Lawsonite metasomatism and trace element recycling in subduction zones

Abstract

International audienceAlthough lawsonite-bearing rocks are rare in exhumed high-pressure (HP) terranes, they are considered to exert a primary role in subduction dynamics. Recent observations in natural settings have shown that fluid–rock interaction at HP conditions, including metasomatism, may lead to unusually high lawsonite amounts even in rocks that originally contained little or no lawsonite. This process may therefore bear important implications for element recycling in subduction zones. A detailed characterization of the geochemical fingerprints associated with lawsonite metasomatism is presented in this contribution. The studied rocks belong to the HP terranes of Alpine Corsica (France), which is the largest documented exposure for lawsonite metasomatism. Metasomatic lawsonite displays complex compositional zoning, including high trace element, Cr and Ti content. The trace element content is much higher compared with the average of non-metasomatic lawsonite, and is in line with the re-incorporation of large amounts of trace elements (e.g. REE, Sr, Pb, Th) in the rock during metasomatism, as shown by mass transfer calculations. Our data suggest that serpentinites represented the main fluid source for the metasomatism, with concurrent contribution of other, possibly Ca-rich lithologies, such as mafic or meta-sedimentary rocks. We propose that the breakdown of metasomatic lawsonite may contribute to the genesis of magmas and their characteristic geochemical signatures

Fluid pathways and high‐P metasomatism in a subducted continental slice (Mt. Emilius klippe, W. Alps)

“…A similar alteration trend is reported in Alpine Corsica where a Permian orthogneissic body exhibits microstructural replacement features and geochemical imprints similar to ASZ rocks (Martin et al ., ). In their case, subduction‐related fluid–rock interaction led to the formation of lawsonitites (see also Brovarone et al ., ). In the Alps, lawsonite veins have been also reported in the Monviso massif associated with intra‐slab fluid transport under eclogite facies conditions (Angiboust et al ., ).…”

Electrical Investigation of Natural Lawsonite and Application to Subduction Contexts

“…The rare occurrence of lawsonite in exhumed HP‐LT rocks has been explained by overprint processes during decompression (Vitale Brovarone, ). Lawsonite may therefore be a common mineral at depth in subduction, especially in old subducted basaltic crust where temperatures are less high than in young slabs, at metamorphic conditions ranging from low‐grade to ultrahigh pressure conditions (e.g., Vitale Brovarone, Alard, et al, ), and its physical properties are relevant to the interpretation of geophysical data in subduction zones.…”

Ultrahigh‐pressure and high‐P lawsonite eclogites in Muzhaerte, Chinese western Tianshan

“…Lawsonite is an important hydrous mineral that carries water and trace elements (e.g., REE, Sr, Pb, Th) into the deep Earth along cold geotherms (e.g., Martin et al, ; Schmidt & Poli, ; Tsujimori, Sisson, Liou, Harlow, & Sorensen, ; Vitale Brovarone, Alard, Beyssac, & Picatto, ). The stability relationships of lawsonite‐bearing mineral equilibria in metabasic rocks have been intensively studied by means of experimental and thermodynamic modelling for a representative mid‐ocean ridge basalt (MORB) composition (Clarke, Powell, & Fitzherbert, ; Forneris & Holloway, ; Okamoto & Maruyama, ; Rebay, Powell, & Diener, ; Schmidt & Poli, ; Wei & Clarke, ).…”