Prediction of water–rock interaction and porosity evolution in a granitoid-hosted enhanced geothermal system, using constraints from the 5 km Basel-1 well

Numerical simulations based on plans for a deep geothermal system in Basel, Switzerland are used here to understand chemical processes that occur in an initially dry granitoid reservoir during hydraulic stimulation and long-term water circulation to extract heat. An important question regarding the sustainability of such enhanced geothermal systems (EGS), is whether water–rock reactions will eventually lead to clogging of flow paths in the reservoir and thereby reduce or even completely block fluid throughput. A reactive transport model allows the main chemical reactions to be predicted and the resulting evolution of porosity to be tracked over the expected 30-year operational lifetime of the system. The simulations show that injection of surface water to stimulate fracture permeability in the monzogranite reservoir at 190 °C and 5000 m depth induces redox reactions between the oxidised surface water and the reduced wall rock. Although new calcite, chlorite, hematite and other minerals precipitate near the injection well, their volumes are low and more than compensated by those of the dissolving wall-rock minerals. Thus, during stimulation, reduction of injectivity by mineral precipitation is unlikely. During the simulated long-term operation of the system, the main mineral reactions are the hydration and albitization of plagioclase, the alteration of hornblende to an assemblage of smectites and chlorites and of primary K-feldspar to muscovite and microcline. Within a closed-system doublet, the composition of the circulated fluid changes only slightly during its repeated passage through the reservoir, as the wall rock essentially undergoes isochemical recrystallization. Even after 30 years of circulation, the calculations show that porosity is reduced by only ∼0.2%, well below the expected fracture porosity induced by stimulation. This result suggests that permeability reduction owing to water–rock interaction is unlikely to jeopardize the long-term operation of deep, granitoid-hosted EGS systems. A peculiarity at Basel is the presence of anhydrite as fracture coatings at ∼5000 m depth. Simulated exposure of the circulating fluid to anhydrite induces a stronger redox disequilibrium in the reservoir, driving dissolution of ferrous minerals and precipitation of ferric smectites, hematite and pyrite. However, even in this scenario the porosity reduction is at most 0.5%, a value which is unproblematic for sustainable fluid circulation through the reservoir.

Constraints on the thermal evolution of the Adriatic margin during Jurassic continental break-up: U–Pb dating of rutile from the Ivrea–Verbano Zone, Italy

The Ivrea–Verbano Zone (IVZ), northern Italy, exposes an attenuated section through the Permian lower crust that records high-temperature metamorphism under lower crustal conditions and a protracted history of extension and exhumation associated partly with the Jurassic opening of the Alpine Tethys ocean. This study presents SHRIMP U–Pb geochronology of rutile from seven granulite facies metapelites from the base of the IVZ, collected from locations spanning ~35 km along the strike of Paleozoic fabrics. Rutile crystallised during Permian high-temperature metamorphism and anatexis, yet all samples give Jurassic rutile U–Pb ages that record cooling through 650–550 °C. Rutile age distributions are dominated by a peak at ~160 Ma, with a subordinate peak at ~175 Ma. Both ~160 and ~175 Ma age populations show excellent agreement between samples, indicating that the two distinctive cooling stages they record were synchronous on a regional scale. The ~175 Ma population is interpreted to record cooling in the footwall of rift-related faults and shear zones, for which widespread activity in the Lower Jurassic has been documented along the western margin of the Adriatic plate. The ~160 Ma age population postdates the activity of all known rift-related structures within the Adriatic margin, but coincides with extensive gabbroic magmatism and exhumation of sub-continental mantle to the floor of the Alpine Tethys, west of the Ivrea Zone. We propose that this ~160 Ma early post-rift age population records regional cooling following episodic heating of the distal Adriatic margin, likely related to extreme lithospheric thinning and associated advection of the asthenosphere to shallow levels. The partial preservation of the ~175 Ma age cluster suggests that the post-rift (~160 Ma) heating pulse was of short duration. The regional consistency of the data presented here, which is in contrast to many other thermochronometers in the IVZ, demonstrates the value of the rutile U–Pb technique for probing the thermal evolution of high-grade metamorphic terrains. In the IVZ, a significant decoupling between Zr-in-rutile temperatures and U–Pb ages of rutile is observed, with the two systems recording events ~120 Ma apart.

