Geochemistry of Site U1309 Gabbros, IODP Expeditions 304/305 at the Atlantis Massif, Mid-Atlantic Ridge 30°N

IODP Site U1309 was drilled at Atlantis Massif (western rift flank of Mid-Atlantic Ridge 30°N; Expeditions 304 and 305), a 1.5-2 Myr old oceanic core complex, through an exposed detachment fault in the shallow part of the dome. The main hole, Hole U1309D, penetrated 1415.5 mbsf and recovery averaged 75%. Over 96% of Hole U1309D is made up of gabbroic rock types, which comprise amongst the most primitive as well as freshest plutonic rocks known from the ocean floor. The most abundant rock type is from the gabbro group, comprising 55.7% of the core recovered. This group spans a wide range in modal composition, including minor amount of olivine, Fe-Ti oxides, and/or orthopyroxene. Olivine gabbro is the second most abundant rock type recovered (25.5%), with modal olivine varying widely (5 to 50%). The modal composition of this rock type is highly variable and locally grades into troctolitic gabbro and troctolite. The gabbros and olivine gabbros from Site U1309 have compositions that are among the most primitive sampled by drilling along the MAR (23°N and 15°20N) and on the Southwest Indian Ridge (ODP Hole 735B). This is reflected in Mg numbers ranging from 67 to 87, low TiO 2 (< 0.72 wt%), Na 2 O, and trace element contents. Site U1309 gabbroic rocks are interpreted as cumulates related to the basalt and diabase cross-cutting the gabbroic section through crystal fractionation processes and a common parental magma. Olivine-rich rocks with relatively low modal plagioclase and clinopyroxene are grouped as olivine-rich troctolite. They represent 5.4 % of the recovered rocks in Hole U1309D, with the thickest interval between 1092 and 1236 mbsf. The olivine-rich troctolites contain >70% olivine by mode, and are commonly intercalated with olivine and troctolitic gabbro. Olivine-rich rocks may represent the most primitive cumulates of the gabbroic sequence. However, they overlap in composition with the impregnated peridotites drilled at 15°20N along the MAR with high Mg numbers (83-90) and Ni contents (1200-2400 ppm) and may have been formed by the same process. Oxide gabbro ( >2% modal Fe-Ti oxides) makes up 7% of the rocks. Many oxide gabbros contain apatite and zircon as accessory minerals. Common occurrences of oxide gabbros were also noted in ODP Hole 735B, Atlantis Bank; and at 15°N at the MAR. Minor felsic ("leucocratic") dikes cross-cut the gabbroic rocks. Oxide gabbro and leucocratic dikes are distinguished by high trace element contents with Zr of 23 to 1180 ppm and Y of 22 to 300 ppm compared to values <5-30 ppm in gabbros and olivine gabbros. They represent the most evolved gabbroic rocks sampled at Site U1309.