"DETRITAL" ZIRCONS ORIGINATING BY MAGMATIC EROSION OF PRE-EXISTING INTRUSIONS AT THE SOUFRIÈRE VOLCANIC CENTER (SAINT LUCIA, LESSER ANTILLES)

Zircons from dacite lava of the late Pleistocene Terre Blanche dome (Soufrière Volcanic Center, Saint Lucia, Lesser Antilles) yield U-Th ages between ~30 ka and ~250 ka. These ages are determined by ion microprobe U-Th analysis of the outermost ~5 µm of unadulterated crystal faces on 33 grains, at a spatial resolution about 6 times higher than is achievable in lateral dimensions by conventional ion microprobe analysis of polished grains. In about 25% of the zircon rim analyses, ages overlap within uncertainty with the (U-Th)/He zircon eruption age of ~30 ka. The remaining zircons lack evidence of near pre-eruptive zircon growth, even in the outermost ~5 µm. For zircon antecrysts, the rim age distribution is polymodal and displays prominent peaks at ~40 and ~80 ka. Scattered older ages up to ~250 ka are present. The oldest ages overlap with published K-Ar ages for mid Pleistocene domes at Soufrière (complied in Lindsay et al., 2001). Xenocrystic zircons derived from late Miocene volcanic country rocks are absent.

Ti-in-zircon temperatures adjusted for sub-unity TiO₂ activity (aTiO₂ = 0.34; calculated from Fe-Ti oxide compositions and temperatures for Terre Blanche dacite) are as low as ~710ºC, approximately 100ºC lower than temperatures calculated from Fe-Ti oxide thermometry.

Based on these results, we favor zircon recycling from preexisting intrusions where zircon crystallized at temperatures lower than the eruptive temperature of the Terre Blanche dacite magma. The lack of young zircon growth in the majority of zircon crystals analyzed argues against a long-lived crystal mush, and suggests that the zircons were incorporated into the dacite magma by magmatic erosion of preexisting intrusions just prior to the eruption, potentially following thermal rejuvenation by magma recharge. Such a scenario is supported by the presence of granodiorite xenoliths in Soufrière domes. The "magmato-derital" zircon record offers unique insights into the crystallization history within the plutonic roots of the Soufrière Volcanic Center and may reveal intrusive recurrence in arc volcanic systems.

Lindsay, J. M., David, J., Shepherd J., and Ephraim J., (2002) Scientific Supplement to the Volcanic Hazard Assessment for Saint Lucia, Lesser Antilles. Seismic Research Unit, The University of the West Indies, St. Augustine, Trinidad and Tobago: 1-63.