Compositional zoning of calcite in a high-pressure metamorphic calc-schist: clues to heterogeneous grain-scale fluid distribution during exhumation

Calcite in former aragonite–dolomite-bearing calc-schists from the ultrahigh-pressure metamorphic (UHPM) oceanic complex at Lago di Cignana, Valtournanche, Italy, preserved different kinds of zoning patterns at calcite grain and phase boundaries. These patterns are interpreted in terms of lattice diffusion and interfacial mass transport linked with a heterogeneous distribution of fluid and its response to a changing state of stress. The succession of events that occurred during exhumation is as follows: As the rocks entered the calcite stability field at T=530–550 °C, P ca. 1.2 GPa, aragonite occurring in the matrix and as inclusions in poikilitic garnet was completely transformed to calcite. Combined evidence from microstructures and digital element distribution maps (Mn-, Mg-, Fe- and Ca–Kα radiation intensity patterns) indicates that transformation rates have been much higher than rates of compositional equilibration of calcite (involving resorption of dolomite and grain boundary transport of Mg, Fe and Ca). This rendered the phase transformation an isochemical process. During subsequent cooling to T ca. 490 °C (where lattice diffusion effectively closed), grains of matrix calcite have developed diffusion-zoned rims, a few hundred micrometres thick, with Mg and Fe increasing and Ca decreasing towards the phase boundary. Composition profiles across concentrically zoned, large grains in geometrically simple surroundings can be successfully modelled with an error function describing diffusion into a semi-infinite medium from a source of constant composition. The diffusion rims in matrix calcite are continuous with quartz, phengite, paragonite and dolomite in the matrix. This points to an effective mass transport on phase boundaries over a distance of several hundred micrometres, if matrix dolomite has supplied the Mg and Fe needed for incorporation in calcite. In contrast, diffusion rims are lacking at calcite–calcite and most calcite–garnet boundaries, implying that only very minor mass transport has occurred on these interfaces over the same T–t interval. From available grain boundary diffusion data and experimentally determined fluid–solid grain boundary structures, inferred large differences in transport rates can be best explained by the discontinuous distribution of aqueous fluid along grain/phase boundaries. Observed patterns of diffusion zoning indicate that fluid was distributed not only along grain-edge channels, but spread out along most calcite–white mica and calcite–quartz two-grain junctions. On the other hand, the inferred non-wetting of calcite grain boundaries in carbonate-rich domains is compatible with fluid–calcite–calcite dihedral angles >60° determined by Holness and Graham (1995) for a wide range of fluid compositions under the P–T conditions of interest. Whereas differential stress has been very low at the stage of diffusion zoning (T > 490 °C), it increased as the rocks were cooling below 440 °C (at 0.3–0.5 GPa). Dislocation creep and the concomitant increase of strain energy in matrix calcite induced migration recrystallisation of high-angle grain boundaries. For that stage, the compositional microstructure of recrystallised calcite grain boundary domains indicates significant mass transport along calcite two-grain junctions, which at the established low temperatures is likely to have been accomplished by ionic diffusion within a hydrous grain boundary fluid film (“dynamic wetting” of migrating grain boundaries). Received: 10 January 2000 / Accepted: 10 April 2000     

Pressure–volume–temperature equations of state: a comparative study based on numerical simulations

 The P–V–T equation of state (EoS) models of Birch–Murnaghan, Vinet and Poirier–Tarantola have been compared with one another and discussed in the light of their ability to reproduce thermoelastic functions and parameters by means of fitting to pressure–volume–temperature data artificially generated for spinel, corundum and forsterite. Numerical simulations relying upon semi-empirical potentials, lattice dynamics and the quasiharmonic approximation have been used to generate P–V–T data. The results obtained indicate that all the P–V–T EoSs tested predict bulk modulus at ambient conditions with errors confined, at worst, within a few percent, and reproduce correctly its dependence on temperature. The derivatives of the bulk modulus versus P and PT are less satisfactorily modelled. The bulk thermal expansion is determined by EoSs within a few percent error, but the deviations increase significantly if the approximation of linear dependence of EoS on temperature is used (linearised thermal pressure model). Received: 30 January 2001 / Accepted: 16 June 2001     

