LEAD ISOTOPE CHARACTERIZATION OF COPPER INGOTS FROM SARDINIA (ITALY): INFERENCES ON THEIR ORIGINS

ABSTRACT The provenance of the materials making up the metal artifacts represents a fundamental question for archaeological research. The complex processes necessary to extract the metals from the minerals cause considerable changes in their chemical composition. By contrast, the ratio of the different lead isotopes in artifacts is not influenced by metallurgic processes. Therefore, the raw mineral material and the extracted metal exhibit the same isotope "footprint". Over the course of history, Sardinia has played a major role in maritime routes. Its considerably rich mineral resources, with copper, lead and iron mineralizations, moreover complemented its strategic importance. During the 10th and 9th centuries BC Sardinia, because of its strategic location, was to become directly involved in the "precolonial" Phoenician expansion. Two repositories of copper ingots recovered at different levels within a nuragic village in northwest Sardinia, can be placed within this historical context of intense traffic and exchange of goods between native Sardinian and Phoenician communities. The ingots were made in different shapes: plane-convex, biconvex, truncated cone and irregular. The analyzed ingots exhibit a considerable lead isotopic variability, although no systematic differences in isotope composition were revealed between the ingots from the two different repositories. Moreover, no systematic isotopic variations were observed between the different shapes in which the material was found. Overall, the ingots exhibit a linear distribution in the lead/lead diagrams. The group of ingots with the lowest isotope ratios project onto the area defined by the northwestern Sardinian mineralizations. The ingot located on the other extreme end of the straight line in the Pb diagrams overlaps the area defined by the southern Sardinian deposits. The isotope footprints of the intermediate samples seem to indicate that they stem from the mixing of two components from two different mining areas of Sardinia, one in the north, the other in the south. Therefore, widespread exchange of metal must have taken place throughout the island, and such an exchange does not seem to have been hindered by the advent of the Phoenicians in Sardinia. The results of the present investigation indicate that the majority of the examined ingots was produced with metal from the northwest of the island, and that these mines were therefore known and exploited in nuragic times. Such findings shed new light on the complex issues involved in proto-historic Sardinian metallurgy

Pre-metamorphic melt infiltration in metasediments : geochemical, isotopic (Sr, Nd, and Pb), and field evidence from Serie dei Laghi (Southern Alps, Italy)

Gradual transitions from K-feldspar free gneisses to K-feldspar bearing augengneisses are sometimes observed in metamorphic terranes. They have been explained with metasomatic porphyroblastic growth connected with regional metamorphism, or with pre-metamorphic presence of magmatic megacrysts. A transition of this kind can be observed in the Serie dei Laghi (Southern Alps, Italy), where coarse-grained metaarenites (Cenerigneiss) grade into Ceneri augengneisses with large K-feldspar porphyroclasts, and banded amphibolites of the \u2018\u2018Strona Ceneri Border Zone\u2019\u2019 grade into Hbl augengneisses rich in K-feldspar. The Ceneri augengneisses are chemically indistinguishable from the Cenerigneiss, but have higher 87Sr=86Sr (0.7256\u20130.7258 vs. 0.7215\u20130.7233), similar to those of the Ordovician granites that were intruded, before the regional metamorphism, into the protoliths of both Cenerigneiss and amphibolites. The Cenerigneiss contains two types of zircons: (1) highly luminescent, rounded grains or fragments, yielding U\u2013Pb SHRIMP ages from 0.43 to 1.0 Ga; (2) euhedral grains with oscillatory zoning (magmatic), with U\u2013Pb SHRIMP concordant ages of 466 13 Ma. This age coincides with the Rb\u2013Sr whole rock emplacement age of the Ordovician granitoids (466 5Ma).The Hbl augengneisses form three groups with distinct geochemical patterns, whose distributions on inter-element diagrams trend towards the Ordovician metagranites and meta-aplites. In addition, the Hbl augengneisses have higher 87Sr=86Sr (0.7132\u20130.7147 vs. 0.7031\u20130.7046) and lower 143Nd=144Nd (0.51214\u20130.51219 vs. 0.51273\u20130.51297) than the amphibolites, suggesting the addition of an isotopically evolved component. The observed chemical and isotope patterns, as well as the vicinity of the augen gneisses to the Ordovician intrusions, lead us to conclude that the Ceneri augengneisses and Hbl augengneisses are the result of infiltration of residual hydrous magmas into the protolith of both the Cenerigneiss and the amphibolites at the time of Ordovician granite emplacement, long before the regional metamorphism in the Serie dei Laghi

Source contamination and mantle heterogeneity in the genesis of Italian potassic and ultrapotassic volcanic rocks: Sr-Nd-Pb isotope data from Roman Province and Southern Tuscany

The Tyrrhenian border of the Italian peninsula has been the site of intense magmatism from Pliocene to recent times. Although calc-alkaline, potassic and ultrapotassic volcanism overlaps in space and time, a decrease of alkaline character in time and space (southward) is observed. Alkaline ultrapotassic and potassic volcanic rocks are characterised by variable enrichment in K and incompatible elements, coupled with consistently high LILE/HFSE values, similar to those of calc-alkaline volcanic rocks from the nearby Aeolian arc. On the basis of mineralogy and major and trace element chemistry two different arrays can be recognised among primitive rocks; a silica saturated trend, which resulted in formation of leucite-free mafic rocks, and a silica undersaturated trend, charactrerised by leucite-bearing rocks. Initial 87Sr/86Sr and 143Nd/144Nd values of Italian ultrapotassic and potassic mafic rocks range from 0.70506 to 0.71672 and from 0.51173 to 0.51273, respectively. 206Pb/204Pb values range between 18.50 and 19.15, 207Pb/204Pb values range between 15.63 and 15.70, and 208Pb/204Pb values range between 38.35 and 39.20. The general εSr vs. εNd array, along with crustal lead isotopic values, clearly indicates that a continental crustal component has played an important role in the genesis of these magmas. The main question is where this continental crustal component has been acquired by the magmas. Volcanological and petrologic data indicate continental crustal contamination to be a leading process along with fractional crystallisation and magma mixing. Considering, however, only the samples thought to represent primary magmas, which have been in equilibrium with their mantle source, a clearer picture emerges. A large variation of εSr vs. εNd is still observed, with εSr from −2 to +180 and εNd from + 2 to −12. A bifurcation of this array is observed in the samples that plot in the lower right quadrant, with mafic leucite-bearing Roman Province rocks buffered at εSr = + 100 whereas the mafic leucite-free potassic and ultrapotassic rocks point to strongly radiogenic Sr compositions. We may argue that mafic leucite-bearing Roman Province rocks point to εSr and εNd values similar to those of Miocene carbonate sediments whereas mafic leucite-free potassic and ultrapotassic rocks point to a silicate upper crust end-member. Lead isotopes plot well inside the field of island arcs, overlapping the values of pelagic sediments as well, but bifurcation between the samples north and south of Rome is observed. The main characteristic for the mantle source of Italian potassic and ultrapotassic magmas is the clear upper crustal signature acquired prior to partial melting through metasomatic agents released by the subducted slab. In addition, one lithospheric mantle source in the north and an asthenospheric mantle source, pointing to an HIMU reservoir, in the south were recognised. The chemical and isotopic differences observed between the northern and southern sectors of the magmatic region were possibly due to the presence of a carbonate-rich component in the crustal enriching agent in the south. One crustal component might have been generated by melting of silicate metasedimentary rocks or sediments from an ancient subducted slab. The second one might reflect the activity of mostly CO2-rich fluid released more recently by the incipient subduction of carbonate sedimentary rocks