Impure marbles from the UHP Brossasco‐Isasca Unit (Dora‐Maira Massif, western Alps): evidence for Alpine equilibration in the diamond stability field and evaluation of theX(CO₂) fluid evolution

Abstract

Two impure ultrahigh-pressure (UHP) marbles, a calcite marble with the peak assemblage Grt + Phe + Cpx + Rt + (Arg) and a dolomite marble with the peak assemblage Crn + Chl + Rt + Dol (±Arg), from the same lens from the polymetamorphic complex of the Brossasco-Isasca Unit (BIU) (southern Dora-Maira Massif) have been petrologically investigated and modelled by calculating P-T phase-diagram projections for H 2 O-CO 2 mixed-volatile systems. Thermobarometric data obtained from the calcite marble suggest Alpine peak conditions in the diamond stability field (4.0 GPa at 730°C), and allow reconstruction of the earlier portion of the Alpine retrograde P-T path, which is characterized by a significant decompression coupled with a moderate and continuous cooling to 650°C at 2.50 GPa. The modelled fluid compositions at peak conditions point to 0.025 £ X(CO 2 ) £ 0.10 and X(CO 2 ) £ 0.0012 in the calcite marble and dolomite marble, respectively, suggesting fluid heterogeneity at the local scale and an internally buffered fluid evolution of the studied impure marbles. The lack of micro-diamond in the BIU marbles is explained by the very-low X(CO 2 ) values, which favoured relatively high fO 2 -conditions, preventing the formation of diamond at the UHP peak metamorphic conditions.

Geological map of the ultra-high pressure Brossasco-Isasca unit (Western Alps, Italy)

“…It is interesting to note that since the BIU discovery (Chopin, 1984) the inferred peak T did not change, whereas the peak P progressively increased from about 3.3 GPa within the coesite stability field (e.g., Compagnoni et al, 1995) to about 4.3 GPa within the diamond stability field. Even though microdiamonds have never been found, the P increase of about 1 GPa has been inferred from both experimental petrology (Hermann, 2003) on representative BIU rocks and from a number of different geobarometers and thermodynamic modelling (e.g., Castelli et al, 2007;Groppo et al, 2007a). The lack of diamond may be explained with the sluggishness of the graphite-diamond reaction at the relevant peak T.…”

“…(e) In metagranitoids, granoblastic polygonal quartz aggregate pseudomorphous after the former igneous quartz crystals (Figures 2a, 3a, 4c), interpreted as evidence of the former presence of UHP coesite (Biino & Compagnoni, 1992). (f) The presence of aragonite instead of calcite in BIU marbles is not to be expected (e.g., Castelli et al, 2007;Groppo et al, 2007b) because of the high speed of the aragonite to calcite inversion at the relatively high-T of the BIU metamorphism. (a) Phengite with high celadonite (Si .…”

“…The application of these general criteria to the BIU and further detailed petrologic studies led to constraining not only its peak P-T conditions (e.g., Castelli et al, 2007;Groppo et al, 2007a and references therein) but also its prograde (e.g., Ferrando et al, 2009) and retrograde (e.g., Groppo et al, 2006) trajectories. It is interesting to note that since the BIU discovery (Chopin, 1984) the inferred peak T did not change, whereas the peak P progressively increased from about 3.3 GPa within the coesite stability field (e.g., Compagnoni et al, 1995) to about 4.3 GPa within the diamond stability field.…”

“…3.50 a.p.f.u.) and low paragonite (, 10 mol%) substitutions (e.g., Castelli et al, 2007;Groppo et al, 2007a). (b) Phengite with unstrained talc and quartz lamellae evident at high-resolution transmission electron microscope (HRTEM) (Ferraris, Chopin, & Wessicken, 2000).…”

“…After the discovery of coesite (Chopin, 1984) in the southern sector of the intermediate Polymetamorphic unit of Vialon (1966), the scientific interest for the southern DMM increased rapidly and a number of detailed petrologic studies were produced (e.g., Castelli, Rolfo, Groppo, & Compagnoni, 2007;Chopin, Henry, & Michard, 1991;Chopin & Schertl, 2000;Compagnoni & Hirajima, 2001;Compagnoni, Hirajima, & Chopin, 1995;Ferrando, Frezzotti, Petrelli, & Compagnonilli, 2009;Groppo, Castelli, & Compagnoni, 2006;Groppo, Lombardo, Castelli, & Compagnoni, 2007a, Groppo, Rolfo, & Castelli, 2007Michard, Henry, & Chopin, 1995;Nowlan, Schertl, & Schreyer, 2000;Schertl, Schreyer, & Chopin, 1991;) aimed at determining the peak metamorphic conditions. The UHP (ultra-high pressure) unit has also been mapped in great detail, and special attention has been paid to recognize the relics of both pre-Alpine and early-Alpine structures and minerals which are essential to understand the nature and relationships among lithologies and to check the internal coherency of the unit.…”