Chemical Composition of Pacific Sediments near 20°S: Changes with Increasing Distance from the East Pacific Rise

Abstract

Six GEOMETEP cores sampled along a transect about 100 km long centered on the East Pacific Rise are compared with six DSDP Leg 92 cores sampled along a transect 350 to 1250 km west of the crest of East Pacific Rise. The influence of hydrothermal precipitation on the chemical character of the samples from both transects is strong and decreases with distance from the East Pacific Rise. Both the amount and the chemical composition of the hydrothermal precipitates change with distance. Near the rise, the precipitates are enriched in manganese and silica; with increasing distance from the rise the precipitates contain higher proportions of iron. The boron enrichment in the hydrothermal precipitates may outweigh the boron enrichment in marine clay minerals.Hydrothermal production of barite and apatite at the East Pacific Rise is somewhat masked by contributions from the dissolution of calcareous tests. Hydrothermal and residual apatite can be distinguished by their yttrium contents. INTRODUCTIONDuring cruise SO 26 of the GEOMETEP program (geothermal metallogenesis east Pacific) on board the German research vessel Sonne, massive sulfides were dredged in a spreading center of the East Pacific Rise (EPR) at about 20°S Gundlach et al., 1983; Backer et al., in press). Also on that cruise, a 100-km-long transect of six sediment cores was sampled on either side of the EPR at the same latitude. In this chapter, this short GEOMETEP transect is compared with the longer transect (350 to 1250 km west of the EPR) of six DSDP holes from Leg 92 (Fig. 1).In addition to massive sulfides, the EPR in this area produces large amounts of hydrothermal oxide precipitates, which are mainly transported westward by deep water currents (Marchig et al., 1984). This combined transect gave us the opportunity to observe the changes in hydrothermal deposition with time and distance from the EPR. DESCRIPTION OF CORESOf the six GEOMETEP cores, the four farthest from the EPR were sampled with a 4-m-long box corer, and the two cores closer to the EPR were sampled with short piston corers. The box cores consisted of a maximum of 280 cm of sediment, and the piston cores consisted of from 10 to 26 cm of sediment. We do not know whether the collected cores represent the complete profiles down to the basaltic basement, because the box corers may have been stopped by a tephra layer.Six sites drilled with a hydraulic piston corer during DSDP Leg 92 were investigated: Sites 600, 601, and 602Leinen, M., Rea, D. K., et al., Init. Repts. DSDP, 92: Washington (U.S. Govt. Printing Office).

Geochemical Studies on Selected Sediment Samples from the Lau Backarc Basin: Evidence for Hydrothermal Ponded Sediments

“…17) distinguishes four groups of samples having characteristic Fe/Mn ratios of 45.9 ± 2.6 (4 samples), 25.3 ± 2.8 (6 samples), 12.8 ± 2.0 (8 samples), and 3.55 ±1.4 (42 samples), including those with the highest OFE and Mn values. This latter average value for the Fe/Mn ratios is similar to that determined on the bulk sediments of the EPR (Dymond, 1981;Ruhlin and Owen, 1986;Marchig and Erzinger, 1986) and on the surficial sediments from 0 to 720 k.y. from the Lau Basin (Riech et al, 1990).…”

“…The majority of the samples have OFE values that range from 100 to 350 mg/kg and Mn values that range from 1 to 15 g/kg. These values are comparable to the major deposits of hydrothermal sediments from the East Pacific Rise (Heath and Dymond, 1977;Lyle et al, 1986;Marchig and Erzinger, 1986;Schrader et al, 1980). Samples from Sites 834 and 835, which have been previously identified in Figure 12, show higher OFE and Mn values that range from 350 to 850 mg/kg and from 15 to 60 g/kg, respectively.…”

Rare Earth, Major and Trace Elements in the Kunimiyama Ferromanganese Deposit in the Northern Chichibu Belt, Central Shikoku, Japan

“…Thus, the positive correlation between Al 2 O 3 and SiO 2 , as shown in Figure 4a, reveals that silica is also derived from detrital components such as postArchean average Australian shale (PAAS) and MORB. The CaO contents are not significant in the Kunimiyama ores (2.88 wt% on average), which is quite different from very high CaO contents of the ferromanganese sediments on the East Pacific Rise (>40 wt%; Marchig and Erzinger, 1986). This is probably because the Kunimiyama ores precipitated on seafloor deeper than carbonate compensation depth.…”

“…55, no. 4, 2005 Major, Trace and REE in Kunimiyama Fe-Mn (Marchig and Erzinger, 1986); MOR ferromanganese crust (Kuhn et al, 1998); back arc Ferich sediment (Binns et al, 1993); arc Fe-rich sediment (Savelli et al, 1999); hydrogenetic ferromanganese crust (Wen et al, 1997); PAAS (Taylor and McLennan, 1985); N-MORB (Boström and Bach, 1995).…”

Co-diagenesis of iron and phosphorus in hydrothermal sediments from the southern East Pacific Rise: Implications for the evaluation of paleoseawater phosphate concentrations

“…Lyle, 1986;Marchig and Erzinger, 1986). The sediments at all three sites are lithologically simple, composed primarily of hydrothermal precipitates derived from SEPR ridge crest vents, and carbonate (Lyle, 1986).…”