Complete sequencing of the Fugu WAGR region from WT1 to PAX6: Dramatic compaction and conservation of synteny with human chromosome 11p13

The pufferfish Fugu rubripes has a genome ≈7.5 times smaller than that of mammals but with a similar number of genes. Although conserved synteny has been demonstrated between pufferfish and mammals across some regions of the genome, there is some controversy as to what extent Fugu will be a useful model for the human genome, e.g., [Gilley, J., Armes, N. & Fried, M. (1997) Nature (London) 385, 305–306]. We report extensive conservation of synteny between a 1.5-Mb region of human chromosome 11 and <100 kb of the Fugu genome in three overlapping cosmids. Our findings support the idea that the majority of DNA in the region of human chromosome 11p13 is intergenic. Comparative analysis of three unrelated genes with quite different roles, WT1, RCN1, and PAX6, has revealed differences in their structural evolution. Whereas the human WT1 gene can generate 16 protein isoforms via a combination of alternative splicing, RNA editing, and alternative start site usage, our data predict that Fugu WT1 is capable of generating only two isoforms. This raises the question of the extent to which the evolution of WT1 isoforms is related to the evolution of the mammalian genitourinary system. In addition, this region of the Fugu genome shows a much greater overall compaction than usual but with significant noncoding homology observed at the PAX6 locus, implying that comparative genomics has identified regulatory elements associated with this gene.

Chloroplast small heat shock proteins: Evidence for atypical evolution of an organelle-localized protein

Knowledge of the origin and evolution of gene families is critical to our understanding of the evolution of protein function. To gain a detailed understanding of the evolution of the small heat shock proteins (sHSPs) in plants, we have examined the evolutionary history of the chloroplast (CP)-localized sHSPs. Previously, these nuclear-encoded CP proteins had been identified only from angiosperms. This study reveals the presence of the CP sHSPs in a moss, Funaria hygrometrica. Two clones for CP sHSPs were isolated from a F. hygrometrica heat shock cDNA library that represent two distinct CP sHSP genes. Our analysis of the CP sHSPs reveals unexpected evolutionary relationships and patterns of sequence conservation. Phylogenetic analysis of the CP sHSPs with other plant CP sHSPs and eukaryotic, archaeal, and bacterial sHSPs shows that the CP sHSPs are not closely related to the cyanobacterial sHSPs. Thus, they most likely evolved via gene duplication from a nuclear-encoded cytosolic sHSP and not via gene transfer from the CP endosymbiont. Previous sequence analysis had shown that all angiosperm CP sHSPs possess a methionine-rich region in the N-terminal domain. The primary sequence of this region is not highly conserved in the F. hygrometrica CP sHSPs. This lack of sequence conservation indicates that sometime in land plant evolution, after the divergence of mosses from the common ancestor of angiosperms but before the monocot–dicot divergence, there was a change in the selective constraints acting on the CP sHSPs.

Tectono-sedimentary evolution of the ’Numidian Formation’ and Lateral Facies (southern branch of the western Tethys): constraints for central-western Mediterranean geodynamics

The origin of the Numidian Formation (latest Oligocene to middle Miocene), characterized by ultra-mature quartzose arenites with abundant well-rounded frosted quartz grains, remains controversial. This formation, sedimented in the external domain of the Maghrebian Flysch Basin, displays three characteristic stratigraphic members with marked longitudinal (proximal–distal) and transverse (along-chain) variations with palaeogeographical importance. The origin of the Numidian supply is related to the outward tectogenetic propagation when a forebulge evolved in the African foreland, leading to the erosion of African cratonic areas rich in quartzose arenites (Nubian Sandstone-like). The ages of the Numidian Formation checked by Betic, Maghrebian and Southern Apennine data suggest a timing for the accretionary orogenic wedge, earlier in the Betic-Rifian Arc (after middle Burdigalian), later in the Algerian-Tunisian Tell (after late Burdigalian) and afterwards in Sicily and the Southern Apennines (after Langhian). A geodynamic evolutionary model for the central-western Mediterranean is proposed.

