwas collected.

5. Mineral composition of Tihany xenoliths

No significant major element zonation was observed in the rock-forming minerals of the Tihany peridotites (Table 2). Olivines, occur-ring either as interstitial phases or as crystal inclusions in orthopyrox-ene show mg# [100×Mg/(Mg2++Fe2+)] of 0.90–0.91 and have ahigh NiO (0.30–0.43 wt.%) content (Table 2). However, the lithologi-cal classification is not in correlation with the mineral chemistry—minerals in both the harzburgites and the lherzolites yield a wideand overlapping compositional range (Fig. 3).

Similar to the olivines, the mg# of orthopyroxenes is high (0.91–0.92) and is independent of the abundance of fluid inclusions in thexenolith. However, the major element content of orthopyroxenes inthe NOFLUID and FLUIDPOOR xenoliths shows consistently lowAl2O3 (2.66–3.05 wt.%), Na2O (0.01–0.08 wt.%), and elevated Cr2O3

(0.55–0.59 wt.%) relative to the FLUIDRICH xenoliths, where higherAl2O3 (up to 6.86 wt.%), (Fig. 3A, Table 2) and Na2O (up to 0.16 wt.%)(Fig. 3B, Table 2) were observed. Clinopyroxenes also show a highmg#, ranging from 0.90 to 0.93. In the NOFLUID xenoliths the clino-pyroxenes are homogeneous in composition and show low Al2O3

(2.66–3.27 wt.%), TiO2 (0.03–0.11 wt.%), Na2O (0.39–0.74 wt.%) andhigh Cr2O3 (0.73–0.90 wt.%). An increase in the Al2O3 (3.25–6.86 wt.%),TiO2 (0.07–0.31 wt.%) and Na2O (0.16–1.15 wt.%) concentration is ob-served in clinopyroxenes from FLUIDPOOR to FLUIDRICH xenoliths rela-tive to clinopyroxenes from the NOFLUID xenoliths (Fig. 3C,D; Table 2).

Equilibrium formation temperature was calculated based on themajor element compositions of the constituent minerals, using thetwo-pyroxene thermometer (Brey and Köhler, 1990) at 1.5 GPa pres-sure (approximating the spinel lherzolite stability field). The Tihanyperidotites show a wide range of equilibrium temperature of between925 and 1165 °C (±16 °C) (Table 1), which is a wider range thanthat of peridotites collected from other localities in BBHVF (e.g.,Embey-Isztin et al., 2001).

6. Trace elements

6.1. Xenoliths — evidence for cryptic metasomatism

Because in dry (hydrous phase free) spinel peridotites clinopyrox-ene is the main carrier of trace elements (e.g. Rampone et al., 1991;Roden and Shimizu, 1993),we focused on thismineral tomonitormeta-somatic imprints of the peridotites. Please note that fluid inclusionsoccur mainly in orthopyroxene that was also analyzed in order to pro-vide background for fluid trace element analyses (shown later).

No zonation of trace elements was observed from core to rim inpyroxenes. With the exception of the highly incompatible trace ele-ments (Rb, Th, U, Ba, Nb), the clinopyroxenes from the same rocksample are uniform in composition (Fig. 4). Compositions, however,vary among the studied xenoliths (Table 4; Fig. 4).

Clinopyroxenes in the NOFLUID xenoliths show the ‘U-shaped’REE-Y (rare earth elements and yttrium) chondrite-normalizedpattern (Fig. 5A). As a general rule, the NOFLUID clinopyroxenes areless enriched in trace elements, especially in MREE (Sm, Nd; Fig. 5)and HFSE (Zr, Hf, Ti; Fig. 4) relative to the FLUIDPOOR and FLUIDRICHxenoliths (Table 4; Fig. 4). In the NOFLUID clinopyroxenes the HFSE(Zr, Hf, Ti; Fig. 4) andMREE concentrations are highest in Tih 0503, low-est in Tih 0304, and have intermediate values in Tih 0305 (Fig. 4). Thelatter two xenoliths have concentration of Zr, Hf, Ti similar to that ofFLUIDPOOR xenoliths. In comparison to the NOFLUID xenoliths, theREE distribution in clinopyroxenes in the FLUIDPOOR and FLUIDRICHxenoliths have a slightly enriched pattern with no depletion in MREE(relative to the NOFLUID ones), but a slight enrichment can be observedin LREE (Fig. 5B). Clinopyroxenes in the FLUIDPOOR xenoliths areenriched in Ce, Nd, Sm, Eu, Zr and Hf compared to the NOFLUID xeno-liths (Fig. 4). The primitive mantle-normalized trace element pattern

