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The prevalence of calcium as a major cation in surface and oceanic environments, the necessity of calcium in the functioning of living cells and bone growth, and the large spread in mass between calcium isotopes all suggest that calcium isotope biogeochemistry can be an important avenue of insight into past and present biogeochemical cycling processes. In the following chapter, we review the main areas of research where Ca isotope studies have been pursued and detail recent research results in biogeochemical applications. In marine environments, biogenic fractionation of Ca isotopes during biomineralization produces predictable offsets in some organisms, which facilitate the reconstruction of seawater δ44/40Ca over geologic timescales. In terrestrial studies, observed Ca isotope fractionation between soil and various components of vegetation enables the construction of a local Ca budget and provides a partial explanation for the scale of Ca isotopic variability within a single watershed. The research reviewed in this chapter provides a foundation for future investigations into the macro- and microscopic processes and biochemical pathways dictating the distribution of this essential nutrient using stable Ca isotope ratios.

Ocean island volcanoes and large igneous provinces, both of which represent mantle plume volcanism, are attractive drilling targets because of compelling scientific issues, the layered structure of lava accumulations, and the demonstrated capability to retrieve a high percentage of core in many environments. Although the programs supporting on-land scientific drilling — ICDP, and in the U.S., NSF-Continental Dynamics — are nominally about continental drilling, the primary dichotomy is between on-land drilling and ship-based offshore drilling (IODP). Mantle plume volcanic rocks are accessible to on-land drilling in many oceanic settings as well as on continents. Drilling and coring ordered, datable sequences of lavas can address questions about the mechanisms of magma generation and transport, the geochemical and geophysical structure of mantle plumes and their relationship to the structure and composition of the deep mantle, the structural evolution of large volcanoes, the subsidence history of volcanic piles, the flexural properties of the lithosphere, and the hydrology, alteration history and geobiology of subsurface volcanic environments.

The interpretation of the isotopic data on continental basaltic rocks is complicated by factors that are similar to those associated with the continental magmatic arcs. In contrast to the continental margin arcs, however, intermediate composition lavas are relatively rare in comparison to basalt. Because it is more difficult in general to reconcile crustal contamination with the chemical composition of basalt, the basalt provinces present some unique problems of interpretation. These provinces are separated into three types here for the purpose of discussion. Flood basalt provinces are large volumes of lava generally having compositions that are tholeiitic or transitional to alkali basalt (Turner and Verhoogen 1960). A second type of province is the continental rift, where the lavas are usually more alkalic and of smaller volume. The third type, which is not entirely distinct from the second, comprises mafic lavas of alkalic and often extremely silica-undersaturated compositions, which are erupted in small volumes over broad areas, generally in the form of cinder cones with small numbers of associated lava flows.

The Nd isotopic data provide an important addition to petrogenetic modeling studies of igneous rocks because they provide previously unavailable information about the chemical composition of magma sources. Oceanic basalts present the possibility of applying the isotopic data for this purpose in a relatively straightforward way because there is little likelihood that the magmas are modified by contamination with old continental crust between the time they are generated in the mantle and when they are erupted at the surface. The Nd isotopic ratio (eNd value) measured on a lava can therefore be safely assumed to represent the ratio in the mantle where the magma originated. Using an appropriate model the eNd value can be used to infer some of the trace element characteristics of the mantle magma source, in particular the Sm/Nd ratio. This can, in turn, be compared to the Sm/Nd ratios of the lavas to evaluate petrogenetic models (Chap. 5). This approach requires assumptions, but is valuable because it allows the internal consistency of the models to be evaluated. The trace element partitioning models provide the basis for evaluating the relationship between the Sm/Nd ratio of a basalt magma and its source in the mantle. A sampling of data for oceanic basalts is shown in Fig. 8.1.

The Sm-Nd system is useful for determining crystallization ages of igneous and metamorphic rocks. It has proven especially valuable for dating mafic and ultramafic rocks that are, in general, difficult to date using other methods. The principles of Sm-Nd dating are identical to those of Rb-Sr dating, which have been discussed in detail elsewhere (e.g. Lanphere et al. 1963; Faure 1977). The underlying assumption is that at some time Tx in the past, all the minerals in a rock have identical values of the ratio 143Nd/144Nd, but have different ratios of 147Sm/144Nd. This starting point is shown in Fig. 2.1 by the open circles representing minerals A, B, and C. The open circle labelled “TR” represents the whole rock, which is just the sum of the minerals, and thus must also have the same 143Nd/144Nd. As time progresses 147Sm atoms decay to 143Nd atoms, so 147Sm/144Nd decreases and 143Nd/144Nd increases. The positions of the minerals and rock on this diagram move along the arrows shown, which have a slope of -1.

In this chapter the Nd isotopic data on continental igneous rocks are examined with emphasis on the implications for petrogenesis. The interpretations are based mainly on two concepts. One is a general model of isotopic variations of Nd and Sr in the crust and mantle, and the other is the systematics of mixing between the products of the different magma sources in the crust and mantle.

The chondritic meteorites probably provide the best samples of the building blocks from which the terrestrial planets were constructed (e.g. Ringwood 1975). They are primitive objects composed of condensates from the solar nebula that have suffered little further modification. As discussed in Chaps. 1 and 3, the inference that Sm and Nd condensed from the solar nebula at a high temperature implies that their relative proportions in chondritic meteorites, and indeed in all of the planets, should be the same as in the solar nebula (i.e., the sun). Because the chondrites have not been affected by magmatic processes, they provide an estimate of the bulk-earth values of Sm/Nd and 143Nd/144Nd.

With the development of Sm-Nd studies, and shortly thereafter Lu-Hf, it has become increasingly apparent that the relationships between the isotopic variations of different elements are extremely important. This has become even more obvious with the added information provided by the rare gas isotopes 3He, 40Ar, and 129Xe (e.g. Clarke et al. 1969; Craig et al. 1978a, b; Kurz et al. 1982a, b; Lupton 1983; Staudacher and Allegre 1982; Hart et al. 1979). The correspondence between Nd isotopic compositions, and the isotopes of other elements are considered in this chapter.

The principles underlying the application of the Sm-Nd radioactive system as a petrogenetic tracer are similar to those of Rb-Sr and U-Th-Pb (Faure 1977). To illustrate, an example is presented of the relationships between isotopic effects, petrogenetic processes, and time.

The behavior of Sm, Nd, and the other rare-earth elements in magmatic processes can be conveniently illustrated by theoretical models of partial melting, fractional crystallization, and mixing. With regard to partial melting, the models that will be described here carry the assumption that equilibrium is always maintained between the magma and the residual solid material in the melting rock.

The Sm-Nd isotopic data have been used as a basis for testing models of the chemical evolution of the earth which lend themselves to mathematical description. The Sm-Nd system offers a particularly powerful constraint because of the ability to predict the Sm/Nd ratio of the total earth. This characteristic also applies to the Lu-Hf system. The models that have been considered are relatively crude, considering the large number of complex processes that they must attempt to represent. By comparison, for instance, to stellar evolution models, they are simple back-of-an-envelope calculations as opposed to hour-long computer calculations. The geochemical earth models have treated only mass transport, and only in a quasi-equilibrium manner. Various parts of the earth’s interior are considered to be “boxes”, each generally homogeneous within, and only the transfer of material between these boxes has been considered. There has been little attempt to concurrently model the other aspects of the physical evolution of the reservoirs, such as heat transfer and the fluid dynamic problems associated with the retention of “homogeneity” in the boxes. Instead, there have been attempts to describe the mass transport in sufficiently general terms to allow some relatively robust first-order conclusions to be drawn.