Mineralogy of gravity concentrates

Whereas detrital Ba is contained as a minor component in mineral lattices, especially potassic feld pars and mica, biogenic and hydrothermal Ba is present as pure barite. Since Ba concentrations do not exceed 2000 ppm on a bulk or 6000 ppm on a carbonate-free basis (cf.

Fig. 8.3), barite is below the detection limit of conventional X-ray diffraction. In order to con-firm its presence, a heavy mineral fraction has been eparated in elected sample with highest bulk Ba concentration . A concentrated solution of sodium tungstate (NaW0₄₎ with a density of 3.0 g/cm³ was u ed as heavy liquid. The heavy fraction was frozen with liquid nitrogen to improve the efficiency of the density eparation; light and heavy fractions were then separated and repeatedly washed with distilled water to remove NaW0₄^• Gravity concentrates, u ually some mg for a bulk ample of approximately 30 g, were meared onto a silicon disc for X-ray analy i . X-ray diffractogram ·are hown in Fig. 8.5 (following pages).

109

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The heavy mineral fraction is composed of pyrite, sphalerite (?), barite and clinopyro-xenes. The presence of phases with densities< 3.0 g/cm³ such as calcite, quartz, and feldspars indicates that gravity separation was not wholly efficient; it is conceivable that these latter phases have been present as parts of aggregates with overall densities> 3.0 g/cm³. Pyrite is the domi-nant phase in samples Ac.91 KS04-l23 and especially KF 16-470. It might be of hydrothermal origin, or it could have formed during decomposition of organic matter. The latter hypothesis is compatible with highest amounts of organic carbon in core KF 16. Sphalerite was tentatively identified based on a single well-defined reflection at 3.13

A.

This suggests the pre ence of a cubic mineral, since XRD patterns of cubic substances show few high inten ity reflection . While other sulfide phases, e.g. geerite (Cu~.60^{S), have very} similar X-ray parameter , phale-rite is by far the most common mineral with a principal reflection at 3.13

A.

^Thi should reflect a distal hydrothermal contribution derived from fine-grained sulfide particles which may be late-rally dispersed in plumes in the water column. The presence of barite is clearly indicated by a sequence of diagnostic reflections. In spite of con iderable enrichment compared to bulk samples, barite is a minor phase strongly "diluted" by abundant pyrite in gravity separate of amples Ac.91 KS04-123 and KF 16-470. Clinopyroxenes, which also have densities greater than 3.0 g/cm³, are present in variable but minor amounts; they are presumably of volcanicla tic origin.

8.2.5 Relationship between Sr, CaC0³ and Ba contents have been observed in barite particles eparated from South Atlantic pclagic ediments.

Depending on the abundance of discrete barite in each core, trontium correlates either with barite or with carbonate, or shows an intermediate signal in the e cdiments (Gingele 1992).

Figure 8.6 shows scatter plots of Sr vs. CaC0₃ and Ba in four sediment core in the Mid-Atlantic Ridge study area. Sr concentrations are nearly alway more elevated than those of Ba and up to ten times higher in carbonate-rich surface ediments. Thi and the clo e correlation between Sr and CaC0

3 indicate that Sr is dominantly as ociated with the carbonate phase.

Ac.91 KS04 (EPSHOM)

!

As higher Ba concentrations occur during glacial period with lower amounts of CaCO,, Sr i inversely correlated with Ba, which is simply a result of dilution by biogenic carbonate. Thi relationship is less clear in the case of core Ac.91 KS04, because carbonate in thi core i controlled by varying amounts of volcaniclastic detritu in addition to glacial-interglacial

nuctuations. eamount and hydrothermal site are characterized by numerou intervals of coarse, frequently

normally grain-sorted redeposited material containing abundant volcanic glass. It is very feasible that such turbidites could also contain hydrothermal precipitates.

Further arguments for a contribution from redeposited primary hydrothermal precipitates are based on the elevated content of other elements commonly associated with hydrothermal ulfides, namely Fe, Cu and Zn (Table 8.1). Data from surface sediments in the vicinity of the Lucky Strike seamount are compared with the composition of volcaniclastic sediments in the same area (sample KG 18-13; this study), proximal hydrothermal sediments at the Galapagos Rift (Marchig et al. 1987) and TAG (Mid-Atlantic Ridge at 26°N; German et al. 1993, Mills et al. 1993) areas, and average pelagic clay (Turekian & Wedepohl 1961).

Fe Mn AI Mg Cu Zn Ba p _{V As}

(%) (%) (%) (%) (pp m) (pp m) (pp m) (pp m) (pp m) (pp m)

KG18-S

7.90 0.29 7.85 4.19 650 224 1875 2290 275 66

KF 7-S

6.91 0.29 8.18 3.90 214 146 1030 2005 244 <30*

KS

10-S

6.55 0.28 8.17 4.56 187 106 620 1850 241 <30*

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5.77 0.11 9.13 4.83 95 73 144 990 225 15 Galapagos

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near vent 30.6 0.15 1.51 0.74 9400 3100 3820 3930 228 152

±5.5 ±0.16 ±0.47 ±0.46 ±4800 ±2200 ±1900 ±1270 ±114 ±76 5-JOOm 8.60 1.55 4.35 1.48 310 390 9260 1660 191 5.3 from vent ±1.8 ±0.28 ±0.52 ±0.25 ±90 ±270 ±188 ±350 ±50 ±3.6

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German et al. 32.2 0.23 1.65 0.83 3360 2130

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Mills et al. 36.0 0.23 ---

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average 6.50 0.67 8.40 2.10 250 165 2300 1500 120 13 pelagic clay

Table 8.1: Geochernical data from surface sediments on the flanks of the Lucky Strike seamount compared to sample KG 18-13 ("local volcaniclastic end member"), data from proximal hydrothermal sediments at the Galapagos Rift (Marchig et al.

