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Molybdenum isotopes in three deep and well-characterized weathering profiles – a saprolite formed on meta-diabase from South Carolina, USA, and two ferruginous bauxites formed on Columbia River Basalts in Oregon and Washington, USA – elucidate Mo isotope behavior during continental weathering. The saprolite records an overall loss of Mo relative to the fresh bedrock, as indicated by negative τMoTi, which defines the loss of Mo relative to the relatively immobile element Ti. The saprolites are also isotopically light: δ98Mo values range from −0.89‰ (relative to NIST 3134) to −0.05‰, mean δ98Mo = −0.40‰, compared to +0.55‰ NIST3134 for the underlying unweathered bedrock. By contrast, the ferruginous bauxites generally record addition of Mo relative to the fresh bedrock (zero to positive τMoTi) and generally have higher δ98Mo values than the parental basalts: δ98Mo of the bauxites range from −0.14‰ to +0.38‰ compared to −0.33‰ and +0.02‰ for the unweathered parental basalt. Low δ98Mo values in the saprolites likely reflect preferential retention of isotopically light Mo adsorbed onto accessory Fe-oxy-hydroxides and clays during weathering, whereas the high δ98Mo values in the bauxites reflect the addition of isotopically heavy Mo from groundwater. When the three profiles are combined, there is a positive correlation between τMoTi and δ98Mo, suggesting that when Mo is lost during continental weathering, the resulting regolith is isotopically light, whereas groundwater addition can shift the regolith to heavier values. Because saprolites are a more common weathering product than bauxites, we conclude that, in general, continental weathering fractionates Mo isotopes such that the weathered upper crust retains isotopically light Mo. In contrast, the groundwater that leaches Mo from the weathered crust is isotopically heavy. Thus, chemical weathering of continents generates the isotopically heavy riverine signature observed globally, and partially contributes to the isotopically heavy seawater signature. Finally, these data, in conjunction with previously published data for glacial diamictites, can be used to assess changes in the crustal Mo isotope signature over the last 2.9 Ga.

Magnesium isotopes have been shown to fractionate significantly during continental weathering, however, the degree and direction of fractionation varies from one study to the next, and the main factors that control Mg isotope fractionation during weathering have yet to be delineated. Here, we report Mg contents and isotopic compositions for two ∼10 m deep drill cores through bauxites developed on Columbia River Basalts (CRBs). Samples from these two drill cores have very low MgO contents (0.12–0.25 wt.%) and variable but high δ26Mg values (from −0.1 to up to +1.8, the heaviest isotopic composition ever reported for Mg) relative to the average values of fresh CRBs (δ26Mg = −0.23 ± 0.07 and MgO = 5.9 wt.%). These features reflect the near complete loss of Mg in the isotopically heavy regolith. The most highly weathered bauxites at the tops of the profiles have the lowest δ26Mg values, reflecting the addition of isotopically light eolian dust. Excluding these samples, δ26Mg in bauxites displays a weak, positive correlation with gibbsite abundance, suggesting that gibbsite preferentially retains 26Mg in the bauxites. The integrated Mg isotopic fractionation factors between bauxite and fluid (Δ26Mgbauxite-fluid), inferred from Rayleigh fractionation calculations, vary from 0.05‰ to 0.4‰, which is similar to the range inferred from previous studies of igneous rock weathering profiles, though none of these previous studies found the extremely heavy δ26Mg and massive Mg depletion seen here. Collectively, studies of weathering profiles suggest loss of isotopically light Mg from the continents, which must contribute to the very light δ26Mg seen in river water and seawater.

Relations among geothermal system dynamics, evolution of an 8-m thick biomorphic mound, and requirements to protect critical habitat of Gila boraxobius , a fish protected under the Endangered Species Act of 1973, were explored at Borax Lake, a shallow thermal lake in the tectonically active northwestern Basin-and-Range Province. Neutral pH, thermal water at ~ 105 °C discharges within a carrot-shaped vent at a depth of 29 m and rapidly mixes within the vent and lake which is mostly − 1 and proximity of seismic sources indicates the mound may be susceptible to liquefaction and slope failure during dynamic earthquake loading. Borax Lake is the unique product of the geothermal system and the biological communities it supports. Portions of the mound may have failed in the past during seismic loading, eroded, and been rebuilt by processes similar to those active today.

The Deer Butte Formation accumulated during the middle Miocene in fault-controlled basins in an extensional setting. The basins developed as regional faults asserted influence after eruption of ash-flow sheets and collapse of calderas of the Lake Owyhee volcanic field. The sequences of Hurley Flat, Dry Creek, and Oxbow Basin contain a lower basalt tephradominated unit formed by basalt hydrovolcanism overlain by fine-grained fluvial and lacustrine volcaniclastic sedimentary units. The sequence of Freezeout Creek was deposited in an erosional valley that was incised into older units and cut across the concurrently active Wall Rock Ridge fault zone. The sequence of Hurley Flat and Dry Creek contain alkaline tholeiitic basalt flows and tephra deposits, whereas the sequences of Freezeout Creek and Oxbow Basin contain subalkaline calcalkaline basaltic andesite. The compositional change occurred after local uplift due to faulting along the Wall Rock Ridge fault zone. The youngest unit, well-sorted, medium-grained, muscovite-bearing arkose of the arkose of Dry Creek Buttes, was deposited in a large river that drained westward from source areas in western Idaho. The Deer Butte Formation was deposited between approximately 15 and 12.6 Ma, while basin and range-type faulting dominated regional structural patterns.

Opal-CT-rich fillings in tree molds and interstitial to breccia clasts in flow-top breccia reduce porosity and contain textures indicative of colloidal deposition. The opal-CT occurs in uniformly textured layers and porous laminated layers of particles and agglutinates cemented by chalcedony. Celadonite, smectite, clinoptilolite, quartz, chalcedony and minor calcite are associated with the opal-CT. These minerals occur in two settings: (1) within interstitial spaces between particles of opal-CT; and (2) within particles and agglutinates of opal-CT. Concentrations of trace and major elements are closely related to the textural setting of the secondary minerals within the layers of opal-CT. Layers of opal-CT that are enriched relative to other layers in REE and Th are red-brown in color, uniformly cryptocrystalline with some microcrystallinity, and lack secondary minerals interstitial to opal-CT particles.