jadeitite from the monviso meta-ophiolite, western alps: occurrence and genesis

Jadeitite from the Monviso meta-ophiolite, western Alps:

occurrence and genesis

ROBERTO COMPAGNONI1,2,*, FRANCO ROLFO1,2 and DANIELE CASTELLI1,2

1 Dipartimento di Scienze Mineralogiche e Petrologiche, Universita degli Studi di Torino, Torino, Italy2 CNR – IGG, Via Valperga Caluso 35, 10125, Torino, Italy*Corresponding author, e-mail: [email protected]

Paper dedicated to the memory of Prof. Shoei BANNO

Abstract: A block is described, which is exposed in the antigorite serpentinite of Vallone Bule, belonging to the Basal SerpentiniteUnit of the Monviso massif (Piemonte Zone of calcschists with meta-ophiolites). The block consists of a quartz-jadeite rock core and ajadeitite rim, very similar to the rocks used by prehistoric men to make stone axeheads. In spite of their different bulk-rockcompositions, both core and rim show the same trace and rare earth elements patterns, suggesting the same protolith. The quartz-jadeite rock exhibits a major, trace and rare earth elements composition consistent with that of oceanic plagiogranite, most likely adyke cutting across upper mantle peridotites, later hydrated to serpentinites. Conversely, the jadeitite, which consists mainly of zonedjadeite crystals progressively enriched in the diopside component from core to rim, is significantly depleted in Si but enriched in Mgand Ca with respect to the quartz-jadeite rock. The trace and rare earth elements similarities and the ubiquitous presence of smallzircons suggest that the jadeitite and the quartz-jadeite rock both derive from a plagiogranite; however, jadeitite would haveundergone a metasomatic process involving a significant desilication and Mg- and Ca-enrichment, connected to the host peridotiteserpentinization. The process, responsible for the transformation of the plagiogranite into a jadeitite, should have occurred duringprograde Alpine high-pressure (eclogite-facies) metamorphism, since the first Na-pyroxene formed is jadeite, corroded and partlyreplaced during the metasomatic process by a progressively more omphacitic pyroxene. Because similar rocks – mostly jadeitites, buteven their plagiogranite protoliths – are reported from other localities from the Western and Maritime Alps, it is likely that the rawmaterials of most jadeitites used to make stone axeheads, which are spread all over the Western Europe, have a similar origin andderive from the western Alps as long suggested.

Key-words: jadeitite origin, Monviso massif, western Alps, oceanic plagiogranite.

1. Introduction

From the early Neolithic to the Bronze Age, i.e., inWestern Europe from the sixth to the second millenniumB.C., prehistoric men used axes and chisels made ofpolished stones, collectively known in the archaeologicalliterature as ‘‘green stones’’, because of their colour (e.g.,Ricq-de-Bouard & Fedele, 1993). The ‘‘green stones’’mainly consist of serpentinite and a number of high densityrocks (r . 3) mostly consisting of fine-grained eclogites,and related high-pressure (HP) metamorphic lithologiessuch as Na-pyroxenites, which include omphacitite, jadei-tite and mixed omphacite-jadeite rocks (e.g., D’Amicoet al., 1995; Compagnoni et al., 1996).

The first author, who reported jadeite from Monviso, wasDamour (1881): he described and analysed a sample fromthe Pisani’s collection (labelled ‘‘Gruner Jaspis von M. Visoin Piemont’’), whose composition (his analysis ‘‘L’’)resulted ca. Jd80. However, the real discovery of jadeite inthe western Alps is due to Franchi (1900, 1903, 1904), who

described a number of jadeite findings from both the westernAlps and Ligurian Apennines and compared them withimplements from the Italian archaeological sites.

More recently, petrographic studies of Neolithic axesfrom archaeological sites of southern France and northernItaly (e.g., Nadeau et al., 1993; Ricq-de-Bouard, 1996,Fig. 39; D’Amico et al., 1995; Compagnoni et al., 1996)have given, on the basis of the percentage of the HP rocksamong polished stone implements, a further evidence thatthe source area of the raw material should have beenlocated in the western Alps. However, whereas primary(in outcrop) or secondary (in sediments) serpentinites,fine-grained eclogites and omphacitites were known in anumber of localities from the Piemonte Zone of calcschists(or ‘‘schistes lustres’’) with meta-ophiolites of the westernAlps, as well as in the alluvial post-orogenic Oligocene toQuaternary conglomerates, only very scanty and tinyoccurrences of jadeitites were reported up to recently,and in most cases with petrographic characteristics differ-ent from those observed in the archaeological implements.

Jadeitite:new occurrences, new data,

new interpretations

0935-1221/11/0023-2164 $ 4.95DOI: 10.1127/0935-1221/2011/0023-2164 # 2011 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

Eur. J. Mineral.

