Figure F57. Histograms for molar Mg# in gabbroic rocks from sites drilled during this leg, Site 923 in the Kane Fracture Zone (MARK) area (Agar and Lloyd, 1997), and Site 735B along the Southwest Indian Ridge (Natland and Dick, 2002). Cumulate oxide gabbros and oxide gabbronorites dominate the gabbroic rocks recovered during Leg 209, especially at Sites 1270 and 1275. Diabases and miarolitic gabbros from Sites 1275 and 1272 have higher Mg#s than the oxide gabbros and closely approximate liquid compositions (see the "Site 1272" summary and Fig. F58). Site 1275 troctolites are probably hybrid rocks, formed by interaction between migrating melt and residual mantle olivine. Only at Site 1268 were gabbroic rocks with primitive cumulate compositions sampled. These rocks were gabbronorites. However, extensive alteration makes it difficult to be certain that their high Mg# is a primary igneous feature. In general, the Leg 209 gabbro samples, and particularly the gabbroic rocks from Site 1275, are the most evolved suite of plutonic rocks recovered by ODP drilling along a mid-ocean ridge. However, although some primitive gabbroic cumulates were sampled at Sites 921, 923, and 735B, the abundance of evolved gabbros leads to average compositions with intermediate Mg#s at these other sites. Thus, with the possible exception of Site 1268 gabbronorites and of impregnated peridotites from, for example, Sites 1271 and 1275, no plutonic suite recovered by ODP drilling provides an example of the refractory cumulates (80% < Mg# < 90%) required to balance crystal fractionation of MORB.
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