Magmatic remnants in plutonio rocks

Séance spécialisée : Granitea océaniques et granites continentaux Paris, 11-12 mars 1991

Bali. Soc. géoì. France, 1993. t. 164, n° 2, pp. 229-242

Magmatic remnants in plutonio rocks by JACQUES L.R. TOURET and MARIA LUCE FREZZOTTI* Kc\ - Magmatic remnants, Granites, Immiscibility, Silicate melt inclusions. Abstract. - Direct evidence of magmatic activity in plutonio rocks is generally obliteratcd by hydrothermal alteration. Systematic studies on shallow intrusives, especially those emplaced in water-rich environments, led to thè common belief that thè transition from magmatic lo submagmatic stages is not visitale. However. remnants of magmatic inclusions, obvious in effusive rocks, havc been preserved in some deeper rocks, especially when they cooled slowly in a dry environmcnt. Such remnants, under thè name of "storie cavities" have been described by Sorby in 1858 in a number of granites, notably from Cornwall and Scotland (St. Austell granite). The interpretation of magmatic remnants in plutonio rocks is not easy because of advanced evolution. The following investigation steps are proposed and discussed : (1 ) Identification of magmatic textures in rock-forming minerai» and selcction of favourable samples ; (2) detailcd petrographic study of possible traces of formcr melt inclusion and characterization of post-trapping inclusion evolution, including melt crystallization and eventual interaction with late aqueous fluids ; (3) microthermometric studies al high tempcraturcs (800-1000°C) and electron-microprobe analyses on quenched inclusions ; chemical compositions should be compared with bulk rock analyses. More than magmatic trends, melt inclusions may record magmatic immiscibility phenomena (silicate melt/brincs, notably), which cannot be detected by any other method. Two examples (Mount Genis granite, SE Sardinia, Italy and Sybille monzosyenite, Laramie anorthosite complex, Wyoming, USA.) are briefly reported and discussed.

Traces d'activité magmatique dans les roches plutoniques Mota clés. - Restes magmatiques, Granites, Immiscibilité, Inclusions vitreuses. Résumé. - Dans les roches plutoniques, notamment granites, toute trace directe d'activité magmatique est en generai oblitérée par l'altération hydrothermale. Les études sur les intrusions a faible profondeur, mises en piace au niveau des eaux météoriques, ont conduit a penser généralement que la transition magmatique-submagmatique n'est plus visible. Cependant, des restes d'inclusions magmatiques, bien connues dans les roches effusives, peuvent ètre préservés dans des roches profondes, surtout lorsque celles-ci se soni refroidies lentement dans un environnement sec. Sous le nom de "stone cavities", ils ont été décrits par Sorby en 1858 dans un certain nombre de granites de Cornouailles et d'Ecosse (St. Austell granite). Leur interpretation n'est pas simple, en raison de leur evolution avancée. Pour leur elude, les étapes suivantes soni proposées et discutées : (1) Identification des textures magmatiques dans les roches plutoniques et sélection des échantillons favorables ; (2) Elude pétrographique détaillée des restes possibles d'inclusions magmatiques et caractérisation de leur evolution post-capture, notamment les phénomènes de dévitrification et les interactions avec des fluides hydrothermaux tardifs ; (3) microthermométrie a haute temperature (80f>1000°C) et analyse (microsonde électronique) des inclusions homogénéisées et trempées. Les analyses sont comparées a celles de la roche totale, doni elles diffèrent en generai de facon significative. Plus que la reconstruction de l'évolution magmatique, les restes d'inclusions vitreuses peuvent permettre de déceler des phénomènes d'immiscibilité magmatique (surtout magma silicaté/saumures), qui ne peuvent étre reconnus par aucune autre technique. Deux exemples sont présentés et discutés : granite de Mte Genis, SE Sardaignc, Italie, et monzosyenite de Sybille, complexe anorthositique de Laramie, Wyoming, USA.

I. - INTRODUCTION

Experimental studies on granitic systems have provided importarti tools in modelling magmatic processes fe.g. Keevil, 1942; Tuttle and Bowen, 1958; Luth et al., 1964; Jahns and Burnham, 1969; Whitney, 1975a, 1975b; Burnham, 1979; Burnham and Ohmoto, 1980; Burnham and Nekvasil, 1986]. They stress thè role of parameters such as temperature, pressure and volatile activities (e.g. water and uncondensable gases, i.e. CCn) during magma crystallization (e.g. Burnham and Jahns, 1962; Kilink and Burnham, 1972; Kadik and Eggler, 1974; Whitney, 1977, 1984, 1988; Andersen and Lindsley, 1985; Price, 1985; Nekvasil and Burnham, 1987; Bodnar and Cline, 1990; Ebadi and Johannes, 1991].

Although theory and experiments allow us to model thè crystallization mechanisms, they can never approach a naturai System. They must therefore be completed with observations on naturai rocks. A direct study of magmatic phenomena can be carried out on evolved melt inclusions which are present in many minerals of plutonic rocks. Silicate melt inclusions are droplets of thè silicate melt (magma or lava), enclosed in thè diffèrent minerals during their growth [Vogelsang, 1867; Deicha, 1955]. Such microsystems preserve an unique information on thè physico-chemical conditions of thè magma at thè moment when thè inclusion was formed. Their study has greatly progressed with thè development of modern technologies such as hightemperature microthermometry [e.g. Yermakov, 1950] and in situ non-destructive microanalysis [Ciocchiatti, 1975].

* Instituut voor Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085. 1081 HV Amsterdam (The Netherlands). Manuscrit déposé le 17 octobre 1991, accepté le 12 octobre 1992. Bull. Soc. aéol. Fi:, 1993, n° 2

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Basic principles bave recently been reviewed in a number of papers, notably by Roedder [1984]. Inclusions may provide information on thè chemistry of melt and magmatic evolution [Clocchiatti, 1971, 1975; Roedder, 1979; Xia, 1984], on thè temperature of lavas, [Sobolev and Kostyuk, 1975; Naumov, 1979, 1988; Hansteen, 1991], and on volatile/magma interactions [Anderson, 1974; Anderson et al, 1989; Metrich and Clocchiatti, 1989], etc... Although first recognized more than 100 years ago [Sorby, 1858], magmatic remnants received little or no interest in rocks from plutonic environments. One might wonder why thè study of silicate melt inclusions in plutonic rocks is not included in thè normal scope of petrological investigations. There are many possible reasons, but one point might account for ali : thè idea that magmatic stage evolution cannot be preserved in plutonic rocks, especially in those emplaced in water-rich environments. In these shallow intrusives stable isotopes very often indicate strong meteoric signatures and complex fluid-rock interactions, confirmed by fluid-inclusion studies [e.g. Weisbrod, 1981]. In thè present review we provide an account of silicate melt inclusion investigations in plutonic rocks as a basis for understanding thè potential interest of these microsystems. Some important results on lavas or very shallow intrusives, in which inclusions are generally well preserved, are discussed. Some generai suggestions are then given for thè reconnaissance, thè analysis and thè interpretation of magmatic remnants in rocks emplaced at greater depths.

