Document XX6z2DOeKEKnZ4jBajzVxrX4

Nat. Hazards Earth Syst. Sci., 11, 1267-1280,2011 www.nat-hazards-earth-syst-sci.net/11/1267/2011/ doi:10.5194/nhess-11-1267-2011 Author(s) 2011. CC Attribution 3.0 License. Natural Hazards and Earth System Sciences Linking rock fabric to fibrous mineralisation: a basic tool for the asbestos hazard G. Vignaroli1, F. Rossetti1, G. Belardi2, and A. Billi2 1Dipartimento di Scienze Geologiche, Universita Roma Tre, Largo S.L. Murialdo 1, 00146 Rome, Italy 2Istituto di Geologia Ambientale e Geoingegneria, CNR, Area della Ricerca di Roma 1, Via Salaria Km 29, 300-00015, Monterotondo Stazione, Rome, Italy Received: 17 November 2010 - Revised: 3 March 2011 - Accepted: 6 March 2011 - Published: 9 May 2011 Abstract. In recent years, many studies have addressed the effect on human health caused by asbestos exposures. As asbestos is a group of fibrous minerals that mainly occurs in mafic and ultramafic rocks (ophiolitic sequences), a close relationship between asbestos occurrence and the geological history of host rocks should be expected. By reviewing the existing literature and presenting characteristic examples, it is proposed a direct correspondence exists between the pres ence of fibrous minerals in ophiolites and the rock fabric systematics due to the combined activity of deformation, meta morphism/metasomatism, and rock/fluid interaction. Under standing the geological factors that may be at the origin of the nucleation/growth of fibrous minerals constitutes a necessary requirement for developing a methodological and analytical procedure to evaluate asbestos hazard (Ah) in the natural prototype (ophiolitic rocks). A parameterisation of the Ah in function of the main geological processes that produce the rock fabric systematics in different tectonic/geodynamic set tings is discussed. A geological multidisciplinary approach (based on geological-structural field evidence combined with textural, mineralogical, petrological, and geochemical inves tigations) is proposed as the prerequisite for the evaluation of Ah in natural environments. This approach, in particular, can provide a robust basis to formulate a procedural protocol finalised to the mitigation of asbestos effects in environments where these effects are still a real threat. Correspondence to: G. Vignaroli (vignarol@geo.uniroma3.it) 1 Introduction Asbestos is the commercial term commonly used for six sil icate minerals (World Health Organization, 1986; Gunter et al., 2007; Gunter, 2010). Due to its useful manufacturing properties, the use and commercialization of asbestos started already in archaeological ages. Since the advent of modern industry in the 19th century, asbestos has been mined and used all around the world (e.g. Ross and Nolan, 2003, and Kazan-Allen, 2005, for a review), while its negative effects on human health were increasingly being defined (e.g. asbestosis, mesothelioma, and lung cancer; Doll, 1955; Mossman et al., 1990; Hughes and Weill, 1991; Wagner, 1991; Rey et al., 1994; Cattaneo et al., 2006). At present, asbestos is listed as a Group I human carcinogen matter by the inter national world health authorities (IARC, 1987). Although new fibrous minerals have been and are being discovered (e.g. Compagnoni et al., 1983,1985; Gianfagna et al., 2003; Meeker et al., 2003; Belluso et al., 2006; Sullivan, 2007), in several countries asbestos is usually classified into two main mineral groups for legislative purposes: serpentine (chrysotile) and amphibole (amosite, crocidolite, anthophyllite, tremolite, and actinolite). Despite the fact that regulatory agencies attempt to control the environmental impact of these minerals (e.g. Gibbons, 1998; Ross and Nolan, 2003; KazanAllen, 2005; de Grisogono and Mottana, 2009; Strohmeier et al., 2010) and that information on the asbestos hazard has multiplied in the last years (e.g. Gunter et al., 2007), asbestos is still an important threat to human health, particularly in ur ban settings where excavation, milling, and transportation of asbestos-bearing rocks during engineering activities (such as quarrying, tunnelling, and railways construction) potentially induce environmental risks for both workers and residents (e.g. Rohl et al., 1977; Schreier, 1989; Ross and Nolan, 2003; de Grisogono and Mottana, 2009). Published by Copernicus Publications on behalf of the European Geosciences Union. 