Fe-sulfide

glass film

Opx

B

1 3 5 7

keV

O

Fe

Mg

Si

S

S Fe

Fe

Fe-sulfide

Cou

nts

1 μm

1 2keV

Cou

nts

C

O

Ga

Mg

Al

Si

OpxMgs

Opx

Mgs

GC

M accum

ulation

glass film

A

500 nm

1 3 5 7

Cou

nts

keV

Opx

glass

O

Fe

Mg

Si

FeCa

Al

Ga

Opx

glass film

C

GC

M

accumulation

300 nm

vesicles

Fig. 2. Daughter phases in fluid inclusions identified by FIB-SEM. A— secondary electron image and corresponding EDS spectra of the host orthopyroxene (Opx) and themagnesite (Mgs)step-daughter phase that is attached to the inclusion wall. The magnesite is in direct contact with the host orthopyroxene and there is no glass film between them. B — backscatteredelectron image of an Fe-bearing sulfide daughter phase and its EDS spectrum. C — secondary electron image of the glass film showing numerous ovoid-shaped vesicles on the surface.EDS spectra of glass film and host orthopyroxene are also shown. GCM — gallium contaminated material.

12 M. Berkesi et al. / Earth and Planetary Science Letters 331-332 (2012) 8–20

of clinopyroxenes in the FLUIDPOOR and FLUIDRICH xenoliths is similarto that of amphibole spinel lherzolites found in the same volcanic field,in which coexisting silicate melt and fluid inclusions are present (Hidaset al., 2010; Szabó et al., 2009).

The orthopyroxenes of the FLUIDPOOR and FLUIDRICH xenolithsare also enriched in LREE and MREE relative to the NOFLUID xeno-liths, and also have higher concentrations of Ce, Nd, Sm and Eu com-pared to the NOFLUID xenoliths (Table 3), similar to the pattern forthe clinopyroxenes.

6.2. Trace elements associated with fluid inclusions — the potentialmetasomatizing agent

Trace elements associated with high density fluid inclusionswere analyzed by in situ LA–ICP-MS analysis of samples Tih 0310

and Tih 0509, belonging to the FLUIDRICH xenolith group. Forthese analyses, a laser spot size large enough to include the entireinclusion plus some of the surrounding host mineral was used.This approach assured that any elements associated with thetrapped fluid, whether they remained in the fluid phase or were de-posited on the walls of the inclusion during quenching, would be in-cluded in the analytical volume. A clear increase in intensity,compared to the intensity for the host orthopyroxenes, was ob-served for some trace elements during LA–ICP-MS analysis of inclu-sions (Fig. 6). The intensity increase is clearly associated with fluidinclusions. By analyzing the CO2-fluid inclusions in situ, LIL elementssuch as K, Rb and Ba, as well as Sr, Ti, Nb, Ca and Al, all showed an in-crease in signal intensity when the fluid inclusions were being ablat-ed (Fig. 6). The duration of the mixed host-fluid inclusion transientsignal was typically 35–50 s (Fig. 6).

0

1

2

3

4

5

6

7

mg#

Al 2O

3 (w

t%)

Opx Opx

Na 2O

(w

t%)

mg#

Al (a.p.f.u.)

Ti (

a.p.

f.u.)

Al (a.p.f.u.)

Na

(a.p

.f.u.

)

0.25

0.20

0.15

0.10

0.75

0

0.15

0.10

0.05

Cpx Cpx

A B

DC0.015

0.010

0.005

0.00

0.00.80 0.85 0.90 0.950.75 0.80 0.85 0.90 0.95

0.05

0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4

Tih 0304 harzburgite

Tih 0305 lherzolite

Tih 0501 lherzolite

Tih 0503 lherzolite

Tih 0310 harzburgite

Tih 0504 harzburgite

Tih 0506 harzburgite

Tih 0507 harzburgite

Tih 0509 lherzolite

LEGEND

NOFLUID xenoliths

FLUIDPOOR xenoliths

FLUIDRICH xenoliths

Fig. 3. Composition of orthopyroxene (A and B) and clinopyroxene (C and D) in the Tihany peridotites from this study (Bakony-Balaton Highland Volcanic Field, Central Pannonian Basin,Hungary). On the figures ortho- and clinopyroxene compositions from NOFLUID, FLUIDPOOR, FLUIDRICH xenoliths are shown separately. Abbreviations: Opx — orthopyroxene, Cpx —

clinopyroxene, a.p.f.u. — atoms per formula unit, mg# — mg-number [Mg2+/(Mg2++Fe2+)], wt% — weight percent.

13M. Berkesi et al. / Earth and Planetary Science Letters 331-332 (2012) 8–20

The absolute element concentration of the CO2-rich fluid inclu-sions cannot be determined because of lack of any internal standardsfor the fluid inclusions. However, trace element ratios in CO2-rich, C–O–H–S fluid inclusions can be estimated (Hidas et al., 2010). For aninternal standard, Ba was selected and its concentration was set at100 ppm (Table 5; Fig. 7). Furthermore, because Cr is containedonly in the host orthopyroxene and was not detected in the fluid in-clusion, this element was used to remove the orthopyroxene contri-bution to the mixed fluid inclusion–host signal. The trace elementratios in the “residual signal” were calculated as described aboveto produce semi-quantitative trace element compositions (Hidas

et al., 2010) characteristic of the fluid inclusions (Table 5). In addi-tion to the trace elements that are linked to the fluid inclusionsbased on the intensity versus time transient signal, Pb and Zr havealso been identified as being associated with the fluid inclusions(Table 5).

