The Fe-C system at 5 GPa and implications for Earth’s core

Earth’s core may contain C, and it has been suggested that C in the core could stabilize the formation of a solid inner core composed of Fe₃C. We experimentally examined the Fe-C system at a pressure of 5 GPa and determined the Fe-C phase diagram at this pressure. In addition, we measured solid metal/liquid metal partition coefficients for 17 trace elements and examined the partitioning behavior between Fe₃C and liquid metal for 14 trace elements. Solid metal/liquid metal partition coefficients are similar to those found in one atmosphere studies, indicating that the effect of pressure to 5 GPa is negligible. All measured Fe₃C/liquid metal partition coefficients investigated are less than one, such that all trace elements prefer the C-rich liquid to Fe₃C. Fe₃C/liquid metal partition coefficients tend to decrease with decreasing atomic radii within a given period. Of particular interest, our 5 GPa Fe-C phase diagram does not show any evidence that the Fe-Fe₃C eutectic composition shifts to lower C contents with increasing pressure, which is central to the previous reasoning that the inner core may be composed of Fe₃C.     

Limitations of the ferrozine method for quantitative assay of mineral systems for ferrous and total iron

The quantitative assay of clay minerals, soils, and sediments for Fe(II) and total Fe is fundamental to understanding biogeochemical cycles occurring therein. The commonly used ferrozine method was originally designed to assay extracted forms of Fe(II) from non-silicate aqueous systems. It is becoming, however, increasingly the method of choice to report the total reduced state of Fe in soils and sediments. Because Fe in soils and sediments commonly exists in the structural framework of silicates, extraction by HCl, as used in the ferrozine method, fails to dissolve all of the Fe. The phenanthroline (phen) method, on the other hand, was designed to assay silicate minerals for Fe(II) and total Fe and has been proven to be highly reliable. In the present study potential sources of error in the ferrozine method were evaluated by comparing its results to those obtained by the phen method. Both methods were used to analyze clay mineral and soil samples for Fe(II) and total Fe. Results revealed that the conventional ferrozine method under reports total Fe in samples containing Fe in silicates and gives erratic results for Fe(II). The sources of error in the ferrozine method are: (1) HCl fails to dissolve silicates and (2) if the analyte solution contains Fe³⁺, the analysis for Fe²⁺ will be photosensitive, and reported Fe(II) values will likely be greater than the actual amount in solution. Another difficulty with the ferrozine method is that it is tedious and much more labor intensive than the phen method. For these reasons, the phen method is preferred and recommended. Its procedure is simpler, takes less time, and avoids the errors found in the ferrozine method.     

Sulfur in peridotites and gabbros at Lost City (30°N, MAR): Implications for hydrothermal alteration and microbial activity during serpentinization

Variations in sulfur mineralogy and chemistry of serpentinized peridotites and gabbros beneath the Lost City Hydrothermal Field at the southern face of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) were examined to better understand serpentinization and alteration processes and to study fluid fluxes, redox conditions, and the influence of microbial activity in this active, peridotite-hosted hydrothermal system. The serpentinized peridotites are characterized by low total sulfur contents and high bulk δ³⁴S values close to seawater composition. Low concentrations of ³⁴S-enriched sulfide phases and the predominance of sulfate with seawater-like δ³⁴S values indicate oxidation, loss of sulfide minerals and incorporation of seawater sulfate into the serpentinites. The predominance of pyrite in both serpentinites and gabbros indicates relatively high fO₂ conditions during progressive serpentinization and alteration, which likely result from high fluid fluxes during hydrothermal circulation and evolution of the Lost City system from temperatures of ∼250 to 150 °C. Sulfate and sulfide minerals in samples from near the base of hydrothermal carbonate towers at Lost City show δ³⁴S values that reflect the influence of microbial activity. Our study highlights the variations in sulfur chemistry of serpentinized peridotites in different marine environments and the influence of long-lived, moderate temperature peridotite-hosted hydrothermal system and high seawater fluxes on the global sulfur cycle.     

