Figure F6 compares compositions of olivine and plagioclase at all depths in the core (data compilation of Dick et al., Chap. 10, this volume). All mineral analyses from individual samples are plotted. The general trends in Mg# presented thus far are mimicked by variability in olivine compositions. The bimodal contrasts between the olivine-gabbro and oxide-gabbro suites are clearly evident in the upper 520 m of the core. The shift toward more differentiated olivine gabbro compositions below 520 mbsf can be seen in trends for both olivine and plagioclase. Wiggles in trends for bulk compositions in Zones 3a-3c show up here in trends of mineral compositions, as well.
Liquid Mg#s (= MgL) calculated from olivines using the formulation of Roeder and Emslie (1970) are indicated at the top of Figure F5A. The most primitive liquids involved in production of gabbros in Hole 735B were actually fairly strongly differentiated, with MgL no higher than ~61. Values of melt MgL down to 50, which would correspond to nearly the least MgL in glasses from abyssal tholeiites from either the Central or Southwest Indian Ridges in the Indian Ocean (Natland and Dick, 2001), represent only a small fraction of the core, all of it in the lower sequences of Series 1 and Series 2. Olivine gabbros of Series 3 crystallized from more differentiated (MgL = 46-35) ferrobasaltic liquids of types commonly found along the fast-spreading East Pacific Rise, but not to 98% of basalts erupted along slowly spreading ridges in the Indian Ocean.
The still more differentiated gabbros, including all of the oxide gabbros, crystallized from even more differentiated magma (e.g., Natland et al., 1991; Natland and Dick, 2001). Along the Southwest Indian Ridge, their evolution is locked up entirely in abyssal gabbros such as those from Hole 735B. Since oxide minerals and, in many cases, low-Ca pyroxenes were on the liquidus, comparison with experimental data on abyssal tholeiites (Walker et al., 1979; Juster et al., 1989) indicates that the relevant liquids were at least ferroandesitic in composition, although this depends on the precise character of the later development of the liquid line of descent. Analogy to experimental data suggests that magma temperatures were from ~1180° to 1150°C during the earliest stages of crystallization of the upper two plutons. From there they ranged down to perhaps 1100°C for the remaining olivine gabbros. Ilmenite and magnetite then reigned on the liquidus to below 1050°C if the environment was fairly dry. Trondhjemite at or near the "basalt solidus" crystallized at ~1000°C (Dixon and Rutherford, 1979).