Title
High-pressure meta-ophiolite boulders and cobbles from northern Italy as possible raw material sources for “greenstone” prehistoric tools: petrography and archaeological assessment
Running title:
Characterization of HP meta-ophiolite boulders and cobbles
Benjámin Váczi1,*, György Szakmány1, Elisabetta Starnini2, Zsolt Kasztovszky3, Zsolt Bendő1, Flavio A. Nebiacolombo4, Roberto Giustetto5 and Roberto Compagnoni5
1 Department of Petrology and Geochemistry, Eötvös Loránd University, Pázmány sétány 1/C, Budapest, H-1117, Hungary
2 Department of Civilizations and Forms of Knowledge, University of Pisa, via dei Mille 19, 56126, Pisa, Italy & School of Humanistic Sciences, Department of Historical Sciences, University of Turin, via S. Ottavio 20, 10124, Torino, Italy
3 Centre for Energy Research, Hungarian Academy of Sciences, Konkoly Thege Miklós út 29-33, 1121 Budapest, Hungary
4 Independent Researcher, Recco (GE), Italy
5 Department of Earth Sciences, University of Turin, Via Valperga Caluso 35, 10125 Torino, Italy *corresponding author, email: vbeni9305@gmail.com
Abstract;
Recent research and field surveys, performed on the Monviso massif as well as in the Po and Curone valleys, revealed the presence of high-pressure (HP) meta-ophiolites – namely “greenstones” – in the form of blocks extracted from primary outcrops or erratic cobbles/boulders in the alluvium, respectively. These rare lithotypes are important, as they may have been used by prehistoric people as raw materials for the manufacture of polished stone tools, in particular axes/adzes, blades and chisels, found all over western Europe and along a wide corridor running from southern Italy to the British Isles. The bulk chemistry and microstructural features of “greenstone” HP-meta-ophiolites collected during the geological surveys were characterized by
polarizing microscopy and SEM-EDX on thin sections, as well as bulk prompt-gamma activation analysis. By comparing the most significant aspects recurring in specimens from a
particular site of collection, a chart was provided indicating the distinctive features of each provenance. These discriminant features were also compared with those of Neolithic polished stone tools from archaeological sites, thus providing interesting outcomes about the supposed sources of their raw materials. In particular, the results suggest a possible local origin for the raw materials used in the Neolithic workshops for the production of “greenstone” artefacts of Rivanazzano (in the Staffora valley) and Brignano Frascata (Curone valley), collected as cobbles/boulders from the secondary deposits of HP-meta-ophiolites found in the same Curone valley and/or other adjacent sites. Our results contravene the results of previous explorations, performed in the same Curone and Staffora valleys, which failed in finding HP-metamorphic lithotypes in the alluvial detritus of the local streams, eroding the Oligocene Conglomerates. These lithotypes are probably relicts from the dismantling of HP-meta-ophiolites from the Voltri Massif (or an analogous palaeo-unit, now
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eroded) during the Oligocene, then redeposited as alluvial sediments in Pleistocene and more recent times.
Key-words: Tertiary Piemonte Basin conglomerates; high-pressure meta-ophiolites; prehistoric
polished stone tools; archaeometry; provenance; secondary alluvial sources; artefact; eclogite;
jadeitite. Text;
1. Introduction
1.1 General specifics of HP meta-ophiolites
These rocks, all with different shades of green colour, were widely used by prehistoric populations for the production of “greenstone” polished stone tools, from the Neolithic (ca. 7 ky ago) until the Bronze Age (Ricq-de-Bouard et al., 1990; D’Amico et al., 2002; D’Amico & Starnini, 2006a; Giustetto et al., 2008). Different rock types are grouped under the term “greenstone”. From a mineralogical point of view, C. Daryll Forde, since 1930, stated that a
sharp distinction should be made between “jade” (nephrite) and jadeite. “Jadeite jade” is a rock made of jadeite, the NaAlSi2O6 pyroxene (specific gravity 3.30 to 3.35); “nephrite jade”,
on the other hand, is a rock made of the lime magnesia silicate [Ca2(Mg,Fe)5Si8O22(OH)2],
usually an amphibole of the tremolite–ferro-actinolite solid-solution series (specific gravity 2.95 to 3.00). According to a recent classification (Giustetto & Compagnoni, 2014), these rocks can
be divided into two main groups: i) ‘Na-pyroxene rocks’, comprising jadeitite (mostly made of the mineral jadeite: 95-100 vol.%), omphacitite (mostly made of omphacite: 95-100 vol.%) and mixed-Na-pyroxenites (with intermediate modal amounts of both jadeite and omphacite); ii) ‘Na-pyroxene + garnet rocks’, comprising eclogite (made of omphacite and garnet in the mutual modal range 25-75 vol.%), garnet-omphacitite (with omphacite – 25-75-95 vol.% – prevailing over garnet) and omphacite-garnetite (with garnet – 75-95 vol.% – prevailing over omphacite). The latter ‘omphacite-garnetite’ term has been introduced only for classification purposes, but no Neolithic implement with such a composition has ever been found so far. However, other lithologies having the same colour but different mineralogy, such as serpentinite, amphibolite, “nephrite”, chloritite, green tuffite and prasinite, have also been included under the term “greenstone” and occasionally used for the manufacturing of prehistoric tools.