II. - SILICATE MELT INCLUSIONS IN EFFUSIVE ROCKS AND SHALLOW INTRUSIVES : SOME BASIC FEATURES

Silicate melt inclusions represent ideai unbiased samples of thè magma trapped at a specific step of thè crystallization history, on condition that they remained closed (i.e. no interactions with thè host minerai and/or circulating fluid phases). Petrographic investigations are fundamental for thè characterization of these microsystems in volcanic rocks. Complete studies require high-temperature (800-1000°C) microthermometry (determination of phase transitions on heating under thè microscope) and chemical analysis of thè glass. [Specific principles of these techniques have been reviewed in a number of publications ; e.g. Clocchiatti, 1975 ; Clocchiatti and Massare, 1985; Roedder, 1979, 1984; Xia. 1984 and will not be further discussed]. Thus, thè analysis of silicate melt inclusions (fig. 1A to D) in effusive rocks might provide information on : (1) thè temperature of magma trapping, by determination of homogenization temperature (Th), temperature at which silicate melt, gas bubble and eventually, daughter minerals become a single, homogeneous phase ; (2) thè composition of thè glass, including major, minor and trace elements [Webster and Duffield, 1991] and volatiles (H2O ; CO2 ; CI; S; etc.) by electron-, ion- microprobe and Raman or infrared microspectroscopy ; (3) thè identification of magmatic phenomena, in particular heterogeneous trapping (solid/melt) and immiscibility (melt/melt; fluid/melt). Bull. Soc. géol. Fr., 1993, n° 2

A) Magmatic immiscibility Immiscibility processes frequently occur during thè magmatic evolution of igneous rocks ; different types including silicate-carbonate melt, silicate-sulfide, silicate-silicate and silicate-fluids immiscibility, have been identified [Roedder, 1984, p. 27 and reference therein; 1992; Amundsen, 1987; Andersen et al., 1988; Dalabakis, 1987]. When two (or more) fluid phases coexist at thè time of thè inclusion formation, this heterogeneous mixture will be trapped with very different ratios of thè coexisting phases in contemporaneous inclusions (= heterogeneous trapping). Roedder and Weiblein [1970, 1979] reported spectacular silicate melt immiscibility in plagioclases and pyroxenes from lunar basalts. Two contrasting glasses (a low index "high-silica melt" and a high-index brownish "high-iron melt") coexist in thè same inclusion, separated by a sharp meniscus : they represent two mechanically trapped immiscible magmas. Silicate melt immiscibility within single inclusions may also occur after thè formation of thè inclusions (= "in situ" immiscibility ; thè magma was trapped as a homogeneous fluid phase and immiscibility occurred during thè subsequent evolution). It will result in Constant proportions of thè different phases, as reported by Clocchiatti and Krasov [1979] in plagioclase phenocrysts from andesitic lavas (Karimsky volcano, Kamchatka, USS) : early plagioclase crystallization led to thè formation of two immiscible silicate melts of contrasting composition, occurring at room temperature as globules of dark pyroxenite glass enclosed in transparent acid silicate glass. "/n situ" immiscibility is a rather common phenomenon in silicate melt inclusions. It has often been described in alkali-basalts, in which globules of sulfide melt unmix from thè silicate glass [Roedder, 1965; Clocchiatti et al., 1979; De Vivo et al., 1988]. In a comparatale view, if thè magma was oversaturated in volatiles at thè moment of thè inclusion formation, these will contain variatale proportions of glass and fluids. This specific case of heterogeneous trapping corresponds to "magma boiling" [Burnham, 1979]. A classical example has been reported by Roedder and Coombs [1967] in granitic blocks from Ascencion Island. Silicate melt inclusions coexist with fluid inclusions ; in some cases mixed silicate glass + brines are observed (fig. 1B). These indicate that thè host K-feldspar and quartz crystallized in a volatilesaturated magma. Thus, they allow to determine thè composition of magmatic fluids. These results have later been confirmed in other hypo-abyssal granites and porphyries [e.g. Reyf and Bazheyev, 1977; Clocchiatti and Nativel, 1984; Harris, 1986; Clocchiatti et al., 1990; Cline, 1991]. They show that concentrated brines (sai. > 50% eq. wt) can be directly exsolved from crystallizing magmas, as it will be discussed below. B) Volatile content of thè magma Silicate melt inclusions have provided some fundamental information on thè evolution of volatile phases in effusive rocks. Volatiles may be dissolved in thè glass and/or occurring as free phases. As discussed above, fluid phases may result from magmatic immiscibility (heterogeneous trapping of silicate melt and fluids). More generally, how-

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Fio. 1. - Silicate meli inclusions in volcanic rocks and shallow intrusives : A) silicatc meli inclusions in a quartz crystal from a volcanic rock, showing incipient recrystallization (arrow). B) mixed silicate glass + brines inclusion in a quartz crystal from thè syenite of Ascencion Island (Roedder and Coombs, 1967]. C) decrepitatcd, high volatile silicate meli inclusion in a quartz crystals of thè microtonalite porphyry System of Calabona (Sardinia, Italy) [Frezzotti et al., 1992], The peculiar shape of thè cavity limited by a series of circle arcs convex towards thè centre of thè inclusion (arrow I) suggest epitaxial quartz growth on Ihe walls under some relatively high internai pressure. Arrow in inset (II) indicates thè expelled aqueous fluids reorganized as fluid inclusions. Length of thè bar in (im. D) Silicate meli inclusion in a clinopyroxene phenocryst, in alkalibasalt from Mount Etna. Fio. 1. - Inclusioni vitreuses silicatées dans les roches volcaniques et intrusives peu profondes. A) inclusion vitreuse dans un quartz d'une roche

de /'inclusion, suggère une recristallisation épitaxiale du quartz sur les parois sous une pression interne relativement élevée. La flèche (II) ìndique les fluides magmaliques réorganisés en inclusions fluides dans le minerai hòte. Longueur du segment en lini. D) inclusion vitreuse silicatée dans un clinopyroxene d'un basa/te alcalin du M. Etna.