2012, 24, 333–343

Published online October 2011

For such reasons, a systematic search of jadeitites wascarried out in the Piemonte zone, especially in theQuaternary deposits – derived from the erosion of themain bodies of HP meta-ophiolites – where they areexpected to be relatively abundant because of their tough-ness, hardness and low alterability with respect to otherlithologies of the primary rock association. The systematicsearch started around the Monviso massif, because it is cutby a number of mountain streams whose drainage mostlyconsists of meta-ophiolite. With the active and determinantcollaboration of two mineral collectors, Franco Manavellaand Franco Salusso, incited by Giorgio Peyronel, the latelamented mineral curator of Museo Regionale di ScienzeNaturali of Torino, the first jadeitite block was discoveredin the summer 2002 at an elevation of about 2400 m a.s.l.just to the north of Punta Rasciassa. This first discoverywas preliminary reported by Compagnoni & Rolfo (2003)and studied by Compagnoni et al. (2007). The petrographicstudy of this jadeitite block, about one cubic meter in size,has shown an unexpected variability in both microstructureand mineralogy (Compagnoni et al., 2007), and character-istics quite different from those found among the archae-ological implements, e.g. the sporadic presence of garnet.

Following the discovery of the Punta Rasciassa jadeitite(Compagnoni et al., 2007) a further, extensive search in thesurrounding area led to the discovery of about a dozenjadeitites, most of them occurring as loose blocks with ashort transport story and three of them in place in theoutcrop, in Vallone Bule and Vallone Chiot del Porcoleading upstream to Punta Rasciassa. The petrographicexamination of these new jadeitite blocks, showing micro-structural and mineralogical characteristics closer to thoseof some archaeological jadeitites, also revealed the pre-sence of a peculiar sample (OF3039), whose characteris-tics were most helpful in understanding the nature of itsprotolith and its metamorphic evolution.

This paper deals with a study of the petrography, mineralchemistry and whole-rock geochemistry of sampleOF3039, leading to the conclusion that the Monviso jadei-tite was derived from an oceanic plagiogranite protolith.

2. Geological framework

The Monviso metamorphic ophiolite composite unit (cf.Lombardo et al., 2002), in the following referred to as‘‘Monviso meta-ophiolite’’, is a north-south trendingbody, 35 km long and up to 8 km wide (Fig. 1 and 2),structurally sandwiched between the underlying Dora-Maira thrust units and the dominantly metasedimentaryunits of the ocean-derived Piemonte Zone of calcschists(or ‘‘schistes lustres’’) with meta-ophiolites, in the oldliterature known as ‘‘Pietre Verdi’’, i.e. Green Stones(Fig. 1). The Monviso meta-ophiolite consists of fragmentsof the former Tethyan oceanic lithosphere (meta-ophiolites)and its Mesozoic sedimentary cover (calcschists), that dur-ing the Alpine orogeny experienced Alpine subductionpeaking under eclogite-facies conditions at T�600 �C

(e.g., T ¼ 620 � 50 �C: Messiga et al., 1999; T ¼ 580 �40 �C: Schwartz et al., 2000; T � 550 �C: Groppo &Castelli, 2010) and P � 2.0 GPa (e.g., P ¼ 2.4 GPa:Messiga et al., 1999; P ¼ 1.9 � 0.2 GPa: Schwartz et al.,2000; P ¼ 2.5–2.6 GPa: Groppo & Castelli, 2010).

Locally, the eclogites of the Monviso meta-ophiolite arecut by omphacite metamorphic veins, which have been thesubject of detailed structural, petrographic, fluid inclu-sions, and stable isotope studies suggesting they formedby local circulation of fluids at eclogite-facies conditions(Philippot & Selverstone, 1991; Nadeau et al., 1993;

Fig. 1. Simplified structural sketch-map of the Western Alps. (1)Helvetic Domain, external Penninic Domain and Prealpine decolle-ment nappe system. The dashed line contours the ExternalCrystalline Massifs (vertical ruling) of the Helvetic Domain (AR:Argentera, P: Pelvoux, BD: Belledonne, MB: Mont Blanc –Aiguilles-Rouges, AG: Aar-Gotthard). SB: Grand St. BernardZone, LPN: lower Penninic nappes. (2) Internal CrystallineMassifs of the Penninic Domain (MR: Monte Rosa, GP: GranParadiso, DM: Dora-Maira, V: Valosio). (3) Piemonte Zone (VM:Voltri Massif) and (a) main bodies of meta-ophiolites. (4)Austroalpine Domain (DB: Dent Blanche nappe, ME: MonteEmilius nappe, SZ: Sesia Zone) and (a) Southalpine Domain (SA:Southern Alps). (5) Helminthoid Flysch nappes (EU: Embrunais-Ubaye, AM: Alpes maritimes). (6) Swiss Molasse (SM), Po Plainand Piemontese-Ligurian Tertiary basin. CL: Canavese line; SVL:Sestri-Voltaggio line, SF: Subalpine frontal thrust, PF: Penninicfrontal thrust. Rectangle is the southern portion of the Monvisomassif enlarged in Fig. 2.

334 R. Compagnoni, F. Rolfo, D. Castelli

Philippot, 1993). Two retrograde metamorphic stages havebeen inferred, with equilibration under blueschist- andgreenschist-facies conditions, respectively (Lombardoet al., 1978).

