Sahami, Ali Reza; (1991) Dating phosphorite deposits using strontium isotopes. Doctoral thesis (Ph.D.), University College London.

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Abstract

The primary mineral in sedimentary phosphorite deposits is francolite. Episodes of phosphogenesis have been dated in six offshore phosphorite deposits, by comparing the 87Sr/86Sr in francolites with 87Sr/86Sr evolutionary curves for sea water. 87Sr/86Sr of coexisting calcite and francolite in well dated replacement phosphorites have been measured and show that Sr in francolite is wholly derived from sea water during replacement of carbonate, and is not inherited from the carbonate precursor. This suggests that replacement francolites may be dated using Sr isotopes. Pore water 87Sr/86Sr have been measured in both a phosphogenic and a non-phosphogenic region, and are within error of the ratio in modem sea water (±3x10-5). Authigenic francolite precipitating within sediments is therefore incorporating Sr with a 87Sr/86Sr equal to that of sea water, not one modified by early diagenesis. However the 87Sr/86Sr in Recent authigenic francolites from the Namibian margin have been affected by early diagenesis of anoxic pore waters in which they formed. Sr isotope stratigraphy is therefore of no use in dating authigenic francolites which formed in pore waters in which early diagenesis of 87Sr/86Sr has occurred. Offshore phosphorite deposits on the South African margin (1.2 ±1.2 Ma and 5.9 ±0.9 Ma), Namibian margin (0.7 ±0.5 Ma and 4.7 ±0.4 Ma), Chatham Rise (5.0 ±0.7 Ma), Californian Borderlands (5.2 ±0.8 Ma), Blake Plateau (5.2 ±0.4 Ma, 22.5 ±2.0 Ma and 27.9 ±2.7 Ma) and the Moroccan margin (5.5 ±0.6 Ma, 16.0 ±2.4 Ma, 18.9 ±0.5 Ma and 66.0 ±3.0 Ma) have been dated. One global episode of phosphogenesis affecting all six deposits occurs at 5.6 ±1.3 Ma. Episodes of phosphogenesis have been correlated to global sea level high stands and warmer oceanic palaeotemperatures. Geochemistry of francolites show that these offshore phosphorites have not undergone significant post-formational alteration, and that variations in francolite geochemistry are primarily the function of the mechanism of phosphogenesis. The substituent composition of replacement francolites have not changed significantly over the past 30 Ma. This suggests that, either; 1) francolite composition is insensitive to the composition of the sea water from which it precipitated, or 2) that the composition of sea water, in relation to the elements Sr, F, S, Na and Mg, has not changed significantiy over the last 30 Ma.

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