The primitive mantle-normalized pattern of the semi-quantitativetrace element abundances for the fluid inclusions from the Tihany xeno-liths are similar to those published byHidas et al. (2010) (Fig. 7), with re-spect to Nb, K, Pb, Sr and Zr (Fig. 7). Additionally, a lower Naconcentrationwas found for the Tihany samples (Fig. 7), and abundancesof Rb, U, Th, La, Ce, Nd and Hf were below detection limits.

0.001

1

10

100

Rb Th U Ba Nb La Ce Pb Sr Nd Hf Zr Sm Eu Ti Y Yb Lu Cr

Rb Th U Ba Nb La Ce Pb Sr Nd Hf Zr Sm Eu Ti Y Yb Lu Cr0.001

0.01

0.1

1

10

100

Cpx

/prim

itive

man

tleC

px/p

rimiti

ve m

antle

Tih 0304 harzburgiteTih 0305 lherzoliteTih 0501 lherzoliteTih 0503 lherzolite

0.01

0.1

NOFLUID xenoliths

Tih 0504 harzburgiteTih 0506 harzburgiteTih 0507 harzburgite

FLUIDPOOR xenoliths

FLUIDRICH xenolith

amph-bearing spl lhz (Szabó et al., 2009)

A

B

Fig. 4. Primitive mantle normalized trace element patterns for clinopyroxenes from theTihany peridotites. Symbols connected by lines indicate the average trace elementvalues, the error bars indicate the standard deviation. A: Clinopyroxenes in NOFLUIDxenoliths, B: clinopyroxenes in FLUIDPOOR and FLUIDRICH xenoliths. For comparison,trace element patterns of amphibole-bearing spinel lherzolites (abbreviated as“amph-bearing spl lhz” in this figure, see also Szabó et al., 2009) from the same volca-nic field are also shown. Note that clinopyroxene compositions reported by Szabó et al.(2009) have wider ranges, and only the composition with the lowest normalized value(xenolith sample Szg07) is shown. Primitive mantle composition is after McDonoughand Sun (1995). Cpx — clinopyroxene.

14 M. Berkesi et al. / Earth and Planetary Science Letters 331-332 (2012) 8–20

Combining the mineral chemistry, the presence of the fluid inclu-sions and the trace elements found to be associated with the fluidinclusions, it is highly likely that the metasomatizing agent was thefluid entrapped in the studied inclusions.

7. Raman spectroscopic analysis of daughter minerals in the fluidinclusions

Fluid inclusions from the Tihany peridotites are CO2-rich andcontain small amounts of H2O and H2S (Berkesi et al., 2009;Fig. 8A in this study). Here we discuss only the analysis and signif-icance of the solid phases, i.e., daughter phases. The solid phase thatwas visible within the orthopyroxene-hosted fluid inclusions(Fig. 1F) was identified as magnesite (Fig. 8C) in xenoliths Tih0310 and Tih 0509. Raman analyses of most fluid inclusions inwhich no solid phases were visible under the microscope con-firmed the presence of magnesite, based on peaks at 1094±1.8,738±1.8 and 330±1.8 cm−1 (Fig. 8C). Additionally, α-quartz co-exists with magnesite in the orthopyroxene-hosted fluid inclusionsbased on the peak at 464±1.8 cm−1 (Fig. 8B).

8. Focused ion beam-scanning electron microscopy (FIB-SEM)

Because polishing material partly or entirely filled open fluidinclusion cavities after preparing the polished thin sections, andbecause some material contained in the inclusions was likely lostfrom exposed inclusions during polishing, it was not possible to an-alyze the exposed fluid inclusions by SEM to determine the compo-sitions of the daughter phases. Focused ion beam-scanning electronmicroscopy (FIB-SEM) has already been shown to be an appropriatemethod for studying submicron-sized samples (e.g., Anderson andMcCarron, 2011; Dégi et al., 2010; Dobrzhinetskaya et al., 2003,2006; Wirth, 2004, 2009). We applied this technique using step-wise exposure of the fluid inclusions to determine the daughterphases of unopened fluid inclusions located beneath the samplesurface.