Spreading change of Africa–South America plate: insights from space geodetic observations

The results derived from geological data show that the half-spreading rate between the African and South American plates has remained relatively constant at 2 cm/year over the past 80 million years (Silver et al. in Science 279:60–63, 1998). In this paper, we have reestimated a new relative angular velocity of Africa–South America plates using the selected space geodetic station data through a new method. Our angular velocity estimates the spreading rates of Africa–South America plate over several years that are similar in azimuth but significantly slower in rate than the NUVEL-1A predictions averaged over the past 3 million years. The implied rates of deceleration coincide with longer-term trends over the past 35 million years and may reflect the effects of plate interaction and coupling of Africa–South America plates. An erratum to this article can be found at     

On the nature of the excess heat capacity of mixing

The excess vibrational entropy (ΔS _vib^ex) of several silicate solid solutions are found to be linearly correlated with the differences in end-member volumes (ΔV _i ) and end-member bulk moduli (Δκ _i ). If a substitution produces both, larger and elastically stiffer polyhedra, then the substituted ion will find itself in a strong enlarged structure. The frequency of its vibration is decreased because of the increase in bond lengths. Lowering of frequencies produces larger heat capacities, which give rise to positive excess vibrational entropies. If a substitution produces larger but elastically softer polyhedra, then increase and decrease of mean bond lengths may be similar in magnitude and their effect on the vibrational entropy tends to be compensated. The empirical relationship between ΔS _vib^ex, ΔV _i and Δκ _i , as described by ΔS _vib^ex = (ΔV _i  + mΔκ _i )f, was calibrated on six silicate solid solutions (analbite–sanidine, pyrope–grossular, forsterite–fayalite, analbite–anorthite, anorthite–sanidine, CaTs–diopside) yielding m = 0.0246 and f = 2.926. It allows the prediction of ΔS _vib^ex behaviour of a solid solution based on its volume and bulk moduli end-member data.     

Compositions and sources of extractable organic matter in Mesopotamian marshland surface sediments of Iraq. I: aliphatic lipids

Shallow surface sediment samples from the Mesopotamian marshlands of Iraq were collected and analyzed to determine the distribution, concentrations and sources of aliphatic lipid compounds (n-alkanes, n-alkanols, n-alkanoic acids, and methyl n-alkanoates) and molecular markers of petroleum in these wetlands. The sediments were collected using a stainless steel sediment corer, dried, extracted with a dichloromethane/methanol mixture and then analyzed by gas chromatography-mass spectrometry (GC–MS). The aliphatic lipid compounds included n-alkanes, n-alkanoic acids, n-alkanols and methyl n-alkanoates with concentrations ranged from 6.8 to 31.1 μg/g, 4.1 to 5.0 μg/g, 5.9 to 7.7 μg/g and from 0.3 to 5.9 μg/g, respectively. The major sources of aliphatic lipids were natural from waxes of higher plants (24–30%) and microbial residues (42–30%), with a significant contribution from anthropogenic sources (27–30%, petroleum), based on the organic geochemical parameters and indices. Further studies are needed to characterize the rate, accumulation and transformation of various organic matter sources before and after re-flooding of these wetlands.     

The importance of physical and biogenic structure to juvenile fishes on the shallow inner continental shelf

