Geochemistry of abyssal peridotites (Mid-Atlantic Ridge, 15°20′N, ODP Leg 209): Implications for fluid/rock interaction in slow spreading environments

Abstract

Abyssal peridotite from the 15°20'N area of the Mid-Atlantic Ridge show complex geochemical variations among the different sites drilled during ODP Leg 209. Major element compositions indicate variable degrees of melt depletion and refertilization as well as local hydrothermal metasomatism. Strongest evidence for melt-rock interactions are correlated Light Rare Earth Element (LREE) and High Field Strength Element (HFSE) additions at sites 1270 and 1271. In contrast, hydrothermal alteration at Sites 1274, 1272, and 1268 causes LREE mobility associated with minor HFSE variability, reflecting the low solubility of HFSE in aqueous solutions. Site 1274 contains the least-altered, highly refractory, peridotite with strong depletion in LREE and shows a gradual increase in the intensity of isochemical serpentinization; except for the addition of H 2 O which causes a mass gain of up to 20 g/100 g. The formation of magnetite is reflected in decreasing Fe 2+ /Fe 3+ ratios. This style of alteration is referred to as rock-dominated serpentinization. In contrast, fluid-dominated serpentinization at Site 1268 is characterized by gains in sulfur and development of U-shaped REE pattern with strong positive Eu anomalies which are also characteristic for hot (350 to 400 °C) vent-type fluids discharging from black smoker fields. Serpentinites at Site 1268 were overprinted by talc alteration under static conditions due to interaction with high a SiO 2 fluids causing the development of smooth, LREE-enriched patterns with pronounced negative Eu anomalies. These results show that hydrothermal fluid-peridotite and fluid-serpentinite interaction processes are an important factor regarding the budget of exchange processes between the lithosphere and the hydrosphere in slow spreading environments.

The variability of peridotite composition across a mantle shear zone (Lanzo massif, Italy): interplay of melt focusing and deformation

“…9). Refractory peridotite from the Oman ophiolite or the Mid-Atlantic Ridge (Godard et al 2000(Godard et al , 2008 displays low Ce, Th and Nb values contrary to harzburgite from the Mid-Atlantic Ridge (Paulick et al 2006), which presents high Ce, Th and Nb contents, particularly for ODP leg sites 1270-1271. The most depleted peridotite from the Mid-Atlantic Ridge (Godard et al 2008) recorded a high degree of partial melting and melt extraction, similar to the peridotites from ODP leg 210 (Müntener and Manatschal 2006).…”

A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab

“…A contribution from metasediments is also indicated by the elevated LREE and LILE contents in the zoisite eclogites (Spandler et al, 2003(Spandler et al, , 2004, even though some of these fluid-mobile elements can also be enriched in serpentinites (Paulick et al, 2006;Scambelluri et al, 2001) and may be mobilized during channelized fluid flow in subducted oceanic crust (Zack and John, 2007).…”

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

“…This is also reflected in a high LOI content of our peridotites ( (Luguet et al, 2003;Boschi et al, 2006;Paulick et al, 2006;Morishita et al, 2009 …”

“…An additional loss of sulfur seems to be caused by subsequent talc alteration of serpentinized peridotites (Section 4.3; cf. Paulick et al, 2006;Harvey et al, 2014).…”