8.1. Daughter crystals in the exposed fluid inclusions

During the stepwise exposure and analysis of fluid inclusions(n=10), several solid phases, includingmagnesite (Fig. 2A) and quartz,were detected within the fluid inclusions, in accordance with results ofour Raman analyses. The crystals range from 200 to 2000 nm and occuras clusters on the inclusion walls. Both the magnesite (Fig. 2A) andquartz show euhedral to subhedral shapes and are in direct contactwith the host orthopyroxene. Furthermore, an S-bearing solid phasehas also been identified. Its size ranges between 400 and 1000 nm andshows a subhedral shape. Based on SEM imaging, this phase is identifiedas Fe-bearing sulfide (Fig. 2B). On the basis of the SEM images, taken atequal sectioning steps, the volume percent of the magnesite and quartzwithin the inclusion cavity range from 3.3 to 5.0 (3.7–5.0 vol.% in sam-ple Tih 0310 harzburgite and 3.3–3.9 vol.% in sample Tih 0509 lherzo-lite), and from 2.6 to 4.2 vol.% (3.1–4.2 vol.% in sample Tih 0310harzburgite and 2.6–3.4 vol.% in sample Tih 0509 lherzolite), respec-tively, whereas the sulfide ranges between 1.3 and 1.9 vol.% (only insample Tih 0310 harzburgite), estimated using the method describedby Anderson and McCarron, 2011.

8.2. Thin glass film at the fluid inclusion wall

One of the most important features observed during FIB-SEManalyses was a thin glass film covering the wall of the fluid inclu-sions (Fig. 2A,C). The thickness of this film is ≈100–200 nm andis not visible using optical microscopy. This film has no crystal mor-phology, however it does contain numerous spherical-shaped holeson its surface (referred to as vesicles on Fig. 2C). The size of thesevesicles was up to ~50 nm, but the majority of them are of severalnanometer size. Vesicles are known to be formed by the exsolutionof volatiles from a melt phase, mostly during rapid quenching (e.g.,Mungall et al., 1996). The lack of crystal morphology together withthe presence of vesicles clearly indicates that the thin coat observedin the inclusion is a glass film. The EDS analyses indicated that thisglass is richer in Fe, Ca and Al, and has higher Si/Mg compared to thehost orthopyroxene.

Importantly, where carbonate and quartz daughter phases occuron the wall, the glass film does not exist between the host orthopyr-oxene and the daughter phases. The glass also covers the carbonateand quartz daughter phases.

9. Discussion: complexity of CO2-rich fluids in the Earth's mantle

9.1. Negative crystal shape as an indicator for fluid entrapment at mantledepth

The negative crystal shape of fluid inclusions in the Tihany xeno-liths (Fig. 1E,F) suggests textural equilibrium between the fluid andits host phase (Viti and Frezzotti, 2000). The development of a

La Ce Nd Sm Eu Gd Dy Y Er Yb Lu0.1

1

10

Cpx

/cho

ndrit

e

100

0.1

1

10

La Ce Nd Sm Eu Gd Dy Y Er Yb Lu

100

FLUIDPOOR and FLUIDRICHxenoliths

NOFLUID xenoliths

A B

Tih 0304 harzburgite

Tih 0305 lherzolite

Tih 0501 lherzolite

Tih 0503 lherzolite

Tih 0310 harzburgite

Tih 0504 harzburgite

Tih 0506 harzburgite

Tih 0507 harzburgite

Tih 0509 lherzolite

LEGEND

NOFLUID xenoliths

FLUIDPOOR xenoliths

FLUIDRICH xenoliths

Fig. 5. Chondrite-normalized rare earth element and yttrium (REY) pattern for clinopyroxenes (Cpx) in the Tihany peridotites. Diagram A is for pyroxenes in the NOFLUID xenoliths,and diagram B represents pyroxenes in the FLUIDPOOR and FLUIDRICH xenoliths. Chondrite composition is after Anders and Grevesse (1989).

15M. Berkesi et al. / Earth and Planetary Science Letters 331-332 (2012) 8–20

negative crystal shape through maturation (Bodnar et al., 1989)likely takes a longer period of time than the duration of transportof the xenoliths from the upper mantle to the surface (Dégi et al.,2009; Roedder, 1984; Szabó and Bodnar, 1996). Moreover, therapid pressure change during uplift does not favor the formationof negative crystal morphology (Bodnar et al., 1989). Thus, the

1

10

100

1000

10000

100000

0 100 200 300 400

Inte

n si

ty

BaTi

Tim

background

opx

opx+FI

Al

K

Nb

Fig. 6. Representative LA–ICP-MS intensity versus time signals during ablation of an orthopyrmixed signal from both the host orthopyroxene and the fluid inclusion. In contrast, the paorthopyroxene.

possibility of the entrapment of the studied fluid inclusions duringtransport to the surface by alkali basalt is unlikely. The high CO2 densi-ty (e.g. ~1 g/cm3 in each xenolith studied) within the fluid inclusionssuggests that the entrapment of thefluids occurred at lithosphericman-tle conditions prior to sampling by upwelling alkali basalt (Berkesi et al.,2009).