Geochronologic data on the Monviso meta-ophiolitesuggest a Tertiary age for the Alpine HP metamorphism(Monie & Philippot, 1989; Duchene et al., 1997; Cliffet al., 1998), which is in line with the radiometric agesobtained from other portions of the Internal PiemonteZone. The most recent geochronologic study of theMonviso meta-ophiolite is a SHRIMP U-Pb determinationon zircons from a syn-eclogitic metamorphic vein, which

yielded an Eocene age of 45� 1 Ma (Rubatto & Hermann,2003).

The Monviso meta-ophiolite comprises at least six dif-ferent units (Fig. 2), the stratigraphy of which has beenrecently reviewed by Lombardo et al. (2002). All jadeititesare found in the Basal Serpentinite Unit, a 400-m-thick beltof antigorite serpentinite, which most likely derives fromformer mantle lherzolites, as suggested by the commonpresence of relict peridotitic clinopyroxenes escaped tothe pervasive serpentinization, with only minor harzbur-gite and dunite. Folded and boudinaged dykes of rodingi-tized metagabbros and metabasalts locally cut the

Fig. 2. Location of the studied sample and associated lithologies, in the framework of the southern portion of Monviso meta-ophiolite(geologic map after Lombardo et al., 1978, modified).

Jadeitite from the Monviso meta-ophiolite, western Alps 335

serpentinites. Lens-shaped bodies of foliated metagabbro,eclogite and metaplagiogranite occur in the upper part ofthe unit. On the divide between Valle Po and Valle Varaita,just west of Colle di Luca (Fig. 1 and 2), i.e., close to thecontact with the overlying Lago Superiore Unit and some 4km north of the large metaplagiogranite body of Verne(Val Varaita: Castelli et al., 2002; Lombardo et al.,2002), the basal serpentinites become progressively morefoliated and include a number of rounded blocks of HProcks (Fig. 3). These blocks, from a few dm to about 100 macross, consist of all the eclogite-facies lithologies exposedin the area, including a variety of coarse- and fine-grained,undeformed to sheared, eclogite and metagabbro, massive-and fine-grained omphacitite, often with lozenge-shapedzoisite pseudomorphs after former lawsonite, and rarejadeitites. Most blocks show a dark green, usually a fewcm-thick, reaction rim with the host antigorite serpentiniteconsisting of a chlorite � actinolite aggregate (Fig. 3).

Jadeitites are massive rocks, very-pale grass green incolour, and show a fine to very fine (,0.5 mm across)grain-size. They occur as blocks from a few dm to about 1meter across within massive or sheared antigorite serpen-tinite. However, most blocks do not occur in their primarysetting (i.e., still included within the host serpentinite) butare found as loose blocks among the debris at the bottom ofthe basal serpentinite slopes. In some jadeitites, an irregu-lar transition to a fine-grained eclogite is locally evident.The locations of the most significant jadeitite bodies areshown in the map of Fig. 2.

The loose jadeititeblocks,which typicallyshowaroundishshape, are rarely retaining a cm-thick coating of the originalhosting serpentinite,usuallyenriched inchloriteorchloriteþactinolite, and are often surrounded by an irregular darkerretrogression margin up to 10–20 cm thick (Fig. 4).

The surface of the studied jadeitite block is cut by anirregular network of grooves from less than 1 mm to

several cm wide and from less than 1 cm to several cmdeep with roundish rims and floors, which appear to haveformed by ablation possibly driven by tectonic erosion(Fig. 4). Locally, the surface is also furrowed by mm-sized cracks, recalling the desiccation mud cracks.

2.1. Petrography and mineral chemistry of jadeitites

Minerals from the studied jadeitite were analysed at theDepartment of Mineralogical and Petrological Sciences,University of Torino, Italy with a Cambridge ScanningElectron Microscope with Energy Dispersive System(SEM-EDS), operating at 15 kV accelerating voltage, 50s counting time, 2.70–2.80 A beam current, and 2 mm spotsize. SEM-EDS quantitative data were acquired and pro-cessed using the Microanalysis Suite Issue 12, INCA Suiteversion 4.01. The raw data were calibrated on natural andsynthetic mineral and oxide standards and the �rZ correc-tion (Pouchou & Pichoir, 1988) was applied. Analyses ofminerals were processed by using the software of Ulmer(1986). Representative mineral analyses are reported inTable 1, while compositional variations of clinopyroxenesare reported in Fig. 7.

The studied sample OF 3039, found as a loose block,was collected in the Vallone Bule along the path toColle di Luca (Luca Pass) at an elevation of about2300 m a.s.l. This block, about 50 cm in diameter,consists of a slightly deeper green rim (OF 3039r) anda paler-green to almost-whitish core (OF 3039c), corre-sponding to a jadeitite and a quartz-jadeite rock,respectively (Fig. 4). The geographic coordinates wherethe jadeitite block has been collected are the following:N 44� 38’ 26.7’’ – E 7� 9’ 12.7’’.