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ever, volatile saturation occurs after thè inclusion formation, during post-trapping cooling ("in situ" immiscibility). In this case thè internai evolution of an inclusion approaches that experienced by thè magma during crystallization [see e.g. Burnham, 1979; Whitney, 1975b]. Solidification and/or crystallization of daughter minerals increases thè volatile content of thè residuai melt. When saturation is reached, free fluid phases or new secondary daughter minerals will forni inside thè gas bubble. Volatile enrichment in thè residuai glass after thè formation of daughter minerai phases may be extreme. Silicate melt inclusions in clinopyroxene phenocrysts of alkali-basaltic lavas from Mount Etna [Frezzotti et ai, 1991] show chlorine contents as high as 1 w t % in thè residuai inclusion glass. These values might nave been higher, as Cl-rich apatite crystals are present in thè shrinkage bubble. If thè trapped fluid pnase is water-rich, oversaturation within single inclusions will be accompanied by a great pressure increase and/or by thè crystallization of JHbO-bearing minerals. During cooling, thè pressure of newly formed aqueous fluids can be sufficiently high to cause decrepitation (Pini. » Pext.). This will result in thè expulsion from thè initial inclusion of an aqueous fluid reorganized as tiny secondary fluid inclusions oriented around thè cavity apices (fig. le). In most cases, however, fluid phases are difficult to detect in thè inclusions, especially condensable gases. The density of a fluid phase depends mainly on thè quantity of gas and thè pressure at thè time of trapping. As thè temperature in magmatic systems is very high (800-1000°C), fluid densities can only be significant at relatively high pressures (V = RTP"1). At low pressures (i.e. trapping at shallow depths ; P < 1 kbar) thè resulting fluid density is so low that even sophisticated analytical techniques will not detect any free volatile. An indirect way to identify thè presence of fluid phases is to judge thè relative dimension of thè bubble with respect to thè glass. A volatile-rich magma yield to relatively large bubbles at room temperature (shrinkage bubble + gas). Theoretically thè following distinction can be achieved by simple microscopie observation (fig. 2) : 1 ) Undersaturated magma : inclusions are characterized by high and Constant glass (Vl)Tbubble (V2) volume ratios (i.e. > 0.9 : shrinkage bubble). 2) Saturateci magma : a) homogeneous trapping : inclusions are characterized by Constant and relatively low glass (Vl)/bubble (V2) volume ratios (i.e. « 0.9). Daughter minerals and low-density fluids are often observed in thè bubble ; b) heterogeneous trapping : inclusions are characterized by extremely variable glass (Vl)/bubble (V2) volume ratios. Condensable gases (notably COz) are generally present. Observations must rely on a statistically representative number of inclusions. Bubbles in silicate melt inclusions correspond to thè contraction coefficient of glass, depending mainly on its chemical composition and volatile content. However, Constant and relatively low glass/bubble volume ratios in silicate melt inclusions might also be thè result of post trapping evolution (i.e. formation of internai fractures, epitaxial growth of thè host minerai on thè cavity walls, see next paragraph). Heterogeneous trapping (b) is Bull. Soc. géol. Fr., 1993, n° 2

o

V2

a +

C

bo

e •

FIG. 2. - Magmatic immiscibility as recorded by silicate melt inclusions. Left side : different bubble dimensioni according to thè volatile content. a) undersaturated magma, b) and e) saturated magma : homogeneous (b) and heterogeneous (e) trapping. Righi side : plot of thè glass (Vl)/vapor bubble (V2) volumetrie ratios in different inclusions. Homogeneous trapping is characterized by Constant V1/V2 ratios in different inclusions. Fio/ 2. - Mise en évìdence d'immìscibìlilé magmatique dans les inclusions vitreuses. A gauche : bulles de taille variable, en fonction de la leneur en vola! i Ics du magma, a) magma sous-saturé b) et e) magma sature : captare homogène (b) et hétérogène (e). A droite : diagramme des rapports volumétriques du verre ( V I ) et de la bulle gazeuse (V2) dans differente! inclusions. Une caplure homogène est caractérisée par un rapport V1/V2 Constant.

stili thè best criterion to characterize magmatic immiscibility with thè typical association silicate melt inclusions + high density CCh inclusions + ali stages of thè transition melt-fluid inclusions (in basic magmas) and silicate melt inclusions + fluid H2O + NaCl ± CCh (in acid magmas). III. - SILICATE MELT INCLUSIONS IN PLUTONIC ROCKS : IDENTIFICATION OF POSSIBLE

REMNANTS

A) Early studies Investigations on silicate melt inclusions in plutonic rocks were started by Sorby in 1858. This classica! paper is often cited for fluid inclusions, but was in fact more devoted to thè study of magmatic remnants. Observations concern essentially two rock types : lavas (Vesuvius) and granites and porphyries from Cornwall (in particular St. Austell) and Scotland. Sorby indicated that some differences should be expected between magmatic remnants in lavas and in deeply emplaced granites. He proposed a fundamental distinction between "glass cavities" in effusive rocks and "stone cavities" (fig. 3) in plutonic rocks, and he drew a continuous evolution for silicate melt inclusions from volcanic to plutonic environments. Recrystallized inclusions in granitic rocks ("stone cavities" ; fig. 3), were correlated to melt inclusions ("glass cavities") in volcanic environments ; their "stony" aspect was interpreted as thè result of crystallization of glass originally present in thè inclusions. Sorby proposed two main investigation steps : (1) thè Identification of thè inclusion discriminating petrographic characters and (2) thè characterization of their evolution after trapping. These points are stili relevant today, and they should be taken into account for any further study.

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be lost. Interaction with aqueous fluids changes thè chemical composition of thè glass : in extreme cases inclusion cavities are filled with argillic minerals. Such phenomena, rather rare in lavas, are frequently observed in subvolcanites [Clocchiatti, 1975]. However, inclusions may be preserved in subvolcanic rocks, even if they have experienced a prolonged hydrothermal fluid circulation. A systematic petrographic investigation of quartz and plagioglase phenocrysts from drill-hole samples (max. depth 500 m) of a microtonalitic porphyry copper system in Sardinia (Calabona) has revealed thè presence of silicate melt inclusions at ali depths. Extremely evolved inclusions occur in thè deeper zones [potassic zone of Lowell and Guilbert, 1970] where extensive circulation of brines occurred for a long lime at high temperatures (> 500°C), whereas well preserved magmatic remnants (transparent and isotropie glass, see fig. 1C) occur in thè quartz crystals of thè most external zones (propilitic alteration), where colder and diluted water dominated [Frezzotti et al., 19921.