500 600 700 800 900 1000

Ca

Si

Rb

Cr

e (sec)

LA-ICP-MS time vs. intensity signals

oxene-hosted fluid inclusion. The part of the spectrum labeled as ‘opx+FI’ represents art of the spectrum labeled ‘opx’ represents signal collected exclusively from the host

0.01

0.1

1

10

100

1000

Rb

Sam

ple/

prim

itive

man

tle

Th U Ba Nb K La Ce Pb Sr Nd Na Zr Hf

opx-hosted fluid inclusions

cpx-hosted fluid inclusions

Tih 0310 (harzburgite) fluid inclusionsTih 0509 (lherzolite) fluid inclusions

FLUIDRICH xenoliths

fluid inclusions from amphl-bearing spl lherzolite

(Hidas et al., 2010) NOTE: to display element ratios only

Fig. 7. Primitive mantle normalized trace element distribution of fluid inclusions from xenoliths Tih 0310 harzburgite and 0509 lherzolite (both belonging to the FLUIDRICH group).Compositions were estimated using the method of Hidas et al. (2010). It should be noted only element ratios are shown. For comparison, the element ratios reported by Hidas et al.(2010) are also shown. Primitive mantle composition is after McDonough and Sun (1995).

16 M. Berkesi et al. / Earth and Planetary Science Letters 331-332 (2012) 8–20

9.2. Post-entrapment carbonation reaction

The major divalent cations of the carbonates detected by Ramanspectroscopy are always the same as that of the host phases (forexample magnesite was found in Mg-rich orthopyroxene-hosted

200 400 600 800

Magnesite

Orthopyroxene (host)

738

685

401

345

330

23921

3

662

Raman shift (cm-1)

Inte

nsity

C

2500 2550 2600 2650

H S2

2608

A

Fig. 8. Representative Raman spectra of phases within the studied fluid inclusions. All sp2608 cm−1 corresponding to H2S. B) Raman spectrum showing the peak at 464 cm−1 corrorthopyroxene close to the fluid inclusions using the same analytical conditions as were useof magnesite and CO2 in the fluid inclusion. Peak positions (numbers above the peaks) wer

fluid inclusions, Figs. 2A and 8C). Moreover, carbonate and quartzwere found exclusively in the fluid inclusions, i.e., these phaseswere not observed as solid inclusions in the xenolith minerals.These observations imply that magnesite and quartz are the resultof a post entrapment carbonation reaction, which can be described

α-Quartz

380 400 420 440 460 480 500 520 540 560

1000 1200 1400 1600

CO2

1286

1385

1010

1094

1033

464

B

ectra were obtained at room temperature. A) Raman spectrum showing the peak atesponding to α-quartz. The dashed lines in B correspond to spectra taken on the hostd for the fluid inclusions. C) Raman spectrum of a fluid inclusion showing the presencee determined by fitting the peaks using a combined Gaussian–Lorentzian function.

17M. Berkesi et al. / Earth and Planetary Science Letters 331-332 (2012) 8–20

by the relevant MgO–SiO2–CO2 system (Koziol and Newton, 1995;Matas et al., 2000) as:

MgSiO3 orthopyroxene ¼ enstatiteð Þ þ CO2 fluidð Þ¼ MgCO3 magnesiteð Þ þ SiO2 quartzð Þ:

ð1Þ

Since magnesite and quartz contain components from both fluidand host they can be considered as “step-daughter crystals” (e.g.,Scambelluri and Philippot, 2001). Considering that the studied fluidinclusions contain a small amount of H2O (up to 11.7 mol%, Berkesiet al., 2009), the reaction given in Eq. (1) is illustrated on Fig. 9 byreaction curves for various bulk fluid compositions between XCO2=1and XCO2=0.5 in the CO2–H2O fluid system. The curves were calculatedusing the Perple_X 6.6 software (Connolly, 2009) applying the databaseand solution models of Holland and Powell (1990) for the solid phases,and the model of Connolly and Trommsdorff (1991) for the fluid mix-tures (different XCO2, Fig. 9). In themodel the MgO–SiO2–CO2–H2O sys-tem Schreinemakers projection was used to create a P (pressure)–T(temperature) plane, excluding all phases except for enstatite, magne-site, quartz and the CO2–H2O fluid mixture (Fig. 9).

Fig. 9 shows that the maximum experimentally determined(Koziol and Newton, 1995) pressure of Eq. (1) at 890 ±2 °C is1.79 GPa (±0.02 GPa), which corresponds to the garnet lherzolitestability field (O'Reilly and Griffin, 1996). The calculated range inequilibrium temperature for the upper mantle beneath the studyarea (Table 1, dash-dotted rectangle on Fig. 9), however is in the spi-nel lherzolite stability field, and far from any reaction curve for Eq. (1)(Fig. 9). Therefore, the reaction that produced the magnesite and the

P (

GP

a)

T (oC)

