HR: 16:30h
AN: V22H-03 INVITED     [PDF]
TI: Igneous Crystallization Beginning at 20 km Beneath the Mid-Atlantic Ridge, 14 to 16 N
AU: * Kelemen, P
EM: peterk@whoi.edu
AF: WHOI, MS 8, Woods Hole, MA 02543 United States
AB: ODP Leg 209 drilled 19 holes at 8 sites along the Mid-Atlantic Ridge from $14\deg$43 to $15\deg$44 N. All sites were previously surveyed by submersible, and were chosen to be $<$ 200 m from peridotite or dunite exposed on the seafloor; outcrops of gabbroic rock were also near some sites. One primary goal of Leg 209 was to constrain melt migration and igneous petrogenesis in this region where residual peridotites are exposed on both sides of the Ridge axis. At Sites 1269 and 1273, we penetrated 112 m of basaltic rubble; recovery was poor (3.7 m) and holes unstable, so drilling was terminated. Lavas form nearly horizontal surfaces overlying cliffs exposing peridotite and gabbro. At 6 other sites, we drilled a mixture of residual peridotite and gabbroic rocks intrusive into peridotite. We penetrated 1075 meters at these 6 sites, and recovered 354 m of core. Drilling at Sites 1268, 1270, 1271 and 1272 recovered 25% gabbroic rocks and 75% residual mantle peridotite. Core from Site 1274 is mainly residual peridotite, with a few m-scale gabbroic intrusions. Core from Site 1275 is mainly gabbroic, but contains 24% poikilitic lherzolite interpreted as residual peridotite "impregnated" by plagioclase and pyroxene crystallized from melt migrating along olivine grain boundaries; these impregnated peridotites were later intruded by evolved gabbros. Impregnated peridotites are also common at Site 1271, and present at Sites 1268 and 1270. The overall proportion of gabbroic rocks versus residual peridotites from these 6 sites is similar to previous dredging and submersible sampling in the area. The proportion of gabbro is larger than in"amagmatic" regions on the ultra-slow spreading SWIR and Gakkel Ridges. Impregnated peridotites from Site 1275 have "equilibrated" textures and contain olivine, 2 pyroxenes, plag and Cr-rich spinel. Their whole rock Mg\#, Cr\# and Ni are high, extending to residual peridotite values. 87 MORB glasses from 14 to $16\deg$N with Mg\# from 60 to 73 [from PetDB] could be plagioclase lherzolite saturated at 0.54 GPa ($\pm$0.14 GPa, 2$\sigma$) and 1220$\deg$C ($\pm$16$\deg$C, 2$\sigma$) [Kinzler \& Grove, JGR 92]. Impregnated peridotites and olivine gabbronorites at other sites contain all or most of these minerals, have similar compositions, and record similar conditions. Melts entered the thermal boundary layer beneath the Mid-Atlantic Ridge at about 20 km depth [e.g., Sleep, JGR1975; Reid \& Jackson, MGR 82; Grove et al JGR 92; Cannat JGR 96; Michael \& Chase CMP 97; Braun et al., EPSL 00], and began to crystallize within impregnated peridotites and as discrete plutons intruding peridotite. 25% gabbro in the upper 20 km of an oceanic plate would correspond to 5 km of "normal" oceanic crust. 25% gabbro (7.2 km/s) + 75% peridotite (8.2 km/s) yields a "mantle" Vp (8 km/s). Residual mantle peridotites from Leg 209 Sites N and S of the $15\deg$20 Fracture Zone are among the most depleted from the mid-ocean ridges. No regional compositional gradient is evident. Most gabbroic rocks are evolved gabbronorites that are not complementary to MORB; instead, they result from complete, near-fractional crystallization of migrating melt at depth. Site 1268 gabbronorites, together with impregnated peridotites, may be primitive cumulates complementary to MORB. As reported elsewhere at this meeting, high temperature shear zones and faults accomodated nearly all of the subsolidus deformation associated with corner flow and exhumation of residual peridotites and high pressure igneous rocks.
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting