Inferred High Primary Volatile Contents in Lavas Erupted in an Ancient Back-Arc Basin, California

Abstract

Several recent studies have demonstrated that high volatile contents are characteristic of many lavas erupted in modern back-arc basins. The Josephine ophiolite is believed to have formed in a back-arc basin in the Late Jurassic, and high primary volatile contents are inferred for pillow lavas of the ophiolite based on the presence of pillows with very vesicular margins. Although the vesicular pillows could be the result of eruption in shallow water, geologic evidence suggests eruption in very deep water. This evidence includes the following: (1) a near absence of vesicles in some pillow lavas; (2) depths of modern back-arc basins are generally greater than 2200 m; (3) siliceous pelagic rocks overlie the ophiolite, suggesting deposition below the CCD; and (4) trace fossils in the flysch sequence overlying the pelagic rocks suggest deposition in abyssal or perhaps lower bathyal depths. The occurrence of isolated- and broken-pillow breccias in the ophiolite suggests that they may form in deep water, and their formation in deep water may depend on a high volatile content in the lava.