Rt. Watkins et al., RARE-EARTH ELEMENTS IN PHOSPHORITE AND ASSOCIATED SEDIMENT FROM THE NAMIBIAN AND SOUTH-AFRICAN CONTINENTAL SHELVES, Marine geology, 129(1-2), 1995, pp. 111-128
Rare earth elements (REE) and major elements have been determined in t
wenty-nine phosphorite and associated sediment samples from the southw
estern coast of Africa, including thirteen samples of unconsolidated s
ediment collected during recent cruises to phosphate-rich areas on the
Namibian shelf. The concentrations of REE in various phosphorite type
s from southwestern Africa differ by more than two orders-of-magnitude
. The samples mostly exhibit REE patterns which are similar to that of
shale, having no Ce anomaly. The Ce content appears to have been cont
rolled by the palaeoredox conditions of the localised environment of d
eposition or diagenesis and not to have been influenced by marine upwe
lling, or by the general palaeoredox conditions of the water column. T
he environments in which REE were incorporated into the francolite app
ear to have been anoxic, or sub-oxic, in the case of both Namibian unc
onsolidated authigenic phosphorites, and the predominantly 'diagenetic
' phosphorites off the South African coast. The ultimate REE source in
both cases appears to have been (mainly eolic) continental detritus.
Pelletal phosphorites are significantly enriched in REE relative to no
n-pelletal varieties. If sequestration of REE dissolved in the water c
olumn contributed to the high concentrations of REE in pelletal phosph
orites, then the main carrier phase appears to have been inorganic det
ritus or biogenic silica equilibrated with oxygen-depleted bottom wate
rs but not fish debris. The contention that onland phosphorites are en
riched in REE relative to their offshore counterparts is, however, ref
uted in the case of southwestern Africa, and the effects of groundwate
r REE enrichment are not obvious in the majority of onland phosphorite
s analysed in this study.