Gaston Godard

Ultrahigh-Pressure (60 Kbar) Magnesite-Bearing Garnet Peridotites from Northeastern Jiangsu, China

The very high P/T metamorphic belt in Chinas Shandong and fiangsu provinces, which has drawn much attention because of its numerous coesite-bearing eclogites, also contains little-studied massifs of garnet peridotites. Three samples, a chromite-bearing garnet peridotite (DZ01), a spinel-free garnet peridotite (DZ02) and a garnet websterite (DG01) from northeastern Jiangsu are studied here. Olivine compositions in peridotites are restricted to

Asthenospheric metasomatism beneath the mid-ocean ridge: Evidence from depleted abyssal peridotites

FO_{90-92}

Geochronology and metamorphic P-T-X evolution of the Eburnean granulite-facies metapelites of Tidjenouine (Central Hoggar, Algeria): witness of the LATEA metacratonic evolution

. The spinel phase in DZ01 is mostly Fe-rich chromite, with increasing magnetite toward its rim. Garnets are pyropic; their rims are relatively enriched in almandine. Orthopyroxene is extremely poor in Al, reflecting the very high P conditions. Clinopyroxene is Cr-rich, Al-poor diopside. Magnesite and phlogopite in DZ01 are believed to have formed in the mantle but somewhat later than the other primary phases. Magnesite, with very low Sr, Ba, and K contents, displays breakdown reactions producing dolomite, calcite, and periclase. Titanoclinohumite and pentlandite have been observed. Amphibole and chlorite occur as secondary phases. Geothermobarometry yields extremely high P of equilibration for these rocks (65-45 kbar). The P-T paths followed by the two garnet peridotites were considerably lower in T than the normal continental geotherm. Evidence against partial melting during uplift of the peridotites is clear. Metasomatism and oxidation of the mantle source by a

Environmental factors controlling the precipitation of Cu-bearing hydrotalcite-like compounds from mine waters. The case of the “Eve verda” spring (Aosta Valley, Italy)

H_{2}O

Hydrothermal origin of manganese in the high-pressure ophiolite metasediments of Praborna ore deposit (Aosta Valley, Western Alps)

-bearing fluid are demonstrated. Oxygen fugacity in DZ01 evolves from +0.45 to + 1.37 log units relative to the FMQ buffer, with decreasing P and T. These results suggest that the garnet peridotites were derived from a mantle wedge overlying a subducted slab.

First report of ultrahigh-temperature sapphirine + spinel + quartz and orthopyroxene + spinel + quartz parageneses discovered in Al–Mg granulites from the Khanfous area (In Ouzzal metacraton, Hoggar, Algeria)

The Southwest Indian Ridge, between 618 and 688E, is one of the deepest, slowest, and coldest sections of the global mid-ocean ridge system, and the composition of the basaltic crust suggests an extremely low degree of melting (Meyzen et al., 2003). In contrast to normal expectations, the subaxial mantle contains peridotites highly depleted in clinopyroxene (,2%), the lowest-melting mineral phase, but anomalously enriched in orthopyroxene (modal olivine/orthopyroxene 2). Furthermore, orthopyroxene grains host mineral inclusions enriched in OH, Na, K, P, S, and light rare earth elements never previously reported in residual mineral assemblages of abyssal peridotites; these inclusions are primarily Na-, Cr-rich diopside variably associated with apatite, amphibole, mica, albite, and sulfides. Surprisingly, these metasomatic minerals do not occur within the other mineral phases (olivine, spinel) or as interstitial phases or veinlets. We conclude that the metasomatic mineral inclusions represent traces of a fertile mantle component that locally escaped extraction during decompression beneath the ridge. Our observations (1) imply interactions of the suboceanic asthenosphere with incompatible element‐rich melts, and (2) provide evidence for refractory mantle blobs in the suboceanic mantle that have compositional similarities to continental lithosphere.

Partial melting of ultramafic granulites from Dronning Maud Land, Antarctica: Constraints from melt inclusions and thermodynamic modeling

Abstract Central Hoggar, within the Tuareg shield to the east of the West African craton, is known for its complexity owing to the interplay of the Eburnean and Pan-African orogenies. The Tidjenouine area in the Laouni terrane belongs to the LATEA metacraton and displays spectacular examples of granulite-facies migmatitic metapelites. Here, we present a detailed petrological study coupled with in situ U–Pb zircon dating by laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) that allows us to constrain the relative role of the Eburnean and Pan-African orogenies and hence to constrain how the LATEA Eburnean microcontinent has been partly destabilized during the Pan-African orogeny; that is, its metacratonic evolution. These metapelites have recorded different metamorphic stages. A clockwise P–T evolution is demonstrated on the basis of textural relationships, modelling in KFMASH and FMASH systems and thermobarometry. The prograde evolution implies several melting reactions involving the breakdown of biotite and gedrite. Peak metamorphic P–T conditions of 860±50 °C and 7–8 kbar (M1) were followed by a decrease of pressure to 4.3±1 kbar and of temperature to around 700 °C, associated with the development of migmatites (M2). After cooling, a third thermal phase at c. 650 °C and 3–4 kbar (M3) occurred. U–Pb zircon laser ablation ICP-MS analysis allows us to date the protolith of the migmatites at 2151±8 Ma, the granulite-facies and migmatitic metamorphisms (M1–M2) at 2062±39 Ma and the medium-grade metamorphic assemblage (M3) at 614±11 Ma. This last event is coeval with the emplacement of large Pan-African granitic batholiths. These data show that the main metamorphic events are Eburnean in age. The Pan-African orogeny, in contrast, is associated mainly with medium-grade metamorphism but also mega-shear zones and granitic batholiths, characterized by a high temperature gradient. This can be considered as typical of a metacratonic evolution.