Figure 1. here
Recently, the JADE international research project led by French scholars (Pétrequin et al., 2005, 2006b, 2012a; Pétrequin & Errera, 2017) involved the first systematic archaeological field
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survey of possible primary outcrops on the Voltri Group (Liguria) and the Monviso Massif (MM, Piemonte), where high-elevation workshops of “jade” have been discovered and ascribed to prehistoric human activity. The presence of a jadeitite boudin at 2,400 m a.s.l., precisely on the northern side of Punta Rasciassa, one of the alleged prehistoric raw material sources according to Pétrequin et al. (2012c), was indeed discovered by Compagnoni & Rolfo (2003a, b) and later studied in detail by Compagnoni et al. (2007, 2012). Rolfo et al. (2015) rightly concluded that the MM meta-ophiolite is indeed one of the most possible source location of the stone-implements raw materials, though the petrographic data are still insufficient to establish the exact source location. Moreover, similar HP
metamorphic rocks occur in other localities in the western and Maritime Alps (e.g., Castelli et al., 2002). On the other hand, Pétrequin et al. (2012a, d) not only considered most of the HP-meta-ophiolitic rocks as “jades/jadeitites”, but also claimed that all polished stone artefacts found in northwestern Europe, especially the often ultra-polished axe blades exceeding 135 mm in length, should derive from Alpine primary sources at high elevation, where these rocks occur as boudins in serpentinite. These authors, based on ethnographic extrapolations and spectroscopic studies, proposed that quarries at high elevation in the western Alps were exploited during Neolithic (Pétrequin & Pétrequin, 1993; Pétrequin et al., 2006a), a fact apparently supported by signs of extraction found on “greenstone” boulders especially in the Monviso and, to a minor extent, in the Voltri Group (Pétrequin et al., 2005, 2006b, 2008). These long axes should embody a symbolic, ceremonial significance – a quality particularly enhanced by the special provenance environment of the raw materials.
1.2. HP-LT meta-ophiolites in Europe
In Europe, the most important primary sources of HP-meta-ophiolites are located in the Alpine region, and precisely in the “Outer” and/or “Inner” belts of the Piemonte Zone (Fig. 1), comprising the Monviso and Voltri massifs (Compagnoni, 2003). There are tiny outcrops in Brittany (Ricq-de-Bouard & Fedele, 1993, p. 3), namely in Vendée (France: Peucat et al., 1982), moreover, in the Armorican Massif of N-E France, and in the Betic Cordilleras (Dominguez-Bella 2016, Puga et al. 1989, 2013). Secondary sources are represented by Oligocene conglomerates of the Piemonte Basin (northwestern Italy), which are exposed in a large zone of the Upper Po Valley comprising Piemonte, northern Liguria and western Lombardy (Fig. S1 in Supplementary Material linked to this article and freely available at https://pubs.geoscienceworld.org/eurjmin). 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112
Besides the exploitation of primary outcrops at high elevations, another aspect is represented by the potential prehistoric utilization of the secondary ‘greenstone’ sources, especially the alluvium, in which tenacious lithotypes (such as those at issue) tend to be naturally enriched. Such a source of supply apparently seems the easiest way to find raw materials for the Neolithic populations of the Po Valley and surrounding areas, who probably exploited the Oligocene sediments. On this assumption, Ricq-de-Bouard & Fedele (1993, p. 16) predicted that actual ‘quarries’ and ‘factories’ of raw materials (especially eclogites) should be found in the Langhe Hills and west-central Ligurian Apennine area. This was indeed the case of the Rivanazzano axe workshop, discovered some years ago by one of the authors (FN) in the Staffora Valley, Pavia Province. This site consists of hundreds of by-products produced by testing natural pebbles, shaping and turning them into axe/adze/chisel roughouts with bifacial percussion technique (D’Amico & Starnini, 2006b, 2012a). Ethnographic observation of the last stone-adze makers of the Langda village in Irian Jaya, Provinsi Papua, Indonesia, firstly described by the Italian Giancarlo Ligabue in 1984 (Toth et al., 1992), offers a good example of exploitation of secondary sources – though the exploited rocks are not HP-meta-ophiolites, but nephrites. In this case, the raw material sources of both utilitarian and extra-long adze/axe blades are in fact boulders and cobbles of different sizes, collected in the Ey River bed (Stout, 2002) during proper expeditions. A large set (about 200 items) of semi-finished pieces from Rivanazzano was also studied with petrographic methods (D’Amico & Starnini, 2006b, 2012a) and the constituent rocks attributed to local provenances, i.e. cobbles and boulders eroded from the Oligocene Conglomerates and redeposited into the Quaternary alluvial/fluvio-glacial sediments (Fig. 2). Similar conclusions were recently obtained by an analogous study of the “greenstone” industry of Brignano Frascata (AL) –, in the Curone valley, dating back to the early-to-middle Neolithic (D’Amico et al., 2000; Giustetto et al., 2017) – in which several artefacts still show raw shapes and surfaces of fluvial pebbles. A series of field surveys, carried out in the past years in the Curone, Staffora and Lemme valleys (Giustetto et al., 2018), south-eastern Piemonte, led to the discovery of large blocks of HP-meta-ophiolites, whose presence further demonstrates the presence of ‘greenstone’ cobbles and boulders in secondary deposits large enough to produce not only tools for daily-uses, but also bigger implements having symbolic purposes.
The current study is aimed at finding significant mineralogical and/or geochemical similarities between those rocks used by our ancestors for manufacturing polished stone tools and analogous geological specimens of known provenance. Such evidence should hopefully shed light about location of the supply sources of HP-meta-ophiolite raw materials. For this purpose,
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‘greenstone’ geological specimens from the primary outcrops of the Monviso massif and the alluvial deposits of the Po River and Curone stream were collected and analysed. The rate of recurrence of these rock types in each locality is not comparable, because of the different sampling strategy, but thanks to the in-depth petrographic and geochemical determinations, characteristic features for each source can be pointed out.