1268 ASBESTOS G. Vignaroli et al.: Rock fabric and asbestos hazard starting engineering works involving ophiolitic rocks both in urban settings and in natural environments. Fig. 1. Flowchart correlating the asbestos hazard to mitigation ef fects in natural environments. Research on asbestos hazard has hitherto been based mainly on measurements of concentrations of dispersed fi bres in air (e.g. Sebastien et al., 1982; Lange et al., 1996; Zakrzewska et al., 2008), soils/water (e.g. Burilkov and Michailova, 1970; Metintas et al., 2002; Emmanouil et al., 2009), and indoors (e.g. Hardy et al., 1992). Results from these measurements have been mainly used to align the as bestos concentrations to the incidence of diseases, or to sup port national laws regulating the asbestos content in natu ral environments. It follows that policy makers and emer gency responders have so far focussed more on the symp toms of the problem (mitigation effects) rather than on the cause (Fig. 1). On the other hand, only few studies have ad dressed the potential risks posed to health by asbestos min erals occurring in their natural settings (e.g. Gabrielse, 1960; Gibbons, 2000; Ross and Nolan, 2003; Van Gosen, 2007; Hendrickx, 2009; Thompson et al., 2011). As asbestos is a mineral typical of mafic and ultramafic rock sequences (i.e. ophiolites), the cause for asbestos concentrations and haz ard most commonly resides in ophiolitic rocks. However, only some of these rocks bear minerals and associated soils hazardous to human health. Understanding what geological history may lead to the formation of hazardous minerals in ophiolites is therefore fundamental to mitigate the asbestos hazard at its origin. The aim of this paper is to investigate the range of the geological processes involved in the development of the asbestos-bearing ophiolitic rocks in order to promote some operative indications for procedures to be adopted before 2 Asbestos in ophiolitic rocks: when and why? Deposits of asbestos occur world-wide and as such asbestos there have been concerns for non-occupational exposure (e.g. Phillips, 1927; Schreier, 1989; Karkanas, 1995; Ross and Nolan, 2003; Van Gosen, 2007; Hendrickx, 2009). Although large asbestos deposits have been documented in metavolcanics (Gianfagna et al., 2003) and metamorphosed iron for mations (Van Gosen, 2007), loci of intensive asbestos mines (both active and inactive) correspond to exposures of ophiolitic rock sequences. A complete ophiolite suite consists of basal peridotites showing different olivine-clinopyroxene-orthopyroxene as sociations (harzburgite, lherzolite, pyroxenite, dunite, etc.) and a main granular-to-porphyroclastic microtexture; over laying intrusive gabbros (often exhibiting cumulus tex tures) and dykes; effusive complex mostly comprising pil low basalts; tectonosedimentary breccias (ophicalcites); and pre-to-syn-rift sedimentary cover (e.g. Mrintener and Piccardo, 2003). Within this wide spectrum of ophiolitic rock types, chrysotile deposits are concentrated in the ultramafic rocks (peridotites, serpentinized peridotites, serpentinites), whereas amphibole asbestos may be present in gabbros, serpentinoschists, and various metasedimentary rocks. What follows is a review of the geological aspects that per tain to asbestos formation, starting from the geological pro cesses associated with the genesis and exposure of ophiolites in continental areas, where they interact with the human pop ulation. 2.1 The geodynamic settings Ophiolites are remnants of paleo-oceanic lithosphere and their present-day exposures are a direct manifestation of the Wilson cycle of plate tectonics, from oceanic construction at divergent plate boundaries to oceanic consumption at conver gent plate margins during orogenesis and crustal growth (e.g. Dilek and Robinson, 2003, and references therein). Creation of ophiolites dominantly occur at oceanic spreading centres in consequence of rifting processes during fragmentation of either continental or oceanic lithosphere (Coleman, 1977; Moores, 1982), although increasing importance has been at tributed to ophiolites formed in supra-subduction zone en vironments (Dilek and Robinson, 2003). Theoretical mod els concerning exposure of mafic and ultramafic suites at the sea-floor in spreading centres include both simple- (e.g. McKenzie and Bickle, 1988) and pure-shear (e.g. Whitmarsh et al., 2001) criteria of lithospheric extension. In the first case (Fig. 2a), ophiolite formation is linked to the activity of lowangle normal fault systems that enhance hydrothermal alter ation and chemical contamination due to pervasive seawater Nat. Hazards Earth Syst. Sci., 11, 1267-1280,2011 www.nat-hazards-earth-syst-sci.net/11/1267/2011/ G. Vignaroli et al.: Rock fabric and asbestos hazard 1269 3) Ophiolite formation and hydrothermal metamorphism (serpentinization) Lithospheric detachment setting Mid-oceanic ridge setting seawater level spreading center 1 e ^ at a*1 tfc xmxxjcxxxxxi XXXXXXXXXXXXJ I 1I ~ ^^^7VA7CA7V7V?' ............cxxxxxxxxx .txxxxxxxxxxx: .xxAxAxAxx/Vxr\rx- x XXXXcontinental crustXXXXXXXX> , _ x, 'xxxxxxxSH.'x; JMHM;xxxxxxxxx xx A serpentinization lithospheric mantl^ asthenosphere b) Ophiolite emplacement in subduction-exhumation cycle frontal _ njxX*.