The Gabbro/Dike transition in oceanic crust generated at slow- and ultraslow-spreading ocean ridge systems
Final Report Abstract
In contrast to fast-spreading ridges, crust formed at slow- and ultraslow-spreading ridges is highly variable, ranging from emplacement of the mantle directly to the seafloor over broad regions, to local highly magmatic spreading that forms layered crust broadly similar to that from fast-spreading ridges. Due to the lack of systematic investigations, the nature of the gabbro/dike transition of slow- and ultraslow-spreading ridge systems is only poorly constrained. In this study we performed a systematic review of the extensive collections of dredges and dive suites of the Woods Hole Oceanographic Institution (WHOI) containing rocks from the sheeted dike rooting zone from slow- and ultraslow-spreading ridges. For this study we describe gabbro/dike transition suites from four localities on slow and ultraslow spreading ridges. Two localities are on the ultraslow-spreading SWIR, while one each is on the ultraslow Gakkel Ridge and the slow-spreading Mid-Atlantic Ridge. Each represents crustal accretion at different rates of magma supply and tectonic setting. Physical characteristics and thermal histories vary from one location to the next, but there are common characteristics that sharply distinguish the dike-gabbro transition at slower spreading ridges from that at faster spreading ridges. From this sample pool, 564 samples were characterized in terms of igneous and metamorphic assemblage, intrusion relationships, deformation textures, veining, and contact relationships. In all investigated sample we observed relatively simple cutting relations of the type that diabase dikes cut gabbro. One typical observation from fast-spreading ridges that gabbro intrudes into metamorphically transformed sheeted dikes, was not discovered. We believe that the observed simple cutting relations can be regarded as a general rule for slowand ultraslow-spreading ridges, and that this criterion can also be used for the discrimination of ophiolites to either fast- or slow-spreading type. We used amphibole geothermometry for an evaluation of the equilibrium temperatures within the gabbro/dike transition of the individual sites. Since our evaluation is based on a large number of samples (164 thin sections) and on a statistically significant number of microprobe analyses (146 amphibole averages in total), we believe that this outcome is robust and well-suited to characterize the thermal regime of the sheeted dike rooting zone of slow- and ultraslow spreading ridges. Based on amphibole formation temperatures, we observed in three working areas a continuation in metamorphic temperatures from granulite, via amphibolite down to greenschist facies indicating an unexpected high heat flux in the gabbro/dike transition. Only in one working area (SWIR 21°E) we observed a bimodal distribution of metamorphic temperatures. As a characteristic feature of the gabbro/dike transition from all working areas, we identified a new type of contact-metamorphic process at the interface between the magmatic and metamorphic regime, not observed so far in any mid-ocean ridge: the transformation of a primary dry melt of diabase intruding gabbro into a hydrous melt, with the potential to crystallize brown amphibole from such a melt and/or to recrystallize the frozen diabase to a granoblastic granulite-facies paragenesis. We assume that the water activity necessary for this process is derived from an interaction of the melt of the intruding diabase with the gabbroic host which was obviously hydrated due to hydrothermal circulation before the diking event.
Publications
- (2013): The Gabbro/Dike transition in oceanic crust generated at slow- and ultraslow-spreading ocean ridge systems: first results. DMG annual meeting, University Tübingen
Bähre, L., Koepke, J., Dick, HJB., Mengel, K.
- The Dike-Gabbro Transition Down in the Roots of the Hydrothermal System (2014). Workshop Tongij University
Dick, HJB., Koepke, J.
- (2015): Hydrothermal activity triggering partial melting in gabbros from slow-spreading ridges with focus on Atlantis Bank. Workshop on Indian Ocean Crust and Mantle Drilling, Woods Hole Oceanographic Institution, Woods Hole, MA
Koepke, J., Wolff, PE., Feig, ST.
- (2015): The Origin and Emplacement, of Atlantis Bank (2015). Workshop on Indian Ocean Crust and Mantle Drilling, Woods Hole Oceanographic Institution, Woods Hole, MA
Dick, HJB.
- Down in the Source: The Dike – Gabbro Transition at Fast and Slow Spreading ridges. InterRidge Workshop on Seafloor Massive Sulphides, China 2015
Dick HJB.