Pseudotachylytes of the Tonale nappe (Italian Alps): petrogenesis, 40Ar-39Ar geochronology and tectonic implications

In the Cu–Fe-sulphide mining district of Servette-Chuc (Saint-Marcel, Aosta Valley, Italy), a streambed called Eve Verda and covered with a colloidal blue-green precipitate has been known since the 18th century. X-ray and chemical analyses reveal that the precipitate is a mixture of a nanocrystalline Cu–Al-sulphate hydrotalcite-like compound close to hydrowoodwardite (Cu1− x Al x [SO4] x /2[OH]2· m H2O) (49 ± 10 vol.%), and amorphous Al and Si oxyhydroxides. The Al/(Al + Cu) ratio of hydrowoodwardite ( i.e ., x ) ranges between 0.10 and 0.30 with a mean value of 0.17 ± 0.04. The average formula may be expressed as follows: Cu0.81Zn0.02Al0.17(SO4)0.08(OH)2·1.10 H2O. In order to construct phase diagrams, we estimated the thermodynamic properties of the hydrowoodwardite solid solution starting from calorimetric measurements available in the literature, i.e ., those for Mg, Co, Ni and Zn hydrotalcites. At least for x < 0.35, hydrowoodwardite may be treated either as an ideal or as a non-ideal solid solution between the two end-members Cu(OH)2 (spertiniite) and Al(SO4)0.5(OH)2 (aluminite). For x = 1/3 ( i.e ., woodwardite sensu stricto following IMA nomenclature) and m = 0, the ideal solid solution model provided a Gibbs free energy of formation (Δ G f) of −684.4 kJ/mol and an enthalpy of formation (Δ H f) of −795.0 kJ/mol, while the non-ideal solution model yielded Δ G f = −658.6 kJ/mol and Δ H f = −767.5 kJ/mol. The estimated data agree well with the solubility measurements reported for hydrotalcites. Since hydrowoodwardite precipitates where mineralised, acidic waters from an ephemeral stream are mixing with alkaline, diluted waters of a perennial spring, we calculated pH– X w pseudosections ( i.e ., isochemical phase diagrams), where X w is the molar proportion of the two mixing waters. The thermodynamic modelling indicates that the formation of hydrowoodwardite is related to a geochemical barrier represented by the perennial spring waters. It also demonstrates that the composition of hydrowoodwardite ( i.e ., x ) is mainly a function of pH, while the modal composition of the precipitate depends instead on X w . This explains the variations observed in the precipitate composition at Eve Verda . Riassunto: Nel distretto minerario di Servette-Chuc (valle di Saint-Marcel) nella Valle d’Aosta, dove era coltivata in passato una mineralizzazione a solfuri di rame e ferro, e presente un piccolo corso d’acqua, il cui letto appare ricoperto da un precipitato colloidale blu-verde. Questo torrente, chiamato nei diversi idiomi Acqua Verde, Eve Verda o Eaux vertes , venne descritto per la prima volta da vice balivo Joseph Rambert nel 1749. Circa cinquanta anni dopo, Horace-Benedict de Saussure, il grande precursore della geologia alpina, forni la prima analisi chimica del precipitato nel suo Voyages dans les Alpes (1796). In questo articolo presentiamo nuove approfondite analisi del precipitato blu-verde per determinarne la natura e la genesi. Le analisi chimiche e lo studio ai raggi X mostrano che il precipitato e una mistura d’idrowoodwardite nanocristallina (responsabile della colorazione blu-verde) e ossidrossidi d’alluminio e silicio amorfi (responsabili della colorazione biancastra). Per quanto riguarda la genesi, dimostriamo, grazie alla modellazione termodinamica, che l’idrowoodwardite si forma come conseguenza del disgelo estivo, quando le acque di drenaggio della soprastante miniera di Servette, acide e ricche in sali disciolti, si uniscono in piccola parte (circa il 6,5 %) alle acque alcaline e scarsamente mineralizzate di una sorgente perenne. Fungendo da barriera geochimica, le acque alcaline causano la precipitazione dell’alluminio e del rame in soluzione sotto forma d’idrowoodwardite, minerale dal colore blu-verde. Durante l’inverno, il gelo impedisce alle acque acide di miniera di scorrere a valle e viene dunque a mancare l’apporto in sali necessari per stabilizzare l’idrowoodwardite. Di conseguenza, il precipitato invernale risulta arricchito in idrossido d’alluminio e assume una colorazione tendente al bianco. Considerato da Rambert nel 1749 come un disastro ecologico, Eve Verda e invece un fenomeno eccezionale e spettacolare, che deve essere visto come una bella curiosita del patrimonio naturalistico della Valle d’Aosta.

Quantitative X-ray Map Analyser (Q-XRMA): A new GIS-based statistical approach to Mineral Image Analysis

The 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% Mn 2 O 3 ) 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.

UV and VIS Raman spectra of natural lonsdaleites: Towards a recognised standard

Abstract The Archaean to Palaeoproterozoic Khanfous area from the Archaean In Ouzzal granulite terrane (Western Hoggar, Algeria) preserves exceptional thermal-peak (1150<T < 1300 °C) mineral parageneses, consisting of orthopyroxene+spinel+quartz, sapphirine+spinel+quartz and sapphirine+orthopyroxene+quartz, in quartz-rich Al–Mg granulites. Reaction textures coupled with P–T FMASH pseudosections indicate that rocks experienced complex multi-stage evolution. Our results suggest that the Khanfous area, as well as the entire northern In Ouzzal metacraton, experienced ultrahigh-temperature crustal metamorphism attributed to a 2 Ga Palaeoproterozoic event, followed by exhumation along a clockwise P–T path. The extreme temperatures attained suggest delamination of the lithosphere and ascent of the asthenosphere after crustal thickening.