1987) and TAG hydrothermal field (MAR, 26°N; German et al. 1993, Mills et al.

1993) and average pelagic clay (Turekian & Wedepohl 1961 ). All data recalculated on a carbonate-free basis.

*

Arsenic data based on detection limit of 12 ppm in bulk sediment samples Concentrations of Cu, Ba and, to a lesser extent, Fe and Zn increase with proximity to the Lucky Strike hydrothermal site in the sequence KSlO - KF7 - KG18. Iron is also derived from vo1caniclastic sediment sources; however, the Fe/Al-ratio is significantly higher than in underlying pure volcaniclastic sediments in box core KG 18 (sample KG 18-13), and is also teadily increasing in the same direction. Sample KG 18-13 is composed almost exclusively of volcanic glass, with a CaC0₃ content of only 0.40%, and can be considered a "local volcanicla-stic end member". As discussed in Chapter 6.4.1, its composition is representative of enriched

I 15

MORB, its Ba content, while higher than in average mid-ocean ridge basalt, cannot explain the elevated Ba content of surface sediments in the same area. A diagenetic origin of enrichments in Fe, Cu and Zn can be ruled out at least in core KG I 8. While yellowish-brownish colors show that surface sediments are oxidized, underlying volcaniclastic sediments should not provide a source of labile metals, and manganese is ab ent from porewaters (C. Rabouille, unpublished data), indicating that no early diagenetic remobilization of metals has occurred.

Metal concentrations are clearly lower than in highly metalliferous sediments from the Galapagos Rift and TAG areas, and concentrations of AI and Mg, elements typically of detrital or volcaniclastic origin, are two to five times higher and almost comparable to the volcaniclastic sample KG 18-13. This demonstrates that hydrothermal input in Lucky Strike surface sediments represents only a minor contribution, and that volcaniclastic input predominates on a carbonate -free basis. No Ba data have been reported for TAG sediments by German et al. (I 993) and Mills et al. ( 1993). Barite has not been observed in hydrothermal deposits of the TAG field (e.g. Rona et al. 1986, Thompson et al. 1988, Tivey et al. I 995), in contrast to hydrothermal sites at the East Pacific Rise and Juan de Fuca Ridge and to the Lucky Strike and Menez Gwen sites discovered more recently in the area of the present study.

Concentrations of Mn, which is typically associated with the hydrothermal oxyhydro-xide fraction, and P, V and As, which are scavenged from the water column in hydrothermal particle plumes, are fairly low in surface ediments at the Lucky Strike searnount. Low and highly variable manganese concentrations have also been observed at TAG and in near vent-sediments at the Galapagos Rift (data in Table 8. I). This is consistent with observations in hydrothermal particle plumes that most of the total manganese remains in solution during buo-yant plume rise and initial lateral dispersal (e.g. Trocine & Trefry I 988, Mottl & McConachy

1990). It has been suggested that bacterial Mn scavenging only occurs in more distal parts of the plume when concentrations of dissolved manganese have been trongly diluted by ambient sea-water (Cowen et al. 1986, 1990). This process results in fractionation of Fe vs. Mn in ridge-crest sediments. P, V, and As show some enrichment with proximity to the Lucky Strike hydro-thermal site, even though V is almost comparable to volcaniclastic sediments, i.e. about 80% of total V in sample KG 18-S could be derived from volcaniclastic sources. Since concentrations of P and As are significantly higher than in average pelagic clay (Turekian & Wedepohl I 966), hydrothermal plume faJiout appears to be present at the Lucky Strike seamount, but again as a minor component. The composition of ,average pelagic clay" according to Turekian & Wede-pohl (I 961) includes data from Pacific Ocean sediments which tend to have higher concentra-tions of a number of elements compared to the Atlantic Ocean. For example, the barium value reported by these authors represents the mean of data for Pacific pelagic clays (4000 ppm) enriched in biogenic barium (Goldberg & Arrhenius I 958) and Atlantic deep-sea clays (700 ppm Ba). Similarly, data on various metals are based entirely (Fe) or partially (Mn, Cu) on slowly accumulating pelagic clays from the Pacific Ocean which are hydrogenetically enriched in metals. Therefore, data from the study area cannot be directly compared with Fe, Mn, Cu, and Ba concentration of "average pelagic clay".

8.3.2 Geochemical and paleoceanographic context of Ba peaks in distal pelagic

Im Dokument THOMAS RICHTER SEDIMENTARY FLUXES AT THE MID-ATLANTIC RIDGE (Seite 116-124)