Fish-habitat relationships on the shallow inner continental shelf were quantified with video sled and metered beam trawl on Fenwick and Weaver shoals offshore of Maryland and Delaware, U.S. These areas provide megascale physical relief and habitat complexity, but for juvenile fishes, mesoscale and microscale habitat is very important particularly as refuge from predation. At these smaller scales, much of the relief on the inner continental shelf is contributed by bedforms or sand waves and biogenic structures such as tubes, shell beds, or pits. A quantitative association for juvenile fishes between and within benthic habitats was found and related primarily to bedform size and amount of biogenic structure. The incidence of fishes was about four-times higher for large bedforms (> 30 cm wavelength and about 10 cm crest height) relative to smaller bedforms (<30 cm wavelength and about 5 cm crest height). For biogenic structure, going from high patch-mat tube densities to lower densities or no biogenic structure increased fish incidence by 5.4 and 3.3 times, respectively. The significant relationships of fishes with bedform size and density of biogenic structure indicated that seemingly small differences in physical structure of a habitat can make the difference between unacceptable and essential habitat for juvenile fishes. Proximity of complex and simple habitats was important in the diel use of habitat and in balancing pressure of refuge from predation provided by complex habitats with foraging for increased resources available in simpler habitats. During the day, spatially complex habitats comprised ofDiopatra andAsabellides tube mats had about twice as many fishes relative to bare sandy habitats (8.3–9.9 versus 4.0–4.1 fishes 100 m⁻², respectively). At night, the pattern was reversed with more fishes present in the bare sandy habitats (12.4–13.5 versus 5.6–8.7 fishes 100 m⁻²). Some fish, such asAmmodytes spp., were very habitat specific and occurred only on dynamic coarser sands near the top of the shoals. Others, such asUrophycis regia, showed less habitat preference and occurred in all habitats during both day and night. Combining the effects of physical relief and biogenics, the habitat with the highest incidence of fishes had large bedforms with some biogenic structure. More emphasis needs to be placed on quantifying the relationship between fishes and their habitats for the fisheries management concept of essential fish habitat to develop into an effective tool on the inner continental shelf. The juvenile life history stages need to be emphasized because fish-habitat interactions are the strongest for these stages and may be the most ecologically important.     

Characterization of non-equilibrium and equilibrium occurrences of paragonite/muscovite intergrowths in an eclogite from the Sesia–Lanzo Zone (Western Alps, Italy)

 Coexisting muscovite and paragonite have been observed in an eclogite from the Sesia–Lanzo Zone (Western Alps, Italy). The P-T conditions of this eclogite reached 570–650 °C and 19–21 kbar and the rocks show several stages of mineral growth during their retrograde path, ranging from the subsequent lower-P eclogite facies to the blueschist facies and then the greenschist facies. Muscovite and paragonite are very common in these rocks and show two texturally different occurrences indicating equilibrium and non-equilibrium states between them. In one mode of occurrence they coexist in equilibrium in the lower-P eclogite facies. In the same rock muscovite ± albite also replaced paragonite during a greenschist-facies overprint, as evidenced by unique across – (001) layer boundaries. The chemical compositions of the lower-P eclogite-facies micas plot astride the muscovite – paragonite solvus, whereas the compositions of the greenschist-facies micas lie outside the solvus and indicate disequilibrium. The TEM observations of the textural relations of the greenschist-facies micas imply structural coherency between paragonite and muscovite along the layers, but there is a sharp discontinuity in the composition of the octahedral and tetrahedral sheets across the phase boundary. We propose that muscovite formed through a dissolution and recrystallization process, since no gradual variations toward the muscovite – paragonite interfaces occur and no intermediate, homogeneous Na-K phase has been observed. Because a solid-state diffusion mechanism is highly unlikely at these low temperatures (300–500 °C), especially with respect to octahedral and tetrahedral sites, it is assumed that H₂O plays an important role in this process. The across-layer boundaries are inferred to be characteristic of such non-equilibrium replacement processes. The characterization of these intergrowths is crucial to avoiding erroneous assumptions regarding composition and therefore about the state of equilibrium between both micas, which in turn may lead to misinterpretations of thermometric results. Received: 3 February 1999 / Accepted: 19 October 1999     

C2/c pyroxene phenocrysts from three potassic series in the Neogene alkaline volcanics, NE Turkey: their crystal chemistry with petrogenetic significance as an indicator of P–T conditions