2. Materials and sampling
We examined HP-metamorphic rocks collected from three localities: i) the Monviso Massif
area (as primary source), ii) the Po River, and iii) the Curone stream (as secondary sources). The search for pyroxene rocks’ (i.e., jadeitite, omphacitite and mixed Na-pyroxenite) and ‘Na-pyroxene + garnet rocks’ (eclogite and garnet-omphacitite) was in the crosshair, since these lithologies represent the main raw materials used for manufacturing the polished stone tools in the archaeological collections of NW-Italy. Furthermore, the same lithologies can also be found, though in minor percentages, in other archaeological collections across Europe (Pétrequin et al., 2013). Other non-eclogite facies rocks – such as metabasite, mica-quartzite, basalt – that also occur in some of the investigated sites, will no longer be considered here. The list of the analysed samples and the applied analytical methods are shown in Table S1. In the Monviso Massif area, samples were collected from the immediate surroundings of Punta Rasciassa. The other two localities are Quaternary alluvial deposits. The Po river sediments contain boulders and cobbles, which originated from the Monviso and other eclogite-facies meta-ophiolites of the western Alps, while the Curone stream cuts through the Oligocene conglomerates to the north of the Voltri Massif (Fig. 2). The alluvial beds of both watercourses contain large amounts of eclogite-facies metamorphic rocks.
Figure 2. here
3. An assessment of pros and cons of different protocols used to study ‘greenstone’ artefacts and geologic materials
An ideal protocol for the study of archaeological finds, such as these prehistoric
‘greenstone’ implements, should possibly respect the primary need of preserving the artefacts integrity. Unfortunately, if strict compositional data are necessary, such a requirement can hardly be met. In contrast, this is not needed when studying analogous geological specimens for comparative purposes, aimed at possibly tracing the provenance and supply sources of the raw materials. To this end, however, it is advisable to use similar analytical protocols on both artefacts
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and geological samples, in order to provide reliable comparisons. Basically, two different
“schools of thoughts” have been followed for the study of these ‘greenstone’ tools so far:
3.1. The first opts for a totally non-invasive approach – being based on sheer visual appearance (to the naked eye; Pétrequin et al., 2012b; Pétrequin & Errera, 2017) and spectroradiometry, a method borrowed from the field of remote sensing (Errera et al., 2007, 2008, 2012a). The supporters of this protocol fulfil the goal to preserve the tools integrity. Also, they claim that this approach is reliable for characterization as well as provenance studies, being able, at times, to trace the origin of single artefacts back to their individual free-standing blocks of outcropping jadeitite (Sheridan et al., 2011, p. 414). Such an origin might be traced by directly comparing the spectroradiometric features of a given stony artefact – using the ‘matrix effect’ and proper statistical methods – with those of thousands of similar tools and geologic specimens from known outcrops. This database, constituting the reference collection of Alpine greenstones for the ‘JADE’ project, counts almost 2500 spectra collected in the past years during field prospections in the Western Alps, whose features have been grouped in different spectrofacies as per nature and provenance (Errera et al., 2012a). This protocol was used to characterize the constituent materials and establish the geological provenance of most “prestige” glassy polished implements in museum collections (Pétrequin et al., 2012c).
3.2. The second preferably uses a more conventional mineral/petrographic approach, including – together with non-invasive methods (density measures, stereo-microscopy observations in reflected light) – also micro-destructive techniques, such as X-ray powder diffraction, polarizing microscopy and SEM-EDS on thin sections (e.g., Compagnoni et al., 2006; Giustetto et al., 2008). Scientists following the second approach object that a so precise attribution (traceable to the very same raw material boulder) – although exciting – sounds risky and yet questionable from an archaeometric point of view. They claim that the precise identification of the source of a rock type involves a thorough study (e.g., D’Amico, 2005; D’Amico et al., 2003), implying the application of a serial, multi-analytical mineral/petrographic protocol. Such a procedure, in their opinion, is the only one capable of describing, on thorough scientific basis, the heterogeneity of the HP-meta-ophiolites constituent of these tools, hence providing a sharp lithotype determination. This becomes particularly important for archaeometric purposes, as it represents the best approach to allow
reliable comparisons with analogous data (collected by using the same protocol) on geological
materials of known provenance, aimed to locate the raw-materials supply sources. The most
important weakness of such an approach – that is, undermining (albeit slightly) the artefacts
integrity – might be partly counterbalanced by the sacrifice of very small specimens, in the form of
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isolated splinters or tiny fragments directly extracted – whenever possible – from broken tools or roughouts, of less importance for archaeologists.
4. Analytical methods
In the current study, we opted for a multi-analytical approach, which is detailed hereafter.
This protocol has been conceived with the minimum possible (micro-)invasiveness, in order to preserve as much as possible the integrity of the analysed items, and be able to apply the same approach to the study of prehistoric implements held in museum collections.
The rock samples were selected based on their macroscopic appearance. Thin section
examination was then performed by means of a polarizing microscope and scanning electron
microscopy (SEM) equipped with an energy-dispersive spectrometer (EDS). The petrographic
study allowed to recognize the main, minor and accessory mineral phases, estimate their modal amounts and evaluate the micro-structural heterogeneity and chronological relationships.