*>^ thrusting ta,t<XXXX)?X> . y y V- y y % / y y X; caonatinacnataXl .caruastXX crustal thinning accretionary j ' hydrated lithospheric mantle XXXXXXXXXXXX ---------------* C) Ophiolite emplacement during obduction unmetamorphic units mctamorphic sole thrust Fig. 2. Schematic scenarios of tectonic settings for ophiolites. (a) Mantle exhumation and ophiolite formation during two main crustal mechanisms: lithospheric detachment (e.g. McKenzie and Bickle, 1988, redrawn and modified) and coaxial stretching at the mid-oceanic ridge (e.g. Whitmarsh et al., 2001, redrawn and modified). (b) Subduction-exhumation setting illustrating possible tectono-metamorphic events experienced by ophiolites (after Agard et al., 2009, redrawn and modified). (c) Ophiolite obduction along a regional-scale metamorphic sole thrust (after Searle et al., 2003, redrawn and modified). penetration during uprising (e.g. Giguere et al., 2003; Mevel, 2003). In the second case, divergent plate motion at the oceanic ridge produces dilatation and cracking in the thin oceanic floor with consequent permeation of mantle rocks at surficial conditions. Apart from processes associated with sea floor formation, the majority of the present-day exposures of ophiolitic se quences coincides with the axial (inner) regions of both ac tive and ancient mountain belts (Dewey and Bird, 1970; Coleman, 1971). The classical scenario of mountain belt considers subduction of an oceanic plate followed by con tinental collision and suturing at the termination of the clas sical Wilson cycle (Dewey and Bird, 1970; Cawood et al., 2009, and references therein). In this context, orogenic wedge formation occurs in the supra-subduction zone due to the continuous flux, burial, and exhumation of the ma terial detached from the subduction plate (e.g., Platt, 1986, 1993; Cloos and Shreve, 1988; Jolivet et al., 2003; Agard et al., 2009; Guillot et al., 2009). Meanwhile, ophiolites undergo a series of tectono-metamorphic events within the subduction channel, progressively equilibrated under highpressure/low-temperature (HP/LT) metamorphic conditions (e.g. Peacock, 1996; Hacker et al., 2003; Guillot et al., 2009) with concomitant metasomatism induced by the cir culating fluid phase (e.g. Scambelluri and Philippot, 2001; Schmidt and Poli, 2003; Bebout, 2007) (Fig. 2b). Circulation within the subduction channel results in the development of polyphase deformation fabrics and metamorphic mineral as semblages that modify the original texture, mineralogy, and rheology of pristine ophiolites (Scambelluri et al., 1995; Her mann et al., 2000; Andreani et al., 2005). Orogenic processes may also involve oceanic obduction, which constitutes the tectonic juxtaposition of the oceanic lithosphere onto conti nental margins (e.g. Coleman, 1977). As in the case of the Semail ophiolites in Oman (e.g. Coleman, 1981; Searle et al., 2003, and references therein), the obducted ophiolites consti tute an almost preserved oceanic lithospheric section, overthrusted along a regional-scale sole thrust (Fig. 2c). The lack of penetrative subduction-zone metamorphism within the Semail ophiolite mass is one of the most important evidence www.nat-hazards-earth-syst-sci.net/11/1267/2011/ Nat. Hazards Earth Syst. Sci., 11, 1267-1280, 2011 1270 G. Vignaroli et al.: Rock fabric and asbestos hazard Fig. 3. Qualitative P-T conditions of stability for serpentine and amphibole minerals. Reactions for serpentines are taken from Wunder et al. (2001); Bucher and Frey (2002); and Evans (2004). Amphiboles fields are taken from Otsuki and Banno (1990); Parra et al. (2000); and Bucher and Frey (2002). Mineral abbreviations: Act: actinolite; An: anorthite; Atg: antigorite; Brc: brucite; Brs: barroisite; Ctl: chrysotile; Di: diopside; En: enstatite; Fo: forsterite; Gln: glaucophane; Hbl: hornblende; Liz: lizardite; Rbk: riebeckite; Spl: spinel; Tlc: talc; Tr: tremolite; Win: winchite. of the obduction process, with deformation, metamorphism, and fluid-rock mainly localized within the metamorphic sole thrust (e.g. Gray and Gregory, 2003). Consequently, despite their original stratigraphic se quence, ophiolitic suites usually show complexities in terms of superimposed structures and mineralogical assem blages (i.e. the rock fabric), an inheritance of the tectonometamorphic history from sea floor formation and/or orogenic wedge construction (e.g. Hermann and Muntener, 1996; Boschi et al., 2006; Nuriel et al., 2009). Within this wide range of geological processes, shear deformation and fluid-rock interaction enhance rock alteration and serpentinization, including fibrous minerals, under particular con ditions of temperature and pressure (e.g. Trommsdorff and Evans, 1974; Evans, 2004). 2.2 The P-T metamorphic conditions In metamorphic petrology, the crystallization of a specific mineral assemblage, including also fibrous minerals poten tially hazardous to human health, reflects pressure and tem perature (P-T) equilibrium conditions attained during the metamorphic evolution. Fibrous mineralisation can there fore be seen as snapshots of the progressive crystallization process occurring in the rock mass, in consequence of the experienced tectono/metamorphic evolution. Figure 3 summarizes the P-T conditions for stability of serpentine and some amphibole minerals. Serpentine miner als equilibrate in a wide field of both T and P. Antigorite is stable from 200 to 660 C, over low-grade to eclogitic condi tions (e.g. Bucher and Frey, 2002; Evans, 2004). Antigorite mostly forms by destabilization of pristine peridotitic min eral assemblages