Geochemistry, petrology and ages of the lunar meteorites Kalahari 008 and 009: New constraints on early lunar evolution

Kalahari 008 and 009 are two lunar meteorites that were found close to each other in Botswana. Kalahari 008 is a typical lunar anorthositic breccia; Kalahari 009 a monomict breccia with basaltic composition and mineralogy. Based on minor and trace elements Kalahari 009 is classified as VLT (very-low-Ti) mare basalt with extremely low contents of incompatible elements, including the REE. The Lu-Hf data define an age of 4286 ± 95 Ma indicating that Kalahari 009 is one of the oldest known basalt samples from the Moon. It provides evidence for lunar basalt volcanism prior to 4.1 Ga (pre-Nectarian) and may represent the first sample from a cryptomare. The very radiogenic initial ¹⁷⁶Hf/¹⁷⁷Hf (εHf = +12.9 ± 4.6), the low REE, Th and Ti concentrations indicate that Kalahari 009 formed from re-melting of mantle material that had undergone strong incompatible trace element depletion early in lunar history. This unusually depleted composition points toward a hitherto unsampled basalt source region for the lunar interior that may represent a new depleted endmember source for low-Ti mare basalt volcanism. Apparently, the Moon became chemically very heterogeneous at an early stage in its history and different cumulate sources are responsible for the diverse mare basalt types.Evidence that Kalahari 008 and 009 may be paired includes the similar fayalite content of their olivine, the identical initial Hf isotope composition, the exceptionally low exposure ages of both rocks and the fact that they were found close to each other. Since cryptomaria are covered by highland ejecta, it is possible that these rocks are from the boundary area, where basalt deposits are covered by highland ejecta. The concentrations of cosmogenic radionuclides and trapped noble gases are unusually low in both rocks, although Kalahari 008 contains slightly higher concentrations. A likely reason for this difference is that Kalahari 008 is a polymict breccia containing a briefly exposed regolith, while Kalahari 009 is a monomict brecciated rock that may never have been at the surface of the Moon.Altogether, the compositions of Kalahari 008 and 009 permit new insight into early lunar evolution, as both meteorites sample lunar reservoirs hitherto unsampled by spacecraft missions. The very low Th and REE content of Kalahari 009 as well as the depletion in Sm and the lack of a KREEP-like signature in Kalahari 008 point to a possible source far from the influence of the Procellarum-KREEP Terrane, possibly the lunar farside.     

Calcite precipitation instability under laminar, open-channel flow

We present a 2D numerical model for the growth of calcite from supersaturated aqueous solutions under laminar, open-channel flow conditions. The model couples solution chemistry, precipitation at solution/calcite interfaces, hydrodynamics, diffusion and degassing. The model output is compared with experimental results obtained using an oversaturated calcite solution produced by mixing CaCl₂ and Na₂CO₃. The precipitation rate is observed to increase when the supersaturated solution flows over an obstruction, leading to a growth instability that causes the formation of terraces. At relatively high flow rates, the most important mechanism for this behaviour seems to be hydrodynamic advection of dissolved species either towards or away from the calcite surface, depending on location relative to the obstruction, which deforms the concentration gradients. At lower flow rates, steepening of diffusion gradients around protrusions becomes important. Enhanced degassing over the obstruction due to shallowing and pressure drop is not important on small scales. Diffusion controlled transport close to the calcite surface can lead to a fingering-type growth instability, which generates porous textures. Our results are consistent with existing diffusive boundary layer theory, but for flow over non-smooth surfaces, simple calcite precipitation models that include empirical correlations between fluid flow rate and calcite precipitation rate are inaccurate.     

Lateral structural variability in zone of eocene island-arc-continent collision,Kamchatka

