Petit spot-like volcanoes exposed in Costa Rica

The study of volcanism in the ocean is fundamental to better understand the dynamics of the Earth mantle and plate tectonics. However, our understanding of this volcanism is limited by difficulties to access the roots of ocean volcanoes and the ocean floor. Recent results in Panama shown that exposed accreted volcanoes can provide another way of understanding volcanism in the oceans [1]. Petit spot volcanoes found on the subducting plate off Japan are considered to reflect volcanism in response to plate flexure [2]. However, petit spot volcanoes are very small and it remains unclear if they are a common feature on the ocean floor; documenting new occurrences of petit spot volcanoes is a key multidisciplinary issue. We recognize here ancient petit spot-like volcanoes accreted in Costa Rica based on new geochemical, geological, 40Ar-39Ar, and biochronologic data. Petit spot-like volcanoes acccreted in Costa Rica consists of tectonic stacks of volcano-sedimentary material that includes vesiculated pillow lavas, volcanic breccias and thick radiolarite beds. Igneous sills compositionnally similar to the lavas are common in the radiolarite beds. Major and trace element contents of the igneous rocks indicate an alkalic, moderately fractionated composition, and support very low degrees of partial melting in the garnet stability field. Normalized trace element patterns are very similar to those of petit spot volcanoes in Japan, and distinct from those of typical OIB, MORB and off-axis seamounts. Step-heating 40Ar-39Ar dating on co-magmatic amphiboles gave two ~175 Ma ages of formation for the petit spot-like volcanoes in Costa Rica. Tectonostratigraphic and biochronologic data clearly document a ~110 Ma age of accretion, and indicate that the volcanoes did not formed close to a subduction zone or a mid-ocean ridge. Therefore, we propose that petit spot-like volcanoes may represent a ubiquitous feature on the ocean floor, which can form far from mid-ocean ridges and subduction zones. Possibly, petit spotlike volcanoes exposed in Costa Rica reflect tectonicallyinduced leaking of melts pre-existing at the base of the lithosphere.