Figure F4. Normative anorthite in plagioclase (approximately molar Ca/[Ca+Na]) vs. forsterite in olivine (molar Mg/[Mg+Fe], or Mg#) in poikilitic impregnated peridotites from Site 1275, compared to compositions of plagioclase and olivine in gabbro sample suites from mid-ocean ridges and the Samail and Wadi Tayin massifs of the Oman ophiolite. The broad variation of anorthite content in plagioclase at nearly constant olivine forsterite content is probably due to "reactive fractionation" (i.e., reaction between residual peridotite and crystallizing melt migrating along olivine grain boundaries) (e.g., Kelemen, 1986). Comparative data on mid-ocean-ridge gabbro suites show the correlation of anorthite in plagioclase with forsterite in olivine that is expected for crystal fractionation without reaction with residual peridotite. Sources for comparative data on oceanic gabbro suites: Cayman Rise (Elthon, 1987); Hess Deep, East Pacific Rise (Natland and Dick, 1996); Mid-Atlantic Ridge Kane Fracture Zone (MARK) area Hole 923A (Casey, 1997); Southwest Indian Ridge Hole 735B (SWIR; Dick et al., 2002); and Samail and Wadi Tayin massifs of the Oman ophiolite (Garrido et al., 2001; Kelemen et al., 1997; Koga et al., 2001; Korenaga and Kelemen, 1997, 1998; Pallister and Hopson, 1981).