Dunite from New Caledonia displays three types of serpentine-dominated veins. The earliest, type 1 veins are narrow (50–100μm wide) and rarely extend across more than a single olivine grain. They are ...lizardite, contain abundant brucite and never contain magnetite. Type 2 veins are 0.01 to 0.1mm wide, extend across several olivine grains and cut across the type 1 veins. They are lizardite, contain magnetite, often in vein interiors, and contain less brucite than type 1 veins. Type 3 veins are dominantly chrysotile, cm-scale, have a magnetite-rich core, and extend for meters or more. Analyses of two representative samples indicate that the type 1 veins have relatively Fe-rich serpentine (XMg=0.92) and brucite (XMg=0.82). These minerals are less magnesian than those in the type 2 veins; serpentine has XMg=0.93–0.94 and brucite has XMg=0.84. In the magnetite-rich core to the type 3 vein both serpentine (XMg=0.94–0.97) and one of the two brucite populations (XMg=0.94) are Mg-rich. Opx in harzburgite layers in these samples is cut by serpentine veins that are on the order of 0.05mm wide. The serpentine veins after Opx lack talc or magnetite and, as with veins cutting olivine, the older veins are more Fe rich (XMg=0.84) than the younger veins (XMg=0.90). We conclude that the formation of magnetite was accompanied by the extraction of iron from the early-formed serpentine and brucite.
Thermodynamic calculations suggest that the type 1 veins formed in a rock-dominated system where the activities of FeO, MgO, and SiO2 were dictated by the compositions of olivine and orthopyroxene. In contrast the type 2 veins were formed in a more fluid-dominated system where the infiltrating fluid was relatively oxidizing and out of equilibrium with the original brucite–serpentine assemblage. Reduction of this fluid was accompanied by reaction of brucite and serpentine to magnetite and hydrogen. By producing magnetite, this reaction extracted iron from brucite and serpentine, making them both more magnesian. This would drive the brucite–serpentine–magnetite assemblage to higher oxygen fugacity, progressively decreasing the efficiency of the magnetite-forming reactions.
► Five types of serpentine veins are found in dunite from New Caledonia. ► Early veins, with Fe-rich brucite and serpentine cut single grains of olivine. ► Later through-going veins contain magnetite and Mg-rich brucite and serpentine. ► Early veins were rock-dominated, later veins were fluid-dominated. ► Reduction of fluid was accompanied by production of magnetite and hydrogen.
The depth extent, strength, and composition of oceanic detachment faults remain poorly understood because the grade of deformation‐related fabrics varies widely among sampled oceanic core complexes ...(OCCs). We address this issue by analyzing fault rocks collected from the Kane oceanic core complex at 23°30′N on the Mid‐Atlantic Ridge. A portion of the sample suite was collected from a younger fault scarp that cuts the detachment surface and exposes the interior of the most prominent dome. The style of deformation was assessed as a function of proximity to the detachment surface, revealing a ∼450 m thick zone of high‐temperature mylonitization overprinted by a ∼200 m thick zone of brittle deformation. Geothermometry of deformed gabbros demonstrates that crystal‐plastic deformation occurred at temperatures >700°C. Analysis of the morphology of the complex in conjunction with recent thermochronology suggests that deformation initiated at depths of ∼7 km. Thus we suggest the detachment system extended into or below the brittle‐plastic transition (BPT). Microstructural evidence suggests that gabbros and peridotites with high‐temperature fabrics were dominantly deforming by dislocation‐accommodated processes and diffusion creep. Recrystallized grain size piezometry yields differential stresses consistent with those predicted by dry‐plagioclase flow laws. The temperature and stress at the BPT determined from laboratory‐derived constitutive models agree well with the lowest temperatures and highest stresses estimated from gabbro mylonites. We suggest that the variation in abundance of mylonites among oceanic core complexes can be explained by variation in the depth of the BPT, which depends to a first order on the thermal structure and water content of newly forming oceanic lithosphere.
