A key aim of the ICDP Oman Drilling Project is to constrain magmatic processes beneath fast-spreading mid-ocean ridges. Several drill cores from the Samail ophiolite (Oman), which is regarded as the best-preserved piece of ancient oceanic lithosphere on land, were obtained. Drill core GT1 covers about 400 m from the layered gabbro section between ~1200 and 800 m above the mantle transition zone (maM). The vast majority of the samples recovered are (olivine-) gabbros with a few cm-scale layers of anorthosite, troctolite, and wehrlite. We found a large scale fractionation trend from 800 to 1070 maM in the primary phases olivine, clinopyroxene, and plagioclase that can be subdivided into five smaller trends, each between 25 and 80 m thick. Above 1070 maM, phase compositions change to more primitive compositions over a 15 m thin horizon, revealing decameter-scale fractionation trends between 1090 and 1170 maM. Significant zoning in clinopyroxene, that was absent below, is observed above 1070 maM. These trends are confirmed by bulk rock chemical and mineral trace element data and indicate that fractional crystallization occurred within the layered gabbros. The fabric symmetry varies along the core with significant lineation at the primitive base and top of the core and almost pure foliation dominating the most evolved horizon at about 1070 maM. Variable fabric symmetries and the observed differences in clinopyroxene zoning could result from different liquid/solid ratios caused by on-going fractional crystallization and occasional magma replenishment.
Dominik Mock1,2, David Axford Neave3, Samuel Müller4, Dieter Garbe-Schönberg4, Benoit Ildefonse2, Jürgen Koepke1, Oman Drilling Project Science Team5
1Leibniz Universität Hannover, Germany; 2Université de Montpellier, France; 3University of Manchester, United Kingdom; 4Christian-Albrechts-Universität zu Kiel, Germany; 5Geoscience Institutions worldwide