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Petrogenesis of kelyphites in garnet peridotites: a case study from the ulten zone, italian Alps
No full textSpinel-bearing symplectites around garnet (kelyphite) record the transition from high-pressure garnet peridotite to low-pressure spinel peridotite. In the first part, we review the previous studies that have been devoted to kelyphite. Many authors have identified orthopyroxene, spinel, clinopyroxene and amphibole in kelyphite. They have emphasized the common existence of two concentric coronae: an outer spinel-poor corona on the olivine side and an inner spinel-rich corona on the garnet side. Chlorite and plagioclase have also been reported in low-grade kelyphites, whereas more complex kelyphite-forming associations, including phlogopite, ilmenite, calcite and perovskite, have been observed in kimberlite xenoliths and interpreted as resulting from metasomatic action of the kimberlite magma or deep fluids. Three different origins have been proposed for kelyphite: magmatic origin, exsolution from orthopyroxene and metamorphic reaction between garnet and olivine during retrogression. This last interpretation is by far the most common. Finally, we discuss the origins of other pyroxenes + spinel symplectites and clusters that occur in peridotite. In the second part, we study various kelyphites observed in the Ulten peridotites (Italian Alps). They are composed of two concentric coronae: (i) a narrow outer spinel-free corona, in contact with olivine, made of orthopyroxene ± amphibole ± clinopyroxene; (ii) a wide inner corona, in contact with garnet, which consists of vermicular spinel forming various symplectites with either orthopyroxene, amphibole or clinopyroxene. An attempt to balance the kelyphite-forming reaction is made through a material transfer study, in order to determine element mobilities and to evaluate the opening of the system during the reaction. Two different quantitative treatments are used: the first approach consists in estimating the stoichiometric coefficients by the least square method; the second is based on measurements of the stoichiometric coefficients of the kelyphitisation products by analysis of back-scattered electron and X-ray images. This study shows that the kelyphite-forming reactions range between two extremes: garnet + olivine → orthopyroxene + spinel + clinopyroxene and garnet + olivine + H2O → orthopyroxene + spinel + amphibole. This accounts for strong variations in the relative abundance of amphibole and clinopyroxene, which were correlated with local fluctuations of PH2O. It is demonstrated that the boundary between the two concentric coronae coincides with the former garnet–olivine interface. These coronae did not maintain the garnet and olivine compositions; they exchanged components during their growth. Moreover, the whole kelyphite system was slightly open, with gains in alkalis, water and Mg, and losses of Fe and Al. The behaviour of Mg and Fe is likely explained by their incipient redistribution between ferromagnesian phases during kelyphitisation, in relation with the temperature decrease. Change in garnet composition close to kelyphite strongly supports this hypothesis. Incipient Tschermakitic substitution in neighbouring minerals could also explain the slight opening to Si and Al. Kelyphite formation has many characteristics of diffusion metasomatism at microscale. μ–μ diagrams are used to illustrate some aspects of this metasomatism. Finally, pressure–temperature conditions of kelyphite formation in the evolution of peridotite during exhumation are discussed. Kelyphitisation is mainly related to a drop in pressure, as attested by a strong volume increase (ΔV/V=7.1)
The subducted Tethys in the Aosta Valley (Italian Western Alps
No full textPre-congress Field trip guide-book B0
Hydrothermal origin of manganese in the high-pressure metasedimentary sequence ofPraborna (Aosta Valley, Western Alps)
No full textThe manganese ore deposit of Praborna crops out in the Zermatt-Saas unit of the Western Alps, in the St. Marcel valley. It represents a Jurassic ophiolitic sedimentary cover subducted at high-pressure conditions during the Alpine orogeny. Major- and trace-element analyses of representative samples of the ore and the host metasediments were collected to geochemically characterise the deposit. Selected phases (piemontite, braunite, garnet, clinopyroxene, white mica and manganiandrosite) were investigated with ion and electron microprobes to link the mineral chemistry to the bulk-rock chemistry. Compared to shales, Praborna is enriched in Mn (up to 38.7 wt% Mn2O3) and in many trace elements (Sc, Co, Ni, Cu, Ge, As, Sr, Ag, Sb, Te, Ba, Tl, Pb and Bi). The bulk-rock REE pattern suggests 20% hydrogenous and 80% hydrothermal inputs in the proto-ore. Compared to the shale, the hanging-wall Mn-poor schists share with the Mn ore body the enrichment in Sc, Mn, Co, Sr and Te, suggesting a common enrichment process involving these elements. The REE pattern suggests a sedimentary origin for these schists, which were probably composed of clay mixed with components of volcanic origin.