Chemical and structural data are reported for C2/c pyroxene phenocrysts collected from three potassic series (Group A: basanite-tephrite, Group B: tephrite-phonolitic tephrite, Group C: alkaline basalt-trachybasalt) of the Neogene alkaline volcanics (NAVs) in northeastern Turkey, in order to investigate the evolution of the magmatic plumbing system and the location of magma chamber(s) with crystallization conditions. The rock series hosting the clinopyroxene phenocrysts show generally porphyritic texture and have a variable phenocryst-rich nature (20–58%), with phenocryst assemblages characterized by cpx ± ol ± plag ± foid ± amp ± bio. The clinopyroxene phenocrysts can be chemically classified as Ti- and Fe³⁺-rich Al-diopsides for Groups A and B (AB-cpxs) and Ti- and Fe³⁺-poor Al-diopsides for Group C (C-cpxs). They have poorly variable composition, clustering in the diopside field. Structurally, the diopside groups have nearly similar a (ranging from 9.73 to 9.75 ?), V _cell (437.2–440.9 ?³), and 〈beta〉 angle values (106.01°–106.23°), but some differences in polyhedral parameters and geometries of the AB-cpxs and C-cpxs have been observed. For example, the AB-cpxs are characterized by larger c (5.27–5.30 vs. 5.25–5.28 ?), V _T (2.27–2.30 vs. 2.23–2.28 ?³), and V _M2 (25.53–25.72 vs. 25.41–25.59 ?³) values and smaller b (8.87–8.88 vs. 8.88–8.91 ?) and V _M1 (11.49–11.63 vs. 11.64–11.83 ?³) values with respect to the C-cpxs. In addition, the AB-cpxs show higher values of V _M2/V _M1 (2.20–2.23) due to large V _M2 and small V _M1 compared to the V _M2/V _M1 ratios of the C-cpxs (<2.19). Such differences in the crystal structure of the AB-cpxs and C-cpxs from the NAVs are partly related to different crystallization pressures, but mostly related to variation in melt composition and, possibly, the influence of other crystallizing mineral phases. In particular, R(M2-O1) and R(M1-O2) (i.e. bond lengths) differences in the clinopyroxenes of different groups support the presence of evolved host rocks with different alkaline character (i.e. silica-undersaturated Groups A–B and silica-saturated Group C). Based on the cpx-geothermobarometry, the crystallization pressures for the C-cpxs are lower than 4.5 kbars, but the AB-cpxs have relatively high-pressure values (5.6–10.6 kbars), suggesting that the AB-cpxs crystallized in higher pressure environments. The relatively higher crystallization temperatures of the AB-cpxs also indicate higher cooling rates. The P–T estimates suggest that the source regions of the clinopyroxene phenocrysts from the NAVs were crustal magma chambers in a closed plumbing system at a moderate- to low-pressure regime.     

Aluminous websterite and granulite xenoliths from the Chyulu Hills volcanic field, Kenya: gabbro–troctolitic cumulates subjected to lithospheric foundering

Whole rock major and trace element abundances in aluminous garnet–spinel websterite, sapphirine-bearing Mg–Al granulite and hibonite-bearing Ca–Al granulite xenoliths from the Chyulu Hills volcanic field, Kenya, suggest that the samples represent a meta-igneous suite linked by fractionation. The incompatible major element contents increase from the websterites to the Mg–Al granulites and further to the Ca–Al granulites. High bulk rock Mg#s and very low concentrations of most incompatible trace elements indicate that the rocks are cumulates rather than crystallized melts. Elevated Ni abundances, impoverishment in Cr and HFSE and high contents of normative plagioclase and olivine in the granulites indicate that their protoliths were similar to troctolite. The textures and metamorphic reaction paths recorded in the granulites suggest igneous emplacement in the crust and cooling from igneous to ambient crustal temperatures accompanied or followed by compression. For the websterite xenoliths, there is an apparent contradiction between the results of P–T calculations that suggest high P and T of crystallization of early generation pyroxenes and elevated P–T conditions during final equilibration (1.4–2.2 GPa/740–980°C) on the one hand and the positive Eu anomaly that suggests shallow-level plagioclase accumulation on the other hand. This contradiction can be reconciled by a model of compression of a plagioclase-bearing (gabbroic) protolith to mantle depths where it recrystallized to an ultramafic assemblage, which requires foundering of dense lower crustal material into the mantle.     

