Mineralogical control of rare earth elements in acid sulfate soils

Major, trace and rare earth element concentrations were measured in porewater, surface water and sediments at an acid sulfate soil site. The concentrations of La and Ce in porewater are up to 1-3 ppm. There is a strong correlation between REE concentration and acidity, except that the maximum concentrations were consistently found below the horizon of maximum acidity, associated with an increase in pH (to ca. 4) and change in mineralogy from jarosite-dominated to goethite-dominated mottles. Jarosite replacement by goethite is as expected with the rise in pH, which in turn is due to the occurrence of a fossil shell bed just below. The rare earth element patterns in the porewaters are enriched in the MREE with respect to Post-Archaean Australian Shale (PAAS). Measurements and calculations show that this is in accord with experiments on low-degree partial dissolution of jarosite, even when the jarosite itself is highly enriched in LREE. There is a clear fractionation in the patterns between the clay-rich soil matrix, which is slightly depleted in the LREE when normalized to PAAS (La/Yb_PAAS ∼0.5), and the secondary mineral phase jarosite, which is enriched in the LREE (La/Yb_PAAS = 15-50). The REE pattern in the porewater changes with the transition from jarosite- to goethite-rich mottles, becoming relatively more enriched in the LREE compared to the HREE, which is consistent with the incongruent dissolution of jarosite to form goethite and the release of greater amounts of jarosite REE to solution, including proportionately more of the jarosite-compatible LREE.Maximum surface water REE concentrations in acidic water were 100-200 ppb La and Ce. REE patterns in surface water were very similar to the porewater transition zone, enriched in the MREE, but asymmetric, relatively enriched in the LREE compared to the HREE.     

Hydrochemical evidence for mixing of river water and groundwater during high-flow conditions, lower Suwannee River basin, Florida, USA

 Karstic aquifers are highly susceptible to rapid infiltration of river water, particularly during periods of high flow. Following a period of sustained rainfall in the Suwannee River basin, Florida, USA, the stage of the Suwannee River rose from 3.0 to 5.88 m above mean sea level in April 1996 and discharge peaked at 360 m³/s. During these high-flow conditions, water from the Suwannee River migrated directly into the karstic Upper Floridan aquifer, the main source of water supply for the area. Changes in the chemical composition of groundwater were quantified using naturally occurring geochemical tracers and mass-balance modeling techniques. Mixing of river water with groundwater was indicated by a decrease in the concentrations of calcium, silica, and ²²²Rn; and by an increase in dissolved organic carbon (DOC), tannic acid, and chloride, compared to low-flow conditions in water from a nearby monitoring well, Wingate Sink, and Little River Springs. The proportion (fraction) of river water in groundwater ranged from 0.13 to 0.65 at Wingate Sink and from 0.5 to 0.99 at well W-17258, based on binary mixing models using various tracers. The effectiveness of a natural tracer in quantifying mixing of river water and groundwater was related to differences in tracer concentration of the two end members and how conservatively the tracer reacted in the mixed water. Solutes with similar concentrations in the two end-member waters (Na, Mg, K, Cl, SO₄, SiO₂) were not as effective tracers for quantifying mixing of river water and groundwater as those with larger differences in end-member concentrations (Ca, tannic acid, DOC, ²²²Rn, HCO₃). Received, March 1999 / Revised, July 1999 / Accepted, July 1999     

Zr-in-rutile thermometry in HP/UHP eclogites from Western China

Four Zr-in-rutile thermometry calibrations are applied to eclogites from Western China. Here, we show that if rutile grows in equilibrium with Qtz and Zrn, and is isolated inside garnet, it preserves its Zr composition and does not undergo compositional change due to cation exchange with the host garnet. It thus preserves the composition for the P–T conditions of its formation and the growth zoning of the host garnet. For the HP/UHP metamorphic temperature, the Tomkins et al. (J Metamorph Geol 25:703–713, 2007) calibration yields temperatures that agree well with previous studies, whereas the other three calibrations (Zack et al. in Contrib Mineral Petrol 148:471–488, 2004; Watson et al. in Contrib Mineral Petrol 151:413–433, 2006; Ferry and Watson in Contrib Mineral Petrol in 154:429–437, 2007), which do not include a pressure correction, give systematically lower temperatures. Zr contents of rutile inclusions within garnet show systematic decrease from garnet core to rim. The rutile inclusions in garnet rims contain the lowest Zr content, similar to that in the matrix. Analyses confirm that the pressure plays a significant role in modifying the primary temperature dependence of the Zr content of rutile. Rutiles trapped in garnets are unable to re-equilibrate easily during retrogression, but those in the matrix can do so, providing retrograde P–T path information.     