In order to confirm the hydrothermal origin of the Praborna Mn ore deposit, we built up a database of more than 5000 data of modern hydrogenous and hydrothermal oceanic Mn deposits worldwide, adding data of oceanic Mn-rich sediments and of the Ligurian Mn ore deposits, which are thought to be the unmetamorphosed geological equivalent of Praborna. The classic ternary Mn-Fe-(Cu + Co + Ni) diagram, the agglomerative hierarchical clustering and the principal-component analysis, which takes into account a larger set of elements, strongly support the hypothesis of an oceanic hydrothermal origin for manganese in the Praborna and in the Ligurian ore deposits.
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Dissakisite-(La), a new end-member of the epidote group from the Ulten peridotites (Italian Eastern Alps): Evidence of mantle-crust interactions in subduction zones
No full textVIENN
"Dissakisite-(La)" (Mg-analogue of allanite) in peridotite from the Ulten zone (Eastern Alps): evidence for crustal metasomatism
No full textThe Lovignanaz Cu-Fe sulphide mine: safeguard of a pre-Roman mining site
No full textThe antique Lovignanaz mine (also known as Molina mine) is located in the western side of the Clavalité valley (1350-1425 m a.s.l.), to the South of the Fénis village, in the southern side of the Aosta Valley (Italy). The mine galleries are developed within chloriteschists, talcschists and metagabbros transposed with calcschists belonging to the Piedmont nappe (Zermatt-Saas zone). The rocks hosting the Fe-Cu sulphide mineralization show blueschist to eclogite facies mineral associations like those studied in Saint-Marcel rocks (Martin et al., 2008, and ref. therein). The mineralization is characterized by disseminated chalcopyrite and pyrite associated with minor ilmenite, rutile, pyrrhotite and magnetite (Fénis. Une communauté au fil de l'histoire, 2000 and ref. therein).
The oldest galleries were dug with techniques described by Agricola in De Re Metallica (1556) and have been attributed to the Romans. These techniques consist in excavation along mineralized layers after “fire-setting”, that is after weakening the rocks with a fire (De Re Metallica, book V).
Other galleries have been excavated with more recent techniques, revealing the use of explosives, and have been attributed to XVIII-XIX centuries mining activity (Gerbore E. E., in: Fénis..., op. cit.).
The remnants of a furnace for rock-roasting have been found along the right side of the Clavalité river (lat. N 45° 41’ 46’’; long. E 7° 29’ 51’’). In the same site, slags with charcoal and red burnt soil, probably related to an old activity, were observed during the field work. Several glassy slags, maybe produced during XVIII-XIX centuries mining activity, were found below a large landslide that partly covers the mine adits.
Near the oldest mine entrances traces of old working instrument have been found.
These mines were also used to extract millstones: few metres from the mine entrances in the Mouilé locality (Fénis..., op. cit.), some of these stones are still unexcavated on the rock walls.
In the lower part of the valley, in correspondence of Miserègne village (Fénis), there is a dump where many large slags with charcoal fragments are piled up. Some of these fragments were analyzed with 14C method and one of them has been dated to IV century b.C.. This age is the oldest found for charcoal associated to slags of the Cu mines in the Aosta Valley. Other slags from the Saint Marcel valley yielded early medieval ages (Tumiati et al., 2005).
The Lovignanaz mining site has been destroyed by landslide events, the last one having occurred in 2000, which partly covered mine entrances and other structures linked to mining activity. The presence of structural lineaments and the practice of “fire-setting” during mining activity may have contributed to trigger the landslide.
References:
AAVV, 2000, Fénis. Une communauté au fil de l'histoire., Musumeci Editore;
Agricola [Bauer G.], 1556, De Re Metallica, ed. 1950 by Hoover H. C. and Hoover L. H., Dover Publications, New York, book V, 118-121;
Casartelli P., 2003, Archeometallurgia nella valle di Saint-Marcel (AO): studio preliminare delle scorie di fusione della miniera di Servette, tesi di laurea, Univ. degli studi dell’Insubria;
Lorenzini C., 1995, Le antiche miniere della Valle d’Aosta, Musumeci Editore, 62-64;
Martin S., Rebay G., Kienast J.-R., Mével C., 2008, An eclogitised oceanic palaeo-hydrothermal field from the St. Marcel valley (Italian Western Alps), Ofioliti, 33 (1), 49-63;
Tumiati S., Casertelli P., Mambretti A., Martin S., 2005, The ancient mine of Servette (Saint-Marcel, Val d’Aosta, Western Italian Alps): a mineralogical, metallurgical and charcoal analysis of furnace slags, Archaeometry 47, 2, 317-34
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