A new finding of Cu-Au alloy in association with rodingite minerals in the Kaa-Khem ophiolitic belt,Tuva

A new Cu-Au alloy occurrence is located at the southeastern flank of the Malye Kopty massif of ultramafic rocks in the Vendian-Early Cambrian Kaa-Khem ophiolitic belt. Lithic clasts with Cu-Au alloy segregations (up to 15 mm in size) intergrown with other minerals were found in alluvium of the Kara-Oss Creek valley, which extends along the fault zone crosscutting ultramafic rocks. Cu-Au alloy occupies the main volume of clasts and fills the network of veinlets in grained aggregates consisting of andradite (2–18% grossular component) and diopside (X _Fe = 0.01–0.05). Cu-Au alloy contains small ingrowths of andradite (up to 43% grossular component), diopside (X _Fe = 0.14–0.19), chlorite (penninite), chalcocite that contains up to 1.5 wt % Au, Cu-bearing greenockite (6.07–13.67 wt % Cu, 0.48–1.56 wt % Zn, and 0.76–1.06 wt % Au), and magnetite. The chemical composition of Cu-Au alloy is nonuniform. The central parts of large Cu-Au alloy segregations consist of Ag-bearing tetraauricupride (AuCu) blocks (3.2–6.4 wt % Ag). They contain veinlet-shaped AuCu zones with 13.3–14.5 wt % Ag. The AuCu blocks are cemented by late Cu-Au alloy, whose composition is close to auricupride (AuCu₃). Taking into account the limits of component miscibility in the Au-Ag-Cu system, the temperature of the Cu-Au alloy formation was estimated at 350–600°C. This temperature corresponds to the formation conditions of garnet-pyroxene rodingite mineral assemblage (Plyusnina et al., 1993). The studied Cu-Au alloy samples from the Malye Kopty massif are very similar to Cu-Au alloy minerals hosted in the Alpine-type ultramafic rocks of the Karabash massif in the southern Urals. This similarity is confirmed by identical chemical compositions of pyroxene, garnet, and chlorite, and similar PT conditions of their formation. The data show that primary ore mineralization of gold-rodingite type occurs in the Kaa-Khem ophiolitic belt.     

Assessment of natural attenuation of aromatic hydrocarbons in groundwater near a former manufactured-gas plant, South Carolina, USA

 Shallow, anaerobic groundwater near a former manufactured-gas plant (MGP) in Charleston, South Carolina, USA, contains mono- and polycyclic aromatic hydrocarbons (MAHs and PAHs, respectively). Between 1994 and 1997, a combination of field, laboratory, and numerical-flow and transport-model investigations were made to assess natural attenuation processes affecting MAH and PAH distributions. This assessment included determination of adsorption coefficients (K _ad ) and first-order biodegradation rate constants (K _bio ) using aquifer material from the MGP site and adjacent properties. Naphthalene adsorption (K _ad =1.35×10^–7 m³/mg) to aquifer sediments was higher than toluene adsorption (K _ad =9.34×10^–10 m³/mg), suggesting preferential toluene transport relative to naphthalene. However, toluene and benzene distributions measured in January 1994 were smaller than the naphthalene distribution. This scenario can be explained, in part, by the differences between biodegradation rates of the compounds. Aerobic first-order rate constants of ¹⁴C-toluene, ¹⁴C-benzene, and ¹⁴C-naphthalene degradation were similar (–0.84, –0.03, and 0.88 day^–1, respectively), but anaerobic rate constants were higher for toluene and benzene (–0.002 and –0.00014 day^–1, respectively) than for naphthalene (–0.000046 day^–1). Both areal and cross-sectional numerical simulations were used to test the hypothesis suggested by these rate differences that MAH compounds will be contained relative to PAHs. Predictive simulations indicated that the distributions of toluene and benzene reach steady-state conditions before groundwater flow lines discharge to an adjacent surface-water body, but do discharge low concentrations of naphthalene. Numerical predictions were "audited" by measuring concentrations of naphthalene, toluene, and benzene at the site in early 1997. Measured naphthalene and toluene concentrations were substantially reduced and the areal extent of contamination smaller than was both observed in January 1994 and predicted for 1997. Measured 1997 benzene concentrations and distribution were shown to be relatively unchanged from those measured in 1994, and similar to predictions for 1997. Received: 26 June 1997 · Accepted: 25 August 1997     

Decoding near-concordant U–Pb zircon ages spanning several hundred million years: recrystallisation,metamictisation or diffusion?