Fig. 3. Two eclogite (Ecl) blocks exposed within foliated antigoriteserpentinite (lighter rocks) belonging to the Basal serpentinite Unit oftheMonvisometa-ophiolite (cf.Fig.2). In theeclogiteblockto the right,a cm-thick chlorite reaction rim with the hosting serpentinite is visible.The Basal serpentinite Unit, besides eclogites and omphacitites, alsoincludes jadeitite blocks such as that shown in Fig. 4. Colle di Luca, onthe divide between Val Varaita and Vallone Bule (Valle Po).

Fig. 4. Mesoscopic view of the composite sample OF 3039. Theweathered surface of the studied block, red-brownish in colour, is cutby an irregular network of grooves with roundish rims and bottoms.The outer (lower) portion of the block is a massive jadeitite, whosegreenish colour darkens towards the exterior because of a progres-sive jadeite retrogression to albite þ aegirine-augite. The inner(upper) portion, pale greenish in colour, is a very well preservedquartz-jadeite rock. The rock fragment is about 50 cm wide.


2.2. Jadeitite (OF3039r)

Under the microscope, the block rim shows a massiveisotropic microstructure and consists almost totally of ajadeite-rich pyroxene with accessory zircon and apatite.Locally, the Na-pyroxene includes a cloudy core, crowdedwith very fine-grained, oriented inclusions of titanite(Fig. 5a, b). This dusty core, which is also present inother HP mafic meta-ophiolites of the Piemonte Zonesuch as eclogites and metagabbros, is interpreted as a relictportion, derived from the former igneous pyroxene whoseoriginal Ti content is still preserved as oriented titaniteneedles (e.g., Compagnoni et al., 1996). The distinctiveaccessory zircons, a few micrometers across, which com-monly show a blocky prismatic shape, may occur as bothtiny single crystals and crystal aggregates (Fig. 6d).

2.3. Quartz-jadeite rock (OF3039c)

The block core mainly consists of a jadeite-rich pyroxeneand quartz with accessory zircon (Fig. 6d) and apatite.Although the pyroxene occurs as crystals of shape andsize similar to those of jadeitite, quartz, with local subgrainmicrostructure (Fig. 5d), occurs as poikiloblasts up to morethan 1 cm across, which include several tens of jadeitecrystals (Fig. 5c, d). This bimodal mineral grain size pat-tern suggests that quartz statically recrystallized to large

poikiloblasts after jadeite growth. Therefore, the quartzsubgrain microstructure was acquired certainly after thejadeite crystallization, most likely late in the rock tectono-metamorphic evolution. The jadeite-rich pyroxenes aresystematically surrounded by a double retrogression cor-ona, consisting of pure albite (Ab95–100) on the inner sidetowards the relict jadeite and a Jd-poor Na pyroxene,usually an aegirine-augite (Fig. 7), on the outer side atthe contact with quartz (Fig. 6c). The orientation of theaegirine-augite crystals, consistent all around each jadeiterelict, suggests its epitaxial growth after the original sodicpyroxene (Fig. 6c).

Chemical compositions of HP pyroxenes, determinedwith SEM-EDS, are very complex and range from almostpure jadeite (Jd98) to ferrian omphacite (up to� Jd45) witha gap between ca. Jd60 and Jd80, most likely correspondingto the well-known miscibility gap (e.g., Green et al., 2007).All spot analyses plot along a line with an almost constantand high Quad/Aeg ratio (Fig. 7). The compositional varia-bility of the analysed pyroxenes, especially evident on theBSE images, may be tentatively interpreted on the base ofmicrostructural relationships (Fig. 6a–c).

Clinopyroxene crystals of the jadeitite are characterizedby the systematic presence of a jadeite-rich core (Jd95–98)and an overall omphacitic rim (Jd45–60), both of whichbeing chemically and optically inhomogeneous (Fig. 6aand 7). The boundary between crystal core and rim is

Table 1. Representative microprobe analyses of clinopyroxene (px) and feldspar (pl) of sample OF 3039 from Vallone Bule. Structuralformulae have been calculated on the basis of 4 cations and 6 oxygens for pyroxene and 5 cations and 8 oxygens for feldspar. Structuralformula of pyroxenes is calculated according to Ulmer (1986), following the assumptions of Wood & Banno (1973). (-) not analysed. (*)analyses with oxides recalculated to 100.00 %.

Analysis 5 13 16 19 20 21 23 29 47 48 49 32 33

corona jd-z13rr jd-zo16c jd-z19in simpl-20 simpl-21 jd-cr23c jd-29c Jd47rr Omp48 Aeg49Ab vs.

JdAb vs.