FIG. 3. - Sorby's representalion of silicata meli and fluid inclusions in granitic rocks. [Redrawn from Sorby, 1858J. Fio. 3. — Représentation par Sorbv d'inclusions vitreuses et fluides dans les granites [redessiné d'après Sorby, 1858].

As a matter of fact, it is remarkable that so little has been done since Sorby. B) Evolution of melt inclusions from volcanic plutonio environments

to

During and after magma solidification, a number of processes take piace in thè inclusions, depending on thè cooling history of thè rock. In volcanic environments, silicale melt inclusions show little or no sign of internai evolution. These generally consist of glass ± one or more gas bubbles ± daughter minerai phase(s) (fig. 1A). Glass is transparent and isotropie, and has a homogeneous composition. On slow cooling, however, crystallization of glass may occur to variatale extent : incipient processes cause thè formations of microcrystals (fig. 1À), more advanced processes lead to thè formation of single crystals or groups of daughter minerals and/or to an epitaxial growth of thè host minerai on thè cavity walls (fig. 1C). This internai evolution may occur without exchanges with thè external environment [magmatic evolution, Clocchiatti, 1975]. The crystallization processes are strongly dependent on thè inclusion size : in a given sample, smaller inclusions will be more easily preserved than larger ones [Roedder, 1979]. If circulating fluids interact during cooling with existing inclusions, their originai chemical magmatic signature will

A'

B'

C'

D'

Fio. 4. - Schematic drawing of typical after-trapping evolution of lowvolatilc silicate melt - (A) and high-volatilc silicate melt inclusions (A') in plutonic rocks. The AD sequence (a quartz) is less evolved than thè A'D' sequence (P quartz). During cooling of thè rocks, glass devitrifies to a different extent and daughter minerals may form in thè inclusions (B-D). These overall processes brings to a variation in thè inclusion aspect and shape (D and D'). (Further explanations in text). FIG. 4. - Représentation schématìque d'évolution post-magmatique d'un magma pauvre (A) ou rìche (A') en volatila, respectivernent. La sèrie AD (quartz a) est beaucoup moins évoluée que la sèrie A'D' (quartz P). Ali cours du refroidissement le verre se dévitrifie de fafon variable et des minéraux fils peuvent se former (B-D). Ces processux entraìnent une variation de l'aspect et de la forme des inclusions (D et D'j. (Pour le détail de la discussion, cf. text). Bali. Soc. géol. Fr.. 1993, n° 2

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As a slow cooling history enhances ali evolution processes, it can be expected that an extremely complex after-trapping evolution will characterize magmatic remnants in plutonio rocks. Additional effects, notably thè reequilibration of thè host minerals during post-magmatic evolution (i.e. recrystallization of quartz, perthitization of K-feldspar) may lead to a change of shape or to thè almost complete disappearance of magmatic remnants. As a result, rests of silicate meli inclusions in intrusive rocks are very different from those in lavas, and this might be one of thè reasons why they are ignored in most studies. Figure 4 schematically represents this evolution : during successive steps of post-magmatic evolution thè glass becomes more irregular and brownish ("devitrification"). The growth of microcrystals may cause an irregular anisotropy (note that, in ali cases, glass isotropy is very difficult to recognize in thick sections (> 30 (J.m), because of thè anisotropy of thè host minerai). This evolution may lead to thè disappearance of thè gas bubble, otherwise thè only unambiguous criterion to identify meli inclusions (immobile gas bubble, as opposed to mobile bubbles in fluid inclusions). A second, less obvious criterion is thè shape, which generally tends to approximate thè negative crystal shape of thè host minerai. The sequence AD indicate a less advanced evolution than in A'D', and then a lower volatile content. In A'D' thè high- volatile content might also be achieved during thè post-trapping evolution (A —» A' transition due to internai crystallization, fluid saturation and than expulsion). C) Identification of former melt inclusions in plutonic rocks : some basic principles With some experience, thè petrographic microscope is sufficient to locate possible magmatic remnants. As these are often very small (< 1-30 p,m), high magnifications objectives (25 and/or 50 x) should be used, on doublé polished thick sections (from 100 to 300 iim), identical to those used for fluid-inclusion studies. Thicker sections allow observations in three dimensions and thè recognition of negative crystal shapes, often difficult to characterize in a two-dimensional section. In "evolved" rocks, thè identification of former melt inclusions takes two steps : first recognition of magmatic textures, then location of former melt inclusions in magmatic minerals.

Fio. 5. - Magmatic textures in granites. A) quartz-feldspar intergrowth. Image width 5 mm. B) miarolitic cavity. Image width 3 mm. C) rests of corrosion gulfs in quartz (arrow). Image width 5 mm. D) preserved magmatic stage zone (A) in a quartz grain (B). Image width 0.5 mm. Fio. 5. - Textures magmatiques dans les granites. A) intercroissance quartz feldspath. Largeur de la figure : 5 mm. B) cavile miarolitique. Largeur de la figure : 3 mm. C) traces de « goìfes de corrosion » dans le quartz (flèche). Largeur de la figure : 5 mm. D) domain magmatique préservé dans un cristal de quartz.

1) Identification of magmatic textures In effusive rocks or shallow intrusives, magmatic textures are obvious and eventually characterized by : a) thè idiomorphic shape of ali minerals growing at thè liquidus of thè System; this often results in large

Fio. 6. - Magmatic remnants in intrusive rocks. A) typical devitrified silicate melt inclusions in quartz from a Hercynian leucogranite in Sardinia (Italy). B) extremely evolved silicate melt inclusion in a Hercynian leucogranite in Sardinia (Italy). C) magmatic remnants (empty arrows) in a high-temperature quartz crystal from a a Hercynian leucogranite in Sardinia (Italy). Late trail-bound inclusions are also observed (black arrows). D) generai view at 900°C of silicate melt inclusions in a quartz crystal terminating in a miarolitic cavity, from Mount Genis granite in SE Sardinia (Italy). Silicate meli (L2) coexists in some of thè inclusions with immiscible hydrosaline melt (LI). Some silicate melt inclusions have already homogenized. Inclusions are at different focus depths [Frezzotti, 1992]. C) quenched magmatic remnant, after heating at = 900°C [Frezzotti, 1992]. D) salt inclusions in K-feldspar, Sybille monzosyenite, Laramie, Wyoming (USA) [Frost and Touret, 1989]. G = glass; V = vapor; LI = hydrosaline melt; L2 = silicate melt; S = salts. Fio. 6. - Restes magmatiques dans les roches intrusives. A) inclusions vìtreuses dévitrìfiées dans un quartz d'un leucogranite hercynien, Sardaigne (Italie). B) reste magmatique très évolué dans un quartz d'un leucogranite hercynien, Sardaigne (Italie). C) restes magmatiques (flèches vides) dans un quartz de haute temperature d'un leucogranite hercynien, Sardaigne (Italie). On observe également des trainées d'inclusions tardives secondaires (flèches noires). D) vue d'ensemble d'inclusions vitreuses a 900"C, quartz dans une cavile miarolitique, Mte Genis, Sardaigne Italie. Dans certaines inclusions, un magma silicate (L2) coexiste avec un magma hvdrosalin immiscible (LI). Certaines inclusions vìtreuses soni déja homogénéìsées (inclusions a diverses profondeurs) [Frezzotti, 1992]. E) restes magmatiques après chauffage a ~ 90ff>C [Frezzotti, 1992], F) inclusion de NaCl dans feldspath des monzosyénites de Sybille, Laramie, Wyoming, USA [Frost and Touret, 1989]. G = verre ; V = vapeur ; LI = magma hydrosalin ; L2 = magma silicate ; S = sels. Bull. Soc. géol. Fr., 1993, n° 2