by water activity-dependent reactions (e.g. Trommsdorff and Evans, 1974; Evans, 1977; Hermann et al., 2000; Bucher and Frey, 2002; Andreani et al., 2007), such as: forsterite + water = antigorite + brucite (1) enstatite + water = antigorite + talc (2) forsterite + talc + water = antigorite (3) Chrysotile and lizardite are (meta)stable phases at tempera tures lower than 300 C (Evans, 2004), following the reac tion: antigorite + brucite = chrysotile/lizardite. (4) Complete destabilization of antigorite in favour of chrysotile/lizardite occurs at T < 200 C with produc tion of talc: antigorite = chrysotile/lizardite + talc. (5) It is important to note that chrysotile does not crystallize directly from early peridotitic assemblages, but it develops from the destabilization of former antigorite. The stability fields of amphiboles depend on the variation in chemical composition of sodic- and calcic-rich species (Evans, 1990; Dale et al., 2005). Nucleation of tremolite in peridotites can occur following the destabilization of olivine, orthopyroxene, and clinopyroxene in presence of aqueous fluid (Bucher and Frey, 2002): forsterite+enstatite+anorthite+water = tremolite+spinel(6) enstatite + diopside + water = tremolite + forsterite. (7) Tremolite is stable from temperature lower than 800 C, down to the greenschists facies field (i.e. 300-400 C). Crys tallization of actinolite and crocidolite (the asbestos variety of riebeckite) attains at T < 400 C over medium-to-high pressure conditions (Otsuki and Banno, 1990; Fig. 3). From the above notions, it can be inferred that the nucleation of both serpentine and amphibole asbestos is ex pected in ophiolitic rocks that experienced cooling during decompression from high-grade metamorphic conditions, af ter destabilization of former minerals. This can be attained by following different metamorphic retrograde paths (i.e. to wards surficial P-T conditions) that can span from (i) a tra jectory characterized by nearly isobaric cooling and, then, nearly isothermal decompression, to (ii) a trajectory with first nearly isothermal decompression and, then, nearly isobaric cooling. Such retrograde paths are representative of the ex humation processes attained by the metamorphic rocks dur ing the orogenic cycle (e.g. Platt, 1993; Spear, 1993). Nat. Hazards Earth Syst. Sci., 11, 1267-1280,2011 www.nat-hazards-earth-syst-sci.net/11/1267/2011/ G. Vignaroli et al.: Rock fabric and asbestos hazard 1271 Table 1. Summary of the deformation fabrics hosting fibres in ophiolites. Rheological regime Ductile Semi-ductile/semi-brittle Brittle Structures Mylonitic shear zones Syn-metamorphic veins Tensile/shear fractures Fault gouges Fault surfaces Preferential site of fi bre formation On the schistosity Within vein walls Within fracture walls On the slip planes On the fault surface Common meso- and micro-scale features Schistosity, stretching lineation Cross- and slip-fibre veins Cross- and slip-fibre fractures Slip kinematic indica tors Slip kinematic indica tors Considered refe rences [1], [2], [4], [5], [6], [7], [8], [9], [10], [12], [18], [19] [1], [2], [3], [7], [8], [11], [13], [14], [15], [16], [20], [21] [3], [7], [8], [12], [17], [19] [6], [7], [8], [12], [17] [6], [7] [1] Trommsdorff and Evans (1974); [2] Evans (1977); [3] Wicks and Wittaker (1977); [4] Piccardo et al. (1988); [5] Scambelluri et al. (1991); [6] Hoogerduijn Strating and Vissers (1994); [7] Karkanas (1995); [8] Hermann et al. (2000); [9] Reinen (2000); [10] Li et al. (2004); [11] Bellot (2008); [12] Andreani et al. (2005); [13] Barker et al. (2006); [14] Auzende et al. (2006); [15] Compagnoni and Groppo (2006); [16] Groppo et al. (2006); [17] Hirose et al. (2006); [18] Hirauchi and Yamaguchi (2007); [19] Giacomini et al. (2010); [20] Groppo and Compagnoni (2007); [21] Phillips (1927). 2.3 Rock fabric evolution Within orogenic belts, ophiolites define large, but usually discontinuous bodies confined by regional-scale tectonic contacts responsible for the tectonic juxtaposition onto base ment rocks (gneiss, granites) (Fig. 4a). The fabric of de formed ophiolites corresponds to different size deformation structures (from centimetre- to regional-scale), usually dis secting the original stratigraphic setting and obscuring the pristine rock fabric (Fig. 4b, c). Fibrous texture occurrence is documented both in shear deformation structures such as mylonites and fault gouges; and in non-shear deformation structures such veins, developed in both ductile and brit tle regimes (Table 1). Processes involved in formation of shear structures include reduction in grain-size, progressive development of pervasive foliation, and crystallization of deformation-related minerals in presence or absence of aque ous fluids (Trommsdorff and Evans, 1974; Scambelluri et al., 1991). Schistose serpentinites (Fig. 5a, b) define the com mon example of large amount of localised ductile shearing, as they accommodate fault slip surfaces by the combined effect of kinematics and crystallization (e.g. Norrell et al., 1989; Hoogerduijn Strating and Vissers, 