Qtzouter outer outer outer outer outer outer inner inner inner inner inner inner

Mineral px px px* px px px px* px px px px pl pl

SiO2 58.31 58.82 59.48 57.60 58.12 57.34 57.52 59.04 58.98 57.87 54.87 68.29 68.96TiO2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.70 1.70 1.03 0.00 0.00Al2O3 14.27 19.35 24.59 12.91 14.45 10.76 14.16 24.15 19.58 12.16 1.87 18.44 18.73Fe2O3 4.28 3.99 0.01 4.82 3.90 4.49 3.89 0.00 3.72 4.05 17.26 0.47 0.00FeO 0.53 0.00 0.43 0.00 0.70 0.62 0.72 0.72 0.01 1.00 2.81 – –MnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 – –MgO 5.62 2.19 0.00 6.31 5.49 8.08 5.39 0.00 1.73 5.91 5.84 – –CaO 7.90 3.14 0.33 9.09 7.99 11.90 7.91 0.44 2.31 7.95 9.01 0.48 0.00Na2O 10.58 13.54 15.15 9.92 10.52 8.26 10.42 14.93 14.04 10.57 9.01 11.67 11.73K2O – – – – – – – – – – – 0.00 0.00Total 101.49 101.03 100.00 100.65 101.17 101.45 100.00 99.28 101.07 101.21 101.7 99.35 99.42

Cations assuming stoichiometry and charge balanceSi 2.008 2.002 2.007 2.007 2.007 1.999 2.010 2.010 2.005 2.010 2.013 3.008 3.031Ti 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.018 0.044 0.028 0.000 0.000Al 0.579 0.776 0.978 0.530 0.588 0.442 0.583 0.969 0.785 0.498 0.081 0.957 0.970Fe3+ 0.111 0.102 0.000 0.126 0.102 0.118 0.102 0.000 0.095 0.106 0.477 0.016 0.000Fe2+ 0.015 0.000 0.012 0.000 0.020 0.018 0.021 0.021 0.000 0.029 0.086 – –Mn 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 – –Mg 0.289 0.111 0.000 0.328 0.283 0.420 0.281 0.000 0.088 0.306 0.319 – –Ca 0.292 0.115 0.012 0.339 0.296 0.445 0.296 0.016 0.084 0.296 0.354 0.023 0.000Na 0.707 0.894 0.991 0.670 0.705 0.558 0.706 0.985 0.925 0.712 0.641 0.997 0.999K – – – – – – – – – – – 0.000 0.000


irregular but sharp and microstructural relationships sug-gest that the jadeite-rich core has been strongly corrodedand replaced by the omphacitic rim. Locally, the jadeite-rich core is surrounded by a zone, with intermediate com-position (Jd80–85), that appears to corrode the core, but inits turn it is corroded and locally totally replaced by theomphacitic rim (Fig. 6a, b and 7).

In contrast, the HP pyroxene crystals of the quartz-jadeite rock consist of a jadeite-rich core (up to Jd98),which is surrounded and corroded by younger rims withthe jadeite-component progressively decreasing from ca.Jd88 to ca. Jd80 (Fig. 7, Table 1). Locally, rare inclusions ofquartz and blebs of omphacite (Jd50) are observed injadeite, which may be also injected along cracks by albitederived from the retrogression corona (Fig. 6c).

2.4. Cathodoluminescence (CL)

A representative thin section of the quartz-jadeite rock hasbeen observed with an optical polarizing microscopeOlympus BH 2 equipped with a cathodoluminescenceCITL 8200 mk3. As evident from the image of Fig. 6d,CL colours are peculiar to each mineral: quartz is very darkblue to almost black, jadeite is bright blue, the exsolutionomphacite blebs within jadeite are pink, the retrogradeaegirine-augite coronas around jadeite are red, retrogradealbite coronas around jadeite are black to very dark blue,and the relict igneous zircons are bright white. Zircon,apparently homogeneous while observed with the optical

polarizing microscope in transmitted light, in CL shows apeculiar spongy microstructure (Fig. 6d), that is similar tothat observed by Lombardo et al. (2002) in cloudy- topatchy-zoned zircons from the metaplagiogranite ofVerne, and interpreted as the result of a later metamorphicevent affecting the primary igneous zircons.

2.5. Geochemistry of jadeitite and quartz-jadeite rock

Twoportionsof thestudiedblock, taken fromthe jadeitite rim(OF 3039r) and the quartz-jadeite rock core (OF 3039c),respectively, were chemically analysed at ALS Chemex,Vancouver (Table2).Bulk-rockchemicaldatawereobtainedby means of ICP-AES and ICP-MS techniques for major andother elements, respectively. Ferrous iron was determined byHCl-HF acid digestion and titrimetric analysis. Detectionlimit for major elements is 0.01 wt% oxide, those for traceand rare earth elements (REE), collected following the ME-MS81 Chemex protocol, are detailed at the ALS Chemexweb page (http://www.alschemex.com/learnmore/periodic_mems81.htm). Chemical data are reported in Table 2. Major,minor and trace elements, REE included, are compared in thediagrams of Fig. 8.

As expected from the modal composition of the two por-tions, the jadeitite has a silica content about 76 % of thequartz-jadeite rock and, conversely, all the other oxides,P2O5 excluded, are higher (Fig. 8a and Table 2). However,the oxide increase in the jadeitite compared to the quartz-jadeite rock is variable, ranging from over 6 times for MgO,

Fig. 5. (a, b) Photomicrographs of the outer portion of the studied sample OF 3039: this portion is a jadeitite consisting of a granoblasticjadeite aggregate locally showing cloudy (c) cores. ((a) Plane Polarized Light – PPL; (b) Crossed Polarizers – XPL). (c, d) Photomicrographsof the inner portion of the same sample OF 3039: this portion is a quartz-jadeite rock, consisting of granoblastic jadeite (Jd) and largepoikiloblastic quartz (Qtz) with incipient subgrain microstructure (c: PPL; d: XPL).


http://www.alschemex.com/learnmore/periodic_mems81.htm

http://www.alschemex.com/learnmore/periodic_mems81.htm

and more than 4 times for CaO, to ca. 1.6 for Na2O, and lessthan 1.5 for Al2O3 and TiO2 (Fig. 8a and Table 2).