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phenocrysts, containing generally idiomorphic magmatic minerals (zircon, apatite, etc.), which can delimitate successive periods of growth (mantled inclusions) ; b) thè possible existence of miaroles (open cavities), in which idiomorphic minerals are particularly well developed. Some of these features may also be preserved locally in rocks which have slowly crystallized at depth (fig. 5). Any magmatic minerai may in principle contain magmatic remnants. These will be best preserved in less alterable minerals, in thè first piace quartz, but other minerals can be useful as well : feldspars, apatite and topaz [Thomas, 1991], zircon [Zhaolin, 1987]. The subsolidus, post-magmatic evolution of quartz essentially consists of thè formation of subgrains, limited by triple junctions at 120° (annealing), but often respecting thè initial overall magmatic shape. Therefore, even in deep-seated rocks, magmatic quartz appears as relatively large grains, approximately with thè dipyramidal idiomorphic shape of high-quartz ((3 quartz). Most typical is thè occurrence of "corrosion gulfs" (long, narrow canals ending on thè outer surface of thè grain, fig. 5c) [Clocchiatti, 1975; Anderson, 1991], which correspond in fact to domains where thè growth of thè crystal has been inhibited by a particle adhering at thè surface. This "corrosion" feature occurs in many granites and is by far thè best way to identify magmatic quartz in these plutonic rocks. As thè adhering particle was often glass, a former meli inclusion is almost systematically present at thè end of thè embayment. Unfortunately, these inclusions were not protected against later alteration and they are generally useless.

IV. - INTERPRETATION PLUTONIC ROCKS

OF MAGMATIC

REMNANTS

IN

The study of magmatic remnants in plutonic rocks is thè same as for ordinary silicate melt inclusions in volcanic rocks. Three sets of data are available : 1) microthermometry (experimental melting under thè microscope) ; 2) chemical analyses (electron microprobe) ; 3) specific magmatic features (i.e. magmatic immiscibility). A) High-temperature microthermometry The theoretical P-T evolution of a magmatic remnant in a granitic rock with increasing temperature is represented in figure 7. At room temperature (TO) thè inclusion consists of crystalline aggregates, glass and a gas bubble (S + G + V). On heating, thè crystalline phases disappear at (TI), and thè inclusion becomes progressively more transparent (S + G + V — > G + V). A sort of reorganization can be seen under thè microscope, but without any clear melting, so that thè bubble shows a more regular shape. Beginning of melting (T2) occurs when thè inclusion crosses thè solidus of thè System (G + V -^ G + V + L). Note that in

2) Characterization of magmatic remnants In most plutonic rocks, silicate melt inclusions at room temperature consist of microcrystalline aggregates ± glass rests ± one or more gas bubble. Under thè microscope, they have a typical dark and opaque granular aspect (fig. 6A, B, C). Water films on thè cavity wall (e.g. by interaction with fluid phases) appear completely black (fig. 6B), because of thè light reflection on thè cavity walls. There are two features which univocally distinguish silicate melt inclusions from other solid inclusions in magmatic minerals : (i) coloured glass rests which typically show a spotted anisotropy due to devitrification, and (ii) thè presence of one or more gas bubbles, even if completely deformed. Minerals frequently occur near thè glass : quartz, feldspar, zircon, apatite, etc.. A discrepancy, even small, between one of these solid inclusions and thè corresponding cavity in thè host minerai may be a good indicator for thè existence of a former melt inclusion.

Fio. 7. - P, T evolution of melt inclusions in granitic systems (cf. text). TO : room temperature. TI : devitrification of glass. T2 : beginning of melting (solidus of thè System). T3, T3' : end of melting. In thè case of undersaturated magmas (T3'), final melting corresponds to thè inclusion homogeniz.ation (disappearance of thè gas bubble). Solidus, saturatcd liquidus (s.), and dry liquidus (d.) curves for granitic System adapted from Tuttle and Bowen [1958] and Kadik and Eggler [19741.

Magmatic remnants occur isolated (fig. 6C) or in planar arrays, often corresponding to growth zones of thè crystals (i.e. primary features). Their size is extremely variable. Large inclusions (up to 100 (ini) are sometimes identified, but these are generally completely altered. Best preserved are smaller inclusions in thè 10-20 jim size range.

Fio. 7. - Evolution P, T des inclusions vitreuses dans les systèmes granitiques (cf. text). TO : temperature ambiante. TI : devitrification du verre. T2 : début de fusion (solidus du systeme). T3, T3' : fin de la fusion. Si le magma était sous-saturé (T3'), la fin de la fusion correspond a l'homogénéisation de l'inclusion (dìsparition de la bulle gazeuse). Le solidus, le liquidus sature (s.) et le liquidus sec (d.) indiqués d'après Tuttle & Bowen [1958] et Kadik and Eggler [1974].

Bull. Soc. géol. Fi:, 1993. n"

MAGMATIC REMNANTS IN PLUTONIC ROCKS

contras! with hydrothermal solutions, this temperature is very difficult to observe, because of progressive melting. It can thus be recorded only approximately. Temperatures of initial melting (T2) can be used to determine thè solidus of thè System (i.e. maximum possible temperature values). Final melting (T3) corresponds to thè crossing of thè liquidus of thè System. In granite-type systems, aqueousrich magmas have a liquidus approximately parallel to and not too distant from thè solidus : (T3) will be close to (T2). If thè System is vapor-undersaturated, however, thè liquidus will have a different slope (fig. 7) and thè discrepancy between (T2) and (T3') will be much larger. If thè magma was undersaturated at thè lime of melting, final melting will also correspond to homogenization (disappearance of thè gas bubble, L + V —» V). At room temperature, such inclusions will have a shrinkage bubble, following thè hypothesis of Sorby [1858]. If at (T3) thè magma was "boiling", final melting will not correspond to total homogenization. This will only occur at much higher temperatures, and will be variatale, according to variable gas/glass ratios ; it has no direct geological significance.

