Late-Alpine rodingitization in the Bellecombe meta-ophiolites (Aosta Valley, Italian Western Alps): evidence from mineral assemblages and serpentinization-derived H₂-bearing brine

Abstract

We report on Alpine metamorphic and fluid inclusion evolution of a polyphase rodingite occurrence within the Bellecombe antigorite-serpentinite, exposed in the Piemonte zone of Aosta Valley, NW Italy. Fine-grained rodingitic rocks, derived from a protolith of basaltic dike(s), are cross-cut by a network of at least six vein generations, consisting of chlorite, diopside, and grossular garnet (Type I), andradite-grossular garnet + diopside (Type II), andradite-rich garnet + chlorite (Type III), grossular-rich garnet (Type IV), vesuvianite (Type V), and chlorite (Type VI). The fine-grained rodingite and associated veins reveal a tectono-metamorphic history similar to that of the hosting serpentinite and characterized by an earlier high pressure metamorphism, followed by decompression/re-equilibration under greenschist facies conditions and by final cooling. The fluid inclusion study, performed on primary fluid inclusions in vesuvianite from Type V veins and on secondary fluid inclusions in andradite-rich garnet from Type III veins, revealed that at P = 0.22 GPa and T = 400°C, an H 2 -bearing brine (6 wt.% CaCl 2 + 6 wt.% NaCl) with traces of was introduced into the rock. This fluid had a composition compatible with the Ca-rich H 2 -bearing, reducing aqueous solutions reported from the serpentinization front. These data point to an important event of rodingitization, probably triggered by hydration of metamorphic olivine, during the late greenschist facies Alpine evolution that was probably triggered by hydration of metamorphic olivine.

Redistribution of Iron and Titanium in High‐Pressure Ultramafic Rocks

“…Sources of Ti in ultramafic rocks include the breakdown of Ti‐bearing primary minerals such as Ti‐augite (e.g., Trommsdorff & Evans, ), and/or fluid facilitated transport of Ti either during seafloor alteration or prograde metamorphism. Fluid facilitated addition of Ti could provide sufficient Ti for the formation of Ti‐clinohumite within veins as observed in this study and others (Ferrando et al, ; Groppo & Compagnoni, ; Rebay et al, ; Scambelluri & Rampone, ;). Previous authors attributed such textures to ocean‐floor metamorphism or, alternatively, high‐pressure, low‐temperature conditions.…”

“…A significant effort has been devoted to the interpretation of metamorphic conditions using silicate textures in high‐pressure ultramafic rocks (Ferrando et al, ; Groppo & Compagnoni, ; Li et al, ; Rahn & Rahn, ; Rebay et al, ; Scambelluri et al, ; Scambelluri & Rampone, ), but less work has been carried out on oxide mineral textures. Exceptions include studies of oxide mineral textures in Val Malenco (Burkhard, ; Peretti et al, ) and Cerro del Almirez (e.g., Lopéz Sanchéz‐Vizcaíno et al, ).…”

“…Ti‐rich phases such as Ti‐clinohumite in veins in high‐pressure serpentinites have been described previously but the origin of such veins are controversial (e.g., Rebay et al, ). Some authors attribute Ti‐clinohumite veins to high‐pressure‐low‐temperature (HP‐LT) metamorphism as a product of fluid‐mediated Ti remobilization on a sample scale associated with H 2 O release during serpentine dehydration (Ferrando et al, ; Groppo & Compagnoni, ; Marchesi et al, ; Scambelluri & Rampone, ). The length scale of Ti mobilization is considered to be on a sample scale, although Lopéz Sanchéz‐Vizcaíno et al (2009) attributed different generations of Ti‐clinohumite veins to mobility of Ti on different length scales from centimeter to meter scale.…”

Ocean floor and subduction record in the Zermatt‐Saas rodingites, Valtournanche, Western Alps

“…The Zermatt‐Saas Zone (ZSZ) ophiolitic complex, which outcrops in the upper Valtournanche (Val d'Aosta, northwestern Italy), consists of high‐pressure/ultra‐high‐pressure (HP/UHP) metaophiolites in which the petrological and structural evolution of metagabbros, metabasalts and serpentinites can be constrained (e.g. Ernst & Dal Piaz, ; Dal Piaz et al ., ; Barnicoat & Fry, ; Bucher et al ., ; Angiboust et al ., ; Bucher & Grapes, ; Groppo et al ., ; Ferrando et al ., ; Rebay et al ., ; Zanoni et al ., ). The deformation–metamorphism relationships in rodingites have been partly overlooked because of their complex composition and mineralogy (Zanoni et al ., ).…”

Quantitative Raman spectroscopic study of the H2─CH4 gaseous system

“…Although H 2 ‐bearing inclusions are rare, they have been recognized in various geological environments, including some Precambrian uranium deposits, serpentinized peridotite, high‐grade metamorphic rocks, and igneous rocks . In these H 2 ‐bearing inclusions, H 2 generally coexists with CH 4 .…”