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Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron-and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite-to amphibolite-facies alteration is most important, coinciding with brittle^ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift lowtemperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high-and low-temperature alteration are their respective associations with ductile and cataclastic deformation, and an overall decrease downhole with hydrothermal alteration generally 95% in the bottom kilometer. Hole 735B provides evidence for a strongly heterogeneous lower ocean crust, and for the inherent interplay of deformation, alteration and igneous processes at slow-spreading ridges. It is strikingly different from gabbros sampled from fast-spreading ridges and at most well-described ophiolite complexes. We attribute this to the remarkable diversity of tectonic environments where crustal accretion occurs in the oceans and to the low probability of a section of old slow-spread crust formed near a major large-offset transform being emplaced onland compared to sections of young crust from small ocean basins.

We use the remarkable similarity between microstructures preserved in naturally and experimentally deformed quartzites as a basis to evaluate quartzite flow laws and their application to natural conditions. The precision of this analysis is relatively high because of the well-constrained deformation history of naturally deformed rocks from the Ruby Gap duplex, Central Australia. The external state variables during deformation in the duplex are well constrained by a combination of thermochronological, microstructural and structural observations. Using a flow law with the form _ e Af m H 2 O s n exp ÀQ=RT , our analysis indicates that values of log (A)=±11.20.6 MPa ±n /s and Q=135 15 kJ/mol provide the best description of the combined natural and experimental constraints with values of m=1 and n=4. Motivated by the results of our analysis, we also evaluated the influence of water fugacity on strain rate determined in the laboratory. In this case, we concur with a previously published suggestion that the measured effect of water fugacity (_ e G f 2 H 2 O ) is likely a manifestation of a change in deformation process with increasing stress. The results of this study provide further support for the application of quartzite flow laws to understand deformation conditions in the Earth, and emphasize the important insights that can be gained by analyzing deformation microstructures in naturally deformed rocks.

Deformation experiments have been conducted to investigate the effect of melt fraction and grain size on the creep behavior of olivine aggregates in the diffusion creep regime. Both nominally melt‐free and melt‐added samples display stress exponents (n = 1.0 ± 0.1) and grain size exponents (p = −3.0 ± 0.5 for nominally melt‐free, p = −3.2 ± 1.2 for melt‐added) indicative of grain boundary diffusion creep. The activation energy for creep of the nominally melt‐free aggregates is 315±35 kJ/mol. An abrupt change in the rheological behavior of the partially molten aggregates occurs at a melt fraction of 0.05. Below this melt fraction the influence of melt on strain rate is rather modest. For example, at a melt fraction of 0.04 the strain rate of melt‐added samples is enhanced by a factor of ∼3 relative to that of melt‐free aggregates. This result is consistent with previously published theoretical models for solution‐precipitation enhanced grain boundary diffusion creep in which the melt phase is present along three‐grain junction tubules and four‐grain junction corners. A comparison with published diffusion data indicates that deformation is limited by transport of Si along melt‐free grain boundaries under both melt‐free and melt‐present conditions. At melt fractions above ∼0.05, the strain rate enhancement is significantly greater than that predicted by the theoretical models. For example, at a melt fraction of 0.07 the strain rate of melt‐added samples is enhanced by a factor of ∼25 relative to that of melt‐free aggregates. Micro structural observations of both hot‐pressed and deformed aggregates with melt fractions greater than ∼0.05 demonstrate that a significant number of two‐grain boundaries are “wetted” by melt. These boundaries provide rapid transport paths not accounted for in the theoretical models. The presence of wetted grain boundaries at melt fractions less than ∼0.19 indicates that the melt topology in the olivine‐basalt system is affected by anisotropic interfacial energies. There is no difference in the strength of partially molten aggregates deformed with or without added water. This result is consistent with the observation that the solubility of water in basalt is ∼3 orders of magnitude greater than that in olivine and supports the conclusion that deformation is limited by transport along melt‐free grain boundaries at all conditions tested.

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The onset of melting in the Earth's upper mantle influences the thermal evolution of the planet, fluxes of key volatiles to the exosphere, and geochemical and geophysical properties of the mantle. Although carbonatitic melt could be stable 250 km or less beneath mid-oceanic ridges, owing to the small fraction (∼0.03 wt%) its effects on the mantle properties are unclear. Geophysical measurements, however, suggest that melts of greater volume may be present at ∼200 km (refs 3-5) but large melt fractions are thought to be restricted to shallower depths. Here we present experiments on carbonated peridotites over 2-5 GPa that constrain the location and the slope of the onset of silicate melting in the mantle. We find that the pressure-temperature slope of carbonated silicate melting is steeper than the solidus of volatile-free peridotite and that silicate melting of dry peridotite + CO(2) beneath ridges commences at ∼180 km. Accounting for the effect of 50-200 p.p.m. H(2)O on freezing point depression, the onset of silicate melting for a sub-ridge mantle with ∼100 p.p.m. CO(2) becomes as deep as ∼220-300 km. We suggest that, on a global scale, carbonated silicate melt generation at a redox front ∼250-200 km deep, with destabilization of metal and majorite in the upwelling mantle, explains the oceanic low-velocity zone and the electrical conductivity structure of the mantle. In locally oxidized domains, deeper carbonated silicate melt may contribute to the seismic X-discontinuity. Furthermore, our results, along with the electrical conductivity of molten carbonated peridotite and that of the oceanic upper mantle, suggest that mantle at depth is CO(2)-rich but H(2)O-poor. Finally, carbonated silicate melts restrict the stability of carbonatite in the Earth's deep upper mantle, and the inventory of carbon, H(2)O and other highly incompatible elements at ridges becomes controlled by the flux of the former.