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16-Jun-2020 14:23

Modern mid-oceanic basalts plot to the radiogenic side of the geochron, consistent with a progressive increase in U/Pb in their depleted mantle source through time (see Sm-Nd).OIB (or plume) basalts have very radiogenic Pb, consistent with their derivation from recycled oceanic crust.In conjunction with the Geosciences Advisory Unit at the National Oceanography Centre in Southampton we are able to offer this relatively new technique for precisely dating very recent sediments.This is particularly applicable in dating sediment accumulation rates for geohazard analysis in subsea engineering projects.

There is a Pb paradox, in that a complimentary low U/Pb, U-donor reservoir, from which the U has been contributed to the ocean basalts, has not yet been identified (lower crust? A sufficiently large range in the U/Pb ratio is not generated by basaltic magmatic processes to enable precise dating by the whole-rock Pb-Pb method.

The modern-day basalt field overlaps with Geochron but extends to right, indicating progressive gain of U relative to Pb.

Some minerals, such as galena (Pb S), concentrate Pb in their mineral structure and have very low U/Pb.

Furthermore, only one Pb isotope (204Pb) is unaffected by radiogenic decay, so it is not possible to precisely correct Pb isotope ratios for instrumentally induced mass fractionation.

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These factors limit the precision and usefulness of whole-rock Pb-Pb isochrons.

Common-Pb method The Earth is assumed to have an initial Pb composition identical with that of chondritic meteorites.

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