Moreover, quantitative SEM-EDS micro-analyses allowed to define the compositional features of
constituent minerals. These analyses were performed at the Department of Petrology and Geochemistry of Eötvös Loránd University, Budapest, by using a Nikon OPTIPHOT2-POL polarising microscope with COOLPIX DS-Fi1 camera system and the “original surface analytical method” (OSAM) adopted by Bendő et al. (2013) on an AMRAY 1830 instrument, equipped with an EDAX PV9800 energy-dispersive system, respectively. The OSAM has been developed for the
analysis of polished artefacts and allows one to measure mineral composition on the surface of the
stone tools without damaging them. The measurements were made with the same operating conditions as for the Hungarian stone artefacts (Bendő et al., 2013, 2018): 1 nA beam current, 20 kV accelerating voltage and 24 mm working distance. Chemical data were processed with the Moran Scientific Pty Ltd V2.3 software, using ZAF correction and calibrated with international standards: jadeite (Si, Al, Na), omphacite (Ca, Fe), ENM augite (Mg), spessartine (Mn), titanite (Ti)
and chromite (Cr) for sodic pyroxenes; SMS garnet (Si, Al, Fe, Ca), SMS pyrope (Mg), spessartine
(Mn), titanite (Ti) and chromite (Cr) for garnets. Backscattered electron images were made with the Tescan Satellite Version 2.9.9.20 software.
Additionally, non-destructive prompt-gamma activation analysis (PGAA) was done to quantitatively determine the average bulk composition. The method is suitable for quantification of major components (Si, Ti, Al, total Fe, Mn, Mg, Ca, Na, K and H) and some trace elements ( e.g. B, Cl, Sm, Gd) in a few cm3 volume, without causing any damage to the investigated specimens. The experiments were performed at the horizontal cold-neutron guide of the Budapest Research Reactor (Szentmiklósi et al., 2010). The sample is placed in the external neutron beam of 9.6×107 cm-2 s-1 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247
thermal equivalent intensity, without any preparation. A calibrated Compton-suppressed HPGe detector was used to detect prompt gamma spectra. The typical irradiation time was chosen to be around 1 h, in order to collect statistically significant counts in the spectra. The spectrum evaluation is done by using the Hypermet PC software, the so-called k0-method is applied for quantitive determination of the elemental composition (Révay, 2009; Szakmány et al., 2011). The major
components are expressed as oxides, while no trace element data have been used in this study. The relative uncertainties of the measured concentrations are typically 1-3wt.%.
5. Results
5.1. Monviso samples
Samples collected from Monviso are mostly ‘Na-pyroxene rocks’. ‘Na-pyroxene + garnet rocks’, however, are also present, with 5-10 vol.% garnet content; these rocks were classified as ‘garnet-bearing omphacitites’ (garnet up to 5 vol.%) and ‘garnet omphacitites’ (garnet between 5-25 vol.%) by Giustetto & Compagnoni (2014), whereas D’Amico (2003) considered them as ‘eclogites’. ‘Na-pyroxene rocks’ are almost monomineralic rocks composed of usually inhomogeneous sodic pyroxenes, which mostly show a fine-grained, lineated matrix, wrapping around omphacite porphyroclasts. Pyroxenes are zoned showing mild greenish pleochroism on the edges and nearly colourless cores. Relict jadeite cores are abundant (Jd85-96), mostly corroded and containing small omphacite blebs (interpreted as exsolution by Compagnoni et al., 2007), surrounded by an inhomogeneous omphacite-rich corona (Jd30-60 Q20-50 Ae0-30, Table S2). Garnet is rare in both rock types, but usually quite large (500-1000 µm) and characterised by cloudy cores, crowded with small (10-50 µm across) inclusions of mostly pyroxene and ilmenite and surrounded by wider (50-200 µm) idioblastic rims, with few or no inclusions. Garnet poikiloblasts are low in grossular and andradite and show a slight compositional zoning, with decreasing almandine and increasing pyrope components from core to rim (core Alm75-90 Prp10-15, rim Alm60-70 Prp20-25, Table S3). The rocks are quite fresh and show only incipient chloritization of garnet. The main accessory
minerals are ilmenite ( 5 vol.%) and zircon ( 1 vol.%). Ilmenite is usually 200-400 µm long (occasionally > 1 mm), rich in inclusions and surrounded by a few µm-thick titanite corona. Rutile also occurs in most samples, but in smaller quantity and size compared to ilmenite. Based on the habit, two zircon subgroups can be distinguished: i) small crystal aggregates (20-100 µm); ii) larger, almost idioblastic single crystals (200-300 µm across, Fig. 3). In most samples, REE-bearing
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minerals (allanite, xenotime and monazite) also occur in small amounts. Allanite is frequent and
commonly shows elongated single crystals, forming worm-like structures among the alteration
products of monazite; xenotime is rare.
Figure 3. here
5.2. Po Valley samples
pyroxene + garnet rocks’ – eclogites, in particular – are more abundant than ‘Na-pyroxene rocks’ among the alluvium deposits of the Po River. Both rock types show more
variability in grain-size and colour than the Monviso samples.
‘Na-pyroxene rocks’ are mid-, sometimes dark green, fine- to medium-grained and usually contain porphyroclasts up to 800 µm across. Sodic pyroxenes show slight greenish pleochroism, weakening towards the rims, and are chemically inhomogeneous (Jd30-50 Q40-60 Ae10-25) with the transition from core to rim showing no sharp boundary. Rutile and zircon are the most widespread accessory minerals, the former occurring usually as elongated aggregates of tiny crystals, slightly altered to titanite. Ilmenite appears only as oriented lamellae in rutile. In the pyroxene matrix, domains of retrogression minerals such as zoisite, epidote, albitic plagioclase and paragonite
commonly appear, together with ore minerals (magnetite, hematite and pyrite), apatite and
phengite. Zoisite is frequent, but its quantity exceeds 10 vol.% in only one sample (Po 2/2). Some Na-pyroxenite samples from the Po alluvium are undeformed while others show a mylonitic foliation/lineation, defined by the orientation of the elongated sodic pyroxene crystals and rutile aggregates, more pervasive than in the Monviso samples. Exceptionally, scarce isolated garnets (1-2 vol.%) are observed, marked by a thin chloritization of the rim and crowded with pyroxene inclusions; contrarily to the Monviso specimens, paragonite, epidote and zoisite inclusions also occur.
