Igneous Rocks, Meteorites, and Fluid-Rock Interactions

Oxygen is the most important element in common, rock-forming minerals. Earth’s crust and mantle contain about 44 wt. % oxygen, and even with its dense iron core, the bulk Earth is estimated to be approximately 30% oxygen. Considering the low mass and large size of the oxygen atom, the oxygen content is even higher if expressed in terms of vol. % or mol. %. For the above reason, a central problem of stable isotope geochemistry is to explain the distribution of oxygen isotope ratios in rocks. As shown in this chapter, much of the diversity in the abundance ratios is related to interactions of rocks with Earth’s extensive hydrosphere, which is nearly 89 wt. % oxygen. Even though hydrogen is only a minor element in rocks, some discussion of hydrogen isotopes is included here because they provide powerful complementary relationships to evaluate fluid-rock interactions. It has been suspected for centuries, and has now been proven by oxygen isotope data (see later), that Earth and the Moon have very similar origins. In particular, the δ18O values of large rock reservoirs on the Moon and Earth are practically identical. Diverse lunar lithologies have remarkably uniform values ranging only from +5.4 to +6.8 relative to SMOW, with the subset of lunar igneous rocks showing even less variation at +5.7 ± 0.2 (Epstein and Taylor, 1971; Taylor and Epstein, 1973). The same limited range of values is found for the largest lithologic reservoirs on Earth. For example, mid-ocean ridge (MOR) basalts are the most abundant igneous rock type on Earth, and cover practically the entire ocean floor. The δ18O values of these basalts are practically uniform at +5.7 ± 0.5 (Kyser, 1986). Similarly, other mafic lavas, as well as peridotites, pyroxenites, and practically all other mantle materials with the exception of the ophiolites and eclogites, have δ18O values in the restricted range of +5.0 to +8.0. Moreover, no apparent secular trend over geologic time has been found in the bulk δ18O values of these reservoirs. For these reasons, it is likely that the bulk δ18O values of Earth and the Moon are identical and very close to +5.7 ± 0.2.