Characterization of metalliferous sediment from a low-temperature hydrothermal environment on the Eastern Flank of the East Pacific Rise

Abstract

Metalliferous deposits are described from the eastern flank of the East Pacific Rise (EPR) offshore Costa Rica, close to a basaltic seamount called "Dorado high".Based on heat-flow data and porewater profiles, the site is an area of active low temperature hydrothermal discharge. We focus on the mineralogical and chemical analyses from a 124 cm long gravity core (GC50), located on the north-western slope of the 100 m high Dorado high. In this core, the sediments consist of detrital clay minerals as well as authigenic minerals such as zeolites, apatites, and Fe/Mn-rich oxyhydroxides. In contrast, the reference sediments from adjacent areas without hydrothermal activity are olive gray hemipelagic muds composed of volcanic glass particles, clay minerals, siliceous microfossils, and some detrital quartz and feldspar.Bulk sediment chemistry and chemical enrichment factors calculated with respect to the reference sediment indicate that the most important chemical changes occurred at the base of the core from 100 to 124 cm bsf, with strong enrichments in MnO, CaO, P 2 O 5 , and Fe 2 O 3 . These enrichments are correlated with the occurrence of authigenic Fe-oxyhydroxide (goethite) and Mn oxide (todorokite and vernadite, at 100 cm bsf), and hydrothermal apatite (110-124 cm bsf). In the upper section of the core from 12 to 70 cm, the sediment is composed of abundant smectite and authigenic phillipsite, and only minor chemical changes can be observed with respect to the reference sediments.The ubiquitous presence of phillipsite suggests that the entire sedimentary column of core GC50 was first affected by diagenesis. However, below 70 cm bsf, these phillipsites are partially dissolved and Fe oxides occur from 110 to 124 cm, followed 2 upward by Mn oxides at 100 cm. This transition from Fe to Mn rich sediments can be interpreted in terms of an upward increasing redox potential. PAAS-normalized REE patterns of GC50 sediments present clearly negative Ce and positive Y anomalies inherited from seawater at the base of core GC50. These anomalies decrease upward, which we interpret together with the transition from Fe to Mn-rich sediments by an upward migrating low temperature hydrothermal fluid. Thus, after a first stage of diagenesis, the discharge of a low temperature hydrothermal fluid occurred through the sedimentary column, leading to the precipitation of hydrothermal compounds that are lacking towards the surface.

Nd–Sr isotope and REY geochemistry of metalliferous sediments in a low-temperature off-axis hydrothermal environment (Costa Rica margin)

“…Some earlier studies suggested that leaching of marine sediments may separate newly formed minerals from particles of detrital origin, because most newly formed minerals are more soluble in dilute acid than detrital particles (Clauer et al, 1993;Stille and Clauer, 1994;Innocent et al, 1999). Our earlier study on the mineralogy of the Dorado samples showed that the bulk sediment is composed of a detrital fraction and a secondary fraction formed during diagenesis and/or hydrothermal activity (Bodeï et al, 2008). The leaching experiments presented here have been initiated in order to characterize the chemical and isotopic composition of the secondary fraction.…”

Neoproterozoic iron formation: An evaluation of its temporal, environmental and tectonic significance

“…Metalliferous sediments are found in close association with mid-ocean ridges and seamounts and their distribution is widespread with known occurrences in the Red Sea (Pierret et al, 2010), along the East Pacific Rise (Barrett et al, 1987;Bodei et al, 2008;German et al, 1999;Sayles and Bischoff, 1973), along the Mid-Atlantic Ridge (Dias and Barriga, 2006;Dias et al, 2008;German et al, 1993), in the Indian Ocean (Bostrom, 1973), in the Gulf of California (Lonsdale et al, 1980) and on many seamounts (Mascarenhas-Pereira and Nath, 2010). Modern metalliferous sediments however, occur in locations with low…”

“…The absence of a positive Eu anomaly may not diagnose the absence of a hydrothermal component, because hydrothermal plumes interact with numerous rare earth element reservoirs such as seawater (negative Eu* CHUR ), basalt (no Eu* CHUR ) and terriginous sediments (negative Eu* CHUR ) that may dilute the primary hydrothermal signature. For example, the Eu* CHUR of many near-ridge metalliferous deposits which were clearly sourced from a hydrothermal plume are commonly much smaller than those observed in the corresponding hydrothermal fluid (Douville et al, 1999;Klinkhammer et al, 1994), or in some cases negative (Barrett and Jarvis, 1988;Bodei et al, 2008;Gamo et al, A C C E P T E D M A N U S C R I P T…”

Formation of Thetis Deep metal-rich sediments in the absence of brines, Red Sea

“…In open systems without protecting brines, the more soluble Mn remains in the seawater, while Fe precipitates next to places where the hydrothermal fluids emerge into the open sea. Near seafloor, Mn may remain in solution in waters (Boulègue and Hamelin, 1983;Mottl et al, 1995), with a precipitation of Mn-components away from hydrothermal vents (Bodeï et al, 2008).…”