The lateral variability of structural elements in the collision zone of the Cretaceous-Paleocene Achaivayam-Valagin island arc with the northeastern Asian margin is considered. The similarity and difference of Eocene collision structural elements in the north and the south of Kamchatka are shown. In northern Kamchatka, the continent-arc boundary is traced along the Lesnaya-Vatyn Thrust Fault, which completed its evolution about 45 Ma ago. The thin, near-horizontal allochthon of this thrust, composed of island-arc rocks, overlies the deformed but unmetamorphosed terrigeneous sequences of the Asian margin. The general structure of this suture in the Kamchatka Isthmus and southern Koryakia is comparable with the uppermost subduction zone, where a thin lithospheric wedge overlaps intensely deformed sediments detached from the plunging plate. In southern Kamchatka (Malka Uplift of the Sredinny Range), the arc-continent collision started 55–53 Ma ago with thrusting of island-arc complexes over terrigenous rocks of continental margin. However, the thickness of the allochthon was much greater than in the north. Immediately after this event, both the autochthon and lower part of allochthon were deformed and subsided to a significant depth. This subsidence gave rise to metamorphism of both the autochthon (Kolpakov and Kamchatka groups, Kheivan Formation) and lower allochthon (Andrianovka and Khimka formations). The anomalously fast heating of the crust was most likely related to the ascent of asthenospheric masses due to slab breakoff, when the Eurasian Plate was plunging beneath the Achaivayam-Valagin arc.     

Geobarometry for Peridotites: Experiments in Simple and Natural Systems from 6 to 10 GPa

Experiments with peridotite minerals in simple (MgO–Al₂O₃–SiO₂,CaO–MgO–SiO₂ and CaO–MgO–Al₂O₃–SiO₂)and natural systems were conducted at 1300–1500°Cand 6–10 GPa using a multi-anvil apparatus. The experimentsin simple systems demonstrated consistency with previous lowerpressure experiments in belt and piston–cylinder set-ups.The analysis of spatial variations in pyroxene compositionswithin experimental samples was used to demonstrate that pressureand temperature variations within the samples were less than0·4 GPa and 50°C. Olivine capsules were used in natural-systemexperiments with two mineral mixtures: SC1 (olivine + high-Alorthopyroxene + high-Al clinopyroxene + spinel) and J4 (olivine+ low-Al orthopyroxene + low-Al clinopyroxene + garnet). Theexperiments produced olivine + orthopyroxene + garnet ±clinopyroxene assemblages, occasionally with magnesite and carbonate-richmelt. Equilibrium compositions were derived by the analysisof grain rims and evaluation of mineral zoning. They were comparedwith our previous experiments with the same starting mixturesat 2·8–6·0 GPa and the results from simplesystems. The compositions of minerals from experiments withnatural mixtures show smooth pressure and temperature dependencesup to a pressure of 8 GPa. The experiments at 9 and 10 GPa producedandradite-rich garnets and pyroxene compositions deviating fromthe trends defined by the lower pressure experiments (e.g. higherAl in orthopyroxene and Ca in clinopyroxene). This discrepancyis attributed to a higher degree of oxidation in the high-pressureexperiments and an orthopyroxene–high-P clinopyroxenephase transition at 9 GPa. Based on new and previous resultsin simple and natural systems, a new version of the Al-in-orthopyroxenebarometer is presented. The new barometer adequately reproducesexperimental pressures up to 8 GPa. KEY WORDS: garnet; mineral equilibrium; multi-anvil apparatus; orthopyroxene; geobarometry     

Late Precambrian rifting in the geological history of the western slope of the South Urals

The rift-related geodynamic setting of the Late Precambrian geological evolution on the western slope of the South Urals is reconstructed on the basis of localization of lithotectonic complexes of this age, their formation conditions, and the geochemistry of rocks. The Early Riphean stage comprises accumulation of coarse-clastic rocks intercalating with alkaline volcanic rocks of the Navysh Complex, which is a constituent of the Ai Formation, and emplacement of doleritic and picritic intrusions of the Shuida Complex and melanocratic dolerite and gabbrodolerite of the Yusha Complex. The Middle Riphean stage is characterized by wide-spread coarse-clastic terrigenous rocks of the Mashak Formation that intercalate with volcanic rocks of the bimodal basalt-rhyolite association, the Berdyaush pluton of rapakivi granite, the Kusa-Kopan layered intrusive complex, the Lapyshta Complex of dolerites and picrites, and numerous occurrences of gabbrodolerites. The terrigenous rocks of the Vendian stage include conglomerate, gravelstone, and sandstone of the Asha Group, while igneous rocks comprise alkaline volcanics of the Arsha Complex, alkali gabbroids of the Miseli Complex, and melanocratic syenite of the Avashla Complex. The geological evolution of the region is distinguished by local (failed or aborted) rifting. The occurrence of lithotectonic complexes is controlled by dynamic conditions of rifting. A certain inheritance in the evolution may be traced for the Early and Middle Riphean and partly for the Late Riphean and Vendian.     

