HR: 0830h
AN: V11E-0533    [PDF]
TI: Petrological insights of the first recovered chromitites from Site 1271, ODP Leg 209, MAR 15$\deg$N
AU: * ABE, N
EM: abenatsu@jamstec.go.jp
AF: Deep Sea Research Department, Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka, Kanagawa, 237-0061 Japan
AU: ODP Leg 209 Shipboard Scientific Party, .
EM: joy@joiscience.org
AF: Ocean Drilling Program, Ocean Drilling Program, College Station, TX 77845-9547 United States
AB: ODP Leg 209 recovered several podiform chromitites at Site 1271, Mid-Atlantic Ridge close to the 15$\deg$ 20'N fracture zone. These chromitites are the first sampled from the Mid-Atlantic Ridge. Furthermore, Site 1271 is only the second chromitite locality that has been found along any Mid-Ocean Ridges. The first chromitite found, which from near the East Pacific Rise at Hess Deep at ODP Site 895 (Arai and Matsukage, 1996), is a single, elongate, discontinuous train of chromite grains $<$1 cm wide. In contrast, the Site 1271 chromitites are rounded rather than elongate, have sharp contacts with surrounding peridotite, and are clearly massive. It has been proposed that the formation of chromitite occurs during subduction-related arc magmatism only, because the formation requires hydrous magmas (Matveev and Ballhause, 2002). Therefore high Cr\#s have been cited as evidence that most ophiolites with high Cr\# in mantle spinels do not form at normal Mid-Ocean ridges. Based on our results, confirming the Hess Deep observation, it is clear that chromitites are not restricted to arc magmatism. However, there is abundant high-temperature amphibole in core from Site 1271, some of which could be igneous. Based on the of unusually high-Cr\# ($>$0.6) in spinels from harzburgite and dunite dredged from the Mid-Atlantic Ridge in the 14$\deg$ to 16$\deg$N region (Bonatti et al., 1992; Dick and Kelemen, 1992; Sobolev et al., 1992) andthe general observation that spinels in chromitites have higher Cr\# than spinels in residual mantle peridotites (e.g.,Dick and Bullen, 1984), we anticipate that the Cr\#'s Site 1271 chromitites are the highest yet found in spinel from Mid-Ocean ridge setting. High Cr\#s can provide insight into the processes of chromitite formation and may give rise tonew interpretation of ophiolite provenance.
DE: 1025 Composition of the mantle
DE: 1724 Ocean sciences
DE: 3640 Igneous petrology
DE: 7218 Lithosphere and upper mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting