The discovery of a fresh spinifex-textured basalt xenolith in glassy dacite, dredged from the seafloor at Satanic Mills, confirms the interpretations assigned by Shipboard Scientific Party (2002), based on textures, to the altered equivalents encountered in Holes 1189A and 1189B at Roman Ruins. The additional altered xenolith now found in Hole 1188A, described here, extends the lateral range of such occurrences to the Snowcap site, and we can now say with confidence that spinifex-textured xenoliths occur throughout the vertical sequence of lavas by which Pual Ridge is constructed.
No outcropping spinifex-textured lavas have been recorded despite intense dredging and bottom photography along the crest of Pual Ridge or anywhere else in the eastern Manus Basin. Hence, it is more likely that the xenoliths derive from deep within the igneous system. Were they accidentally broken from the quenched margin of a subjacent mafic intrusion, one might expect fragments from more slowly cooled interiors to occur as well.
The possibility is noted above that the fresh spinifex-textured xenolith was relatively fluid, perhaps incompletely crystallized, when incorporated within the dacitic melt. A long-standing puzzle regarding the felsic lavas at Pual Ridge is their apparently high eruption temperatures, at or above their liquidus, as indicated by an essentially aphyric nature and their fluidity as evidenced by lava structures more commonly associated with mafic melts (Waters et al., 1996; Waters and Binns, 1998). The latter include lobate and pillowlike structures and the presence of drained collapse pits. Reheating within a fractionating magma chamber by the introduction of fresh batches of parental basalt magma is the possible explanation. The major and trace element geochemistry of the fresh spinifex-textured xenolith conforms to the requirements of a potential parent magma for the Pual Ridge fractionation sequence (basaltic andesite through andesite and dacite to rhyodacite). Conceivably, the xenolith and its drilled equivalents represent lobes detached from a fresh magma batch and incorporated into a fractionated melt in the chamber, becoming itself quenched while causing overheating of the latter.
Although it contains no hydrous minerals, vesicularity of the fresh spinifex-textured xenolith indicates that the melt from which it crystallized rapidly was distinctly hydrous. An interesting possibility deserving further study, suggested by plagioclase rims on vesicles and miarolitic cavities, is that initial igneous crystallization of the mafic silicate melt caused increase in the proportion of water to such an extent that final crystallization was under supercritical hydrothermal conditions, in a manner akin to the formation of pegmatites in granite bodies.