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Crystallization sequences in all units of both Holes 417D and 418A follow the order: (spinel), plagioclase ± spinel, plagioclase + olivine ± spinel, plagioclase + olivine + clinopyroxene, plagioclase + clinopyroxene + magnetite, and plagioclase + magnetite + quartz ± clinopyroxene ± apatite. Spinel is a common groundmass phase in the least-fractionated unit (418A lithologic Unit 6), but also occurs as rounded grains in some more-fractionated units. Analyzed olivine compositions range from Foes to F079.5, and show a moderate correlation with the Mg/Fe ratio of the host glass. The presence of clinopyroxene is largely controlled by the degree of crystallinity of the sample, but is more abundant as a phenocryst phase in the more fractionated units. Cpx first appears as homogeneous rounded, and possibly resorbed, grains with high mg [ = Mg/(Mg + Fe)]. Later development of cpx is as sector-zoned intergrowths with plagioclase, and still later interstitial grains. There is a continuous decrease in mg (0.88 to 0.46) and an increase in Ti/Al in cpx through this crystallization sequence. Glomeroporphyritic clots of cpx ± plag are cognate and include both earlyand middle-stage cpx compositions. Late-stage subcalcic augite (CaO < 14 wt. %) co-exists with pigeonite (mg = 0.64 to 0.68) in some massive units. Plagioclase phenocryst compositions range from Ami to Ans9 and analyzed groundmass grains range from Am 6 to Amβ. Initiation of pyroxene crystallization is marked in feldspar composition by a change from nearly constant Fe contents to gradually increasing Fe with Ab content. The beginning of crystallization of Fe-Ti oxides is marked by inflections in the feldspar and pyroxene compositional trends. Textural evidence supports the conclusion based on plagioclase and clinopyroxene inflections, that magnetite starts to form only after crystallization has proceeded to where plagioclase has the composition An 60 to 65, and the Mg/(Mg + Fe) ratio of clinopyroxene = 0.72 to 0.75. Local disequilibrium textures and compositions may indicate some mixing of magmas, but the pyroxene compositions generally indicate crystallization from rising and fractionating magmas, possibly with increasing silica activities. Late-stage segregation patches from greater than 95 per cent crystallization contain Naplagioclase (Aniβ to Amβ), quartz, oxides, and possibly apatite. These "trondhjemitic" patches represent the first direct evidence that ocean floor basaltic liquids may evolve to high Si, high Na/K differentiates.

Mineral Compositions and Crystallization Trends in Deep Sea Drilling Project Holes 417D and 418A