As for trace elements (Fig. 8b), their pattern appears sys-tematically enriched in the jadeitite compared to that of thequartz-jadeite rock, with the exception of P (Fig. 8b). Theelement enrichment is uneven, being more significant for Cs,K, Ba and Sr. However, both patterns show evident negativepeaks for Rb, Ba, K, Sr, P, and Ti, and positive peaks centredon Cs, U, Pb, Zr, and heavy REE (Fig. 8c). Also, the REE aresystematically enriched in the jadeitite with respect to thequartz-jadeite rock, but both patterns are very consistent andcharacterized by a strong Eu negative anomaly (Eu/Eu* ¼0.2–0.4, Fig. 8c). The close similarity of the trace-elementpatterns of the jadeitite and quartz-jadeite rock, especiallythat of REE, strongly suggests a genetic relationship.

3. Discussion and conclusions

From the CIPW norm calculations (Table 2), the quartz-jadeite rock exhibits a mineralogy (Q¼ 28.41, Ab¼ 63.47,Or ¼ 0.06, Di ¼ 2.42, Wo ¼ 0.58, Ac (i.e., Aeg) ¼ 1.48, Il

¼ 0.06, Hem ¼ 1.59 and Ti ¼ 0.33, as wt%) similar to thatof a felsic differentiate, consisting primarily of quartz andsodic plagioclase (almost 92 wt%) with only minoramounts of ferromagnesian minerals. Because of its occur-rence within an ultramafic rock belonging to the PiemonteZone of calcschists with meta-ophiolites (Fig.1 and 2a), itis very likely that the protolith of the quartz-jadeite rockwas an ‘oceanic plagiogranite’ dyke (cf. Coleman &Peterman, 1975; Coleman, 1977, p. 50), originally intru-sive into mantle ultramafics. This origin is also supportedby the trace-element pattern (Fig. 8b) and especially theREE pattern, characterized by a marked Eu negative anom-aly (Fig. 8c), when compared to other metaplagiograniteoccurrences from both the Monviso and other Alpine meta-ophiolites.

The Monviso Basal Serpentinite Unit also hosts theaforementioned metaplagiogranite of Verne, which con-sists of a large body of a former leucocratic, Fe-rich quartzdiorite and shows primary contact with eclogitized FeTi-oxide gabbro (Castelli et al., 2002; Castelli & Lombardo,2007). A U-Pb zircon radiometric age of 152 � 2 Ma hasbeen inferred for the emplacement of the quartz diorite

Fig. 6. (a) Back-scattered electron (BSE) image of the outer portion of sample OF 3039, which shows a darker jadeite core (Jd) withexsolution of omphacite, surrounded and corroded by a lighter omphacite rim (Omp) with oriented beads of omphacite of differentcomposition. (b) BSE image of the outer portion of sample OF 3039, which shows at least two jadeite (Jd) generations (dark and mediumgrey) surrounded and corroded by an omphacite (Omp) rim (lighter) with omphacite beads with different composition similar to thoseobserved in (a). (c) BSE image of the inner portion of sample OF 3039, which shows a zoned jadeite (Jd) within quartz (Qtz) surrounded bya double retrogression corona consisting of albite (Ab) on the jadeite side, and aegirine-augite (Aeg) on the quartz side. The aegirine-augitecorona is composed of crystals all with the same crystallographic orientation, suggesting an early epitaxial growth during jadeite retro-gression breakdown. (d) Cathodoluminescence (CL) image of the inner portion of sample OF 3039: the aegirine-augite coronas (Aeg) arered, jadeite (Jd) blue and the omphacite (Omp) exsolutions pink, quartz (Qtz) black and albite (Ab) black to very dark blue. The two brightcrystals are zircons (Zrn). Long side length is 0.5 mm. Numbers refer to the microprobe spot analyses reported in Table 1.


(Lombardo et al., 2002), that is locally cut by late-mag-matic plagiogranite dykes (Castelli et al., 2002; Castelli &Lombardo, 2007). Compositions of these dykes (Fig. 8 andTable 2) are very close to that of the quartz-jadeite rockfrom Vallone Bule, which shows a similar REE patternonly slightly enriched in LREE. The difference in the silicacontents between the quartz-jadeite rock and the Vernemetaplagiogranite suggests that the two rocks, thoughcogenetic, correspond to different degrees of magmaticdifferentiation. However, it cannot be excluded that smallchanges as to major and trace elements in the studiedsample may have occurred even during oceanic alteration.