1994; Vissers et al., 1995; Karkanas, 1995; Hermann et al., 2000; Andreani et al., 2005). Within schistose serpentinites, syn-kinematic fi brous minerals are disposed parallel among them and define a structural preferential orientation (Fig. 5c). Mechanisms promoting fibrous mineralisation during ductile shearing are mainly classified into (i) ongoing (re)crystallization of fibres during syn-kinematic schistosity (Andreani et al., 2005; Hi rauchi and Yamaguchi, 2007), and (ii) re-orientation of pre existing fibres (Reinen, 2000). At more brittle rheological conditions, shear structures (fault gouges, fault planes) lo calize the deformation in the rock mass immediately at the boundary of the main slip surface. Fault gouges produce a cataclastic tectonic melange (Fig. 5d) accommodating sev eral meters of displacement by pervasive fragmentation of the host rock (e.g. Hoogerduijn Strating and Vissers, 1994; Hirauchi and Yamaguchi, 2007). External fluids permeate along the slip planes favouring the mineralisation of fibres. Interconnection between different fault systems can be pro vided by minor brittle structures (secondary faults, fracturing network) that also define a hydraulic pathway for fluid mi gration within the rock volume. Along fault surfaces, fibres tend to occur along the slip planes (Fig. 5e) parallel to the slip direction. Within these semi-brittle deformation condi tions, back-scattered electron (BSE) images reveal that single fibres may be fragmented (Fig. 5f). Micro-cleavage occurs parallel (and along) the fibre length (i.e. the length-wise sep aration), and new formed fibres tend to arrange themselves parallel to the cleavages. It should be noted that, statistically, new formed fibres are shorter and thinner with respect to the pristine mineral from which they originated. Although the fibres maintain a similar aspect ratio (i.e. the length divided by its width), their negative effects on human health may in crease as the particles reach a respirable size (e.g. Gunter et al., 2007). www.nat-hazards-earth-syst-sci.net/11/1267/2011/ Nat. Hazards Earth Syst. Sci., 11, 1267-1280, 2011 1272 G. Vignaroli et al.: Rock fabric and asbestos hazard Fig. 4. (a) View of a tectonic window in Calabria region (southern Italy), where ophiolitic sequences are bounded from overlying continental basement units by tectonic contacts (after Rossetti et al., 2001); (b) metagabbros with sheared zone of foliated chloritoschists (Ligurian Alps, Italy); (c) picture and line-drawing of massive metabasite with pervasive serpentinized fault systems (Ligurian Alps, Italy). Non-shear structures (e.g. syn-metamorphic veins, ten sile fractures; Passchier and Trouw, 2005) can be thought as dilatation sites in rocks where fibrous mineral form in concomitance of fluid circulation (e.g. Wicks and Wittaker, 1977; Karkanas, 1995; Hermann et al., 2000; Auzende et al., 2006; Compagnoni and Groppo, 2006). Syn-tectonic veins are sites of mineral crystallization characterized by confined mass transfer processes (Cox and Etheridge, 1989; Barker et al., 2006). Fibrous serpentine texture in vein can occur in response to progressive, incremental crystallographic orien tation during mineral formation (Andreani et al., 2004). Slipfibres structures present fibrous minerals disposed parallel with respect to the vein walls (Fig. 6a). In cross-fibres struc tures, fibres tend to be disposed roughly perpendicular to the boundaries of the vein (Fig. 6b). In both cases, fibres are confined within the vein body and their length and amount are directly proportional to the displacement rate (slip-fibre veins) and to the width of the vein (cross-fibre veins). Com monly, tensile fractures (fracture mode-1 of Atkinson, 1987) developed within massive mafic rocks display fibres arranged in flaws and dispersed on the fracture surfaces (Fig. 6c). In that case, fibres do not cover the entire fracture surface, but a patchy distribution is more common. At the microscale, fibres appear as confined within the fracture walls, show ing textural equilibrium with the fracture-filled minerals (e.g. quartz or plagioclase) (Fig. 6d). Within this microstructure, fibre length (and abundance) depends on the fracture surface and aperture. 2.4 Fluid-rock interaction Both ductile and brittle deformation patterns induce mechan ical perturbation in the rock volume that corresponds to pref erential pathways for fluid flow in rocks (secondary perme ability creation and maintenance; Oliver, 1996, and refer ences therein). Structurally-controlled fluid flow plays a ma jor role in mineral reactions, mass transfer, and deformation in metamorphic rocks (Ferry et al., 1994). Metasomatism of ophiolitic rocks by contamination due to silica-rich fluids al lows the destabilization of former mineral assemblages (e.g. Nat. Hazards Earth Syst. Sci., 11, 1267-1280,2011 www.nat-hazards-earth-syst-sci.net/11/1267/2011/ G. Vignaroli et al.: Rock fabric and asbestos hazard Shear deformation structures 1273 Fig. 5. Examples of ductile and brittle shearing in ophiolites. (a) Meso-scale ductile mylonitic