Sulfide enrichment at an oceanic crust-mantle transition zone: Kane Megamullion (23°N, MAR)

Abstract

ABSTRACT:The Kane Megamullion oceanic core complex located along the Mid-Atlantic Ridge (23°30'N, 45°20'W) exposes lower crust and upper mantle directly on the ocean floor. We studied chalcophile elements and sulfides in the ultramafic and mafic rocks of the crust-mantle transition and the mantle underneath. We determined mineralogical and elemental composition and the Cu isotope composition of the respective sulfides along with the mineralogical and elemental composition of the respective serpentines. The rocks of the crust-mantle transition zone (i.e., plagioclase harzburgite, peridotite-gabbro contacts, and dunite) overlaid by troctolites are by one order of magnitude enriched in several chalcophile elements with respect to the spinel harzburgites of the mantle beneath. Whereas the range of Cu concentrations in spinel harzburgites is 7-69 ppm, the Cu concentrations are highly elevated in plagioclase harzburgites with a range of 90-209 ppm. The zones of the peridotite-gabbro contacts are even 2 more enriched, exhibiting up to 305 ppm Cu and highly elevated concentrations of As, Zn, Ga, Sb and Tl.High Cu concentrations show pronounced correlation with bulk S concentrations at the crust-mantle transition zone implying an enrichment process in this horizon of the oceanic lithosphere. We interpret this enrichment as related to melt-mantle reaction, which is extensive in crust-mantle transition zones. In spite of the ubiquitous serpentinization of primary rocks, we found magmatic chalcopyrites [CuFeS 2 ] as inclusions in plagioclase as well as associated with pentlandite [(Fe,Ni) indicate that the metal enrichment, increased sulfide modes, and potentially formation of small sulfide deposits could be expected globally along the petrological Moho.

Experimental Investigation on the Transport of Sulfide Driven by Melt‐Rock Reaction in Partially Molten Peridotite

“…Except for the deviatoric stress, a preliminary experimental study (Z. J. Wang & Jin, 2020) proposed that during the melt‐rock reaction, entrainment of sulfide liquid may be enhanced by the reaction infiltration instability (RII) in the partially molten peridotite when a reacting melt percolates through a dissolvable, porous, melt‐mineral mixture. The mechanical transport and enrichment of sulfide during this melt‐rock reaction have been extensively demonstrated in mantle peridotites, which has clarified some geochemical paradoxes of the chalcophile/siderophile elements (Ciazela et al., 2018; Lorand & Luguet, 2016). The RII theory proposed by Chadam et al.…”

“…Except for the deviatoric stress, a preliminary experimental study (Z. J. proposed that during the melt-rock reaction, entrainment of sulfide liquid may be enhanced by the reaction infiltration instability (RII) in the partially molten peridotite when a reacting melt percolates through a dissolvable, porous, melt-mineral mixture. The mechanical transport and enrichment of sulfide during this meltrock reaction have been extensively demonstrated in mantle peridotites, which has clarified some geochemical paradoxes of the chalcophile/siderophile elements (Ciazela et al, 2018;Lorand & Luguet, 2016). The RII theory proposed by Chadam et al (1986) also applies to magmatic systems by introducing compaction of crystal mush and driving a solubility gradient along the flow direction instead of the propagating reaction front (Aharonov, 1995), resulting in positive feedback among the increasing permeability driven by melt-rock reaction and the associated reinforcement of melt flux in reacting regions.…”

“…This melt‐rock reaction is well known as a pervasive process even forming melt‐rich channels with an extremely speedy magma flow due to the positive feedback in partially molten regions of the mantle, which has been broadly demonstrated by geological investigations (e.g., Kelemen et al., 1995; Sundberg et al., 2010), high‐temperature, high‐pressure (HTHP) laboratory works (e.g., Daines & Kohlstedt, 1994; Pec et al., 2015) and theoretical analyses (Aharonov, 1995; Chadam et al., 1986; Spiegelman et al., 2001). During a reaction of melt with a depleted mantle, melt re‐fertilization processes potentially occur, thereby impregnating the mantle with sulfides, which have been widely found in mantle xenoliths, orogenic and abyssal peridotites (e.g., Ciazela et al., 2018; Luguet et al., 2003; Niu, 2004; Z. J. Wang et al., 2009).…”

Experimental Investigation on the Transport of Sulfide Driven by Melt-rock Reaction in Partially Molten Peridotite

“…Except for the deviatoric stress, a preliminary experimental study proposed that during the melt-rock reaction, entrainment of sulfide liquid may be enhanced by the reaction infiltration instability (RII) in the partially molten peridotite when a reacting melt percolates through a dissolvable, porous, melt-mineral mixture. The mechanical transport and enrichment of sulfide during this melt-rock reaction have been extensively demonstrated in mantle peridotites, which has clarified some geochemical paradoxes of the chalcophile/siderophile elements (Ciazela et al, 2018;Lorand & Luguet, 2016). The RII theory proposed by Chadam et al (1986) also applies to magmatic systems by introducing compaction of crystal mush and driving a solubility gradient along the flow direction instead of the propagating reaction front (Aharonov, 1995), resulting in positive feedback among the increasing permeability driven by melt-rock reaction and the associated reinforcement of melt flux in reacting regions.…”

“…This melt-rock reaction is well known as a pervasive process even forming melt-rich channels with an extremely speedy magma flow due to the positive feedback in partially molten regions of the mantle, which has been broadly demonstrated by geological investigations (e.g., Kelemen et al, 1995;Sundberg et al, 2010), high-temperature, highpressure (HTHP) laboratory works (e.g., Daines & Kohlstedt, 1994;Pec et al, 2015) and theoretical analyses (Aharonov, 1995;Chadam et al, 1986;Spiegelman et al, 2001). During a reaction of melt with a depleted mantle, melt re-fertilization processes potentially occur, thereby impregnating the mantle with sulfides, which have been widely found in mantle xenoliths, orogenic and abyssal peridotites (e.g., Ciazela et al, 2018;Luguet et al, 2003;Niu, 2004;Wang et al, 2009).…”