Similar quartz-jadeite rocks, frequently in close associa-tion with eclogite and interpreted as derived from primaryplagiogranite dykes, were also reported from several local-ities from the Western and Ligurian (Maritime) Alps (seeCastelli et al., 2002, with references therein). A few of suchmetaplagiogranites, intruding lherzolites as dykes, havebeen reported from the Voltri Group and the Sestri-Voltaggio Zone of the Maritime Alps (Borsi et al., 1996)(see also Table 2). It is worth noting that the bulk-rockchemistry of the studied quartz-jadeite rock plots withinthe average of trondhjemites (i.e., the most differentiatedplagiogranites) from the non-metamorphic ophiolites ofnorthern Apennines (e.g., Montanini et al., 2006).

All plagiogranites include small zircons with the samesize and shape as those from the studied block from theMonviso massif. Therefore, on the basis of bulk chemicalcomposition of main, minor and trace elements and thepresence of the small blocky prismatic zircons, the studiedquartz-jadeite rock from Vallone Bule, Monviso massif,

can be interpreted as the metamorphic product of a formerplagiogranite dyke, emplaced into mantle peridotites some150 Ma ago, and later boudinaged and metamorphosedunder eclogite-facies conditions during the Alpineorogeny.

The studied block from Vallone Bule, Monviso massif,consists of a jadeitite rim and a quartz-jadeite rock core.The two lithologies share similar REE patterns and theubiquitous occurrence of small zircons with a peculiarblocky prismatic shape, which are typical of oceanic pla-giogranites (e.g., Ohnenstetter & Ohnenstetter, 1980).Similar to other metaplagiogranites from the WesternAlps and Northern Apennines, the block likely representslate stages of the plutonic activity within the Tethyanultramafic oceanic crust and its formation can be modelledby fractional crystallization of evolved tholeiitic magmas(e.g., Borsi et al., 1998; Montanini et al., 2006; Castelli &Lombardo, 2007). However, because of the main elementdifferences between core and rim of the block, it is neces-sary to assume that at some stage during the evolution ofthe former dyke, took place a process, responsible for thechemistry change of the block rim. This metasomatic pro-cess produced not only a significant decrease in the silicacontent (and therefore the complete disappearance ofquartz in the rim), but also an increase in some majorelements, especially Mg and Ca that are over 6 and 4times, respectively, richer within jadeitite compared tothe quartz-jadeite rock.

This metasomatic process most likely occurred duringthe mantle peridotite serpentinization (e.g., Sorensenet al., 2006; Harlow et al., 2007), since the conversionof a silica-undersaturated rock, mainly consisting of oli-vine, into a silica-saturated and hydrated serpentinite isaccompanied by a notable desilication and a mobilizationof Mg and Ca. It is in fact well known the close spatial andgenetic association between the process of peridotite ser-pentinization and the formation of rodingites, which areCa-enriched rocks, derived from mafic dyke protolithsoriginally intrusive into the peridotite. However, thismetasomatism appears to be significantly different froma typical mafic dyke rodingitization process. In particularNa, which is entirely leached out in rodingites, in thestudied jadeitite is increased of about 1.6 times. Na pre-servation may be the evidence that the metasomatismoccurred at HP metamorphic conditions, when the Na-pyroxene jadeite is a stable phase. This interpretation isalso supported by the microstructural relationships of Na-pyroxenes in jadeitite, suggesting that the jadeite wasstable before the arrival from the hosting ultramaficrock of Mg and Ca, responsible for the jadeite corrosionand its conversion to omphacite.

It is interesting to note that the petrographic study ofNeolithic stone implements from southern France andnorthern Italy has shown that most, if not all, jadeititesare characterized by the presence of small zircons (e.g.,Ricq-de Bouard et al., 1990; D’Amico et al., 1995) similarto those observed in the Vallone Bule jadeitite. This pecu-liar mineralogic feature suggests that at least a large part ofjadeitites selected by the Neolithic men to make stone

Fig. 7. Compositions of clinopyroxenes from the inner (quartz-jadeite rock) and outer (jadeitite) portions of sample OF 3039 plottedin the jadeite-aegirine-Quad (normalized wollastonite þ enstatite þferrosilite) diagram of Morimoto et al. (1988). Dashed lines deline-ate the additional compositional fields proposed by Rock (1990); (A)Ferrian omphacite; (B) Aluminian aegirine-augite; (C) Calcianjadeite; (D) Calcian ferrian jadeite; (E) Jadeite; (F) Ferrian jadeite.


implements derives from oceanic plagiogranite protoliths.Similar jadeitites are also found in the Oligocene toQuaternary conglomeratic deposits of Piemonte andLiguria, which derive from the erosion of the HP rocksfrom the Western Alps (Giustetto & Compagnoni, 2004;Compagnoni et al., 2005). A systematic search among thepebbles of the rivers at the boundary between Piemonteand Liguria led to the discovery of jadeitites in Val diLemme, certainly derived from the Voltri massif,Ligurian Alps (Compagnoni et al., 2006). Furthermore, atypical jadeitite has been recently discovered as a looseblock within serpentinites of the Piemonte Zone on the leftside of Valle dell’Orco above Locana (Ivano Gasco, 2008,pers. com.).

