Th: isotopic composition

Figure 11 High-precision Th isotopic compositions (activity ratios) for the silicate rock standard Table Mountain Latite, as measured by thermal ionization mass spectrometry (TIMS) and multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). (From Halliday et al., 1998b.)...

Fig. 1 ( " Th/ " Th)versus " Thplot for whole-rock and mineral separate zero-age samples from Vesuvius. From Capaldi and co-workers. 1944 refers to mineral separates from the lava erupted in 1944 other numbers are laboratory numbers of cumulate ejecta. Most cumulates have large ("" Ra/"" U) ratios so their age is zero in comparison with half-life of " Th. Hyperbolic correlation strongly suggests that Th in Vesuvian lavas is derived by mixing of two components with different Th isotopic compositions (i.e. derived from two sources with different Th/ " U ratios). Because data include whole-rock lavas and different minerals drawn hyperbola is not believed to be mixing curve of magmas...

As was discussed previously, Th in lavas from Vesuvius, Stromboli, Etna and Mt. St. Flelens contains a component (recorded in mineral phases) with a higher Th isotopic composition (lower A") than that of the host whole rocks. Because most Th in the w hole rocks probably was fed to the magma from an interstitial, metasomatic mantle component, the lower K com-... 

Element has no stable nuclides the value given in parentheses is the atomic mass number of the isotope of longest known half-life. However, three such elements (Th, Pa and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated. 

In these situations, addition of a tracer of unique isotopic composition is required, and the nature of the tracers added depends on the measurement technique. For example, short-lived and Th (with respective half-lives of 70 and 1.9 years) are commonly used as a tracer for alpha spectrometric analysis of U and Th, whereas longer-lived... 

Table 1 lists MORE studies to date and Figures 2 and 3 present data from different areas of ridge. The first study of U-series disequilibria in MORE was the pioneering work of Condomines et al. (1981) (Fig. 2A). These workers analyzed samples having a relatively wide range in composition (Mg 72 to 57) from the FAMOUS region of the Mid-Atlantic Ridge (MAR 37°N) by combined alpha spectrometry (for U and Th isotopic ratios) and mass spectrometry (isotope dilution measurements for U and Th... 

O Hara MJ (1968) The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth Sci Rev 4 69-133 O Nions RK, McKenzie D (1993) Estimates of mantle thorium/uranium ratios from Th, U and Pb isotope abundances in basaltic melts. Phil Trans Royal Soc 342 65-77 Oversby V, Gast PW (1968) Lead isotope compositions and uranium decay series disequilibrium in reeent volcanic rocks. Earth Planet Sci Lett 5 199-206... 

Figure 8. ° Pb7 Pb vs. Th/Us (derived using Eqn. 5 in the text) diagram for mid-ocean ridge and ocean island basalt based on a recent data set with mostly mass spectrometry measurements (Turner et al. 1997 Bourdon et al. 1996 Dosso et al. 1999 Claude-lvanaj et al. 1998, 2001 Sims et al. 2002). The data show a relatively well defined array that intersect a closed-system hne for the bulk Earth starting with an initial lead isotope composition equal to Canyon Diablo (T = 4.55 Ga). This intersect was used by Allegre et al. (1986) to define the Th/U ratio of the Earth.

An important aspect in the preceding discussion is the need to separate the fluid and sediment components spatially and (as we will see) also temporally. Quantitative mass balance estimates (e g., McCulloch and Gamble 1991 Stolper and Newman 1994 Ayers 1998) often conclude that there is as much, or even more, Th and U in the bulk slab componenf (i.e., sediment plus fluid from the altered oceanic crust). However, if the sediment component added is in U-Th isotope equilibrium (or returns to this state prior to fluid addition see Section 5.3), then addition of only 0.02 ppm U in the fluid will result in significant U-excesss in the composite source (e.g., Condomines and Sigmarsson 1993 Turner et al. 1997). 

Ku T-L, Bull WE, Frieman ST, Knauss KG (1979) °Th/ " U dating of pedogenic carbonates in gravelly desert soils of Vidal Valley, Southeastern California. Geol Soc Am Bull 90 1063-1073 Ku TH, Krauss KG, Mathieu GG (1977) Uranium in open ocean concentration and isotopic composition. Deep Sea Res 24 1005-1017... 

It is important to distinguish clearly in this scenario between the general solidification of the Earth s crust, which had the effect of freezing in variations in the U/Pb and Th/Pb ratios, and the specific mineralization event which created the galena (lead sulfide, PbS) deposits, which removed the lead from the uranium and thorium, and effectively therefore froze the isotopic composition of the lead in the galena at the values representative of the time of mineralization. 

Figure 11,25 Th-Pb and U-Pb isotopic compositions of whole rocks from Granite Mountains Formation (Wyoming) based on data of Rosholt et al. (1973). 2.79 Ga isochrons based on Pb-Pb age are superimposed, to highlight effect of differential elemental remobilization.

---