In situ isotopic (U–Pb, Lu–Hf) and trace element analyses of zircon populations in six samples of the intrusive Mawson Charnockite, east Antarctica, emphasise complex zircon behaviour during very high-grade metamorphism. The combination of geochemical data sets is used to distinguish xenocrysts and identify a population of primary igneous zircon in situations where U–Pb data spread close to concordia over a few hundred Myr. The population is filtered to exclude grains with: (1) U–Pb ages >2% discordant, (2) anomalous trace element-content (Th, U, Y, REE) and (3) outlying Hf-isotopic values. Rare metamorphic-type grains were also excluded. Upon filtering the population, minimum emplacement ages for each sample were determined using the oldest grain(s). This approach improves upon age determinations in complex data sets that use weighted mean or isochron methods. Our results suggest that the Mawson Charnockite was emplaced episodically at c. 1145–1140 Ma, c. 1080–1050 Ma and c. 985–960 Ma. Core-outer core-rim and core-rim textures were identified but are not correlated with U–Pb ages. We establish that recrystallisation (mainly of zircon rims) must have occurred shortly following igneous crystallisation and that metamictisation/cracking is a Paleozoic to Recent event. Therefore, intra-zircon diffusion in a high-T, high-strain environment during Meso-Neoproterozoic orogenesis is inferred to have caused the extensive U–Pb isotopic disturbance. Charnockitic magmatism prior to c. 1,000 Ma has not previously been recorded in the Mawson region and indicates that orogenesis may have commenced c. 150 Myr earlier than previously thought. Correlations with similar aged rocks in adjacent regions have implications for supercontinent reconstructions.     

Dauphiné twinning and texture memory in polycrystalline quartz. Part 3: texture memory during phase transformation

Samples of quartz-bearing rocks were heated above the α (trigonal)–β (hexagonal) phase transformation of quartz (625–950°C) to explore changes in preferred orientation patterns. Textures were measured both in situ and ex situ with neutron, synchrotron X-ray and electron backscatter diffraction. The trigonal–hexagonal phase transformation does not change the orientation of c- and a-axes, but positive and negative rhombs become equal in the hexagonal β-phase. In naturally deformed quartzites measured by neutron diffraction a perfect texture memory was observed, i.e. crystals returned to the same trigonal orientation they started from, with no evidence of twin boundaries. Samples measured by electron back-scattered diffraction on surfaces show considerable twinning and memory loss after the phase transformation. In experimentally deformed quartz rocks, where twinning was induced mechanically before heating, the orientation memory is lost. A mechanical model can explain the memory loss but so far it does not account for the persistence of the memory in quartzites. Stresses imposed by neighboring grains remain a likely cause of texture memory in this mineral with a very high elastic anisotropy. If stresses are imposed experimentally the internal stresses are released during the phase transformation and the material returns to its original state prior to deformation. Similarly, on surfaces there are no tractions and thus texture memory is partially lost.     

Trace element distribution in Central Dabie eclogites

Coesite-bearing eclogites from Dabieshan (central China) have been studied by ion microprobe to provide information on trace element distributions in meta-basaltic mineral assemblages during high-pressure metamorphism. The primary mineralogy (eclogite facies) appears to have been garnet and omphacite, usually with coesite, phengite and dolomite, together with high-alumina titanite or rutile, or both titanite and rutile; kyanite also occurs occasionally as an apparently primary phase. It is probable that there was some development of quartz, epidote and apatite whilst the rock remained in the eclogite facies. A later amphibolite facies overprint led to partial replacement of some minerals and particularly symplectitic development after omphacite. They vary from very fine-grained dusty-looking to coarser grained Am + Di + Pl symplectites. The eclogite facies minerals show consistent trace element compositions and partition coefficients indicative of mutual equilibrium. Titanite, epidote and apatite all show high concentrations of REE relative to clinopyroxene. The compositions of secondary (amphibolite facies) minerals are clearly controlled by local rather than whole-rock equilibrium, with the composition of amphibole in particular depending on whether it is replacing clinopyroxene or garnet. REE partition coefficients for Cpx/Grt show a dependence on the Ca content of the host phases, with D _REE ^Cpx/Grt decreasing with decreasing D _Ca . This behaviour is very similar to that seen in mantle eclogites, despite differences in estimated temperatures of formation of 650–850 °C (Dabieshan) and 1000–1200 °C (mantle eclogites). With the exception of HREE in garnet, trace elements in the eclogites are strongly distributed in favour of minor or accessory phases. In particular, titanite and rutile strongly concentrate Nb and Zr, whilst LREE–MREE go largely into epidote, titanite and apatite. If these minor/accessory minerals behave in a refractory manner during melting or fluid mobilisation events and do not contribute to the melt/fluid, then the resultant melts and fluids will be strongly depleted in LREE–MREE. Received: 11 February 1999 / Accepted: 31 January 2000     

