PIXE mapping on multiphase fluid inclusions in endoskarn ...periodicodimineralogia.it/2013_82_2/2013PM0017.pdf*Corresponding author: [email protected] Abstract In this work we

An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYand applied topics on Environment, Archaeometry and Cultural Heritage

DOI: 10.2451/2013PM0017Periodico di Mineralogia (2013), 82, 2, 291-297

PERIODICOdi MINERALOGIAestablished in 1930

Introduction

Magma-derived fluids are important in geologicprocesses (metal sequestration and ore deposition)but are intrinsecally transient. Fluid inclusionstrapped and sealed in crystals are by far the mostrepresentative of fluid compositions that existedduring and after crystal growth (Roedder, 1979).As a consequence fluid inclusion investigation inmany cases represents the unique opportunity toobtain data on these fluids.

In the last years, fundamental informations

about the major and trace element compositionof magmatic hydrothermal fluids have beenprovided by in situ analysis of individual fluidinclusions by several tecniques (PIXE, LA-ICP-MS, Synchrotron X-Ray fluorescence, Heinrichet al., 1992, 2003; Audetat et al., 1998; Ulrich etal., 1999, 2002; Philippot et al., 2001; Vanko etal., 2001; Kamenetsky et al., 2002; Kurosawa etal., 2003). Here we apply PIXE tecnique to fluidinclusions present in endoskarn xenoliths fromAD 472 subplinian eruption of Vesuvius(Fulignati et al., 2001).

PIXE mapping on multiphase fluid inclusions in endoskarn xenoliths of AD 472 eruption of Vesuvius (Italy)

Paolo Fulignati1,*, Vadim S. Kamenetsky2, Paola Marianelli1 and Alessandro Sbrana1

1Dipartimento di Scienze della Terra, Via S. Maria 53, 56126 Pisa, Italy2School of Earth Sciences and Centre for Ore Deposit Research, University of Tasmania,

GPO Box 252-79, Hobart, Tasmania 7001, Australia*Corresponding author: [email protected]

Abstract

In this work we report a microthermometric and proton-induced X-ray emission (PIXE)mapping investigation on multiphase fluid inclusions hosted within nepheline andclinopyroxene of endoskarn xenoliths present in the deposits of the AD 472 eruption ofVesuvius. PIXE mapping on magmatic fluid inclusions repesents a useful tool for thecharacterization of the composition of magma derived fluids, exsolved from active magmachambers. In fluid inclusions we observed the occurrence of widespread solid phases formedby Fe, Pb, Zn, As ± Cu ± Mn, suggesting the good metal transport capability of Vesuviusmagmatic fluids, which interacted with carbonate country rocks leading to the formation ofendoskarn.

Key words: fluid inclusions; PIXE; Vesuvius magma chamber; exsolved fluids.

Fulignati et al._periodico 13/09/13 11:55 Pagina 291

P. Fulignati, V.S. Kamenetsky, P. Marianelli and A. Sbrana292

The aim of this paper is to provide furtherinsights into the physical nature and compositionof magma-derived fluids represented bymultiphase fluid inclusions, present in endoskarnxenoliths.

The AD 472 magma chamber of VesuviusSomma-Vesuvius is a K-alkaline Quaternary

stratovolcano of southern Italy, with a sommacaldera (Mount Somma) and an inner cone(Vesuvius). During the past 18 ky severalexplosive eruptions of different magnitude,including at least four Plinian and six sub-Plinianoutburst, have occurred. All the main eruptionshave both Plinian phases and important phasesdominated by magma-water interaction, whichalways involves fluids hosted in the deepMesozoic carbonate formations or in the shallowvolcanic sequences. The involvment ofhydrothermal fluids from a thermometamorphichalo is sometimes suggested by the presence ofaltered lithics and their fluid inclusions. Thedeposits of each explosive eruption show both falland flow deposits. Fall deposits are variablystratified pyroclastics surges andphreatomagmatic, lithic rich pyroclastic flowsoccur frequently during major events. Thephreatomagmatic phases of subplinian eruptionsgenerally occur during the final stages of thesustained column.

The AD 472 eruption products vary incomposition from early stage leucititic phonoliteto late stage K-phonotephrite (Rosi andSantacroce, 1983). The compositional zoning ofproducts is interpreted to reflect regularwithdrawal from a compositionally and thermallystratified shallow (< 5 km) magma chamber(Cioni et al., 1998). The solidification front of themagma chamber (in the sense of Marsh, 1995)consists of foid-bearing syenites andclinopyroxenites and grades continously intocarbonate country rocks through a skarn shell(Fulignati et al., 2001, 2004). The estimatedtemperatures for the solidification front range

from < 850 °C (upper part of the magma chamber,foid-bearing syenites) to ~ 1150 °C (lower part ofthe magma chamber, cumulate fromclinopyroxenites to olivin clinopyroxenites).

