Serpentinization of New Caledonia peridotites: from depth to (sub-)surface

Abstract

Serpentinization processes occur at geological settings notably during oceanic subduction and 32 obduction, where mantle rocks interact with water. Different types of serpentine minerals form 33 according to temperature and pressure conditions, and potentially chemical exchanges. Therefore, the 34 characterization of serpentine minerals, and the possible occurrence of multiple serpentine 35 generations in mantle rocks provide essential constraints on the conditions of fluid-rock interactions 36 in the mantle. The serpentinite sole of the Peridotite Nappe of New Caledonia (Southwest Pacific) is 37 the result of several superimposed serpentinisation events. The latter were discriminated using 38 mineralogical and geochemical approaches and modeling. 39Lizardite represents more than 80% of the entire serpentine content of the ophiolite. It is 40 crosscut by several veins of other serpentine species in the serpentinite sole. The relative chronology 41 appears as follows: lizardite 1 → lizardite 2 → antigorite → chrysotile → polygonal serpentine. The 42 transition from primary/magmatic minerals to lizardite 1 is almost isochemical. Then, the 43 development of lizardite 2 yields an enrichment in fluid-mobile elements such as Cs, Rb, Ba, U and 44 light rare-earth elements and an apparent increase of the Fe 3+ /FeT ratio. The modeling of δ 18 O values 45 (1.9‰ to 13.9‰) and δD values (88‰ to 106‰) of all serpentine species through Monte-Carlo 46 simulations show that New Caledonia serpentines were mainly formed in equilibrium with fluids 47 released by the dehydration of altered oceanic crust (AOC) during subduction between 250°C and 48 350°C. AOC-derived fluids are not the unique source of fluids since a low temperature (100-150°C) 49 meteoric component is also predicted by the models. Thus, serpentine acts as a tape-recorder of fluid-50 rock interactions into the mantle from depth to (sub-)surface. 51 52 assumed to be one of the most efficient ways to recycle water and fluid-mobile elements (FME) into 67 the deeper mantle (

Induced Polarization and Magnetic Responses of Serpentinized Ultramafic Rocks From Mid‐Ocean Ridges

“…In addition, clear trends are observed between either the magnetic susceptibility or the chargeability and the grain density (or bulk density) of the peridotites. Integrating both induced polarization, magnetic and gravity surveys as complementary geophysical methods has therefore the potential of better characterizing the process of serpentinization at oceanic ridges and exhumed ophiolites (see for instance Ulrich et al., 2020). It could be used to better explain marine magnetic and induced polarization anomalies potentially related to the exhumation rate and local tectonics at oceanic ridges.…”

Hydroxychloride‐Bearing Fluid Inclusions in Ultramafic Rocks From New Caledonia: Implications for Serpentinization in Saline Environments on Earth and Beyond

“…Considering tectonic evolution of the New Caledonia ophiolite from spreading oceanic basin, to subduction zone, and finally to obducted massifs, fluids that might infiltrate the ultramafic rocks include seawater, slab fluids, shallow crustal fluids, and meteoric water. Nevertheless, fluid inclusion assemblages are locally crosscut by matrix serpentine (Figures 2b and 2d) that mainly formed in the presence of slab‐derived fluids (Mothersole et al., 2017; Ulrich et al., 2020). Moreover, only the lherzolite originally formed in a mid‐ocean‐ridge‐like setting (Secchiari et al., 2016; Ulrich et al., 2010), whereas the harzburgite, dunite, and chromitite were subjected to melt activity in the forearc mantle (Marchesi et al., 2009; Pirard et al., 2013; Secchiari et al., 2020; Ulrich et al., 2010).…”

“…Peridotite from the New Caledonia ophiolite has experienced varying degrees (0%–100%) of serpentinization. The base of the ophiolite is an almost completely altered serpentinite sole that is characterized by the development of multiple generations of serpentine (Cluzel et al., 2012; Ulrich et al., 2020). In contrast, peridotite in the upper part of the ophiolite is less altered, with serpentinization initiating along a sequence of veinlets and culminating as meshes and bastites (Frost et al., 2013; Mothersole et al., 2017; Ulrich et al., 2020).…”

Serpentinites of Different Tectonic Origin in an Exhumed Subduction Complex (New Caledonia, SW Pacific)

“…In their study, Cluzel et al (2020) interpret these oxygen and hydrogen data (in concert with 87 Sr/ 86 Sr, Ca contents, and REE values of tremolite) to indicate vein formation at ∼300°C to 450°C in the presence of magmatic fluids; in this case, hydrothermal fluids associated with supra-subduction, slab-derived felsic dikes that formed shortly after subduction initiation. Though a sedimentary influence cannot be ruled out, additional recent stable isotope modeling of serpentinites from the New Caledonia ophiolite indicates limited interaction of slab sediment-derived fluids within New Caledonia's mantle wedge during the initial phases of subduction (Ulrich et al, 2020). Moreover, these authors discern, on the basis of Sr isotopes and low abundances of sediment-derived elements such as As and Sb (Deschamps et al, 2013), that the input of subducted sediments was low at the time of serpentinization of the mantle wedge, and instead the dehydration of altered oceanic crust comprised the dominant fluid source.…”