Key Points
The detachment fault is defined by a 450 m zone of mylonitic deformation
The fault deformed viscously and rooted at depths >7 km and temperatures >700oC
Variation among oceanic faults is due to water content and/or thermal structure
The behavior of Fe during serpentinization largely controls the potential for oxidation-reduction reactions and energy budget for serpentinite-hosted microbial communities. We present Fe isotope data ...for mineral separates from a partially serpentinized dunite from New Caledonia to understand the behavior of Fe during serpentinization processes. Our new Fe isotope data in mineral separates is compared to existing data from whole rock studies of serpentinites, which have generally concluded that Fe mobility during serpentinization is restricted to the highest temperatures of serpentinization in subduction zones. Measurements of mineral separates from New Caledonia show significant Fe isotope fractionations, with serpentine-brucite mixtures having the lowest δ56Fe∼−0.35‰ and magnetite having the highest δ56Fe∼+0.75‰. Olivine, orthopyroxene, and the whole rock composition are all within error of δ56Fe=0.00‰. Fe isotope thermometry between mineral phases reveals two distinct temperatures of equilibration, one for the mantle olivine and pyroxene (∼1325°C), and a second, much lower temperature (∼335°C) for the serpentinite assemblage. The combined isotopic, mineralogical and geochemical data indicate that during the magnetite-forming stage of serpentinization, a pore fluid in equilibrium with the mineralogical assemblage evolves to higher Fe concentrations as serpentinization proceeds. When this pore fluid is removed from the serpentinizing environment, the total abundance of Fe removed from the rock in the pore fluid is much less than the bulk rock Fe and has a minimal effect on the overall rock composition.
We introduce a technique for U–Pb dating of baddeleyite using secondary ion mass spectrometry (SIMS)
in situ analysis of ng-mass crystals that cannot be efficiently extracted by conventional mineral ...separation techniques. Average
207Pb/
206Pb ages for Precambrian baddeleyite crystals are within <
0.3% of the respective isotope dilution thermal ionization mass spectrometry (ID-TIMS) ages.
206Pb/
238U ratios are corrected for instrumental fractionation calibrated through linear regression in a Pb/U relative sensitivity vs. UO
2
+/U
+ calibration plot. Calibration is performed on separated baddeleyite crystals (∼
100–200
μm in maximum dimension) mounted in random crystallographic orientation.
206Pb/
238U ages for baddeleyite from Duluth gabbro (FC4b) and Kovdor are accurate within 1–2% when averaging 15–30 individual spot analyses and relative sensitivities calibrated on Phalaborwa baddeleyite. The relative difference of
206Pb/
238U between large crystals and micro-baddeleyite from FC4b is within ∼
1%. Comparison between silicate glass and baddeleyite, as well as replicate analysis of the same grains in different orientations relative to the incidence direction of the primary beam support previous evidence for bias in Pb/U sensitivity in baddeleyite due to variable crystal orientations. We successfully utilized oxygen flooding and a UO
2
+/U
+-based calibration to significantly reduce orientation dependent bias.
Leucogranitic rocks formed by crustal melting are a prominent feature of collisional orogens of all ages. This study describes leucogranitic gneisses associated with an Archean collisional orogeny ...preserved in the Teton Range of northwestern Wyoming, USA. These leucogneisses formed at 2.68Ga, and initial Nd isotopic compositions suggest they are derived from relatively juvenile sources. Two distinct groups of leucogneisses, both trondhjemitic, are identified on the basis of field relations, petrology, and geochemistry. The Webb Canyon gneiss forms large, sheet-like bodies of hornblende biotite trondhjemite and granodiorite. This gneiss is silica-rich (SiO2=70–80%), strongly ferroan, comparatively low in alumina, and is characterized by high Zr and Y, low Sr, and high REE contents that define “seagull”-shaped REE patterns. The Bitch Creek gneiss forms small sills, dikes, and plutons of biotite trondhjemite. Silica, Zr, Y, and REE are lower and alumina and Sr are higher than in the Webb Canyon gneiss. These differences reflect different melting conditions: the Webb Canyon gneiss formed by dehydration melting in which amphibole and quartz breaks down, accounting for the low alumina, high FeO, high silica content and observed trace element characteristics. The Bitch Creek gneiss formed by H2O-excess melting in which plagioclase breaks down leaving an amphibole-rich restite, producing magmas higher in alumina and Sr and lower in FeO and HREE. Both melt mechanisms are expected in collisional environments: dehydration melting accompanies gravitational collapse and tectonic extension of dramatically thickened crust, and water-excess melting may occur when collision places a relatively cool, hydrous lower plate beneath a hotter upper plate. The Archean leucogranitic gneisses of the Teton Range are calcic trondhjemites and granodiorites whereas younger collisional leucogranites typically are true granites. The difference in leucogranite composition reflects the geochemically immature Archean crust that partially melted in the Teton Range compared to the more geochemically evolved rocks typically involved in younger collisional orogens.
•The regeneration of activated carbon as a mercury sorbent is investigated.•FeCl3 and NaCl are found to be suitable catalysts for HgCl2 decomposition.•Reaction rate can be increased by a factor of ...10.•Activation energy can be reduced by as much as 40%.