Age constraints on the tectonic evolution of the Itremo region in Central Madagascar

The Itremo region in Central Madagascar comprises a deformed metasedimentary sequence (Itremo Group) that has undergone greenschist to lower amphibolite facies metamorphism. During a first phase of deformation (D1) Itremo Group sediments were deformed into a fold-and-thrust belt and transported toward the E to NE on top of migmatitic gneisses rocks of Anatananarivo block. A second phase of deformation (D2) affected both the fold-and-thrust belt and structurally underlying units, and formed large-scale N-S trending folds with steeply dipping axial planes. A Late Neoproterozoic Th–U–Pb XRF monazite age (565±17 Ma) dates the emplacement of a granite that truncates first-phase structures in the Itremo Group, and indicates that the fold-and-thrust belt formed prior to ≈565 Ma. Th–U–Pb electron microprobe dating was applied to elongated monazites that lie within the first-phase foliation of Itremo Group metapelites. The detrital cores of zoned monazites reveal two distinct age populations at ∼2000 and 1700 Ma, the latter age giving a maximum depositional age for the Itremo Group. Statistical analysis of ages determined from the rims of zoned monazites and from unzoned monazites indicates three Late Proterozoic–Early Paleozoic monazite growth events at about 565–540, 500 and 430 Ma. The oldest age population is contemporaneous within error, with the intrusion of the dated granite. The two younger age populations are found both in the Th–U–Pb and Ar–Ar data; together with the perturbation of the Rb–Sr system we interpret both ages as due to alteration related to fluid circulation events, possibly connected to the emplacement of pegmatite fields in Central Madagascar. Syn-D1 tectonic growth of contact metamorphism minerals such as andalusite has been observed locally in metapelites along the margin of Middle Neoproterozoic (≈800 Ma) granites, suggesting that D1 in the Itremo Group is contemporaneous with the intrusion of granites at ≈800 Ma. The N-S trending D2 folds are associated with ≈E-W shortening during the final assembly of Gondwana in Late Neoproterozoic–Early Cambrian times.

Phenotypic divergence along lines of genetic variance

Natural populations inhabiting the same environment often independently evolve the same phenotype. Is this replicated evolution a result of genetic constraints imposed by patterns of genetic covariation? We looked for associations between directions of morphological divergence and the orientation of the genetic variance-covariance matrix (G) by using an experimental system of morphological evolution in two allopatric nonsister species of rainbow fish. Replicate populations of both Melanotaenia eachamensis and Melanotaenia duboulayi have independently adapted to lake versus stream hydrodynamic environments. The major axis of divergence (z) among all eight study populations was closely associated with the direction of greatest genetic variance (g(max)), suggesting directional genetic constraint on evolution. However, the direction of hydrodynamic adaptation was strongly associated with vectors of G describing relatively small proportions of the total genetic variance, and was only weakly associated with g(max). In contrast, divergence between replicate populations within each habitat was approximately proportional to the level of genetic variance, a result consistent with theoretical predictions for neutral phenotypic divergence. Divergence between the two species was also primarily along major eigenvectors of G. Our results therefore suggest that hydrodynamic adaptation in rainbow fish was not directionally constrained by the dominant eigenvector of G. Without partitioning divergence as a consequence of the adaptation of interest (here, hydrodynamic adaptation) from divergence due to other processes, empirical studies are likely to overestimate the potential for the major eigenvectors of G to directionally constrain adaptive evolution.

Inherent size constraints on prokaryote gene networks due to "accelerating" growth

Networks exhibiting accelerating growth have total link numbers growing faster than linearly with network size and either reach a limit or exhibit graduated transitions from nonstationary-to-stationary statistics and from random to scale-free to regular statistics as the network size grows. However, if for any reason the network cannot tolerate such gross structural changes then accelerating networks are constrained to have sizes below some critical value. This is of interest as the regulatory gene networks of single-celled prokaryotes are characterized by an accelerating quadratic growth and are size constrained to be less than about 10,000 genes encoded in DNA sequence of less than about 10 megabases. This paper presents a probabilistic accelerating network model for prokaryotic gene regulation which closely matches observed statistics by employing two classes of network nodes (regulatory and non-regulatory) and directed links whose inbound heads are exponentially distributed over all nodes and whose outbound tails are preferentially attached to regulatory nodes and described by a scale-free distribution. This model explains the observed quadratic growth in regulator number with gene number and predicts an upper prokaryote size limit closely approximating the observed value. (c) 2005 Elsevier GmbH. All rights reserved.