shear zone with development of serpentinites in massive lherzolites; (b) meso-scale pervasive schistosity in serpentinites accomplished by occurrence of free asbestiform minerals (probably tremolite); (c) SEM image of strongly aligned amphibole in foliated microfabric; (d) decimetric-width fault gouges in serpentinites with fine-grained antigorite (the white horizons); (e) antigorite slip fibres developed on fault surface in metagabbros; (f) BSE images of fibrous amphiboles showing fracturing in parallel thinner fibres. Mineral abbreviation: Am: amphibole. olivine, pyroxene) in favour to hydrated ones (e.g. serpen tines and amphiboles) (e.g. Trommsdorff and Evans, 1974; Evans, 1977; Schmidt and Poli, 2003; Bebout, 2007; Bellot, 2008). The continuous feedback between dynamic recrys tallization, strain localization, and fluid channelling implies deep changes into the structural permeability of the rock vol ume (e.g. Etheridge et al., 1984; Barnes et al., 2004). In upper crustal conditions, fault-related fracture systems are the most important mechanism allowing infiltration of external fluids (e.g. Kerrich, 1986; Marquer and Burkhard, 1992). The extreme heterogeneity within the structural archi tecture of fault zones induces hydrodynamic partition within the massive protolith (e.g. Caine et al., 1996). Chemical dise quilibrium between external fluid flow and wallrock has been www.nat-hazards-earth-syst-sci.net/11/1267/2011/ Nat. Hazards Earth Syst. Sci., 11, 1267-1280, 2011 1274 G. Vignaroli et al.: Rock fabric and asbestos hazard Non-shear deformation structures Fig. 6. Examples of non-shear deformation structures in ophiolites. (a) Antigorite slip-fibre and (b) antigorite cross-fibre veins in massive serpentinites; (c) tensile fracture in massive metagabbro with filling mineralisation composed by quartz, plagioclase, and fibrous amphiboles; (d) microphoto of thin section from fracture in (c), showing a plagioclase texture hosting fibrous amphibole crystals. Mineral abbreviations: Am: amphibole; Pl: plagioclase; Qtz: quartz. described as responsible of development of secondary min eralisation in ophiolites (e.g. O'Hanley, 1991; Kyser et al., 1999), including fibrous ones (Karkanas, 1995). Within this "open system" fluid flow process (Oliver, 1996, and refer ences therein), geochemical contamination acts through ma jor fluid-hosted structures and is not pervasive within the un deformed wallrock volume. 3 Discussion The asbestos hazard in ophiolites is intimately connected with the geological properties of the rock mass. The asbestos occurrence is not a casual aspect, but testifies of a series of geological factors that are diagnostics of the geological his tory of the host rocks. A large literature documents the occurrence of both as bestos serpentines and asbestos amphiboles in different mafic and ultramafic lithologies, as well as in metasediments (e.g. Ross and Nolan, 2003; Compagnoni and Groppo, 2006; Van Gosen, 2007; Hendrickx, 2009; Giacomini et al., 2010). It is therefore inferred that the lithological heterogeneity of the ophiolitic rocks is not the main factor controlling the asbestos formation. The following points should be taken into account: - asbestos is not a primary constituent of the ophiolitic rock mass; - asbestos is typically described in specific structural con ditions of the rock mass; - a narrow range of pressure and temperature conditions control the stability field of these minerals; - chemical alteration of the host rock is often responsible for the formation of asbestiform minerals. Through the above-reported comparison between previous works and our experience, we recognise a first-order correla tion between asbestos formation (the asbestos hazard: Ah) in ophiolites with (i) the metamorphic cooling/decompression history (cd), (ii) the interplay between ductile and brittle de formation structures and the derived secondary permeabil ity (sp), (iii) the activity of polyphase deformation (pd), and Nat. Hazards Earth Syst. Sci., 11, 1267-1280,2011 www.nat-hazards-earth-syst-sci.net/11/1267/2011/ G. Vignaroli et al.: Rock fabric and asbestos hazard 1275 obduction no subduction-zone metamorphism, localized deformation (sole thrust) subduction/accretion prograde metamorphism, increase ofP and T, ductile-to-brittle deformation Orogenic cycle exhumation/cratonization retrograde metamorphism, decompression and cooling, ductile-to-brittle deformation Fig. 7. Schematic diagram correlating the asbestos hazard in ophiolites (Ah) to the modality of sea-floor exposure and different orogenic stages. The Ah is expressed as a spectrum of possibilities directly connected to the rock fabric heterogeneities (ductile-to-brittle deformation localization, metamorphism/metasomatism, and fluid flux) that are representative of the tectonic environments. The diagram suggests that Ah increases in ophiolites where a larger variety of deformation structures and associated metamorphic mineralisation can be expected due to the progressive superimposition of chronologically distinct tectono-metamorphic