TABLE I. - Homogenization temperatures in magmatic rcmnants from different granites. TABL. I. - Tempéralures d'homogénéisation d'anciennes inclusioni vitreuses dans divers granites.

Rock

Locality

Syenite of Cilaos

Pantelleria, Italy Mariktikan, U.S.S. Reunion

Ongoryolites

U.S.S.

Various granites Various granites Peralk. granites

China U.S.S. Norway

Various granites

Germany

Calcaik. granites

Sardinia, Italy

Pantellerite Granites

Minerai

Th°C

Quartz Feldspar Quartz

750 850 1000-1150

Quartz

650 - 750

Quartz Feldspar Zircon Quartz Quartz

750 - 840

Quartz Feldspar Quartz

References

.Benhamou & Clocchiatti, 1976 Reyf& Bazheyev, 1977 Clocchiatti & Nativel, 1984 Naumovet al., 1984

800- 900

Zhaolin, 1987 Naumov, 1988 Hansteen & Lustenhouwer, 1990 Thomas, 1991

750-1250

Frezzotti, 1992

940 -1100 550-1250 900- 955

Available data from various authors on microthermometric results are reported in table I (most were done in thè USS ; for comparison with melt inclusions in volcanites see Clocchiatti). Measured initial melting temperatures are reasonably in agreement with experimental solidus determinations [Thompson and MacKenzie, 1967] and vary from = 550°C (high fluorine granites; solidus lowering, Manning [1981]) to = 700°C (solidus determination in thè System Ab-Or-Qtz-H 2 O at 2 kbar; Tuttle and Bowen [1958]), and to = 750°C (solidus in thè System Ab-Or-Qtz-H 2 O-CO 2 , for XH.O =-5; Ebadi and Johannes |1991]). On thè other hand, homogenization temperatures cover a wider temperature interval from 800°C up to 1200°C. Unreasonably high magmatic temperatures are found by various authors (notably Russian workers : see references in table I), often within

.237

thè same samples. This can be due to (i) experimental problems, in particular : interactions between melt and host minerai (melting of inclusion cavity wall during heating) and homogenization kinetics (thè equilibrium gas bubble/liquid is very long to be reached in high-silica melt [Clocchiatti, 1975; Xia 1984]; therefore, high-temperature microthermometry is very different from that on fluid inclusions) ; and/or (ii) melting of originally altered inclusions which have lost their originai chemical magmatic signature. With ali these difficulties in mind, it is however clear that careful Th measurements give a reliable indication of thè chemical composition of thè magma. An extensive compilation by Thomas [1990], based on several hundreds of samples, indicates a very good linear correlation between Th and composition, higher Th being associated to lowSiCh magmas. B) Chemical composition The chemical composition of thè glass can be obtained by electron-microprobe analyses on inclusions opened after heating, in order to homogenize thè inclusion contents. The problems are basically thè same as for lavas : diffusions effects of some chemical elements towards thè host minerai [Roedder, 1984], and/or epitaxial growth of thè host minerai on thè cavity walls [Clocchiatti, 1975]. Additional complications are given by thè small size of thè inclusions, thè internai inclusion evolution (re-melting of altered inclusions), thè consequences of high temperature microthermometry (possible melting of thè host minerai at thè cavity wall), thè relationships between T attained during heating experiment and composition of thè glass (heterogeneous glass). Analyzed glass in quartz crystals of post-kinematic high silica (SiO2 = 74-75 wt%) Hercynian granites from Sardinia [Frezzotti, 1990] are shown in figure 8 in terms of thè system Q-Ab-Or. The solidus minima for thè System Q-AbOr-HaO at 1 and 2 kbar pressures are also indicated. Very few data (field 1, fig. 8) roughly approach thè mother rock composition (black star, fig. 8) either enriched in thè Ab or in thè Q components ; thè wide majority of magmatic remnants shows extremely variable normative compositions. The distribution of thè chemical compositions allows to define two distinct chemical trends. 1) Albitization Some glasses show a consideratale increase of thè Ab component. Formation of albite on K-feldspar is always found in post-kinematic Hercynian leucogranites of Sardinia. The inclusions result from thè re-melting of originally altered silicate melt inclusions : interactions with circulating fluid phases caused thè same chemical exchanges in thè inclusions as in thè host granites. It is important to note that silica depletion is absent. Therefore, interactions between inclusions and thè host minerai are minimal. The often quoted principle [Roedder, 1984, pp. 482-484] that thè inclusion contents should be depleted in elements forming thè host minerai does not hold, at least in this case. Bull. Soc. gèni. Fr., 1993, n° 2

238

J. TOURET AND M.L. FREZZOTTI

In conclusion, some indications on homogenization temperatures and chemical compositions might be obtained also from thè study of magmatic remnants in plutonic environments. However, according to our experience, favourable cases are not very common, and evolutionary trends are difficult to be interpreted in some detail. Much more promising is thè possible reconnaissance of thè typical magmatic phenomena, notably immiscibility, which are unambiguous and cannot be obtained by any other method. C) Magmatic immiscibility : thè cases of Mount Genis granite (SE Sardinia, Italy) and Sybille monzosyenite (Laramie anorthosite complex, Wyoming, USA)

50

Ab

50

Or

Hic. 8. - Q-Ab-Or normative diagram of meli inclusions in quartz from Mount Genis granite. Only a few analyses (1) can be reprcsentative of thè rock composition (black star). The overall distribution defines two different trends : (2) albitization and (3) Si-enrichment (See text). [Q-Ab-Or plot after Tuttle and Bowen, 1958; Luth et al., 1964]. Fio. 8. - Diagramme Q-Ab-Or (normei d'inclusions vitreuses dans les quartz du Mte Genis granile. Très peu d'analyses (1) correspondent a la roche totale (éìoile noire). La distribution des compositions definii deux tendances : (2) albitization et (3) silicification. /Q-Ab-Or d'après Tuttle and Bowen, 1958: Luth et al., i964].

