Origin of spongy textures in clinopyroxene and spinel from mantle xenoliths, Hessian Depression, Germany

Summary Spongy textures are observed in anhydrous Group 1 mantle xenoliths (harzburgite, lherzolite and wehrlite) hosted in Tertiary alkali basaltic lavas from the Hessian Depression, Germany. These textures are developed only on clinopyroxene and spinel, and occur as rims or cross-cutting veinlets and patches showing optical continuity with the host grain. They are often associated with pools of amorphous glassy material. There is no preferential development of spongy domains against the xenolith-lava contact suggesting that the host magma did not play any significant role in their formation. Spongy clinopyroxene and spinel occur in all rock types, but, are more pervasive in wehrlite. Chemically, spongy domains of clinopyroxene and spinel are more refractory than unaffected areas, which is consistent with their formation through a partial melting event. The associated glassy material shows chemical characteristics which suggest that the melt pools are genetically related to the development of the spongy textures. The partial melting event was probably triggered by the infiltration of a low-density fluid. The fluid may have evolved from a silicate melt responsible for the metasomatic Fe-enrichment recorded in wehrlite. In this context, the more pervasive development of spongy clinopyroxene in wehrlite may be explained by a higher concentration of the evolved fluid phase at proximity to its silicate melt source. Received March 15, 2000; revised version accepted September 6, 2001     

Mo isotope composition in Mo-rich high- and low-T hydrothermal systems from the Swiss Alps

We analyzed the molybdenum (Mo) isotope compositions (IC) of 59 samples from two molybdenite mineralizations (Alpjahorn and Grimsel) and from a Mo-rich hydrothermal breccia (Grimsel) from the Aar Massif, Switzerland. The formation temperature of the Late Paleozoic Mo mineralizations (300-600 °C) is much higher than that of the Pliocene breccia (100-160 °C). The Mo IC of the molybdenites varies over 1.35‰. Even in a single hand specimen it spans 0.45‰, indicating that fractionation processes during molybdenite precipitation can vary on a cm scale. The Mo IC of most molybdenites analyzed here are significantly heavier than that of the host rock (δ^98/95Mo = (0.05 ± 0.1)‰) and show a bimodal distribution centered around δ^98/95Mo ≈ 1.1‰ and 0.2‰. This result rules out single stage Rayleigh fractionation as the relevant formation mechanism and instead, redox variations are suggested to be a main factor controlling the Mo IC of the studied high-temperature Mo deposits. The range of the Mo IC in one single deposit, the Alpjahorn, overlaps with the variation range of almost all other published values for Mo IC in Mo deposits. Compared to the molybdenites, the breccia shows an even wider variation of 3.0‰ (δ^98/95Mo between −1.6‰ and +1.4‰). In contrast to the high-T molybdenite deposits, here the Mo was transported via oxidized surface waters into the breccia system, where it was reduced and precipitated. This indicates that oxidation and reduction of Mo complexes may lead to highly variable Mo IC in hydrothermal systems.     

Presumed magnetic biosignatures observed in magnetite derived from abiotic reductive alteration of nanogoethite

The oriented chains of nanoscale Fe-oxide particles produced by magnetotactic bacteria are a striking example of biomineralization. Several distinguishing features of magnetite particles that comprise bacterial magnetosomes have been proposed to collectively constitute a biosignature of magnetotactic bacteria (Thomas-Keprta et al., 2001). These features include high crystallinity, chemical purity, a single-domain magnetic structure, well-defined crystal morphology, and arrangement of particles in chain structures. Here, we show that magnetite derived from the inorganic breakdown of nanocrystalline goethite exhibits magnetic properties and morphologies remarkably similar to those of biogenic magnetite from magnetosomes. During heating in reducing conditions, oriented nanogoethite aggregates undergo dehydroxylation and transform into stoichiometric magnetite. We demonstrate that highly crystalline single-domain magnetite with euhedral grain morphologies produced abiogenically from goethite meets several of the biogenicity criteria commonly used for the identi?cation of magnetofossils. Furthermore, the suboxic conditions necessary for magnetofossil preservation in sediments are conducive to the reductive alteration of nanogoethite, as well as the preservation of detrital magnetite originally formed from goethite. The findings of this study have potential implications for the identification of biogenic magnetite, particularly in older sediments where diagenesis commonly disrupts the chain structure of magnetosomes. Our results indicate that isolated magnetofossils cannot be positively distinguished from inorganic magnetite on the basis of their magnetic properties and morphology, and that intact chain structures remain the only reliable distinguishing feature of fossil magnetosomes.     