events. (iv) the fluid flux (ff) experienced during the geological his tory of the rock. Such a relationship is here expressed as: Ah = cd sp pd ff. (8) Equation (8) is a parameterisation of the asbestos hazard in function of the main geological processes that are at the ori gin of the rock fabric heterogeneities in ophiolites, and Fig. 7 represents the probability of asbestos occurrence in qualita tive terms (i.e. low to high probabilities) as a function of the tectonic/geodynamic environment. According to the lit erature on asbestos occurrence and hazard (see references in Table 1), Ah can be expressed as a spectrum of geo logical possibilities that are influenced by superimposition of chronologically distinct tectonic events (from sea-floor to orogenic), interconnection between different deformation patterns (ductile to brittle), multistage dynamic metamor phism/metasomatism, and polycyclic fluid/rock interaction. In processes involved in sea-floor exposure, localized rock deformation along lithospheric detachment and metasomatic fluid accomplish for decompression and cooling providing conditions for nucleation of chrysotile and low-T amphiboles (actinolite) within ductile-to-brittle deformation struc tures (e.g. Hoogerduijn Strating and Vissers, 1994; Vissers et al., 1995). On the other hand, a less pervasive deforma tion is expected for ophiolites adiabatically exhumed at mid ridge spreading setting, where large volumes of these rocks preserve their original fabric (Mrintener and Piccardo, 2003). In processes concerning orogenic events, a large variety of (polyphase) deformation structures and associated metamorphic mineralisation can be considered in ophiolites experi encing the complete subduction-exhumation orogenic cycle (Scambelluri et al., 1995; Hermann et al., 2000; Barnes et al., 2004; Li et al., 2004). In this case, due to the pro gressive superimposition of chronologically distinct tectonometamorphic events, the interaction between progressive de formation (from ductile to brittle conditions), decompres sion/cooling, and fluid circulation enhances the structural permeability in the rock mass (e.g. Oliver, 1996). Defin ing occurrence and possible interaction between rock fabrics derived from different geological processes is fundamental to assess the distribution of the asbestos-bearing localised zones. 4 The geological multidisciplinary approach In order to fix the mode and types of asbestos occurrence in ophiolites, we propose that geological-structural field work aimed at defining the deformation fabrics of the host rock should be the prerequisite of a multidisciplinary geological research program that includes mineralogical, petrological, www.nat-hazards-earth-syst-sci.net/11/1267/2011/ Nat. Hazards Earth Syst. Sci., 11, 1267-1280, 2011 1276 Fig. 8. Flowchart of the proposed multidisciplinary approach cor relating the geological properties of the ophiolitic rocks to the as bestos hazard. and geochemical analyses. The flowchart shown in Fig. 8 delineates such an approach, which is detailed below. The geological-structural survey carried out at the proper scale should focus on the geometry (i.e. spatial distribution, width, and frequency), character (brittle vs. ductile), and at titude of those structures in which fibrous mineralisation is visible at the naked eye. The systematic sampling of dif ferent lithologies, deformation structures, and related min eralisation are critical to a series of laboratory analyses to identify: Nat. Hazards Earth Syst. Sci., 11, 1267-1280,2011 G. Vignaroli et al.: Rock fabric and asbestos hazard 1. the petrographic-structural characteristics. Meso- and micro-scale investigations aided by back-scattered elec tron images acquired at the scanning electron micro scope (SEM) scale will be addressed to define relations between different generations of mineral types and ge ometry and texture of the main deformation structures. For the identification, description, and quantitative de termination of common fibrous minerals, both tradi tional techniques (optical microscopy, X-ray powder diffraction, and infrared spectroscopy) and recent tech niques (^.-Raman spectroscopy) should be used. Op tical microscopy, together with SEM and transmission electron microscope observations, allows characterising the petro-textural and morphological features of the fi bres. Diffraction and infrared spectroscopy provide de termination on the chemistry of the fibrous mineral. The ^.