Crystal-Melt Separation and the Development of Isotopic Heterogeneities in Hybrid Magmas

If a magma is a hybrid of two (or more) isotopically distinctend-members, at least one of which is partially crystalline,separation of melt and crystals after hybridization will leadto the development of isotopic heterogeneities in the magmaas long as some of the pre-existing crystalline material (antecrysts)retains any of its original isotopic composition. This holdstrue whether the hybridization event is magma mixing as traditionallyconstrued, bulk assimilation, or melt assimilation. Once a magma-scaleisotopic heterogeneity is formed by crystal–melt separation,it is essentially permanent, persisting regardless of subsequentcrystallization, mixing, or equilibration events. The magnitudeof the isotopic variability resulting from crystal–meltseparation can be as large as that resulting from differentialcontamination, multiple isotopically distinct sources, or insitu isotopic evolution. In one model, a redistribution of one-thirdof the antecryst cargo yielded a crystal-enriched sample with⁸⁷Sr/⁸⁶Sr of 0·7058, whereas the complementary crystal-poorsample has ⁸⁷Sr/⁸⁶Sr of 0·7068. In other models, crystal-richsamples are enriched in radiogenic Sr. Isotopic heterogeneitiescan be either continuous (controlled by the modal distributionof crystals and melt) or discontinuous (when there is completeseparation of crystals and liquid). The first case may be exemplifiedby some isotopically zoned large-volume rhyolites, formed bythe eruptive inversion of a modally zoned magma chamber. Inthe latter case, the isotopic composition of any (for example)interstitial liquid will be distinct from the isotopic compositionof the bulk crystal fraction. The separation of such an interstitialliquid may explain the presence of isotopically distinct late-stageaplites in plutons. Crystal–melt separation provides anadditional option for the interpretation of isotopically zonedor heterogeneous magmas. This option is particularly attractivefor systems whose chemical variation is otherwise explicableby fractionation-dominated processes. Non-isotopic chemicalheterogeneities can also develop in this fashion. KEY WORDS: isotopic heterogeneity; zoning; hybrid magma; crystal separation; Sr isotopes; aplite; rhyolite     

Comparison of arsenic concentrations in simultaneously-collected groundwater and aquifer particles from Bangladesh,India, Vietnam,and Nepal

One of the reasons the processes resulting in As release to groundwater in southern Asia remain poorly understood is the high degree of spatial variability of physical and chemical properties in shallow aquifers. In an attempt to overcome this difficulty, a simple device that collects groundwater and sediment as a slurry from precisely the same interval was developed in Bangladesh. Recently published results from Bangladesh and India relying on the needle-sampler are augmented here with new data from 37 intervals of grey aquifer material of likely Holocene age in Vietnam and Nepal. A total of 145 samples of filtered groundwater ranging in depth from 3 to 36 m that were analyzed for As (1–1000 μg/L), Fe (0.01–40 mg/L), Mn (0.2–4 mg/L) and S (0.04–14 mg/L) are compared. The P-extractable (0.01–36 mg/kg) and HCl-extractable As (0.04–36 mg/kg) content of the particulate phase was determined in the same suite of samples, in addition to Fe(II)/Fe ratios (0.2–1.0) in the acid-leachable fraction of the particulate phase. Needle-sampler data from Bangladesh indicated a relationship between dissolved As in groundwater and P-extractable As in the particulate phase that was interpreted as an indication of adsorptive equilibrium, under sufficiently reducing conditions, across 3 orders of magnitude in concentrations according to a distribution coefficient of 4 mL/g. The more recent observations from India, Vietnam and Nepal show groundwater As concentrations that are often an order of magnitude lower at a given level of P-extractable As compared to Bangladesh, even if only the subset of particularly reducing intervals characterized by leachable Fe(II)/Fe >0.5 and dissolved Fe >0.2 mg/L are considered. Without attempting to explain why As appears to be particularly mobile in reducing aquifers of Bangladesh compared to the other regions, the consequences of increasing the distribution coefficient for As between the particulate and dissolved phase to 40 mL/g for the flushing of shallow aquifers of their initial As content are explored.