Grt

Spl

Spl

Pl

0

0.5

1.0

1.5

Mgs+Q

tz

En+C

O2

1.1

1.0

0.9

0.8

X =1.0X =0.9X =0.8X =0.7X =0.6X =0.5

200 400 600 800 1000 1200

Fig. 9. Pressure (GPa)–Temperature (°C) diagram showing the inferred formationconditions for the host xenolith, fluid inclusions and daughter minerals. The blackarrow shows an approximate P–T path followed by the upwelling basaltic melt/lavathat transported the xenolith to the surface. The reaction curve of enstatite (En)+CO2=magnesite (Mgs)+quartz (Qtz) is shown in red, estimated from Holland andPowell (1990). The solid lines with numbers between 0.6 and 1.1 correspond to the iso-chores of the CO2 labeled in g/cm3. The dashed-dotted rectangle indicates the possibleP–T ranges where the mantle rocks might have equilibrated in the lithospheric uppermantle, and where the fluid inclusions were originally trapped. This was determinedfrom the equilibrium temperatures of the xenolith using the geothermometer of Breyand Köhler (1990) and by the spinel (Spl)/garnet (Grt) and the plagioclase (Pl)/spinel(Spl) transition curves (O'Reilly and Griffin, 1996). XCO2 is the molar fraction of CO2 inthe hypothetical CO2–H2O fluid system. (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)

quartz inside the inclusion likely did not take place at spinel lherzoli-tic mantle conditions. Rather, magnesite and quartz likely formedinside the fluid inclusions during transport of the xenoliths to the sur-face by the alkali basalt (black arrow on Fig. 9). The path followed bythe xenoliths from mantle depth to the surface was essentially adia-batic (Fig. 9). Along most of the P–T path of the host basalt, the stablephase assemblage is always the reactant (i.e. the host enstatite andthe CO2 fluid, Fig. 9). The reaction to form the step-daughter phasescould have only commenced when the fluid inclusion-bearing xeno-liths cooled to about 400–600 °C (e.g. the intersection of the reactioncurves with the fluid isochore, Fig. 9). It is important to note that as aresult of the reaction (1), some of the CO2 in the fluid is now con-tained in magnesite. As a consideration, the fluid inclusion isochoreused to determine the formation conditions for the inclusions mustbe recalculated by taking into account the volume properties of car-bonates. However, discussion of the assumptions involved in makingthis slight correction to the isochore is beyond the scope of this paper,and has no significant effect on the main conclusions of this study.

At ambient temperature the thermodynamically stable phase as-semblage for the reaction given by Eq. (1) is the carbonation reactionproduct without CO2 (Fig. 9). Under equilibrium conditions the reac-tion should continue until either all of the CO2 is depleted, or all of theorthopyroxene has been depleted. However, CO2 still exists withinthe fluid inclusions even though the host orthopyroxene is in excess.This quasi-discrepancy can be explained by an incomplete or arrestedreaction forming the step-daughter phases. The incompleteness ofthe reaction may be due to the slow kinetics of the reaction betweenthe H2O-poor fluid and the mineral at low temperature (100–300 °C).However, it is also likely that during the reaction of the fluid with thehost orthopyroxene, the formation of the glass phase on the wallscould have acted as a “barrier” between the CO2-rich fluid and thehost orthopyroxene. Our FIB-SEM analyses revealed a glass film cov-ering the entire wall of the inclusions (Fig. 2C), with the exceptionof that part of the wall where step-daughter carbonate and quartzwere present. These step-daughter phases always have direct contactwith the host orthopyroxene (Fig. 2A). Consequently, the glass, repre-senting silicate components that were formerly dissolved in the fluid,started to precipitate later or simultaneously with the magnesite andquartz step-daughter phases.

These textural and chronological observations (Fig. 2) suggest thatthe glass film prevented further growth of the step-daughter phasesby isolating the residual fluid from the host orthopyroxene, resultingin the preservation of CO2 in the fluid inclusions. Carbonates occur-ring together with CO2 is a general phenomenon in mantlexenolith-hosted fluid inclusions (e.g., Andersen and Neumann, 2001and references therein; Frezzotti and Peccerillo, 2007; Frezzotti etal., 2002). This suggests that the presence of a glass film in mantlefluid inclusions might also be a common feature, precluding or atleast arresting the completion of the carbonation reaction and pre-serving the CO2-rich (residual) fluid in the inclusions.

9.3. The role of the silicate components in supercritical CO2-rich fluidtrace element transport in the mantle

Cryptic fluid metasomatism is commonly invoked to explain traceelement enrichment or depletion without a change in modal mineral-ogy in mantle xenoliths (e.g., Malaviarachchi et al., 2010; Young andLee, 2009) and results of this study will help to provide a better un-derstanding of fluid-assisted metasomatic processes that have affect-ed the upper mantle. The good correlation between enrichment of theclinopyroxenes in Al2O3, TiO2 and Na2O (Fig. 3C, D; Table 2), MREEand Zr (Table 4) and the presence of fluid inclusions provides strongevidence for fluid-related metasomatism of the studied peridotitexenoliths. Therefore, the chemistry of the fluid inclusions can be con-sidered to be characteristic of a fluid phase whose composition isthe result of fluid/rock interaction, as is usually assumed for CO2-rich

18 M. Berkesi et al. / Earth and Planetary Science Letters 331-332 (2012) 8–20

fluid inclusions (de Vivo et al., 1988; Hidas et al., 2010; Roedder, 1984;Szabó and Bodnar, 1996). Interactions between metasomatic fluids andmantle rocks appear to have occurred to different extents in differentsamples (Fig. 4), as shown by the chemistry of the xenoliths. Some clin-opyroxenes that show the greatest extent of cryptic metasomatism,based on Sm/Yb and La/Sm chemistry, are FLUIDRICH, whereas othersare FLUIDPOOR or NOFLUID (Fig. 4).