Analyses of melt and fluid inclusions fromjuveniles and foid-bearing syenitic xenolithsreveal that a hydrosaline fluid phase wasexsolved from the upper parts of the magmachamber (Fulignati et al., 2001; Fulignati andMarianelli, 2007) and was involved in skarngenesis in the carbonate-bearing peripheralportions of the magmatic reservoir (Fulignati etal., 2001, 2004; Gilg et al., 2001). In particular,the occurrence of fluid inclusions in skarnxenoliths, from AD 472 eruption, indicatescirculation of a magmatic hypersaline fluid phasein the peripheral upper parts of magma chamber.The multiphase (carbonate-bearing) meltinclusions are thought to result from a magmatic-derived hypersaline fluid that reacts withcarbonate wall rocks (Fulignati et al., 2001),inducing carbonate melting through sintecticreactions (Lentz, 1999). This complex fluid (Na-K-Ca-carbonate-chloride-rich hydrosaline melt)metasomatizes the rigid crust generating atypical endoskarn. The paucity of fluid inclusion-bearing xenoliths suggests that the exsolution ofa hypersaline fluid phase is probably notextensive in the peripheral parts of the AD 472chamber. As a consequence the exsolved fluidsare mainly confined at the solidification front-carbonate wall rock interface of the magmachamber.

Analytical methods

Microthermometric experiments on fluidinclusions were carried out in the fluid inclusionlaboratory of the Earth Sciences Department ofthe University of Pisa with a Linkam TS 1500heating stage and an optical heating stage,designed in the Vernadsky Institute ofGeochemistry, Moscow (described by Sobolev etal., 1980). The accuracy of measurements was

Periodico di Mineralogia (2013), 82, 2, 291-297


PIXE mapping on multiphase fluid inclusions ... 293

around ± 10 °C for both stages, controlled by themelting point of pure silver, gold and K2Cr2O7.SEM-EDS analyses were performed using aPhilips XL30 apparatus equipped with EDAXGenesis (Earth Sciences Department, Universityof Pisa) at 20 kV accelerating energy and about0.1 nA beam current. The inclusions were openedby fracturing the host crystals in air. The sampleswere carbon coated and analyzed immediatelyafter, with the aim to prevent as much as possiblethe lost of the daughter minerals.

Proton-induced X-ray emission (PIXE)methodology have been used for thecharacterization of the composition of our fluidinclusions. PIXE is a non destructive tecniqueconsisting of bombarding the sample with a highenergy beam of protons. Protons eject inner shellelectrons wich produces an X-ray emissioncharacteristic for each element. The focussedbeam penetrates the mineral (around tenmicrometers depth) and interacts with speciescontained in the fluid inclusions without destroingthem. Due to the absorption of soft X-rays by thehost crystal PIXE can be used for analysis ofelements with atomic number higher than 30. Inthis work qualitative PIXE maps of fluidinclusions were obtained by using a 0.3-0.7 nAbeam of 3 MeV protons focussed into ≈ 2µmbeam with new nuclear microprobe CSIRO-GEMOC, Australia (Ryan et al., 2001a, b).

Results

Fluid inclusions were found in rare skarnxenoliths present in the lithic rich layers fromproximal outcrops of the deposits of the AD 472eruption of Vesuvius. These xenoliths areholocrystalline rocks and consist of fassaiticclinopyroxene, phlogopite, nepheline, calcite andapatite. On the basis of previous studies of themagma chamber wall rock interface, weinterpreted these rocks as endoskarn. Theserocks are in fact produced by the interaction ofthe rigid crust of the magma chamber with

magmatic-derived hypersaline fluids, whichhave reacted with carbonate wall rock, givingrise to Na-Ca-K-carbonate-chloride richhydrosaline melt (Fulignati et al., 2001, 2004).

Based on petrographic observation, the shapeand orientation of inclusions is consistent withentrapment during crystal growth, and thus thestudied inclusions are identified as primary,following criteria provided by Roedder (1984)and Goldstein (2003). The studied inclusions arehosted within nepheline and clinopyroxene.They are multiphase fluid inclusionscharacterized by a vapor bubble, deformed bythe occurrence of several daughter minerals, anda very small amount of interstitial liquid that israrely visible under microscope (Figure 1).Chlorides, silicates, sulfates, carbonates, sulfidesand oxides are always present among thedaughter minerals (Figure 2), in some casesfluorides also occur (Fulignati et al., 2001).

Microthermometric experiments on theseinclusions were carried out showing thatdaughter minerals totally melt at 800-830 °C(Figure 3). Vapor bubble homogenization occursonly in a few fluid inclusions between 860-885°C. These homogenization temperatures are


Figure 1. Microphotographs of multiphase fluidinclusion in endoskarn xenoliths. Plane polarizedlight. Scale bar 40 µm.



comparable with the conditions of skarn genesis,as reported in Fulignati et al. (2004), or slightlyhigher, as some homogenization values areprobably overestimated due to the deformationof the cavity of the inclusions during heatingexperiments. Some solids (possiblyheterogeneously trapped silicates) often persistto higher temperature (840-980 °C, Figure 3).

