Inclusions of highly compressed, generally liquefied, nearly pure CO2, usually with glass (presumably basaltic), were found in the minerals of every occurrence of olivine-bearing nodules in basalts studied, from 72 localities throughout the world. They were found in olivine, clinopyroxene, orthopyroxene, and plagioclase, from dunitic, peridotitic, pyrox-enitic and gabbroic nodules. Large numbers of these inclusions are secondary in origin, trapped during the healing of fractures, but many are found that appear to be primary; both types require the existence of immiscible globules of a dense, supercritical CO2 fluid in the basaltic liquid, at high pressures. Under ordinary microscope examination, they appear as opaque dust grains, usually <5 μ in size. A cool microscope stage, high magnification, and strong, convergent light, free of infrared, are needed to recognize them. Their composition was established by five methods. Similar CO2 inclusions were also found in phenocrysts of olivine, titanaugite and oxyhornblende in basalts from 19 occurrences, in some eclogite xenoliths and in kimberjite from a diamond pipe. Thirty-one other ultra-mafic igneous rocks showed evidence of gas inclusions under pressure, but no visible liquid CO2.

The primary inclusions permit the estimation of the pressure at the time of formation of the enclosing minerals. The secondary inclusions record the ambient pressures at the time of breaking and rehealing of the host mineral. Many inclusions of both types in the nodules show a density of filling corresponding to the pressure of 10-15 km of basalt liquid, but inclusions formed at greater depths may have decrepitated on eruption. The fact that many inclusions did not decrepitate upon eruption (or subsequent laboratory heating in air to 1200° C) indicates that olivine has a surprisingly high tensile strength at high temperature. The saturation of basalt magmas with respect to CO2 on a world-wide basis, as evidenced by these inclusions, has considerable significance to a variety of petrologic problems such as vésiculation, the composition of volcanic gases, and the origin of phenocrysts, carbonatites, and perhaps diamonds.

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