The discovery and orbit of Charon

The observations leading to, and confirming the discovery of, 1978 P 1 (Charon) are reviewed. The original interpretation of these observations remains valid. The circular orbit with synchronous revolution and rotation is in agreement with both observation and theory. The expectation that eclipse events are imminent is supported by our recent data.     

Privatizing farm worker justice: Regulating labor through voluntary certification and labeling

This paper assesses the possibilities and limits of efforts to incorporate social accountability into California agricultural production through voluntary certification and labeling, in the context neoliberal governance. We argue that, in its contradictory role as market mechanism, regulatory form, and social cause, certification both resists neoliberalization of the agro-food system and reinscribes neoliberal thinking. Unlike more traditional forms of social justice organizing, which have historically sought to alter power relations between labor, capital, and the state, the very notion that production conditions can be regulated through voluntary, third-party monitoring and labeling embraces several key neoliberal principles: the primacy of the market as a mechanism for addressing environmental and social ills, the privatization of regulatory functions previously reserved for the public sphere, and the assertion of the individual rights and responsibilities of citizen–consumers. Interviews with certification actors lead us to conclude that the strategic embrace of certification is driven by contradictory motivations within the movement for social accountability in agriculture, which can only be understood in relation to the confluence of a broader neoliberal political–economic order with California’s particular arrangements of farm labor politics and agro-food activism. Specifically, agro-food consolidation, rollback of protective labor regulation, the evisceration of the farm worker movement, and the conservative agrarianism of the sustainable agriculture movement intersect to circumscribe the realm of possibility and create conditions that undermine farm worker representation in the governance of agricultural labor practices.     

The beginnings of hydrous mantle wedge melting

This study presents new phase equilibrium data on primitive mantle peridotite (0.33 wt% Na₂O, 0.03 wt% K₂O) in the presence of excess H₂O (14.5 wt% H₂O) from 740 to 1,200°C at 3.2–6 GPa. Based on textural and chemical evidence, we find that the H₂O-saturated peridotite solidus remains isothermal between 800 and 820°C at 3–6 GPa. We identify both quenched solute from the H₂O-rich fluid phase and quenched silicate melt in supersolidus experiments. Chlorite is stable on and above the H₂O-saturated solidus from 2 to 3.6 GPa, and chlorite peridotite melting experiments (containing ~6 wt% chlorite) show that melting occurs at the chlorite-out boundary over this pressure range, which is within 20°C of the H₂O-saturated melting curve. Chlorite can therefore provide sufficient H₂O upon breakdown to trigger dehydration melting in the mantle wedge or perpetuate ongoing H₂O-saturated melting. Constraints from recent geodynamic models of hot subduction zones like Cascadia suggest that significantly more H₂O is fluxed from the subducting slab near 100 km depth than can be bound in a layer of chloritized peridotite ~ 1 km thick at the base of the mantle wedge. Therefore, the dehydration of serpentinized mantle in the subducted lithosphere supplies free H₂O to trigger melting at the H₂O-saturated solidus in the lowermost mantle wedge. Alternatively, in cool subduction zones like the Northern Marianas, a layer of chloritized peridotite up to 1.5 km thick could contain all the H₂O fluxed from the slab every million years near 100 km depth, which suggests that the dominant form of melting below arcs in cool subduction zones is chlorite dehydration melting. Slab P–T paths from recent geodynamic models also allow for melts of subducted sediment, oceanic crust, and/or sediment diapirs to interact with hydrous mantle melts within the mantle wedge at intermediate to hot subduction zones.     