2) SiOì enrichment Practically ali silicate meli inclusions are extremely enriched in SiÓ2 compared to thè granite composition. As thè inclusions occur in quartz, this could be partly due to a host minerai effect during microprobe analysis (mean inclusion size = 30-40 (im). Heating of thè inclusions to high temperatures (= 800°C) might also bave caused melting of thè cavity walls with some SiC>2 increase in thè silicate melt. This latter hypothesis is thè most likely explanation, as ali inclusions define a Q —> Abso trend. It cannot be ruled out that thè inclusions which show only a moderate increases of thè Q component (i.e. no apparent inclusion volume change after quenching to room temperature and/or larger inclusions ; analyses within area 1 in fig. 8) reflect originai melt compositions. It is possible that such melt inclusions represent residuai melt, after thè crystallization of plagioclase (albite to oligoclase), K-feldspar and biotite. The Si enrichment trend is thus representative for thè originally trapped magma to which SiCh has been added, either by fractional crystallization, or during high temperature heating. Thus, thè crossing of thè two opposite trends, assuming a PFLO = 1 kb [Frezzotti, 1990] allow to characterize thè composition of thè initial magma, which is very dose to mother rocks and to thè ternary minimum for thè system Q-Or-Ab in water saturated conditions. Bali. Soc. géol. Fr.. 1993, n° 2

Some direct or indirect evidence of magmatic immiscibility, heterogeneous trapping, or of thè overall volatile evolution can be preserved, even in thè more unfavourable conditions (e.g. deeply emplaced plutons). Two examples are reported : Mount Genis, a post-kinematic Hercynian leucogranite from Sardinia (Italy) [Frezzotti, 1990, 1992] and thè Sybille monzosyenite of thè Laramie anorthosite complex from Wyoming (USA) [Frost and Touret, 1989]. These two examples are complementary, covering different pressure environments from shallow Mount Genis granite (P = 1 kbar), to thè deeper (~ 3 kbar) Sybille monzosyenite. 1) Mount Genis granite [Frezzotti, 1992] The Mount Genis granite is one of thè post-tectonic leucogranite intrusives which are part of thè Hercynian Sardinia-Corsica batholith. It comprises several plutonic rocks (from gabbros to leucogranites) which correspond to a calc-alkaline series [Poli et al., 1989]. Leucogranites represent thè youngest products ; geochronological data indicate emplacement ages of 289 ± 1 Ma [Del Moro et al., 1975]. Mount Genis is a compositionally and texturally homogeneous pluton and consist of K-feldspar (30-35 voi. %), albite (30%), quartz (30-35%) and biotite (« 3%). Miarolitic cavities are ubiquitous : mm-sized interstices were ali magmatic minerals are present. Recrystallized melt inclusions occur in quartz from thè massive rocks and from thè miaroles. A well visible gas bubble is systematically present and has a very variable relative size (up to > 4Óvol.%). Upon heating to high temperatures (= 800-900°C), preserved inclusion melt and two immiscible liquids appear in thè inclusions with vapour bubbles > 40 voi.tot. : one silica rich (L2), thè other being almost pure Na-K chloride (LI). The overall view in one crystal at ~ 900°C is presented in figure 6D. On quenching to room temperature, thè latter liquid crystallized as cubie masses smeared around thè gas bubble, while L2 formed completely transparent glass (fig. 6E). The fact that thè L1/L2 proportions are extremely variable in thè different inclusions (fig. 6D) suggests that magmatic immiscibility did exist between LI and L2, at thè lime of trapping. The inclusions correspond to heterogeneous trapping of this chloride/silicate/melt mixture. Therefore, during late magmatic stages, a hydrosaline fluid exsolved from thè granite magma. The possibility that these brines formed from boiling of a honnogeneous fluid in thè system FhO-NaCl-KCl is unlikely. Fluid inclusions

MAGMATIC REMNANTS IN PLUTONIO ROCKS

do not show traces of fluid immiscibility (i.e. no vapor-rich inclusions). The saline brines were directly exsolved from thè granite magma, they were not concentrated by a subsequent fluid immiscibility.

matic origin. Because these halide cubes were trapped at high temperatures, they were interpreted to represent traces of an immiscible halide melt. Fluid-inclusion studies in quartz show that these hydrosaline melts coexisted with CO2. Primary CCb inclusions (d = 0.7 g/cm 3 ) appear to be co-magmatic and He on isochores (fig. 9) which intersect thè presumed crystallization conditions of thè Sybille monzosyenite. The combined results on solid and fluid inclusions suggest thè existence at magmatic stages of a water-poor, CCbsaturated chloride melt. The primary assemblage consisted of silicate melt, COz-saturated saline melt and COi fluids. These two examples record immiscibility processes between a granite magma and a hypersaline fluid phase and they point to an extremely similar magmatic stage evolution for two plutons which experienced different crystallization histories. The absence of CCh in thè Mount Genis pluton might reflect locai environments and it could tentatively be related to thè shallow emplacement of this pluton. Granites originating in thè lower crust, notably those reworked from granulile terrains, are often rich in CCh. Those formed and/or emplaced near thè surface, where H2O is thè dominant fluid [Touret, 1987] tend to be FhO-rich and CO2poor. But thè presence or absence of CCh does not change thè silicate/salt melt immiscibility. If CO2 had been present, it would have been exsolved as well. Magmatic immiscibility can therefore be recognized in silicate melt inclusions even if they have lost their primary characteristics and give unrepresentative microthermometric or chemical results. Rests of silicate melt inclusions surrounded by crystallographically oriented tiny fluid

2) Sybilie monzosyenite [Frost and Touret, 1989] The Sybille monzosyenite is one of thè late-stage plutons associated with thè Laramie anorthosite complex (1.4Ga). It is a differentiated pluton, monzonite at thè border zones and a main quartz-free monzosyenite intrusion [Fuhrman et ai, 1988], consisting of orthoclase (46 voi. %), plagioclase (10%), clinopyroxene (Wo4o Eni2 Fs4g) (10%), quartz (8 %), olivine (Fa94 Fo4 Tp2) (5 %), ilmenite (Ilmgv) (3 %) and horneblende (1 %). Crystallization conditions are 3 kbar and 950-1000°C. Obvious magmatic remnants are not visible in thè feldspars, but these contain three populations of solid inclusions which were probably trapped during crystal growth : (1) euhedral grains of apatite commonly associated with an opaque phase (generally ilmenite and more rarely graphite), (2) laths of ilmenite and (3) isotropie inclusions associated with small grain of hedenbergite and/or green amphibole. These isotropie grains (fig. 6F) had a distinct cubie shape ; semiquantitative microprobe analyses indicate that they are Na-K chlorides with lesser amount of Ca. Some grains are hypersolvus and were precipitated at temperatures above those of thè consulate point in thè System KCl-NaCl [500°C ; Barret and Wallace, 1954]. Due to their association with amphiboles and other magmatic minerals in thè feldspars thè salt inclusions represent primary inclusions of mag-