Previous workers (e.g., Eggler, 1975; Spera, 1981) have shownthat pure CO2 does not dissolve significant concentrations of traceelements, even at high temperature and pressure (P~2.0 GPa,T~1300 °C). It is therefore unlikely that pure CO2 fluids alone cantransport significant quantities of trace elements in the lithosphericmantle. In contrast to CO2, aqueous fluids can dissolve and transportsignificant amounts of LILE, chlorine and LREE at high P–T (e.g.,Keppler, 1996; Kessel et al., 2005). However, Raman analyses of inclu-sions in this study and studies of fluid inclusions in other mantlexenoliths worldwide have documented that the inclusions containamounts of H2O that are not sufficient to account for observed traceelement abundances in the inclusions. Moreover, the duration of thehost-fluid inclusion transient signals obtained on fluid inclusions dur-ing LA–ICP-MS analyses (Fig. 6) was much longer (35–50 s, Fig. 6)than is expected from releasing of a fluid from an inclusion (2–5 s,Allan et al., 2005; Bertelli et al., 2009). For these reasons, it is not like-ly that the trace elements detected during LA–ICP-MS analysis of fluidinclusions (Figs. 6 and 7, Table 5) are dominantly dissolved in H2O atroom temperature. It is, however, likely that the glass film (Fig. 2C)coating the inclusion walls contributed to the signals from the fluidinclusions during ablation (Fig. 6).

It is possible that the carbonates and perhaps quartz step-daughter minerals also contain some trace elements. It should be em-phasized, however, that the host orthopyroxenes are highly depletedin trace elements of interest (Table 3), hence the source of trace ele-ments incorporated into the daughter phases during reaction be-tween the fluid and the host mineral would have been the originallytrapped CO2- and trace element-enriched fluid. Based on previousstudies, carbonates may contain significant Rb, Ba and Sr (e.g., Kiliaset al., 2006). Moreover, quartz may also contain Al and Ti in detect-able concentrations (Flem et al., 2002; Müller et al., 2003; Rusk etal., 2008), as shown by analyses of quartz from magmatic and hydro-thermal environments. HFS elements, such as Zr and Nb, were foundto be concentrated in the fluid inclusions, and their concentrationsare slightly increased in FLUIDPOOR and greatly increased inFLUIDRICH clinopyroxenes (Fig. 4). Carbonate or quartz cannot be re-sponsible for the elevated Nb and Zr contents. HFS elements preferen-tially relate to the silicate melt (e.g., Green et al., 1989) by orders ofmagnitude more than the coexisted fluid phase (Young and Lee,2009). In addition, Young and Lee (2009) showed that if HFSE areadded to mantle pyroxenes by cryptic metasomatism, the metaso-matic agent must be a silicate melt.

The primitive mantle-normalized trace element pattern of theFLUIDPOOR and FLUIDRICH clinopyroxenes is similar to that of anamphibole-bearing spinel lherzolite from the same volcanic field, inwhich coexisting silicate melt and fluid inclusions have been foundboth in clinopyroxene and orthopyroxene (Hidas et al., 2010; Szabóet al., 2009) (Fig. 4). These workers pointed out that the fluid inclu-sions might have been trapped following high P–T silicate melt-fluid immiscibility and the trace element characteristics of the im-miscible fluids are controlled by the chemistry of the parentalmelt. After applying the same semi-quantitative estimation forthe trace elements associated with the fluid inclusions (Fig. 7), ra-tios for Nb, K, Pb, Sr and Zr that are similar to those reported byHidas et al. (2010) were obtained (Fig. 7; Table 5). Thus, the traceelement characteristics of the clinopyroxene and trace element ra-tios of the fluid inclusions that are both similar to values reportedby Hidas et al. (2010), together with our FIB-SEM measurements,support the idea that the trace element content of supercritical

CO2-rich fluid is mainly controlled by its dissolved silicate compo-nents. It is therefore highly likely that dissolved silicate compo-nents in a supercritical, volatile (C–O–H–S)-rich fluid areresponsible for transport trace elements in the lithospheric mantle.This suggests the potential of CO2-rich mantle fluids to producecryptic metasomatism of the mantle.

Our findings additionally show that reactions within the fluid in-clusions, after entrapment of a single-phase fluid, formed daughterphases and a residual CO2-rich fluid phase. Additionally, the silicatecomponents, dissolved formerly in the CO2-rich fluid, quenchedonto the wall, forming a glass film, during uplift to the surface. Thisglassy material may prevent and/or arrest continued fluid-host min-eral reactions and growth of the step-daughter phases and thus pre-serve the CO2-rich residual fluid.

