The White Mountain granitoid suite represents an isolated window into Cretaceous age magma intruded into Wrangellia terrane basement. Although the total area of exposed granitoid at White Mountain is relatively small (∼1 km 2 ), substantial textural, chemical, and isotopic complexities exist. The granitoid suite consists of six surficially isolated bodies, all of which are calc-alkaline and metaluminous, ranging in composition from hornblendebiotite quartz diorite to biotite granodiorite. Three 40 Ar/ 39 Ar analyses provide cooling ages between 113.3 ± 1.3 and 117.38 ± 0.54 Ma, suggesting at least two pulses of magmatism are represented in the granitoid suite. Two of the bodies, comprising ∼20% of the total exposed granitoid, are enclave-bearing, with the hosts representing the most chemically evolved material at White Mountain and the enclaves among the least evolved. The enclaves typically are <15 cm in size and circular to oval in shape, are dominated by plagioclase and amphibole, and are intermediate in composition (∼54 wt% SiO 2 ). Enclave rare-earth element patterns and isotopic characteristics, and the lack of petrographic evidence for quenched margins, suggest that they are cumulates from liquids chemically similar to but isotopically distinct from their host materials. One granitoid hand specimen exhibits textural and geochemical evidence for mixing at the low MgO end of the compositional spectrum. Although the granitoid suite exhibits a narrow range in whole rock isotopic compositions (ε Nd(115 Ma) 7.2–9.1 and 87 Sr/ 86 Sr (i) 0.7032–0.7043) further suggesting open system differentiation, these compositions do not require an appreciable role for ancient, evolved continental lithosphere in the White Mountain magmatic system(s). Rather, the dominant source reservoir was depleted mantle. This conclusion provides evidence that mid-Cretaceous magmatism in this region was generated in either an intraoceanic island arc or an “immature” (proto-) continental arc tectonic setting.

Experimental studies of Fe 1− x O wüstite and (Mg 0.8 Fe 0.2 )O ferropericlase were performed at room temperature and high pressure using a diamond anvil cell technique. Mössbauer spectroscopy indicated a magnetic ordering transition in FeO at 4.7 GPa, while no evidence of magnetic ordering was detected in ferropericlase up to at least 50 GPa. A structural cubic-to-rhombohedral distortion was observed in both materials using powder X-ray diffraction: at 12 GPa for FeO and at 35 GPa for ferroperi-clase. This suggests that the structural distortion is independent of magnetic ordering in the FeO-MgO system. A complete single-crystal elasticity tensor of FeO was determined up to 10 GPa, and only minor changes of elastic moduli were detected at the magnetic transition pressure, indicating relatively weak magnetoelastic coupling in FeO at high pressures. The existence of a rhombohedral distortion in ferropericlase with mantle composition at high pressures and the absence of magnetic ordering have important implications for the interpretation of seismological data with respect to lower-mantle inhomogeneity.