40Ar/39Ar geochronological constraints on the evolution of lateritic iron deposits in the Quadrilatero Ferrifero, Minas Gerais, Brazil

Weathering profiles overlying the Sapecado, Pico and Andaime iron ore deposits, Quadrilátero Ferrífero (QF), Minas Gerais, Brazil, reach depths of 150–400 m and host world-class supergene iron orebodies. In addition to hosting supergene ore bodies of global economic significance, weathered banded iron-formations at the Quadrilátero Ferrífero and elsewhere (e.g., Carajás, Hamersley) are postulated to underlie some of the most ancient continuously exposed weathering profiles on earth. Laser incremental-heating 40Ar/39Ar results for 69 grains of hollandite-group manganese oxides extracted from 23 samples collected at depths ranging from 5 to 150 m at the Sapecado, Pico and Andaime deposits reveal ages ranging from ca. 62 to 14 Ma. Older Mn-oxides occur near the surface, while younger Mn-oxides occur at depth. However, many samples collected at the weathering–bedrock interface yield ages in the 51–41 Ma range, suggesting that the weathering profiles in the Quadrilátero Ferrífero had already reached their present depth in the Paleogene. The antiquity of the weathering profiles in the Quadrilátero Ferrífero is comparable to the antiquity of dated weathering profiles on banded iron-formations in the Carajás Region (Brazil) and the Hamersley Province, Western Australia. The age versus depth distributions obtained in this study, but not available for other regions containing similar supergene iron deposits, suggest that little further advance of the weathering front has occurred in the Quadrilátero Ferrífero lateritic profiles during the Neogene. The results suggest that weathering in some of these ancient landscapes is not controlled by the steady-state advance of weathering fronts through time, but may reflect climatic and geomorphological conditions prevailing in a remote past. The geochronological results also confirm that the ancient landsurfaces in the Quadrilátero Ferrífero probably remained immune to erosion for tens of millions of years. Deep weathering, mostly in the Paleogene, combined with low erosion rates, account for the abundance and widespread distribution of supergene iron, manganese, and aluminum orebodies in this region.

Clay mineralogical and isotopic (K-Ar, delta O-18, delta D) constraints on the evolution of the North Anatolian Fault Zone, Turkey

This study presents the first attempt to constrain the evolution of the North Anatolian Fault Zone (NAFZ) by age dating and isotope tracing of clay minerals formed during near-surface faulting. Extensive illitic clay mineralisation occurred along the NAFZ related to hydrothermal alteration of the fault gouges and pseudotachylytes. Samples representing the pre-fault protoliths outside the fault zone do not contain authigenic illitic clay minerals indicating that hydrothermal processes were confined to the areas within the fault zone. K-Ar age data indicate that the hydrothermal system and the associated illite authigenesis initiated at similar to 57 Ma. This process is interpreted to reflect the onset of significant strike-slip or transtensional faulting immediately after the continental collision related to the closure of the Neotethys Ocean. Following the initiation of the fault movements in the latest Paleocene-Early Eocene, displacements along the NAFZ have continued, with probably intensified fault activities at similar to 26 Ma and later than similar to 8 Ma. Oxygen isotope compositions of the illitic clays from different locations along the NAFZ are similar, with narrow ranges in delta O-18 values indicating clay precipitation from fluids with similar oxygen isotope compositions and crystallisation temperatures. The delta O-18 and delta D values of the calculated fluid isotopic composition (delta O-18=5.9 parts per thousand to 11.2 parts per thousand, delta D=-59 parts per thousand to -73 parts per thousand) are consistent with metamorphic and magmatic origin of fluids mobilised during active tectonism. The interpretation of the fluid flow history of the NAFZ is in agreement with that reported previously for some well-known large-scale high-angle fault zones, which similarly developed along collisional-type orogenic belts and are commonly associated with significant mesothermal ore mineralisation. (c) 2005 Elsevier B.V. All rights reserved.