Traditionally, mercury sorbents are disposed of in landfills, which may lead to contamination of soil and groundwater. In this work, the regeneration of activated carbon (AC) as a mercury sorbent was investigated. The decomposition of HgCl2 on the surface of pure AC was studied, as well as sorbent which has been treated with FeCl3 or NaCl. In all cases, the sorbent is found to be structurally stable through a single regeneration, which is verified through BET, XRD, and XPS analysis. The desorption of mercury from the sorbent is found to follow Henry's law. Additionally, a kinetic analysis suggests that although the presence of activated carbon lowers the energy requirement for the desorption of mercury, it significantly decreases the rate by decreasing the concentration of the HgCl2. FeCl3 and NaCl both promoted the decomposition of HgCl2, but FeCl3 did so more significantly, increasing the rate constants by a factor of 10 and decreasing the activation energy for the decomposition of HgCl2 by 14% to 40%.
The 2·63 Ga Louis Lake batholith, a calc-alkalic pluton exposed in Wind River Range of western Wyoming, consists of minor diorite, quartz diorite, granodiorite, and granite. At shallow structural ...levels the batholith is pyroxene free, but at deeper levels, all units of the batholith contain pyroxenes. On its northern margin the batholith was emplaced at P = 5–6 kbar, T = 775–800°C, fO2 at FMQ (fayalite–magnetite–quartz) + 1·5 to FMQ + 1·8, and aH2O ∼0·1. Along the southern margin of the batholith the emplacement pressure was ∼3 kbar. The batholith includes two compositional series. The peraluminous high-REE series is rich in K2O, and displays larger FeOt/(FeOt + MgO). The metaluminous low-REE series has less K2O and smaller FeOt/(FeOt + MgO). In the Boulder Canyon area, most of the rocks belong to the low-REE series. The Nd, Pb, and Sr initial isotopic compositions of both series vary and do not correlate with SiO2 abundance. Relatively constant Fe/Mg ratios over a wide range of SiO2 and the large variation in initial isotopic compositions indicate that the sources for the Louis Lake batholith include older Wyoming province crust- and mantle-derived mafic melts. The calc-alkalic nature of the Boulder Canyon charnockites indicates that there is no special C-type magma. Instead, charnockites will form in any rock type, if the water activity is low enough.
Hydrothermal atomic force microscopy (HAFM) provides in situ access to the surfaces of dissolving crystals at temperatures above the ambient boiling point of water. Here, we applied HAFM to the (001) ...surfaces of labradorite and anorthite at temperatures up to 125°C. In HCl solutions (pH 2) we observed the formation of a rough and soft surface layer on both minerals. By applying high loading forces to the scanning tip, the soft layer can be removed and the underlying interface (between the fresh solid and the altered layer) can be observed. In this way, in situ information about the thickness of the altered layer on plagioclase and the morphology of the underlying interface can be obtained. On labradorite, the thickness of this layer does not exceed about 30 nm within the first 5 hr of exposure to acidic solution at 125°C, but on anorthite thicknesses of up to about 300 nm were observed. The uncovered interface on anorthite shows a nonuniform morphology and either appears rough in AFM images or shows a step-like pattern.
On anorthite, etch pits spread underneath the altered layer. This suggests that material must be released and transported through the layer without obvious changes in morphology of the layer’s surface. Based on the rate of spreading of etch pits, the dissolution rate was calculated to be about 2 × 10−6 mol m−2 s−1 at 125°C. This value agrees reasonably well with literature data and supports the suggestion that dissolution mainly takes place underneath the altered layer and not on its surface.
An in situ U–Pb SIMS (IN-SIMS) method to date micro-baddeleyite crystals as small as 3
μm is presented with results from three samples that span a variety of ages and geologic settings. The method ...complements ID-TIMS geochronology by extending the range of dateable crystals to sizes smaller than can be recovered by physical separation. X-ray mapping and BSE imaging are used to locate target grains in thin section, followed by SIMS analysis on a CAMECA
ims 1270, using the field aperture in the transfer column to screen out ions from host phases. Internal age precisions for the method are anticipated to range from 0.1% for Precambrian rocks to 3–7% for Phanerozoic rocks. Results establish a 2689
±
5
Ma age for mafic dikes in the Wyoming craton, USA, a 1540
±
30
Ma age for a subaerial lava flow from the Thelon Basin of northern Canada, and a 457
±
34
Ma age for mafic dikes in the platform sequence of southeastern Siberia. The method is ideal for relatively non-destructive dating of small samples such as extraterrestrial rocks and precious terrestrial samples.
PDF
