Lead Cenozoic

We applied this technique to make an initial estimate of the timing and extent of Cenozoic uplift of the Colorado Plateau (Sahagian et al. 2002a). Because the technique measures paleoatmospheric pressure, it is not subject to uncertainties stemming from the use of proxies that depend on environmental factors other than elevation alone. Vesicular lavas preserve a record of paleopressure at the time and place of lava emplacement because the difference in internal pressure in bubbles at the base and top of a lava flow depends on atmospheric pressure and lava flow thickness. The modal size of the vesicle (bubble) population is larger at the top than at the bottom. This leads directly to paleoatmospheric pressure and thus elevation because the thickness of the flow can easily be measured in the field, and the vesicle sizes can be measured in the lab. All proxies have their limitations and hence are not applicable in all places and at all times. Vesicular basalts are no exception. For a lava flow to record atmospheric pressure, the flow thickness must remain constant between the time the upper and lower crusts (10-20 cm) form, and the time of complete solidification of the flow. 

Further support for the weathering hypothesis for Late Ordovician glaciation comes from the strontium isotope record. A multimillion-year decline in the marine record of Sr/ Sr in limestones is reversed in the early Late Ordovician (Veizer et al., 1999), a likely consequence of collisional tectonics that initiated at this time and continued (as did the rise in Sr/ Sr) through the Silurian (Richter et al, 1992). The increase (from 0.7078 to 0.7088) rivals the Cenozoic rise, which has been attributed to the Himalayan uplift and associated with the progressive cooling leading to Quaternary glaciation (Raymo et al, 1988). 

The polymetallic sulphide deposit at Rudny Altay comprises a number of nearly vertical ore bodies at depths of 450-500 m in a tuff-slate formation. The tuff-slate is covered by dense Mesozoic-Cenozoic clays 40-50 m thick. The lithogeochemical survey does not yield clear anomalies. But the CHIM results for lead delineate satisfactorily the position of the ore bodies and their approximate projection to the surface (Fig. 2-22). 

The late Cenozoic was therefore a time of unusually high organic carbon deposition rates, leading to an increase in the size of the sedimentary organic carbon reservoir. The organic subcycle thus acted as a carbon sink over the course of the Himalayan uplift. There are two possible causes of this evolution. 

Hart and Kyle (1993) also measured isotopic compositions of lead (i.e., the Pb/ Pb ratios) in the Cenozoic lavas of the HaUett volcanic province, the BaUeny Islands, as weU as of Scott and Peter I islands. The 206pb/204pb an(j 8 Sr/ Sr ratios of these rocks outline several overlapping areas in Fig. 16.42 that can be extrapolated to the HIMU component of the magma sources in the mantle (Zindler and Hart 1986 Hart 1988). 

The HIMU component typically has a high 206pb/204pb j. bo between 20.5 and 21.5 and a low Sr/ Sr ratio between 0.7025 and 0.7030 which places it in Fig. 16.42 on an extension of the hyperbolic Sr-Pb isotopic mixing line loosely defined by the Cenozoic volcanic rocks of the Hallett volcanic province. The volcanic rocks on the Balleny Islands and on Scott and Peter I islands have isotopic compositions of strontium and lead that overlap those of the Hallett volcanic province. However, the Sr/ Sr ratios of the Balleny/ Scott islands are low (0.7025-0.7030), whereas those of Peter I Island (0.7038-0.7040) are normal for oceanic island basalts. 

Cenozoic Sediments, Whole-Racks and HCl-Soluble Lead... 

Cenozoic age from various localities. (References to data are given in Section III, part 6 — Cenozoic sediments whole-rock and HCl soluble lead) ... 

Table 20. Selected lead isotope data on basalts and silicic igneous rocks of Cenozoic age in the Rocky Mountain region. United States ...

Lead isotope data are still valuable even when the lead is only partially extracted and its lead isotopic composition is not equal to that of the whole-rock. A Pb-Pb isochron age may still be calculated and a legitimate comparison may be made of lead from possibly isotopically heterogeneous inclusions with that from isotopically uniform host rocks of Mesozoic or Cenozoic age. Cooper and Green (1969) have made a well executed study of this kind on Iherzolite inclusions and host alkali basalt from western Victoria, Australia. Partly volatilized lead appears to always be more radiogenic than the whole-rock value on old rocks. As the volatilized leads analyzed by Cooper and Green from Iherzolite are less radiogenic than those of the isotopically uniform host basalt, the data require separate immediate source regions for the leads in the... 

Appendix D. Isotopic composition of lead in Cenozoic and Cretaceous igneous rocks... 

Lead and strontium isotope studies of Cenozoic volcanic rocks in the Rocky Mountain region — a summary. Colorado School Mines Quart. 63, 149—174 (1968a). 

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