Back front of seismicity induced by non‐linear pore pressure diffusion

Borehole fluid injections are accompanied by microseismic activity not only during but also after termination of the fluid injection. Previously, this phenomenon has been analysed, assuming that the main triggering mechanism is governed by a linear pressure diffusion in a hydraulically isotropic medium. In this context the so‐called back front of seismicity has been introduced, which allows to characterize the hydraulic transport from the spatiotemporal distribution of post‐injection induced events. However, rocks are generally anisotropic, and in addition, fluid injections can strongly enhance permeability. In this case, permeability becomes a function of pressure. For such situations, we carry out a comprehensive study about the behaviour and parametrization of the back front. Based on a model of a factorized anisotropic pressure dependence of permeability, we present an approach to reconstruct the principal components of the diffusivity tensor. We apply this approach to real microseismic data and show that the back front characterizes the least hydraulic transport. To investigate the back front of non‐linear pore‐fluid pressure diffusion, we numerically consider a power‐law and an exponential‐dependent diffusivity. To account for a post‐injection enhanced hydraulic state of the rock, we introduce a model of a frozen (i.e., nearly unchanged after the stimulation) medium diffusivity and generate synthetic seismicity. We find that, for a weak non‐linearity and 3D exponential diffusion, the linear diffusion back front is still applicable. This finding is in agreement with microseismic data from Ogachi and Fenton Hill. However, for a strong non‐linear fluid–rock interaction such as hydraulic fracturing, the back front can significantly deviate from a time dependence of a linear diffusion back front. This is demonstrated for a data set from the Horn River Basin. Hence, the behaviour of the back front is a strong indicator of a non‐linear fluid–rock interaction.     

Modeling acid-gas generation from boiling chloride brines

Background  

This study investigates the generation of HCl and other acid gases from boiling calcium chloride dominated waters at atmospheric pressure, primarily using numerical modeling. The main focus of this investigation relates to the long-term geologic disposal of nuclear waste at Yucca Mountain, Nevada, where pore waters around waste-emplacement tunnels are expected to undergo boiling and evaporative concentration as a result of the heat released by spent nuclear fuel. Processes that are modeled include boiling of highly concentrated solutions, gas transport, and gas condensation accompanied by the dissociation of acid gases, causing low-pH condensate.     

Molluskan species richness and endemism on New Caledonian seamounts: Are they enhanced compared to adjacent slopes?

Seamounts were often considered as ‘hotspots of diversity’ and ‘centers of endemism’, but recently this opinion has been challenged. After 25 years of exploration and the work of numerous taxonomists, the Norfolk Ridge (Southwest Pacific) is probably one of the best-studied seamount chains worldwide. However, even in this intensively explored area, the richness and the geographic patterns of diversity are still poorly characterized. Among the benthic organisms, the post-mortem remains of mollusks can supplement live records to comprehensively document geographical distributions. Moreover, the accretionary growth of mollusk shells informs us about the life span of the pelagic larva. To compare diversity and level of endemism between the Norfolk Ridge seamounts and the continental slopes of New Caledonia we used species occurrence data drawn from (i) the taxonomic literature on mollusks and (ii) a raw dataset of mainly undescribed deep-sea species of the hyperdiverse Turridae. Patterns of endemism and species richness were analyzed through quantitative indices of endemism and species richness estimator metrics. To date, 403 gastropods and bivalves species have been recorded on the Norfolk Ridge seamounts. Of these, at least 38 species (∼10%) are potentially endemic to the seamounts and nearly all of 38 species have protoconchs indicating lecithotrophic larval development. Overall, our results suggest that estimates of species richness and endemism, when sampling effort is taken into account, were not significantly different between slopes and seamounts. By including in our analyses 347 undescribed morphospecies from the Norfolk Ridge, our results also demonstrate the influence of taxonomic bias on our estimates of species richness and endemism.     

