The Gabbro/Dike transition in oceanic crust generated at slow- and ultraslow-spreading ocean ridge systems

• Applicant: Professor Dr. Jürgen Koepke
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2012 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 227716517

The gabbro/dike transition of the oceanic crust is one of the most important interfaces of our planet, where magmatic, metamorphic and hydrothermal processes interfere. At fast-spreading ridges, where the crust is generally accepted to be layered and uniform, the gabbro/dike transition is characterized by a decametre-thick horizon made of hornfels formed under metamorphic conditions up to the granulite facies. This corresponds to the conductive boundary layer (CBL), a thin (<100 m), hot (>650 °C), impermeable zone predicted by theoretical models, which is sandwiched between the axial magma chamber and the sheeted dikes and across which the heat of the axial magma chamber is exchanged. In contrast, 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. It is the aim of the proposed project, to perform in collaboration with Henry Dick from the Woods Hole Oceanographic Institution (WHOI, Massachusetts, US) a systematic review of the extensive collections of dredges and dive suites of the WHOI containing rocks from the sheeted dike rooting zone from slow- and ultraslow-spreading ridges. These will be characterized in terms of igneous and metamorphic assemblage, deformation textures, veining, and contact relationships first in hand specimen. For the subsequent work samples will be selected representative for different tectonic/magmatic environments (tectonic emplacement mode vs. high magmatic productivity). This suite will then be investigated petrographically and with the electron microprobe, in order to unravel the petrographic record of primary magmatic, late magmatic, secondary high-temperature (i.e. granoblastic recrystallization), low-temperature hydrothermal, and tectonic processes. Special focus will be to discover the presence of the characteristic granulite-facies granoblastic overprint in the corresponding dike basalts - the key for the development of a CBL. The database to be obtained will then be compared with that reported from fast-spreading systems to make a generalized comparison of the gabbro/dike interface, and its implications for crustal accretion in these very different spreading environments. The working hypothesis is that for those segments of slow-spreading ridges with stronger magmatic activity it is expected that the gabbro/dike dynamics are similar to fast-spread, EPR-like crust, with a well-defined CBL, while for those segments where crustal accretion is controlled by tectonic processes, it is expected that relative simple cutting relations are developed (basalt cuts gabbro).

• DFG Programme: Research Grants
• International Connection: USA
• Participating Person: Dr. Henry Dick