Studies in “Standard Samples” for Use In the General Analysis of Silicate Rocks and Minerals*

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).

Section: Methods Of Investigation and Resultsmentioning

confidence: 99%

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

Marchig¹,

Erzinger²

1986

“…The initial solutions were prepared by a technique close to that described by Walsh (1980). Determinations were con- (Abbey, 1980) with results of plasma spectroscopy performed at the Geological Institute of the USSR Academy of Sciences. (Abbey, 1980; see Table 1).…”

Section: Methodsmentioning

confidence: 99%

“…Determinations were con- (Abbey, 1980) with results of plasma spectroscopy performed at the Geological Institute of the USSR Academy of Sciences. (Abbey, 1980; see Table 1). The computer (PDP 11/03) contained a series of equations which described the relationship between concentrations of measured elements expressed as percentage of oxides.…”

Section: Methodsmentioning

confidence: 99%

Hydrothermal Deposits of the Galapagos Rift Zone, Leg 70: Mineralogy and Geochemistry of Major Components

Marentsov¹,

Sakharov²,

Drits³

et al. 1983

The hydrothermal deposits that we analyzed from Leg 70 are composed of ferruginous green clays and fragments of manganese-hydroxide crust.Data from X-ray diffraction, IR-spectroscopy, electron diffraction, and chemical analyses indicate that the hydrothermal green clays are composed of disordered mixed-layer phases of celadonite-nontronite.Electron diffraction shows that the parameters of the unit cells and the degree of three-dimensional ordering of mixed-layer phases with 80% celadonite interlayers are very close to Fe-micas of polymorphic modification IMceladonite. In some sections, there is a tendency for the number of celadonite layers to increase with depth.The manganese-hydroxide crust fragments are predominantly composed of todorokite (buserite). An essential feature of hydrothermal accumulation is the sharp separation of Fe and Mn. Ba/Ti and Ba/Sr ratios are typical indicators of hydrothermal deposits. Sediments composing the hydrothermal mounds were deposited from moderately heated waters, which had extracted the components from solid basalts in environments where there were considerable gradients of temperature, eH, and pH. The main masses of Fe and Mn were deposited in the late Pleistocene. Postsedimentary alteration of deposited hydrothermal sediments led to their slight recrystallization and, in the green clays, to celadonitization.Further, factor analysis (by Varentsov) of chemical components from these hydrothermal deposits revealed paragenetic assemblages. Green clays corresponding to a definite factor assemblage were formed during the main stage of hydrothermal mineral formation. Manganese hydroxide and associated components were largely accumulated during an early stage and at the end of the main stage.

“…For Nb concentrations of standards, according to the last review of trace elements in rocks by Abbey (1980), no recommended Nb concentrations exist between 0 and 15 ppm, and there are none for higher concentrations, either; all values are "proposed" concentrations. The good fit in Table 1 between "proposed" concentrations and concentrations found with either a = 2.65 or a = 2.81 demonstrates the coherence of the analytical method between 0 and 115 ppm.…”

Section: Shipboard Trace-element Analysis During Leg 82: a Specific Amentioning

confidence: 99%

Analysis of Trace Elements in Basalts by Shipboard X-Ray Fluorescence Spectrometry: A Discussion of Niobium

Étoubleau¹,

Bougault²,

Rideout³

et al. 1985

X-ray fluorescence spectrometry was used on board the Glomar Challenger during Leg 82 for analysis of "hygromagmaphile" trace elements that are useful to identify so-called "depleted" or "enriched" basalts; the results were used in choosing the new sites to be drilled on the basis of the "enriched" or "depleted" character of previous sites. This chapter describes a hitherto unpublished method for deducing concentrations from intensity measurements; the method yielded the precision (better than 1 ppm for Nb) necessary for purposes of the cruise.The procedure for the matrix-effect correction is described. The case of Nb is discussed in detail because Nb concentrations in basalts are the lowest among the elements investigated (Nb, Zr, Ti, Y, V) and because it is the key element for deducing the enriched or depleted character of the different basalt units. Ta neutron-activation measurements made on shore after the cruise demonstrate, on the basis of the well-known constant Nb/Ta ratio in basalts, that the accuracy of the shipboard measurements of Nb concentrations in basalt units is about 0.5 ppm.