A Top-Cut Model for Deposits with Heavy-Tailed Grade Distribution

In some ore deposits, the grade distribution is heavy-tailed and high values make the inference of first- and second-order statistics nonrobust. In the case of gold data, high values are usually cut and the block estimation is performed using truncated grades. With this method, a part of the deposit is omitted, resulting in a potential bias on resources figures. Ad-hoc corrections can be applied on the final figures to take into account the tail of the grade distribution, but no theoretical guidelines are available. A geostatistical model has been developed to handle high values based on the assumption that for high grade zones, the only tangible information is the geometry. The grade variable z can be split into three parts: the truncated grade ( $\operatorname{Min} (z, z_{\mathrm{e}})$ where z _e is the top-cut grade), a weighted indicator above top-cut grade (1{z≥z _e}), and a residual. Within this framework, the residual is poorly structured, and in most cases is a pure nugget-effect. Moreover, it has no spatial correlation with the truncated grade and the indicator above top-cut grade. This decomposition makes the variographic study more robust because variables (indicator and truncated grade) do not keep high grade values. The underlying hypotheses can be tested on experimental indicator variograms and the top-cut grade can be justified. Finally, the estimation is based on a truncated grade and indicator cokriging. The model is applied to blast holes from a gold deposit and on a simulated example. Both cases illustrate the benefits of keeping the high values in the estimation process via an appropriate mathematical model.     

Oceanic ridge off-axis deep structure in the Mansah region (Sumail massif,Oman ophiolite)

Large scale structural mapping of the Oman ophiolite indicates that the Mansah area (Sumail massif) was a ridge off-axis region at the time the ophiolite was detached. This paper presents a detailed structural mapping of the region. We show that, as opposed to other off-axis areas, it contains plunging lineations, correlated with a thick Moho transition zone and chromite pods, indicative of a mantle diapir. However this diapir has a discontinuous structure, it is bounded by shear zones and types of diabases that are not found elsewhere in Oman; it also has a broken crust, strongly affected by hydrothermal alteration. This suggests that Mansah fossilized an off-axis diapir intruding a cooling lithosphere. It may be a good candidate to be the root of an off-axis seamount such as those found in the East Pacific, and may bring new insights into this particular volcanism which we are only beginning to explore.     

Cumulating processes at the crust-mantle transition zone inferred from Permian mafic-ultramafic xenoliths (Puy Beaunit,France)

Ultramafic and mafic xenoliths of magmatic origin, sampled in the Beaunit vent (northern French Massif Central), derive from the Permian (257 Ma) Beaunit layered complex (BLC) that was emplaced at the crust-mantle transition zone (∼1 GPa). These plutonic xenoliths are linked to a single fractional crystallisation process in four steps: peridotitic cumulates; websteritic cumulates; Al-rich mafic cumulates (plagioclase, pyroxenes, garnet, amphibole and spinel) and finally low-Al mafic cumulates. This sequence of cumulates can be related to the compositional evolution of hydrous Mg basaltic magma that evolved to high-Al basalt and finally to andesitic basalt. Sr and Nd isotopic compositions confirm the co-genetic character of the various magmatic xenoliths and argue for an enriched upper mantle source comparable to present mantle wedges above subduction zones. LILE, LREE and Pb enrichment are a common feature of all xenoliths and argue for an enriched sub-alkaline transitional parental magma. The existence of a Permian magma chamber at 30 km depth suggests that the low-velocity zone observed locally beneath the Moho probably does not represent an anomalous mantle but rather a sequence of mafic/ultramafic cumulates with densities close to those of mantle rocks.