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.

The design of active and robust bimetallic nanoparticle catalysts requires the control of the nanoscale alloying and phase-segregation structures and the correlation between the nanoscale phase structures and the catalytic properties. Here we describe new findings of a detailed investigation of such nanoscale phase structures and their structure-catalytic activity correlation for gold-platinum nanoparticles prepared with controllable sizes and compositions. The nanoscale alloying and phase-segregation were probed as a function of composition, size, and thermal treatment conditions using X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, electrochemical characterization, and density functional theory modeling. The results have provided the experimental evidence in support of the theoretically simulated dependence of alloying and phase segregation on particle size and temperature. More importantly, new insights have been gained into the control of the nanoscale phase properties of this bimetallic system among alloyed, partially alloyed, or partially phase segregated structures. In contrast to the largely alloyed character for the catalysts treated at 300-400°C, the higher-temperature treated catalysts (e.g., 800°C) are shown to consist of a Pt-rich alloy core and a Au shell or a phase-segregated Au domains enriched on the surface. This conclusion is further supported by the electrochemical and electrocatalytic data revealing that the catalytic activity is highly dependent on the nanoscale evolution of alloying and phase segregation. The thermal control of the nanoscale alloying, phase-segregation, and core-shell evolution of the nanoscale bimetallic catalysts provided the first example for establishing the correlation between the nanoscale phase structures and the electrocatalytic activity for oxygen reduction reaction correlation, which has profound implications to the design and nanoengineering of a wide variety of bimetallic or multimetallic nanostructures for advanced catalysts.

This paper reports findings of an investigation of the synthesis of monolayer-capped binary gold-silver (AuAg) bimetallic nanoparticles that is aimed at understanding the control factors governing the formation of the bimetallic compositions. The synthesis of alkanethiolate-capped AuAg nanoparticles was carried out using two related synthetic protocols using aqueous sodium borohydride as a reducing agent. One involves a two-phase reduction of AuCl(4)(-), which is dissolved in organic solution, and Ag(+), which is dissolved in aqueous solution. The other protocol involves a two-phase reduction of AuCl(4)(-) and AgBr(2)(-), both of which are dissolved in the same organic solution. AuAg nanoparticles of 2-3 nm core sizes with different compositions in the range of 0-100% Au have been synthesized. The two synthetic routes were compared in terms of bimetallic composition and size properties. Our new findings have allowed us to establish the correlation between synthetic feeding of metals and metal compositions in the bimetallic nanoparticles, which have important implications to the exploration of gold-based bimetallic nanoparticles for constructing sensing and catalytic nanomaterials.

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