Eclogites are usually dark toned and few samples are mid green; both types show an evident lineation, defined by elongated aggregates of sodic pyroxenes, garnet and rutile. The pyroxene grain size is variable, with coarser, less pleochroic, deformed and corroded (up to 1-2 mm) crystals embedded in a more pleochroic fine-grained matrix. Pyroxene has a typical omphacite composition (Jd40-55 Q20-50 Ae10-20), but rare relict jadeite cores (with composition Jd80-85, Table S2) locally occur. Compared to Monviso, garnet is smaller (usually 100-300 µm across) but more abundant (up to 40 vol.%), commonly with an atoll-like structure and evidence of peripheral chloritization. Their chemical composition shows a slight zoning from core to rim, with decreasing grossular and
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increasing pyrope components, while almandine remains almost constant (core Alm62-68 Prp10-15 Grs14-18; rim Alm62-65 Prp20-22 Grs8-12, Table S3). Similar to ‘Na-pyroxene rocks’, the most common accessory mineral is rutile, slightly altered to titanite; it forms elongated aggregates of small crystals defining the foliation/lineation, together with zircon, phengite and ore minerals; scarce elongated crystals of allanite are also observed. Xenotime was detected once, but monazite is absent. Retrograde minerals such as clinozoisite, epidote, paragonite, blue amphibole, albitic plagioclase, and chlorite aggregates are also present.
5.3. Curone Valley samples
The majority of samples collected from the alluvium of the Curone stream are eclogites,
whereas ‘Na-pyroxene rocks’ are rarer. In addition, both rock types show a stronger alteration (up 50 vol.%) than at the other localities.
‘Na-pyroxene rocks’ have heteroblastic microstructure consisting of small and oriented Na-pyroxene, amphibole and clinozoisite. Pyroxenes have omphacitic- to Fe-omphacitic composition, in the range Jd25-45 Q40-55 Ae10-25, withAe increasing and Jd decreasing from core to rim. Retrograde minerals – including clinozoisite, epidote, blue amphibole, actinolite, albite, paragonite
and chlorite – can reach 50 vol.% of the rock. Rutile is the most common accessory mineral, slightly altered to titanite, whereas ilmenite is nearly absent. The recognized ore minerals are chalcopyrite, hematite and pyrite.
Eclogites show a strong foliation/lineation, defined by layers of sodic pyroxene, blue amphibole, clinozoisite, garnet and rutile. Pyroxenes occur as both small (100-300 µm) oriented crystals in a mylonitic matrix, with greenish pleochroism (Jd30-50 Q40-60 Ae10-20), and large (up to 2 mm) corroded and deformed porphyroclasts, showing slight or no pleochroism (Jd35-45 Q45-55 Ae5-15, Table S2). Amphibole crystals show a strong compositional zoning, with increasing actinolite and decreasing glaucophane component from core to rim. Garnets, which contain small pyroxene and amphibole inclusions, are mostly corroded, partly altered to chlorite and usually tend to form chains aligned parallel to the foliation/lineation. They may be either relatively homogeneous or with a complex zoning (Alm51-71 Prp8-26 Grs9-32 Sps0-8, Table S3). White mica (phengite), amphibole, and epidote (filling the space between garnet crystals) are generally closely associated with garnet. Rutile is the main accessory mineral, forming aggregates slightly altered to titanite. Ilmenite is rare and only forms few µm-wide coronas around rutile. In some samples aggregates are common, which may consist either of retrograde minerals such as epidote, clinozoisite, albitic plagioclase, white mica, and chlorite, or newly formed pyroxene nematoblasts, apatite and rutile. Opaque ores
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are also present, and barite occurs as vein filling in about half of the specimens. REE-bearing minerals are nearly absent; only few sample contain prismatic allanite crystals.
6. Discussion
6.1. Bulk-rock compositions of Curone, Monviso and Po Na-pyroxene rocks and comparison with
Neolithic artefacts
The investigated samples show quite similar pyroxene compositions, independently from
their site of origin (cf. Tables S2). In spite of such similarity, some inferences can be made based
on the observed compositional variations. Pyroxenes in the Po specimens spread out over an
almost continuous range of compositions, which covers both the jadeite and omphacite fields in the related ternary diagram (Morimoto et al., 1988). Conversely, two more distinct pyroxene compositions (jadeite and omphacite, respectively) seemingly characterize samples from the Curone valley. In addition, jadeite appears to be scarcer in the Po samples (Fig. 4, triangles). In order to discriminate the analysed rocks based on their major-element composition, the chemical groups proposed by D’Amico et al. (2003) can be tentatively adopted (i.e., approximate Fe2O3tot, MgO and Na2O wt.%, indicated hereafter). However, in a few samples (MVISO 17/1; PO 2/7), the bulk-rock group and the lithology do not fit. This can be explained by: i) the thin section being not representative for the whole specimen; ii) a basic inaccuracy of the performed analyses; iii) an improper definition of the related chemical groups. Further sampling will allow to confirm these preliminary conclusions.