9.4. Origin of silicate components in CO2-rich fluids: transporteddissolved or formed in-situ?

An important question is whether the incongruent melting oforthopyroxene should be taken into account or excluded when tryingto understand the origin of the silicate component (present now asglass film within the fluid inclusions). It is stated above that the FIB-SEM exposure revealed a chronology of the precipitation of thestep-daughter phases. This shows that first the magnesite (togetherwith quartz) crystallized, followed by quenching of the dissolved sil-icate, as glass film, onto the inclusion wall. The calculated tempera-ture for the beginning of the carbonation reaction is 400–600 °C(Fig. 9). If there had been any orthopyroxene melting, then it wouldhave started in this temperature range (or at lower temperatures),which is highly unlikely. Additionally, the trace elements found tobe associated with the fluid inclusions suggest that the silicate glasscontains trace elements that are extremely incompatible in the hostorthopyroxene (Fig. 6), such as K, Ba, Rb. If the presence of silicateglass was the result of incongruent melting of orthopyroxene, no dif-ference in trace element abundance would be found between the hostorthopyroxene and the fluid inclusion. On the basis of this consider-ation, we exclude melting of pyroxene as the source of the silicateglass. Rather, we propose that silicate components were dissolved inthe CO2-rich fluid at entrapment. A similar hypothesis was describedpreviously (Green, 1979; Hidas et al., 2010; Szabó et al., 2009), andthese workers suggested that various trace elements found to beclosely associated with the fluid inclusions had been dissolved inthe CO2-rich fluid at higher pressure and temperature.

10. Conclusions

C–O–H–S fluid inclusions in mantle xenoliths from the TihanyVolcanic Complex, Bakony–Balaton Highland Volcanic Field, CentralPannonian Basin, western Hungary, contain numerous solid (daugh-ter) phases, documenting the chemical complexity of CO2-rich fluidsin the mantle.

Magnesite and quartz were detected and interpreted to haveformed after entrapment via a carbonation reaction involving theCO2-rich fluid and the host orthopyroxene. The fluids in the inclusionsstarted to react when the mantle xenolith enclosed in the coolingbasaltic lava intersected the reaction curve at about 400–600 °C.

Volume proportions of various daughter phases were preciselydetermined using the FIB-SEM technique. The proportions of themagnesite and quartz step-daughter phases ranged from 3.3 to 5.0and from 2.6 to 4.2 vol.%, respectively; whereas the sulfide rangedfrom 1.3 to 1.9 vol.%. FIB-SEM analyses also revealed a 100–200 nmthin glass film covering the entire wall of the fluid inclusions. Thisfinding supports the hypothesis that CO2-rich fluids in the lithospher-ic mantle are also transporting significant amounts of dissolved sili-cate components. The precipitation of a glass film on the inclusionwalls prevents or at least arrests the completion of the carbonation

19M. Berkesi et al. / Earth and Planetary Science Letters 331-332 (2012) 8–20

reaction. It is likely that the presence of the glass film might be a com-mon feature in mantle fluid inclusions.

The strong correlation between the trace element abundances inpyroxenes and the presence of fluid inclusions indicates metasoma-tism of the peridotite xenoliths by a volatile-rich phase. Entrapmentof a one phase, supercritical C–O–H–S-bearing fluid phase occurredin the lithospheric mantle. This fluid underwent phase separationduring uplift to the surface, resulting in the formation of daughterphases and the precipitation of the glass film onto the wall after thefluid inclusions were trapped in the mantle. Our results show that sil-icate components dissolved in a supercritical, CO2-rich fluid is themost likely explanation of the ability of mantle fluids to transporttrace elements and produce cryptic metasomatism and chemicaloverprinting of the peridotite wall rock.

Supplementary materials related to this article can be foundonline at doi:10.1016/j.epsl.2012.03.012.

Acknowledgments

The authors thank Prof. Jacques Touret and the editor, Dr. BernardMarty, for many valuable comments and suggestions that helped toimprove the quality of this paper. This project was financially sup-ported by the European Union and co-financed by the EuropeanSocial Fund (grant agreement number: TAMOP 4.2.1/B-09/1/KMR-2010-0003) to M. Berkesi, T. Guzmics and C. Szabó and by the FrenchEmbassy in Hungary toM. Berkesi. M. Berkesi, C. Szabó and T. Guzmicswere additionally supported by the Hungarian-French Bilateral Affair(proposal number: FR-10/2008 to C. Szabó) and János Bolyai ResearchScholarship of the Hungarian Academy of Sciences, respectively. Theauthors owe thanks to Luca Fedele (Virginia Tech, USA) for helpduring the LA–ICP-MS analysis, as well as to Gábor Varga (EötvösUniversity, Hungary) for the FIB-SEM analyses. For Raman spectrosco-py, Marie-Camille Caumon (G2R laboratory, Université de Lorrainne,France) and Balázs Vajna (University of Technology and Economics,Hungary) are thanked. Zsolt Bendő (Eötvös University, Hungary) isacknowledged for his help in sample preparation. This publication isno. 58 of the Lithosphere Fluid Research Lab.

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