Fluid Pulses During Stepwise Brecciation at Intermediate Subduction Depths (Monviso Eclogites, W. Alps): First Internally Then Externally Sourced

Abstract

Key Points Mylonitic metagabbros cemented by unfoliated Omp + Grt ± Lws matrices show that pristine brecciation may occur at eclogitic conditions Geochemical variations and sharp increase in fluid content of matrices points to local embrittlement prior to external fluids ingression Structural and geochemical evidence suggest that eclogite‐facies brecciation can control initial strain localization within HP shear zones

Fluid Circulation Along an Oceanic Detachment Fault: Insights From Fluid Inclusions in Silicified Brecciated Fault Rocks (Mid‐Atlantic Ridge at 13°20′N)

“…Therefore, H 2 production in the reaction zone seems to be possible, but may not account for the elevated H 2 /H 2 S ratios analyzed in our fluid inclusions, which are best explained by H 2 derived from ultramafic‐rock serpentinization. Contribution of ultramafic‐derived fluids is further supported by slight Cr and Ni enrichments of both sample ODM218b (Figure 5c) and some chlorites from highly silicified mafic breccias (Tables and ; Angiboust et al., 2014; Boutoux et al., 2014; Locatelli et al., 2019; Spandler et al., 2011).…”

Along-dip variations of subduction fluids: The 30–80 km depth traverse of the Schistes Lustrés complex (Queyras-Monviso, W. Alps)

“…Different generations of veins and eclogitic breccia matrices were characterized (Locatelli et al, 2018) (1): the first J o u r n a l P r e -p r o o f generation of vein (V1) and matrix (M1) is almost exclusively composed of omphacite whereas (2) the second generation of vein (V2) and matrix (M2) is filled by omphacite and garnet; (3) the last matrix generation (M3) is composed of lawsonite pseudomorphs and relics of garnet and omphacite. Thermodynamic modelling and trace elements analysis of omphacite and garnet in these veins and matrices allowed to identify the progressive opening of the system to external fluid infiltrations (Locatelli et al, 2018(Locatelli et al, , 2019b: from metric at peak conditions (V1 veins and M1 matrix) to possibly decametric opening at early retrograde conditions (V2 veins and M2) and hectometric to kilometric opening at retrograde conditions (M3).…”

“…In eclogite-facies metamafics, we used samples of omphacite ± garnet bearing veins (Fig. 4h) and matrices of eclogitic breccias from Monviso massif of LPL unit (see Locatelli et al, 2018;2019b).…”

“…Both correspond to Fe-Ti metagabbro blocks metamorphosed, mylonitized and brecciated under eclogite-facies conditions. Omphacite-bearing veins V1 and V2 (see Locatelli et al, 2018;2019b) crosscutting the mylonitic foliation and M2 matrix of eclogitic breccias were sampled in outcrop 10. M1 matrix of eclogitic breccias was studied in outcrop 11.…”

“…Monviso Fe-Ti-metagabbros allow studying fluid ingression during progressive brecciation and strain localization at eclogite-facies conditions (Angiboust et al, 2012a;Locatelli et al, 2018). Thermodynamic modelling and trace element compositions suggest that omphacite crystallization in V1 veins and M1 matrix reflects final glaucophane and lawsonite breakdown in Mg-rich gabbros, while the omphacite from V2 veins and M2 matrix formed at incipient retrograde conditions without local fluid production and in the presence of some serpentinitederived fluids (Locatelli et al, 2019b). The salinity of V2 and M2 omphacite FI does not deviate significantly, however, from that of V1 veins, suggesting either remobilization and entrapment of local remnant fluids (i.e., FI or grain boundary fluids) or that external fluid input was minimal (consistent with only incipient opening of the system; Locatelli et al, 2019b) and/or had a similar salinity.…”