Figure 4. here
Based on PGAA measurements (Table S4), most of the Monviso samples can be identified as Fe-mixed jades (Fe2O3 7-12 wt.%; MgO 3-4 wt.%; Na2O 9-10 wt.%), but there are also three Fe-jadeitites (Fe2O3 7-11 wt.%; MgO 1-3 wt.%;Na2O 11-12 wt.%), two jadeitites (Fe2O3 2-4 wt. %; MgO 2-4 wt.%; Na2O 10-13 wt.%), one mixed jade (Fe2O3 3-7 wt.%; MgO 4-7 wt.%;Na2O 9-11 wt.%) and one omphacitite (Fe2O3 7-9 wt.%; MgO 7-10 wt.%;Na2O 6-8 wt.%). Based on the same criteria, samples from the Po valley are very different, and include omphacitite, jadeitite, Fe-mixed jade, Fe-eclogite (Fe2O3 12-18 wt.%; MgO 3-7 wt.%;Na2O 5-10 wt.%) and intermediate eclogite. The Curone samples fit only three of the categories described by D’Amico et al. (2003),
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namely Fe-mixed jade, Fe-eclogite and intermediate eclogite. Some of the Curone Valley samples could not be classified properly in any of the defined groups, due to the high amount of retrograde minerals. Quite significant Fe2O3tot amounts are occasionally detected in all investigated sites, though the higher aegirine contents (up to Ae50) are specific to samples from the Curone valley.
As far as garnet is concerned, the Po and Curone samples show quite a similar composition, rich in almandine and pyrope [up to (Alm+Prp)85] with a moderate grossular + andradite content [(Grs+Adr)10-30] and poor in spessartine (even lacking in some of the Curone specimens). The Monviso samples show an even richer almandine + pyrope content [occasionally > (Alm+Prp)90] (Table S3).
The bulk compositions of studied rock samples, coming from primary and secondary sources, have also been compared with data acquired with a similar protocol on prehistoric tools from different archaeological sites in Europe. According to the chemical groups proposed by
D’Amico et al. (2003), a collection of HP-meta-ophiolite “greenstone” polished stone tools found
in Neolithic sites from Hungary, whose raw materials were inferred to originate from the western Alps (Bendő et al., 2014, 2018), are classified as jadeitite, mixed-jade, omphacitite, Fe-jadeitite, Fe-mixed jade, Fe-eclogite, Mg-eclogite and glaucophane schist. This confirms the similarities
between the Hungarian stone tools and the Italian geological samples, allowing even more accurate
comparisons. For example, in addition to their similar bulk-rock chemistry, two jadeitite samples – a geological one, coming from the Monviso and an archaeological implement (n. M6.2010.10B.6380.1, found at Alsonyek) from a museum collection – are also very similar in
terms of mineralogy, microstructure and mineral-chemical features (Fig. 5, c and d).
Figure 5. here
Very high Fe2O3tot contents were reported from omphacitite, mixed Na-pyroxenite and eclogite implements from the Neolithic sites of Castello di Annone and Brignano Frascata, in Piemonte, north-western Italy (Giustetto et al., 2016, 2017). These Fe2O3tot values, up to 13-18 wt.
%, would be distinctive of Fe-omphacitites, Fe-mixed jades and Fe-eclogites by adopting the chemical groups of D’Amico et al. (2003). As such ranges are not typical of other ‘Alpine’ archaeological finds (e.g., prehistoric polished stone tools from central-eastern Europe, attributed to the Monviso area: Pétrequin et al., 2011), the provenance of these tools was tentatively related to the Voltri massif – where unusually high Fe contents have been detected in geological specimens (Giustetto et al., 2017). The outcomes of the current study apparently complicate this scenario. In fact, although the higher Fe2O3 contents still characterize eclogite samples from the Curone 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418
(collecting meta-ophiolite derived from the dismantlement, about 30 Ma ago, of the palaeo-Voltri Massif – or an equivalent unit now eroded away), similar Fe-rich lithotypes are also occasionally observed in geological samples from the Monviso area and Po valley. This indicates how these provenance studies should be considered all but trivial. Only a significant increase in the number of analysed geological specimens – with consequent improvement in the statistics – should allow these comparative mineral-petrographic analyses to become more reliable and accurate, and possibly to identify the fingerprints of the different sources.
6.2. Petrographic, petrological and geochemical comparison of the Curone, Monviso and Po
samples
The main petrographic features of the samples are summarized in Table 1. The HP
meta-ophiolite samples collected from the three investigated geological sources show several
petrographic and geochemical analogies. Subtle differences, however, can also be pointed out and
possibly used to discriminate samples belonging to a particular source.
In all samples, sodic pyroxene and garnet are the main rock-forming minerals. Most of
these rocks, regardless of the sampling site, are fine-grained, a recurrent feature in most prehistoric tools. Such a feature should therefore not be considered specific to the Monviso area (cf. Errera et al., 2012b; Pétrequin et al., 2012b). In terms of differences, corroded relict jadeite cores are abundant in Monviso, whereas deformed omphacite porphyroclasts containing oriented
inclusions are common among the Po and Curone samples. Furthermore, the Monviso samples commonly show no or only slight preferred orientation of constituent minerals, whereas in the
other two sites strong mineral lineation and mylonitic structure are common (especially in Curone samples). The latter features occur also in many Neolithic implements from Brignano Frascata, located in the same Curone valley as well as Rivanazzano, further reinforcing the attributed provenance of their raw materials from local secondary deposits (D’Amico & Starnini, 2012b; Giustetto et al., 2017). Garnet also shows typical features, attributable to each of the investigated locations. For example, garnet poikiloblasts, 1-2 mm across and characterized by idioblastic rims, are observed in the Monviso eclogites. In the Po samples, atoll-like garnets are common, while the Curone eclogites contain medium-sized garnets, usually grouped as small chains, showing a wider chemical variability (Fig. 6).