In a recent archaeological article by Sheridan et al.(2010, Fig. 3 and 4) a quartz-jadeite rock from the Voltrimassif, which contains in addition to jadeite about 50 vol%of quartz, is reported as jadeitite or quartz-jadeitite: mostlikely it is the eclogite-facies product of an original plagi-ogranite, similar to that preserved in the core of the studiedblock from Vallone Bule. This finding supports the ideathat in the western Alps most jadeitites have formed at theexpense of original oceanic plagiogranite dykes through ametasomatic process characterized by a significant desili-cation of the protolith, connected to the peridotite serpen-tinization. It is to point out that the suggested genesis ofthis jadeitite from the western Alps is different from thatof other similar rocks, especially from the famous

Table 2. Chemical compositions and CIPW norms of the studied jadeitite (OF3039) from Vallone Bule and similar lithologies from theMonviso meta-ophiolite (samples OF2729 and OF2746 from Castelli & Lombardo, 2007) and from the Voltri Group (sample Rio Cane 4,from Borsi et al., 1996).

Oxide(wt %)

Jadeitite(OF3039r)

Qtz–Jd rock(OF3039c)

Plagiogranite(OF2729)

Plagiogranite(OF2746)

Plagiogranite(Rio Cane 4) ppm OF3039r OF3039c OF2729 OF2746

Rb 1.8 1 6.6 6.9Sr 40.2 11 24.3 47.4Ba 23.6 8.2 48.5 79.1Cs 0.41 0.06 0.5 0.4

SiO2 57.10 74.60 70.70 71.00 76.10 Pb 2.5 2.5 2.5 9TiO2 0.23 0.17 0.40 0.55 0.26 Y 183.5 127 168 161.5Al2O3 18.10 12.35 13.90 13.70 12.60 Zr 374 259 900 662Fe2O3 3.79 0.32 2.43 2.00 0.73 Hf 21.7 14.6 28 22FeO 1.60 1.09 0.96 1.80 0.62 Nb 3.6 2.2 18 17MnO 0.10 0.03 0.07 0.07 0.04 Ta 0.8 0.4 1.5 1.3MgO 2.77 0.45 0.34 0.46 0.75 Th 2.76 1.89 2 2CaO 4.31 1.03 0.77 1.10 0.32 U 1.38 1.09 0.9 0.8Na2O 12.10 7.70 7.42 7.41 7.81 Ni 151 18 25 8K2O 0.04 0.01 0.89 0.42 0.10 Co 16.6 6.4 2.7 4.7P2O5 0.01 0.02 0.04 0.04 0.03 V 10 14 15 33L.O.I. 0.28 0.13 0.67 0.39 0.77 Cr 300 90 20 20Total 100.43 97.90 98.59 98.94 100.13 Cu 8 13 2.5 2.5

Ga 82.4 53.7 39 36Zn 36 18 91 86La 10.9 7 35 36.2Ce 39.7 28.8 108.5 106.5Pr 7 5.21 15.5 15.9

Normative minerals Nd 30.9 22.7 70 70.4Q – 27.82 22.43 23.46 29.00 Ag 0.5 0.5 0.5 1Or 0.24 0.06 5.26 2.48 0.59 Dy 24.8 17.05 28.9 26.7Ab 47.75 63.47 62.79 62.70 64.26 Er 20 13.65 20.1 18.2An – – 1.99 2.88 – Eu 1.74 0.73 3.7 3.1Ne 24.45 – – – – Gd 13.8 9.36 20 19.6Di 17.03 4.18 1.22 1.90 1.12 Ho 5.95 4.09 5.9 5.8Wo – – – – – Lu 3.95 2.64 3.2 2.9Ol 0.55 – – – – Mo 1 1 1 1Aeg 8.37 1.48 – – 1.61 Sm 11.7 8.73 20.4 19.5Hy – 0.79 0.28 1.09 1.97 Sn 12 4 17 15Mt 1.30 – 2.16 2.90 0.25 Tb 3.68 2.56 4.1 3.9Il 0.44 0.32 0.76 1.04 0.49 Tl 0.025 0.025 0.25 0.25Hem – – 0.94 – – Tm 3.52 2.35 3.1 3Ap 0.02 0.05 0.09 0.09 0.07 W 1 3 1 2

Yb 25.5 17 20.8 18.8


Guatemalan jadeitite jades, as results from the carefulreview by Harlow et al. (2007).

In conclusion, the new occurrences of jadeitites reportedfrom the internal part of the Piemonte zone likely confirmthat most archaeological jadeitite implements from wes-tern and southern Europe were produced using the raw

materials exposed in the western Alps, as long hypothe-sized (Damour, 1881; Franchi, 1900, 1903, 1904).

Acknowledgements: The authors are very grateful to LucaMartire for assistance with the cathodoluminescenceimages. Careful and constructive reviews by TakaoHirajima, an anonymous referee, and George Harlow, theGuest-Editor, significantly improved the paper.

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Received 4 April 2011

Modified version received 29 July 2011

Accepted 23 August 2011


jadeitite from the monviso meta-ophiolite, western alps: occurrence and genesis

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