Trace element behavior during serpentinization/de-serpentinization of an eclogitized oceanic lithosphere: A LA-ICPMS study of the Lanzo ultramafic massif (Western Alps)

Evolution of Fe redox state in serpentine during subduction

“…Numerous studies have shown that most of the major elements, in particular iron, are immobile during the serpentinization processes at the oceanic ridge (Andreani et al, 2013;Godard et al, 2008;Mével, 2003;Paulick et al, 2006) and during prograde metamorphism in subduction zones (Debret et al, 2013a(Debret et al, , 2013bGarrido et al, 2005;López Sánchez-Vizcaíno et al, 2005). In agreement with these previous results, no significant change of Fe content is observed in our serpentinites samples during prograde metamorphism (Table 1).…”

“…The Fe 2 O 3 Tot (BR) content ranges from 7.79 to 9.53 wt% (Table 1) and these slight variations most probably reflect variations in the modal abundances of the protolith minerals; it has previously been shown that the major element compositions of serpentines are mainly controlled by primary mineralogy (Godard et al, 2008;Mével, 2003;Paulick et al, 2006). Furthermore, according to previous geochemical studies on these samples (Debret et al, 2013b;Lafay et al, 2013), the selected serpentinites have primary mineral modes similar to plagioclase/spinel lherzolites. The different Fe 2 O 3 Tot (BR) contents among the selected samples are thus attributed to slight variations in clinopyroxene, spinel and plagioclase contents.…”

“…Most of these samples have already been characterized in previous petrological (Debret et al, 2013a;Schwartz et al, 2013) and geochemical (Debret et al, 2013b;Lafay et al, 2013) studies. The metamorphic conditions and petrographic characteristics of the studied samples are summarized in Table 1.…”

“…With prograde metamorphism, the progressive serpentine phase changes (lizardite → antigorite → olivine/pyroxene) drive fluid release to the mantle wedge (Hattori and Guillot, 2007) until depths of about 150-180 km, where dehydration of serpentine to form olivine, enstatite and chlorite assemblages (Trommsdorff et al, 1998;Fumagalli and Poli, 2005) should be complete (Ulmer and Trommsdorff, 1995;Wunder and Schreyer, 1997). Geochemical observations based on fluid mobile element (B, Li, As, Sb, Ba, Rb and Cs), halogens (F, Cl) and volatiles (S) behavior during prograde metamorphism in subduction zones reveal a direct link between the composition of the fluid released during serpentine phase transitions and those of arc magmas (Debret et al, 2013b(Debret et al, , 2014Savov et al, 2005Savov et al, , 2007Scambelluri and Tonarini, 2012;Vils et al, 2011). Nonetheless, there is no overall consensus on the evolution of the redox state of serpentinites during subduction.…”

“…Indeed, these ophiolites are considered to have been highly hydrated and serpentinized during an oceanic stage (distal continental margin or mid-ocean ridge environment) prior to partial dehydration during prograde metamorphism (Debret et al, 2013a(Debret et al, , 2013bHattori and Guillot, 2007;Lafay et al, 2013;Li et al, 2004;Schwartz et al, 2013;Vils et al, 2011). Here we propose to use the redox state of iron in serpentinites, an abundant element in this type of rock, in order to assess the possible nature of the released fluids during serpentine phase changes in subduction zones.…”