Figure F32. Chemical relationships between gabbros and minerals from Hole 735B. A. Ternary FeO-SiO2-TiO2 diagram for gabbros and minerals of Hole 735B. Small dots = olivine gabbros and troctolites; squares = oxide gabbros; large open circles = average clinopyroxene, amphibole, and olivine for olivine and oxide gabbros; large dots = average ilmenite and magnetite. Data sources are Dick et al. (Chap. 10, this volume) and Natland et al. (1991). The shaded triangle links average olivine gabbro with ilmenite and magnetite. A solid line is drawn from the upper apex of the triangle through the oxide gabbros to a point on the baseline of the triangle. The lever rule applied to this line shows that the proportion of ilmenite to magnetite in oxide gabbros, corresponding to the ratio of the respective lengths of the longer and shorted double-headed arrows, is 4:1. This assumes all FeO is contained in the oxide minerals, whereas some of it is in olivine and pyroxenes. The ratio thus gives the minimum proportion of ilmenite to magnetite in oxide gabbros. Petrographic estimates range from 3:1 to 5:1. B. MgNo (= Mg/[Mg + Fe2+] with Fe2+ determined by titration of least altered olivine gabbros (dots) and troctolites (pluses) vs. their TiO2 contents. Data are from "Appendix B," of Natland and Dick (Synthesis Chap., this volume). TiO2 shows a weak correlation to MgNo (solid line; linear regression correlation coefficient = 0.58) even in troctolites, which have very little clinopyroxene. Among silicates, TiO2 is only present in significant amounts in clinopyroxene (0.6% average throughout the core). The amount of TiO2, which increases with decreasing MgNo, that is accommodated in modally fluctuating amounts of clinopyroxene (average = 29%) is indicated by the dashed shaded line. The effects of adding 1% separately of average ilmenite and magnetite (Natland et al., 1991) to a typical troctolite are given by the curved arrows. Almost all TiO2 thus has to be accommodated in small amounts, but no more than 1%, of ilmenite in any of these rocks. Adding ~0.4 ± 0.3% of ilmenite on a background of modally variable clinopyroxene thus accounts for the weak correlation. This would be accompanied only by ~0.01% of intergrown magnetite (see text for discussion).

Figure F32. Chemical relationships between gabbros and minerals from Hole 735B. A. Ternary FeO-SiO₂-TiO₂ diagram for gabbros and minerals of Hole 735B. Small dots = olivine gabbros and troctolites; squares = oxide gabbros; large open circles = average clinopyroxene, amphibole, and olivine for olivine and oxide gabbros; large dots = average ilmenite and magnetite. Data sources are Dick et al. (Chap. 10, this volume) and Natland et al. (1991). The shaded triangle links average olivine gabbro with ilmenite and magnetite. A solid line is drawn from the upper apex of the triangle through the oxide gabbros to a point on the baseline of the triangle. The lever rule applied to this line shows that the proportion of ilmenite to magnetite in oxide gabbros, corresponding to the ratio of the respective lengths of the longer and shorted double-headed arrows, is 4:1. This assumes all FeO is contained in the oxide minerals, whereas some of it is in olivine and pyroxenes. The ratio thus gives the minimum proportion of ilmenite to magnetite in oxide gabbros. Petrographic estimates range from 3:1 to 5:1. B. MgNo (= Mg/[Mg + Fe²⁺] with Fe²⁺ determined by titration of least altered olivine gabbros (dots) and troctolites (pluses) vs. their TiO₂ contents. Data are from "Appendix B" of Natland and Dick (Synthesis Chap., this volume). TiO₂ shows a weak correlation to MgNo (solid line; linear regression correlation coefficient = 0.58) even in troctolites, which have very little clinopyroxene. Among silicates, TiO₂ is only present in significant amounts in clinopyroxene (0.6% average throughout the core). The amount of TiO₂, which increases with decreasing MgNo, that is accommodated in modally fluctuating amounts of clinopyroxene (average = 29%) is indicated by the dashed shaded line. The effects of adding 1% separately of average ilmenite and magnetite (Natland et al., 1991) to a typical troctolite are given by the curved arrows. Almost all TiO₂ thus has to be accommodated in small amounts, but no more than 1%, of ilmenite in any of these rocks. Adding ~0.4 ± 0.3% of ilmenite on a background of modally variable clinopyroxene thus accounts for the weak correlation. This would be accompanied only by ~0.01% of intergrown magnetite (see text for discussion).