Figure 6. here 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452
The most significant differences, however, concern the distribution of minor and/or accessory minerals. In all samples, Ti-bearing minerals represent the most abundant of them, but their assemblage and relative amounts vary from site to site. In our data set, ilmenite is almost the only Ti-bearing mineral in the Monviso samples, whereas rutile is the main accessory phase in the
other two sites (Po and Curone). However, portions of a few jadeitite boudins from Monviso
(Compagnoni & Rolfo, 2003; Compagnoni et al., 2007, 2012) bear sodic pyroxenes associated with up to 10 vol.% rutile (Compagnoni et al., 2007). In all localities, alteration of rutile and ilmenite to titanite – a feature reported as recurrent in specimens from the Voltri Massif (D’Amico, 2012) – is common, though incipient (e.g., the Curone sample in Fig. 7, BSE images). The occurrence of all these Ti-bearing phases has been documented also in the HP meta-ophiolites used for the manufacturing of Neolithic implements.
Figure 7. here
The REE-bearing minerals are differently distributed in the investigated rocks. Samples
from Monviso commonly contain thin and elongated allanite crystals, less commonly monazite and/or xenotime, whereas the Po specimens contain allanite and xenotime, but in much smaller amounts than the Monviso samples. Only a few of the Curone specimens contain prismatic allanite, whereas xenotime and monazite are absent. With respect to opaque ore minerals, the Monviso specimens contain only ilmenite but no Fe-sulfide or Fe-oxide. By contrast, in the Po and Curone samples numerous ores are present, including chalcopyrite, pyrite, hematite and magnetite.
Moreover, a remarkable difference in the degree of retrogression is observed. The Monviso
samples only sporadically contain retrograde phases (< 5 vol.%, mostly chlorite and/or clinozoisite). In the Po specimens, where the degree of alteration is much stronger, epidote, clinozoisite, chlorite and white mica are common, whereas blue amphibole and sodic plagioclase are rarer. The Curone specimens, in addition to glaucophane, contain up to 50 vol.% of retrograde phases, including actinolite, epidote, clinozoisite, albitic plagioclase, chlorite, and white mica. These features, frequently observed in several greenstone implements from the Neolithic sites of Brignano Frascata and Rivanazzano, can be attributed to meta-ophiolites typical of the Voltri massif (D’Amico, 2012; Pétrequin et al., 2012b) and further support an origin from the conglomeratic and alluvial deposits located nearby (including the Curone stream alluvial bed; Giustetto et al., 2017). These data are at odds with Pétrequin et al. (2012c) conclusions, claiming the lack of jadeitite and
eclogite cobbles/boulders in the Curone (and Staffora) alluvial beds (i.e., in jadeitite and eclogite), a 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486
fact that would have prevented the possibility of a prehistoric exploitation of local secondary raw materials for the production of big axes. Blastoporphyritic aggregates occur only in the Po and Curone samples, and contain retrograde phases, in addition to newly formed Na-clinopyroxene and apatite.
Table 1. here
One may object that the inevitable, residual uncertainty that still affects the exact location of the raw-material supply sources, even after having applied in-depth mineral/petrographic analyses and sacrificed part of the inspected specimens, might cause this sophisticated, micro-destructive protocol to be more trouble than it is worth. It is true that a similar uncertainty would involve those determinations achieved by means of any alternative approach (including the non-invasive one, based on visual appearance and spectroradiometry: Errera et al., 2012a; Pétrequin et al., 2012b; Pétrequin & Errera, 2017). The advantage of preserving the integrity of the studied specimens – an auspicated figure of merit of any well-grounded archaeometric approach – may be gradually reached by progressively smoothing the degree of micro-destructiveness requested by this mineral/petrographic protocol, while preserving the scientific substance of the outcomes. This becomes particularly important when prehistoric ‘greenstone’ artefacts are concerned. The acquisition of rigorous mineralogical and petrographic data, while reducing any potential damage to the sample integrity, might be pursued by operating with particular devices and under specific conditions. In this study, for example, the OSAM setting was tested, capable of collecting reliable SEM-EDS data by operating directly on the polished surface of the ‘greenstone’ implement, with no prior treatment (Bendő et al., 2012, 2013, 2014). In the same vein, state-of-the-art XRD/XRF coupled devices, aimed at studying polycrystalline materials, might allow the collection of data directly from the surface of the artefacts, with no need for preliminary sampling nor crushing (Chiari et al., 2008). Easy and non-invasive methods, based on XRD data, are also available to determine the chemistry of Na-pyroxenes solid solutions in ‘greenstone’ implements (Giustetto et al., 2008). The use of these particular devices and procedures goes more and more in the direction of disposing of rigorously scientific data, while safeguarding the integrity of the archaeological finds.
On the other side, without an accurate reconstruction of the original prehistoric landscape, it is almost impossible to indicate where the possible supply areas were located. What nowadays is exposed (and achievable) might not have been thousands of years ago and 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519
vice versa, after thousands or even millions of years of erosion, especially in such Alpine extreme environment.
