Figures 16 and 17 show the downhole variation of Mg number and TiO2. The least evolved rocks are troctolites that occur between 500 and 520 mbsf. Throughout the entire gabbro section there are numerous thin intervals of Fe-Ti oxide gabbros and felsic rocks that are significantly to strongly differentiated. The Mg number should be used with some caution as a "differentiation index" in the case of the oxide gabbros. Mg numbers of cumulate rocks decrease as a result of the accumulation of iron-rich minerals, such that low numbers overestimate the extent of crystallization. Overall, the gabbros split into two groups: olivine gabbros and troctolites with minor oxides that have high magnesium numbers, which are crosscut by later Fe-Ti-rich oxide gabbros and felsic veins with high TiO2 contents, low Mg number, and relatively sodic compositions that exhibit extreme variability in their composition due to the accumulation of iron oxides. Because of their crosscutting late occurrence and their extreme variability, these rock types have little value for establishing a chemical stratigraphy, which is based entirely on the chemistry of the main gabbro types and principally on the Mg number. Examining the depth profile (Fig. 16), approximately five or six major cycles can be identified of decreasing Mg number going from high Mg at depth to low Mg with increasing TiO2 (roughly 1500 1400, 950 1400, 700 900, 525 700, 250 525, and 0 225 mbsf). Although it projects off the end of the 700 900 mbsf cycle, the overlying unit is chemically distinct. A prominent feature of the observed variation is that the lowermost three units in this cyclic repetition are more iron rich than the upper two, which is consistent with a somewhat more sodic composition and a slightly higher TiO2 content of the rocks.
Mass chemical balance was done to calculate the bulk chemical composition of Hole 735B in 500 m sections. This was done using the average composition of each lithology in the section, the total thickness of intervals for each lithology in the section, and their average densities. This calculation demonstrates large bulk chemical differences between the sections of the hole. From the seafloor to 500 mbsf, the bulk composition has 1.4 wt% TiO2, and a Mg number of 0.67 [Mg x 100]/[Mg + Fe2+]), whereas the next 500 m has 0.7 wt% TiO2, and a Mg number of 0.70, and the lowermost 500 m has 0.4 wt% TiO2, and a Mg number of 0.71. Except for the most highly incompatible elements, the bulk composition of the hole is close to the composition of a primitive MORB with 0.69 wt% TiO2, Mg number around 69.3, and 2.84 wt% Na2O. By contrast, the upper half of the Leg 176 section contains only 0.69 wt% TiO2, but has lower sodium (2.67%) reflecting;the depleted character of the upper olivine gabbros (2.49 vs. 2.87 wt% Na2O in the lower olivine gabbros). Each successive interval down the hole has half as much TiO2 as the interval above it.
This remarkable change in bulk chemistry of the hole is not due to more evolved olivine gabbros at the top of the hole. In the upper 500 m, the olivine gabbros have, in fact, significantly lower titanium and sodium and higher Mg number than the olivine gabbros lower in the hole. Rather, the difference in bulk composition is entirely due to the increasing volume of crosscutting Fe-Ti-rich oxide gabbros and gabbronorites in the upper part of the hole.
The main conclusions that can be drawn from the shipboard chemical analyses from both Legs 176 and 118 are:
1.The main rock type is a moderately fractionated olivine-bearing gabbro having between 0.2 and 1.0 wt% TiO2. Fe-Ti oxide gabbros containing up to 7 wt% TiO2 and up to 20 wt% Fe2O3 occur in centimeter- to decimeter-thick intervals throughout the core. The abundance of Fe-Ti oxide appears to decrease with depth, but this is not related to a decrease in TiO2 of the parental liquids from which the gabbros crystallized. The development of localized concentrations of Fe-Ti-rich gabbro seems to depend on favorable conditions for formation rather than on the TiO2 content of the starting material.
2.Gabbros with similar Mg number, MgO and Ni contents but variable TiO2 content commonly occur together. Hence the differences in TiO2 content cannot result from simple fractional crystallization from a common parental magma. Factors other than cotectic crystallization that affect phase proportions and compositions must have been involved. These are likely to include complex mixing of early cumulates with more differentiated liquids, assimilation-fractional crystallization processes during melt transport through the mass, and redistribution of crystal phases during solution channeling of migrating melts.
3.Within the 1000-m section drilled during Leg 176, six chemical units can be identified. With few exceptions the boundaries of these units coincide with changes in lithologic, metamorphic, and structural properties. The thickness of the separate units varies from 100 to 300 m. Most likely these chemical units represent the scale at which individual magmatic events added to the construction of oceanic Layer 3 at this ultra-slow spreading ridge.
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