On the Ratios between Elastic Modulus and Uniaxial Compressive Strength of Heterogeneous Carbonate Rocks

The ratios M _R = E/σ _c for 11 heterogeneous carbonate (dolomites, limestones and chalks) rock formations collected from different regions of Israel were examined. Sixty-eight uniaxial compressive tests were conducted on weak-to-strong (5 MPa < σ _c < 100 MPa) and very strong (σ _c > 100 MPa) rock samples exhibiting wide ranges of elastic modulus (E = 6100–82300 MPa), uniaxial compressive strength (σ _c = 14–273.9 MPa), Poisson's ratio (ν = 0.13–0.49), and dry bulk density (ρ = 1.7–2.7 g/cm³). The observed range of M _R = 60.9–1011.4 and mean value of M _R = 380.5 are compared with the results obtained by Deere (Rock mechanics in engineering practice, Wiley, London, pp 1–20, 1968) for limestones and dolomites, and the statistical analysis of M _R distribution is performed. Mutual relations between E, σ _c, ρ, M _R for all studied rocks, and separately for concrete rock formations are revealed. Linear multiple correlations between E on the one hand and σ _c and ρ on the other for Nekorot and Bina limestone and Aminadav dolomite are obtained. It is established that the elastic modulus and M _R in very strong carbonate samples are more correlated with ρ−σ _c combination and ε _a max, respectively, than in weak to strong samples. The relation between M _R and maximum axial strain (ε _a max) for all studied rock samples (weak-to-strong and very strong) is discussed.     

A Long-Term Study of Spawning Activity in a Florida Gulf Coast Population of Horseshoe Crabs (<Emphasis Type="BoldItalic">Limulus polyphemus</Emphasis>)

Populations of the American horseshoe crab (Limulus polyphemus) differ in broad areas of their biology. We observed a non-harvested, marked Florida Gulf coast population during their spring spawning (March–May) in 11 years across a 17-year period (1992–2009). Long-term changes occurred in the number of spawning pairs: the population was stable from 1992 to 2000 but increased markedly after 2000. Short-term variation in numbers of spawning pairs, unpaired females, unpaired males, and operational sex ratios was explained by changes over the season and during each week of spring tides and by differences in actual (not predicted) maximum high tide height. Wind direction strongly affected tidal inundation and the number of spawning horseshoe crabs. Tagging individuals revealed that females returned to the nesting beach less often than males and most females were re-sighted only within 1 week of spring tides. No animals were seen across more than 6 years. Implications for management are discussed.     

Modelling phase–assemblage diagrams for magnesian metapelites in the system K2O–FeO–MgO–Al2O3–SiO2–H2O: geodynamic consequences for the Monte Rosa nappe, Western Alps

Magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain P–T conditions in terranes where metamorphism is not easily visible. Phase–assemblage diagrams for natural and model magnesian metapelites in the system KFMASH are presented to document how phase relationships respond to water activity, bulk composition, pressure and temperature. The phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites. It is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a(H₂O). Specifically, the appearance of the characteristic HP assemblage chloritoid–talc–phengite–quartz (with excess H₂O) in the magnesian metapelites of the Monte Rosa nappe (Western Alps) is due to the reduction of a(H₂O). Furthermore, the mineral assemblages are determined by the whole-rock FeO/(FeO+MgO) ratio and effective Al content X _A as well as P and T. The predicted mineral associations for the low- and high-X _A model bulk compositions of magnesian metapelites at high pressure are not dependent on the X _A variations as they show a similar sequence of mineral assemblages. Above 20 kbar, the prograde sequence of assemblages associated with phengite (with excess SiO₂ and H₂O) for low- and high-X _A bulk compositions of magnesian metapelites is: carpholite–chlorite → chlorite–chloritoid → chloritoid–talc → chloritoid–talc–kyanite → talc–garnet–kyanite → garnet–kyanite ± biotite. At low to medium P–T conditions, a low-X _A stabilises the phengite-bearing assemblages associated with chlorite, chlorite + K-feldspar and chlorite + biotite while a high-X _A results in the chlorite–phengite bearing assemblages associated with pyrophyllite, andalusite, kyanite and carpholite. A high-X _A magnesian metapelite with nearly iron-free content stabilises the talc–kyanite–phengite assemblage at moderate to high P–T conditions. Taking into account the effective bulk composition and a(H₂O) involved in the metamorphic history, the phase–assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system KFMASH and therefore may contribute to gaining insights into the metamorphic evolution of terranes. As an example, the magnesian metapelites of the Monte Rosa nappe have been investigated, and an exhumation path with P–T conditions for the western roof of the Monte Rosa nappe has been derived for the first time. The exhumation shows first a near-isothermal decompression from the Alpine eclogite peak conditions around 24 kbar and 505°C down to approximately 8 kbar and 475°C followed by a second decompression with concomitant cooling.M. Frey: deceased     

Link between convection and meridional gradient of sea surface temperature in the Bay of Bengal

We use daily satellite estimates of sea surface temperature (SST) and rainfall during 1998–2005 to show that onset of convection over the central Bay of Bengal (88–92°E, 14–18°N) during the core summer monsoon (mid-May to September) is linked to the meridional gradient of SST in the bay. The SST gradient was computed between two boxes in the northern (88–92°E, 18–22°N) and southern (82–88°E, 4–8°N) bay; the latter is the area of the cold tongue in the bay linked to the Summer Monsoon Current. Convection over central bay followed the SST difference between the northern and southern bay (ΔT) exceeding 0.75°C in 28 cases. There was no instance of ΔT exceeding this threshold without a burst in convection. There were, however, five instances of convection occurring without this SST gradient. Long rainfall events (events lasting more than a week) were associated with an SST event (ΔT ≥ 0.75°C); rainfall events tended to be short when not associated with an SST event. The SST gradient was important for the onset of convection, but not for its persistence: convection often persisted for several days even after the SST gradient weakened. The lag between ΔT exceeding 0.75°C and the onset of convection was 0–18 days, but the lag histogram peaked at one week. In 75% of the 28 cases, convection occurred within a week of ΔT exceeding the threshold of 0.75°C. The northern bay SST, T _N , contributed more to ΔT, but it was a weaker criterion for convection than the SST gradient. A sensitivity analysis showed that the corresponding threshold for T _N was 29°C. We hypothesise that the excess heating (∼1°C above the threshold for deep convection) required in the northern bay to trigger convection is because this excess in SST is what is required to establish the critical SST gradient.     

Crystal chemical significance of chemical zoning in dissakisite-(Ce)

Dissakisites from Trimouns dolomite mine, France, have two kinds of single crystals: chemical-zoned and homogeneous types. Back-scattered electron microprobe (BSE) images of these dissakisites reveal both Ca–Al rich dark zones and Fe-ΣREE rich bright zones. Crystal structures of three dark and two bright zones in a chemical-zoned dissakisite and of a homogeneous zone in unzoned dissakisite were refined to individual R indices (about 3.0–5.0%) based on 1,400 observed [|F ₀| > 4σF ₀] reflections measured with MoKα X-radiation using the single crystal diffractometer. The differences in brightness between their BSE images arise from those in coupled substitutions of the elements occupying A2 and M3 sites. The main reason for these differences is that ten-coordinated A2 polyhedra and M3 octahedra are directly linked through their shared edge, which creates a great potential for making this coupled substitution. This zoning indicates that formation of the whole zoned crystal, where each zone could be grown steadily with its crystallographic axes mutually parallel to each other, may be identified as autoepitaxy.