Figure F46. Weight percent CO₂ vs. weight percent CaO in metaperidotite samples from ODP Legs 153 and 209. Leg 153 data from Cannat, Karson, Miller, et al. (1995) and Casey (1997). The upper diagram shows that most samples with high CaO have high CO₂ and lie along a mixing line toward calcite and aragonite CaCO₂. A few samples with high CaO and low CO₂ are impregnated peridotites, which include igneous plagioclase and clinopyroxene precipitated from melts migrating along grain boundaries. The lower diagram shows more detail for samples with low CO₂ and CaO contents. Although the data are scattered, much of this may be due to analytical uncertainty. Most metaperidotites from Legs 153 and 209 contain appreciable CO₂, mainly in aragonite and calcite. Thus, carbonate alteration accounts for a significant fraction of the whole-rock CaO budget in these rocks. This suggests, but does not prove, that CaO may have been metasomatically added to these rocks along with CO₂ during hydrothermal alteration.