7. Conclusions
Pétrequin et al. (2011) is criticising the Italian approach to stone axe raw material sourcing because of: i) no supervision by Neolithic archaeologists, ii) lack of occurrences, where solid bodies of the raw material are present, iii) such long distance exchange could not have happened if only secondary sources were available. The real problem, in our opinion, is
that before looking for the provenance of an artefact, it is necessary to establish its actual features, starting from the analysis of the artefact and determining the nature of the raw material. This was the aim of a number of studies focussed on the petrologic characterization of more than one thousand prehistoric polished stone artefacts, dating from the early Neolithic to the Bronze Age, mainly from northern Italian sites. The outcomes proved that these assemblages were composed of a variety of HP-meta-ophiolites (mainly eclogites, jadeitites and omphacitites), quite heterogeneous and occurring in different sites in quantities apparently related to their distance from the potential sources of raw materials. From the morpho-technological point of view, it was clear that most artefacts have been worked out from original, naturally-shaped river pebbles. The discovery of the Rivanazzano (D’Amico & Starnini, 2012b) and Brignano Frascata (Giustetto et al., 2017) workshops, along the fringes of the Oligocene formations located to the East of the Voltri massif, confirmed such an evidence. Their origin from Alpine primary sources was considered obvious, but geologically it is extremely difficult – if not impossible – to pinpoint the exact provenance of single artefacts, due to the heterogeneous nature of the HP meta-ophiolites and their large distribution in the western Alpine arc.
The real point is that a clear distinction should be made between every-day functional tools and “symbolic/prestige” items. For the former ones, which represent indeed the largest amount of analysed polished tools from the Italian prehistoric collections, the raw material was likely collected from the closer alluvial deposits – possibly no more than few dozens of km, as the crow flies. The results of our survey seem to prove such an occurrence, which is further supported by the existence, in the Curone stream alluvial bed, not only of suitable-sized pebbles and cobbles, but also of large boulders, that could be exploited and turned into large-sized artefacts. On the other hand, the larger (15-36 cm long), ultra-polished jade ceremonial axes – very rare in the Italian contexts, but more common in late Neolithic sites distributed along a north-western axis, connecting the Alps to Great
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Britain through France – might have been worked at high elevation, perhaps by people living on the western side of the Monviso, where suitable secondary sources were less available.
7. Acknowledgments
This research was funded by the Hungarian National Research, Development and Innovation Office (NKFIH, former OTKA), under the contract No. K100385.
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Table 1. Petrographic features of the investigated geological samples.
Figure 1. Simplified geotectonic map of the Western Alps. The distribution of the areas of the blueschist- and eclogite-facies alpine overprints of the Piemonte Zone – the only ones of archaeometric interest – is highligthed: DM –
Dora-Maira; GP – Gran Paradiso; L – Lanzo; M – Monviso; ME – Monte Emilius; MR – Monte Rosa; V –
Voltri Massif. Other Units: AG – Aar-Gotthard; AM – Alpes maritimes; AR – Argentera; BD – Belledonne; DB – Dent Blache nappe; EU – Embrunais-Ubaye; LPN – Lower Penninic nappes; MB – Mont Blanc-Aiguilles-Rouges; P – Pelvoux; SA – Southalpine domain; SB – Grand St. Bernard Zone; SM – Swiss Molasse; SZ – Sesia Zone;
Figure 2. Map of the localities from where the analysed rock-samples were collected (red) and location of the Voltri Massif. The Po samples originate from two locations, labelled “Po 1” and “Po 2”, respectively; the Curone samples originate from four different alluvial deposits: BF – Brignano Frascata, SSC – San Sebastiano Curone, GR – Gremiasco, FC – Fabbrica Curone.
Figure 3. Photomicrographs of zircon crystals in representative samples, belonging to different subgroups: i) small, elongated crystal aggregates (on the left); ii) larger idioblastic crystal (on the right).
Figure 4. Chemical compositions of pyroxenes (top) plotted on the Q – Jd – Ae ternary diagram (Morimoto et al., 1988) and of garnets (bottom) plotted on the (Alm+Prp) – Sp – (Grs+Adr) diagram. The Monviso samples contain jadeite and omphacite in nearly equal amounts, the Po samples contain mostly omphacite, and the Curone samples contain jadeite and omphacite, but omphacite is prevailing.
Figure 5. (A-B) Bulk-rock chemical composition of the Hungarian jadeitite and Fe-eclogite artefacts (Bendő et al. 2014b) measured by PGAA, compared with the geological jadeitite and Fe-eclogite specimens: the spider diagrams are normalised to UCC (McLennan, 2001). A significant match is evident with the jadeitite and Fe-eclogite compositions of D’Amico et al. (2003), marked by the shaded grey area. (C) BSE image of the M6.2010.10B.6348.1 Hungarian stone artefact (Bendő et al., 2014b). The cloudy pyroxene cores surrounded by a jadeite-rich zone and an omphacite rim are evident. (D) BSE image of the Monviso 12/1 jadeitite sample, whose pyroxene chemical zoning and microstructure are similar to those of the stone artefact.
Figure 6. Photomicrographs (plane-polarized light) showing the appearance of garnet in samples from different geological sources. (A) Garnet from the Monviso 4/3 sample, showing abundant inclusions, a cloudy core and a clear, idioblastic rim. (B) Atoll-like garnets from the Po 2/7 sample. (C) An aligned garnet aggregate in a mylonitic, Na-pyroxene-rich matrix – CUR-NEB 6c sample from Curone. (D) Former idioblastic garnet crystals partly altered to chlorite (sample FRA 1/5).
Figure 7. BSE image showing the microstructural features of the main Ti-minerals from different sample locations. The Monviso samples contain almost exclusively ilmenite, the Po samples rutile, and those from Curone mainly aggregates and/or bands consisting of both rutile and titanite.
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