Reply to ‘Comment on “The beginnings of hydrous mantle wedge melting” by Till et al.’ by Stalder

The comment of Stalder raises three main concerns regarding the interpretation of the experiments presented by Till et al. (2012): (1) our inability to uniquely distinguish between high-pressure hydrous silicate melt and solute-rich aqueous fluid leads to the incorrect interpretation of phase relations, (2) the temperature interval over which hydrous melting takes places is inordinately large and contrary to expectations, and/or (3) the possibility that the system may be above the second critical end point (SCEP) in this H₂O-rich silicate system has been insufficiently discussed. In this reply, we provide clarification on these concerns and argue that with the extent of knowledge available today, the chemical characteristics of our experimental products at 3.2 and 4?GPa evince the presence of a silicate melt at temperatures <1,000?°C and we are below the SCEP in the peridotite–H₂O system at the P–T conditions of our experiments. If in fact the quench observed in our experiments does represent that of a supercritical (SC) fluid, then our data suggest Mg and Fe are highly soluble in SC fluids at the P–T conditions of the base of the mantle wedge below arc volcanoes. Therefore, our results would require a significant change in thinking about the chemical compositional characteristics of SC fluids.     

Critical legal geographies of possession: Antarctica and the International Geophysical Year 1957–1958

This is an article about the politics of territory in Antarctica. It revolves around what at first seems like a very simple geopolitical question: who owns Antarctica? As this article demonstrates, this seemingly simple question is far from easy to answer: it cannot be answered with a straightforward list of states, nor by conventional geopolitical understandings of territorial possession (Agnew and Corbridge, Mastering space: Hegemony, territory, and international political economy, 1995). Struggles between states for territorial possession has characterised much recent geopolitical history; struggles for Antarctica do not entirely follow this pattern, and revolve instead on the nature and the concept of territorial possession itself. The article focuses in particular on the debates about, and changes to, Antarctic legal and geopolitical territories triggered by the 1957–1958 International Geophysical Year: before the IGY Antarctica was an unstable composite of state claims, unclaimed terra nullius, and terra communis or land unavailable to state claim. By the 1959 Antarctic Treaty, this unstable composite legal and geopolitical geography emerged as a new form of territory, one in which the conventional global mode of territory—state possession—was no longer dominant. Understanding Antarctic legal geographies adds depth to critical geopolitical studies which focus on the ways in which space is actively constructed by specific discourses, understandings, and groups.     

Field Application of the Combined Membrane‐Interface Probe and Hydraulic Profiling Tool (MiHpt)

The Membrane‐Interface Probe and Hydraulic Profiling Tool (MiHpt) is a direct push probe that includes both the membrane interface probe (MIP) and hydraulic profiling tool (HPT) sensors. These direct push logging tools were previously operated as separate logging systems for subsurface investigation in unconsolidated formations. By combining these two probes into one logging system the field operator obtains useful data about the distribution of both volatile organic contaminants (VOCs) and relative formation permeability in a single boring. MiHpt logging was conducted at a chlorinated VOC contaminated site in Skuldelev, Denmark, to evaluate performance of the system. Formation cores and discrete interval slug tests are used to assess use of the HPT and electrical conductivity (EC) logs for lithologic and hydrostratigraphic interpretation. Results of soil and groundwater sample analyses are compared to the adjacent MiHpt halogen specific detector (XSD) logs to evaluate performance of the system to define contaminant distribution and relative concentrations for the observed VOCs. Groundwater profile results at moderate to highly contaminated locations were found to correlate well with the MiHpt‐XSD detector responses. In general, soil sample results corresponded with detector responses. However, the analyses of saturated coarse‐grained soils at the site proved to be unreliable as demonstrated by high RPDs for duplicate samples. The authors believe that this is due to pore water drainage observed from these cores during sampling. Additionally, a cross section of HPT pressure and MiHpt‐XSD detector logs provides insight into local hydrostratigraphy and formation control on contaminant migration.