SUMMARY

The major conclusions of this study are as follows:

  1. Despite mineral compositional variation in a given sample, major constituent minerals in Hole 735B gabbroic rocks are, to a first order, in chemical equilibrium. That is, olivine + plagioclase in troctolite, plagioclase + clinopyroxene in gabbro, plagioclase + clinopyroxene + olivine in olivine gabbro, plagioclase + clinopyroxene + olivine + orthopyroxene in gabbronorite, and so on, have all coprecipitated from their respective parental melts (Fig. F1).
  2. Fe-Ti oxides could be in equilibrium with major silicate minerals in ferrogabbros and gabbronorites. However, these oxides in gabbros, olivine gabbros, and troctolitic gabbros are not in chemical equilibrium with silicate minerals but must have formed or been added to the rocks at temperatures below the liquidus of the silicate mineral assemblages (Figs. F3, F4). Disseminated oxides in some rocks may have precipitated from trapped Fe-Ti-rich melts. Oxides that concentrate along bands/zones of shearing probably mark zones of low pressures into which expelled interstitial melt transport/coalesce. Continuous cooling during transport leaves oxides behind tracing the passageways of melt transport.
  3. Although oxide-silicate liquid immiscibility and Fe-Ti-rich melt reaction with wall rock may be invoked to interpret the origin of the felsic veins, we believe that simple fractional crystallization is adequate. SiO2 enrichment in residual melt is the natural consequence of oxide removal/crystallization at a late stage of tholeiitic magma evolution (Fig. F4).
  4. The mineral compositional similarity between fine-grained microgabbros and their coarse-grained gabbroic host (Fig. F5) suggests that most of these microgabbros are, to a first order, in chemical equilibrium with their coarse-grained host. We interpret that the microgabbros represent zones of melt expelled from the host gabbros. This melt must therefore be in equilibrium with minerals in the host (already formed coarse crystals) and minerals crystallizing out of the melt (currently forming fine crystals).
  5. Whereas felsic veins, ferrogabbros, and some microgabbros have meltlike compositions, the bulk composition of Hole 735B is not meltlike, but is rather a cumulate composition. The most primitive olivine in Hole 735B has Fo = 0.842. The melt that is parental to this olivine would have Mg# 0.637 (0.590-0.637, for Kd = 0.27-0.33), which is significantly less than Mg# = 0.714 of bulk Hole 735B. This suggests that a significant mass fraction of more evolved products is needed to balance the high Mg# of the bulk cumulate. A simple calculation shows that in terms of Mg#, 25%-45% of average eastern AII F.Z. basalt is needed to combine with 55%-75% of bulk Hole 735B gabbros to give a melt parental to olivine of Fo = 0.842. This result is not unexpected.
  6. The inferred melt with Mg# 0.637 parental to the most primitive olivine in Hole 735B is still rather evolved to be in equilibrium with residual mantle olivine of Fo > 0.89. This suggests that a significant mass fraction of more primitive cumulate (i.e., high-Mg# dunite and troctolite, etc.) is yet to be sampled. This hidden cumulate could well be deep in the lower crust or simply in the mantle section. We favor the latter because of the thickened thermal boundary layer atop the mantle beneath slow-spreading ridges. It is inevitable that mantle melts that migrate through this cold thermal boundary layer will cool and crystallize whatever minerals on the liquidus, which in this case are chromite/Cr spinel, forsteritic olivine, and anorthitic plagioclase.
  7. Whole-rock compositions of cumulate rocks are controlled by both compositions and modes of the constituent minerals. Abundances of all elements, and particularly highly compatible elements (e.g., Ni and Cr), are controlled by mineral compositions, but slightly compatible elements (Sc, Sr, V, Ti, and Y, etc.) are also controlled by mineral modes. Incompatible and highly incompatible elements (i.e., Zr and Nb) are largely determined by the presence of trapped melt or cumulates of late-stage highly evolved melt (i.e., Fe-Ti oxides). Whole-rock CaO/Al2O3 ratio can effectively describe modal systematics of coarse-grained gabbroic rocks, particularly when the abundances of olivine, orthopyroxene, and Fe-Ti are low in abundance, and thus can be used to evaluate modal controls on trace element systematics.

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