Nuclear microscopy by PIXE was used toimage element distribution of several fluidinclusions within the root zone of the nephelineand clinopyroxene crystals. PIXE maps werecarried out on single multiphase fluid inclusions.PIXE element images of typical inclusions(Figure 4) always show the occurrence of metalsand that individual solid phases were variable interms of metal content. For example, werecognized solids enriched in Fe, Pb, Zn ± As ±Cu ± Mn (Figure 4A, B, C, D); Fe, Mn (Figure 4B); Mn, Zn (Figure 4D); and Mn (Figure 4D).The composition of the liquid phase is undefinedowing to its scarcity at room temperature.

Discussion and Conclusions

Processes of exsolution of an aqueous fluidphase during late magmatic differentiation playan important role either in volcanology and inthe generation of magmatic-hydrothermal oredeposits (Hedenquist and Lowenstern, 1994).Magma-derived fluids are however intrinsicallytransient as regard their chemical composition(Kamenetsky et al., 2002). The rare opportunityto describe their original composition is offeredby fluids exsolved from active magma chambers,which are entrapped during crystal growth asfluid inclusions. These entrapped magmatic-derived hypersaline fluids are thought to be the


Figure 3. Hystograms of: a) temperature of halitehomogenization; b) temperature of vapor bubblehomogenization and last solid disappearance. n = number of measurements.

Figure 2. Scanning electron microscope images ofexposed multiphase fluid inclusions, found inendoskarn xenolith, containing chlorides [halite (Hal)and sylvite (Syl)], sulfates [glaserite (Gla) and arcanite(Arc)], and Na-Ca carbonate (Carb). Scale bar is 20µm.


PIXE mapping on multiphase fluid inclusions ... 295Periodico di Mineralogia (2013), 82, 2, 291-297

Figure 4. Optical images and proton-induced X-ray emission (PIXE)element maps of individualmultiphase fluid inclusions. Colorscale (from dark to white) in eachelement image is normalized to itsown maximum. Increasing intensityis proportional to elementconcentration. Optical images showinclusions before PIXE analysis.Green outlines on element maps markboundaries of fluid inclusions. Solidsenriched in Fe, Pb, Zn ± As ± Cu ±Mn (Figure 4A, B, C, D); Fe, Mn(Figure 4B); Mn, Zn (Figure 4D); andMn (Figure 4D). Scale bar represents 20 µm.



result of the interaction between hydrosalinefluids and carbonate wall rock, giving rise to Na-Ca-K-carbonate-chloride rich hydrosaline melt.

Coupled microthermometry and trace elementanalysis of single fluid inclusions are a usefultool to trace the processes that affect magmaderived fluids exsolved from active magmachambers. The high temperature ofhomogenization of investigated fluid inclusions(860-885 °C) supports a magmatic origin for thefluids involved in skarn reaction at the peripheralportions of AD 472 magma chamber ofVesuvius. PIXE analysis allow mapping traceelement distribution of single fluid inclusionspresent in endoskarn xenoliths. The occurrenceof solid phases formed by Fe, Pb, Zn, As ± Cu ±Mn suggests that these elements were abundantin the fluids exsolved from the AD 472 magmachamber. This testifies the metal transportcapability of Vesuvius magmatic fluids, whichinteracted with carbonate country rocks leadingto the formation of endoskarn, confirming thepreliminary data reported in Fulignati et al.(2001). These results, coupled with recentlypublished data on composition of multiphasemagmatic fluid inclusions, representative of thefluid exsolved from peripheral parts of themagma chamber of AD 79 eruption (Fulignatiet al., 2011), indicate that hypersaline fluidphases released by the crystallizing phonoliticupper parts of the sub-Plinian and PlinianVesuvius magma chambers have the capabilty tocarry ore-forming elements. The exsolution ofmetal enriched fluids from Vesuvius magmachambers well agree with the occurrence ofscheelite, stibnite and galena mineralizationfrequently found in skarn xenoliths from thedeposits of Plinian eruptions (Fulignati et al.,2005). These results represent clear evidence onthe involvment of this metal-enriched magmaticfluids during skarn forming processes atVesuvius, although there are no evidence ofmetal ore deposits around the volcano.

Acknowledgements

Many thanks are due to Masanory Kurosawaand Hans-Joachim Massonne for theirconstructive comments and suggestions thatimproved the quality of the manuscript.

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Submitted, January 2013 - Accepted, May 2013




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PIXE mapping on multiphase fluid inclusions in endoskarn ...periodicodimineralogia.it/2013_82_2/2013PM0017.pdf*Corresponding author: [email protected] Abstract In this work we