Lead isotope uniformity

The question of lead isotope uniformity in an ore body has also been investigated in several other laboratories. Many ore deposits, especially the so-called strata-bound or comformable ore deposits, have a lead isotope composition that varies very little throughout the whole deposit, often by less than 0.3%. Such ore deposits commonly have lead isotope compositions that lie close to a single-stage model evolution curve (28). Such a simple model cannot describe the composition in any real ore body. 

The narrow range of lead isotopic abundances in the Laurion area as shown in this study and the fact that major amounts of lead from this area were mined and used in ancient times indicate that these ratios should serve as a reasonable tracer. The data also justify the assumption previously made, but until now unproved, that the lead output of the Laurion mines was essentially of one uniform and recognizable type. 

The application of lead isotope analyses to provenancing almost pure copper artifacts seems relatively straightforward in most instances, but what of arsenical copper and tin bronze alloys In principle, the arsenic in arsenical copper may have been derived from high-arsenic minerals (such as tennantite or basic copper arsenates) containing lead of a different isotopic composition from that of the copper ore used to produce the copper. However, absolutely no archaeological or other evidence of proves that this was ever done in the Bronze Age Aegean. On the contrary, at Kythnos definite evidence shows that arsenical copper was produced in EBII times from arsenical copper ores of variable (sometimes zero) arsenic content but of uniform lead isotope composition. 

In light of these results, the lead isotope composition of the objects from LMI and LMIII West Crete (Figure 16) is surprising in its uniformity. It is much too early to say that no copper sources other than Lavrion were used in western Crete. Statistically, however, it may be significant that not even one of the 15 objects analyzed so far falls out of the Lavrion field. 

On the other hand, sediments, well-mixed because they are far from their source, tend to be isotopically rather uniform even if the detritus is Precambrian in age, but the lead is still somewhat radiogenic (Doe et al, 1966). The isotopic composition in the insoluble residues of these sediments, all roughly of the same sedimentation age, appears to be more a function of grain size but which may in reality be more a function of K-feldspar content. The coarse fractions are less radiogenic than the silt- or clay-sized fractions, and the lead isotope ratios still reflect the Precambrian ancestry of the detritus (Muffler and Doe, 1968). The finer fractions of detritus approach more closely the isotopic composition of the HCl-soluble fraction. 

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... 

Magmatogenic Ores. Ores in which the isotopic composition of ore lead is the same as that in the presumably related igneous source at the time of crystallization are good candidates for a magmatogenic origin. Data for this fingerprint are available in support of this mechanism. These ores are usually isotopically uniform with Pb/ " Pb< 19.5 and Pb/ Pb < 39. Examples (Fig. 22) are the ores at Butte, Montana (Doe et al., 1968), Bingham Canyon, Utah (Stacey et al., 1968), and the Nelson batholith, British Columbia (Reynolds, 1967). It must be... 

The four isotopes, as those of any element, have the same chemical properties. The four are not, however, uniformly distributed in the earth s crust the occurrence of three of them, in minerals and rocks, is associated with the radioactive decay of isotopes of thorium and uranium. In most minerals and rocks the relative amounts (or the isotopic ratios) of the isotopes of lead (often expressed relative to the amount of stable lead-204) are generally within well-known ranges, which are independent of the composition of the mineral or rock they are, however, directly related to the amounts of radioactive thorium and uranium isotope impurities in them. 

Schoenberg et al. (2008) presented the first set of Cr isotope data for rocks and Cr(ll) rich ores. Mantle derived rocks and chromite ores from layered intrusions have a uniform Cr/ Cr isotope ratio very close to the certified Cr standard NIST SRM 979. The Cr isotope composition of hydrothermal lead chromates is substantially heavier (S Cr from 0.6 to 1.0%o) than the rocks from which the chromium was leached. 

Within ocean sediments, the decay of uranium and thorium isotopes leads to the creation of Rn, which is released to sedimentary pore waters and subsequently diffuses into the over-lying seawater. Near the seafloor, excess Rn can be seen against the background of a natural standing stock of this isotope in the water column, which is produced by in situ decay of Ra, a long-lived and relatively uniformly distributed isotope. Because of its short half-life, the existence of this excess isotope some several hundred meters above the seafloor implies a significant flux into the bottom waters, and the shape of the profiles has been modeled as a vertical diffusive balance with radioactive decay of radon and in situ... 

There are various sources of radiocesium in the environment. The input from atmospheric weapons testing in the mid-twentieth century leads to the most widespread and homogeneous contamination of soil and water. The accumulated contribution of Cs in Europe from this source is estimated to be about 1000 Bq m 2. The fairly uniform deposition of Cs from fallout followuig the nuclear weapons testing has led to the widespread use of this isotope as a tracer of erosion (Ritchie and McHenry, 1990 Agudo, 1998). The accident that occurred at the Chernobyl nuclear plant in April 1986 led to more localized contamination. The events that led to this accident and the consequences have been widely... 

L-Arg (guanido- N2, N, >98%) hydrochloride, L-Trp (indole-Ds, D, 98%), and L-Ile (Uniform- Ce, C, 98%) were purchased from Cambridge Isotope Laboratories, Inc. The enrichment of these compounds (>95%) was reconfirmed by MALDI-TOFMS analysis. All other chemicals were of highest grade commercially available. Complete release of luciferin (1) from Vargula by electric stimulation The individuals were put into 10 mL of artificial seawater in a Petri dish, which was equipped with two carbon electrodes (spare lead). In order to release the luciferin completely, AC 30 V was charged to the electrodes for one second repeatedly (20-30 times), until no bioluminescence was observed after the pulse stimulation. Preparation of the formula bait... 

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