In-situ fractionation at the decameter scale in lower oceanic crust

As the best-preserved fragment of paleo oceanic crust on land, the Samail ophiolite (Sultanate of Oman) allows extensive field studies on fast-spread lower oceanic crust. Drill core GT1 of the Oman Drilling Project in the frame of ICDP (International Continental Scientific Drilling Program) spans about 400 m from the layered gabbros between ~1200 and 800 m above the crust-mantle boundary (maM). The drilled samples are mostly gabbros or olivine gabbros and we applied petrological, geochemical, and microstructural methods to identify those magmatic processes relevant for the lower crust accretion. Mg# (Mg/Mg+Fe x 100; molar basis) in olivine and clinopyroxene and the Ca# (Ca/Ca+Na x 100; molar basis) in plagioclase show parallel fractionation trends from 800 to 1070 maM which can be subdivided into four 25 to 80 m-thick smaller trends. Above 1070 maM, phase compositions change towards more primitive compositions over a 15 m-thin horizon, revealing decameter-scale fractionation over the uppermost 80 m of the core. These trends are confirmed by bulk rock Mg# and Cr/Zr ratios and thermodynamic modeling suggests that they can result from in-situ fractionation by a crystallization degree of less than 30 mol% followed by primitive melt replenishment. Thus, the necessity of removing latent heat which is regarded as critical in terms of in-situ crystallization is only limited, because most melt migrates upwards, without crystallizing. Our results suggest that magma replenishment and fractional crystallization are the key processes responsible for the accretion of the layered gabbro section of the Oman paleocrust.