P

239

(B) J

T conditions

ol c r y s t a l t i z a t i o n

Tj immiscibility i NaCI melts

Inclusion Decrepitation

.o

2-

(A)

rx ^

0.35

-i 200

1

1

1

1 600

1

r

1

1 1000

1

1

1— —P* T °C

Fio. 9. - P, T evolution of thè Sybille monzosyenite (arrow) [from Frost and Touret, 1989] (isochore densities in g/cm3). Immiscibility of saline melts must have occurred before thè end of crystallization of thè monzosyenite. (A) = beginning of magma crystallization (precise temperature unknown). (B) = end of crystallization. At Ti (unknown, but between A and B), immiscibility of NaCI melts, which result in isolated NaCI cubes trapped in thè feldspars. Fio. 9. - Evolution P, T de la monzosyenite de Sybille [Frost and Touret, i989], délerminée d'après /'evolution des inclusions fluides (derisile de* isochores en g/cm'). L'immiscihilité des magmas salins a dù se produire avant la fin de la cristallisation de la monzosyenite. (A) = début de cristal/isation du magma (temperature precise incannile), (B) = fin de cristallisation. A la temperature Ti (inconnue mais située entre A et B), immiscibilité de magmas salins (NaCI), doni le resultai s'observe sous forme de cubes de NaCI iso/és dans les feldspaths. Bali. Soc. géol. Fr., 1993, n° 2

240

J. TOURET AND M.L. FREZZOTTI

inclusions (fig. 1C) are often present in plutonio rocks. Although they do not resemble anymore thè originally formed microsystems (as schematically reported in fig. 4D') and no direct data can be obtained, some indications on thè magmatic fluid can be indirectly obtained by thè study of thè surrounding fluid inclusions. One aspect of thè latter is that they reflect thè chemical composition of thè fluid expelled from thè melt. Their density is not representative of thè formation conditions, but only of thè P and T at thè lime of decrepitation of thè inclusion.

V. - SUMMARY AND CONCLUSIONS

Even in plutonic rocks, remnants of magmatic inclusions can give more reliable information than generally realized. The reasons why thè study of magmatic remnants has not been developed may be thè result of two concomitant factors : thè "a priori" hypothesis that magmatic stages are only rarely preserved in slowly cooling rocks [Weisbrod, 1981] and thè lack of specific knowledge for their identification. This work presents some indications for their recognition and interpretation. The identification of magmatic remnants in most plutonic rocks requires specific microscopie investigations on doubly polished thick sections. Such remnants have a different appearance from melt inclusions in volcanic rocks because post-trapping evolution leads to glass crystallization. The recognition of magmatic textures in thè studied samples indicates potential areas where thè existence of magmatic remnants is possible. The shape, size, and degree of evolution (i.e. presence of a gas bubble and or glass rests, etc.) of thè magmatic remnants should be then characterized. The usuai techniques for melt inclusion studies apply directly for magmatic remnants in plutonic rocks, but thè preservation of thè originai characters is to be expected only in some cases, where thè inclusion evolution has been limited, e.g. in shallow rocks [Hansteen and Lustenhouwer, 1990; Thomas, 1991]. In most cases, phase transitions during heating experiments occur at unrealistically high temperatures (= 1000-1200°C), and thè chemical composition has been considerably modified. But, even with these limitations, magmatic remnants may provide interesting information. First of ali their identifi-

cation is a direct proof for a magmatic origin of thè host rock. Then : — partial chemical information can be obtained from thè analyses of thè inclusion glass; in order to obtain reliable formation temperatures, thè analyses must always be preceded by high-temperature heating to homogenize thè content of thè inclusion ; — eventually, direct or indirect evidence for typical magmatic phenomena, notably immiscibility, can be characterized ; — thè characterization of magmatic fluids is sometimes possible, even in inclusions that experienced irreversible variations of their primary characteristics or were completely obliterated during thè subsolidus evolution of thè host rock; this is done on thè neighbouring fluid inclusions (fluids expelled from thè melt during subsequent evolution). These results may have important consequences for many aspects of magmatic studies, notably on thè role of fluids in thè generation and crystallization of magmas. Experimental investigations on volatile saturation during silicate melt crystallization tend to model fluids as pure or nearly pure H2O [e.g. Tuttle and Bowen, 1958; Burnham, 1979; Whitney, 1984]. Although water is certainly one of thè major constituents of magmatic fluids, especially in acid rocks, other species may also be present. If they do not interact with minerals, they will not be recorded by minerai analyses. The study of inclusions might then be thè only way to infer their presence and to model their influence on magmatic evolution. Acknowledgements. - We dedicate this paper to thè memory of Jean Lameyre, a long time friend of thè senior author. It benefited from many discussions with R. Ciocchiatti, thè pioneer of modern melt inclusion study, D. Massare and J. Weisz who taught high temperature microthermometry to M.L.F. and provided some unpublished analyses on silicate melt inclusions. We owe many thanks to C. Ghezzo for Constant interest and information, notably concerning granites from thè Hercynian Sardinia-Corsica batholith, and thè Calabona example. The authors acknowledge thè perceptive reviews of E.A.J. Burke, R. Clocchiatti and C. Lecuyer. This work was supported by a NATO-CNR fellowship to M.L.F n.Ol 1260.

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ANDERSON A.T. (1991). - Hourglass inclusions; theory and application to thè Bishop Rhyolitic Tuff. - Am. Mìneralogist, 76, 530-547.

ANDERSEX T., GRIEFIN W.L. & O'REILLY S.Y. (1988). - Primary sulfide melt inclusions in mantle-derived megacrysts and pyroxenites. - Lithos, 20, 279-294.

ANDERSON A.T., NEWMAN S., WILLIAMS S.N., DRUITT T.H., SKIRIUS C. & STOLPER E. (1989). - H 2 O, CO2, CI, and gas in plinian and ashflow bishop rhyolite. - Geology, 17, 221-225.

ANDERSEN D.J. & LINDSI.EY D.H. (1985). - New (and f i n a l i ) models for thè Ti-magnetite/ilmenite geolhermometer and oxygen barometer. - Eos, 66, 416.

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ANDERSON A.T. (1974). - Evidence for a picritic volatile-rich magma beneath Mt. Sbasta, California. - J. PetroL, 15, 243-267.

BARRET W.T. & WALI.ACE W.E. (1954). - Studies of NaCl-KCl solid solutions. II. Experimental entropies of formation, lattice spacing,

Bull. Soc. géol. /•>.. 1993, n u 2

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