-Raman spectroscopy helps to quickly identify fibrous minerals, especially those of the serpentine group, di rectly on the rock chip (Rinaudo et al., 2003; Groppo et al., 2006). The goal is the assessment of the linkage be tween rock fabrics and types of fibrous mineralisation; 2. the thermobarometric environmental conditions for as bestos crystallization. Based on quantitative chemical analyses of equilibrium mineral assemblages, P-T esti mates obtained from both inverse and forward petrolog ical modelling techniques (Powell and Holland, 2008) are compared in order to define the P-T trajectory fol lowed by the host rock and recognize the potential P-T regimes compatible with asbestos crystallization; 3. the geochemistry of the fluid/rock interaction processes leading to rock alteration and asbestos growth in ophiolites. A whole-rock geochemical balance (major and trace elements) from the unaltered to the altered rock mass will be performed to derive the volume and chem istry of the circulating fluids. Fluid inclusions analy ses integrated with stable isotopes systematics are used to reconstruct the linkage between deformation history and the paleo-fluid circulation system. These investi gations will determine the source, the volume, and the physical-chemical parameters of the fluids accompany ing fibrous mineralisation in deformation structures; 4. the textural-morphological aspect of fibres investigated at optical microscopy and through SEM analyses; 5. the degree of free asbestos fibre (referred as Release In dex by the Italian Ministerial Decree no. 178, 14 May 1996) that can be produced by application of external stresses on the rock mass (crushing, milling; Bellopede et al., 2009). The aim is to link the geological properties of the rock sample with the type and quantity of parti cles that can be inhaled and thus considered dangerous to human health. The use of experimental apparatus to simulate the effects of crushing and areal dispersion of www.nat-hazards-earth-syst-sci.net/11/1267/2011/ G. Vignaroli et al.: Rock fabric and asbestos hazard 1277 the rock fragments/fibres (e.g. Belardi et al., 2008) may provide a standardised and robust database. The integration and synthesis of the various results should permit establishing correlations existing between (i) the de velopment of deformation structures, the alteration mineral ogy, and growth of asbestos in ophiolites; (ii) intrinsic ge ological properties of the rock mass (mineralogical compo sition, texture, mechanical properties); and (iii) the charac teristics (in terms of shape, dimension, and quantity) of the air-dispersed particles generated by the different physicalmechanical solicitations. The proposed multidisciplinary approach can provide the basis to propose a predictive protocol finalised to the mit igation of effects due to asbestos contamination in environ ments where asbestos still constitutes a problem. These envi ronments can correspond to engineering operativities of ex traction, transport, and storage of ophiolitic rocks devoted to infrastructures or territory planning (e.g. Rohl et al., 1977; Ross and Nolan, 2003; Bandli and Gunter, 2006; Giacomini et al., 2010; Vignaroli et al., 2010). Our approach aims to propose specific techniques supporting regulatory agencies dealing with the natural materials in the framework of the asbestos hazard that are currently based on morphological factors (fibre length/width ratio) and chemical compositions. We are confident that national and international organizations concerned with the environmental impact due to asbestos ex posure (Kazan-Allen, 2005; Lee et al., 2008) will consider such geological studies as a baseline to improve mitigation effects and to help minimize the risk of asbestos exposure to the general population. 5 Conclusions Our proposal aims to be placed at the source of the asbestos topic (see flowchart in Fig. 1). As asbestos naturally occurs in ophiolites, we suggest relating the geological properties of ophiolite suites with the asbestos hazard (Ah) in terms of rock fabric heterogeneities created in response to the dif ferent tectonic/geodynamic settings. Ophiolites in orogenic settings (recording oceanic formation at spreading centres to oceanic destruction at convergent margins) are expected to gain rock fabric systematics particularly prominent for the nucleation and growth of fibrous minerals, due to the feed back between cooling/decompression history (cd), secondary permeability (sp), polyphase deformation (pd), and fluid flux (ff). This correlation should be considered as a first step to outline the structural-metamorphic control on growth of as bestos in ophiolites. Understanding that fibrous mineral oc currence in natural prototype is a record of the structuralmetamorphic history helps to reconstruct an analytical and methodological procedure for investigation of the rock fab ric aimed at evaluating the asbestos hazard. Our synthesis implies that the geological-structural context of a particular geological site defines a first-order aspect to be taken into consideration by asbestos regulatory agencies, especially in connection with habitual engineering operations of extrac tion, transport, and storage of ophiolitic rocks. Acknowledgements. This paper benefited from the advice and contributions in the field by C. Faccenna, M. Malusa, V. Olivetti, R. Polino, G. Spagnolo, and G. Urru. Advice from F. Tecce and T. Theye are also acknowledged. We are grateful to the reviewers (M. E. Gunter and B. S. Van Gosen) for significantly improving the quality of the manuscript and to the Editor (F. Guzzetti) for handling the manuscript. 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