Groundmass crystallization of Mount St. Helens dacite, 1980–1986: a tool for interpreting shallow magmatic processes

The 1980–1986 eruption of Mount St. Helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale. Groundmass plagioclase size measurements are coupled with measured changes in matrix glass, plagioclase and Fe–Ti oxide chemistry to document increasing groundmass crystallinity, and thus to better constrain proposed physical models of the post-May 18, 1980 magmatic reservoir. Measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth (approximately 10^-9 cm/s) of groundmass plagioclase occurred immediately subsequent to May 18. Relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10^-11 cm/s; plagioclase growth rates then decreased to <1x10^-11 cm/s through 1986. Changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst-rim chemistry, although the matrix glass composition appears to have remained approximately constant from 1981–1986 after a rapid compositional change from May 18 until the end of 1980. Plagioclase microphenocrysts show increasingly more complex zoning patterns with time; microphenocryst-core compositions are commonly positively correlated with crystal size. Both of these observations indicate continuous groundmass plagioclase growth through 1986. Magmatic temperatures estimated from Fe–Ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930°C and at log f_O2 of -10.8; by 1985–1986 oxide temperatures decreased to 870°C. Chemical and textural changes can be explained by: (1) rapid degassing and crystallization in response to the intrusion of magma into a shallow (<4.5 km) reservoir toward the end of the May 18, 1980 eruption; (2) continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir. Growth rates (and consequent chemical changes) appear to decrease at the end of 1980—this is coincident with the change in eruption style from explosive eruptions, sometimes followed by dome growth, to solely extrusive (dome-building) events, and can be explained by the expected viscosity increase of both degassing and increasing crystallinity. The model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies (Casadevall et al. 1983; Gerlach and Casadevall 1986 a, b), patterns of crater deformation (Chadwkck et al. 1988) and post-1980 seismicity (Endo et al. 1990), although it does not explain the experimental data of Hill and Rutherford (1989) on the growth rate of amphibole reaction rims. Textural measurements on Mount St. Helens dacite can also be used to evaluate crystallization kinetics in silicic magmas, systems for which experimental data is almost non-existent. Plagioclase growth rates are 5–10 times slower than estimated plagioclase growth rates in basaltic systems, a result consistent with the higher viscosity of a more silicic melt. Furthermore, patterns of textural change (both average crystal size and number density) are similar to those observed during the 1984 Mauna Loa eruption by Lipman and Banks (1987), suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates, such that the rate of crystallization scales with the melt viscosity.     

Silicic recharge of multiple rhyolite magmas by basaltic intrusion during the 22.6 ka Okareka Eruption Episode, New Zealand

Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete rhyolite magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption ( 0.01 km³ magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing rhyolite magma (T1) with a volume of  0.3 km³. Hybrid basalt/rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other rhyolite magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende rhyolite magma (T2) ( 0.3 km³), and a hotter (780 °C), crystal-poor, pyroxene–hornblende rhyolite magma (T3) ( 4.5 km³). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km³ of T3 magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the rhyolite magmas. The eruption sequence and the restricted direct basaltic intrusion into only one magma (T1) is consistent with the rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary magma storage depths of < 8 km. Each of the magmas display quartz crystals containing melt inclusions that are compositionally highly evolved relative to the accompanying matrix glass, and thus point to a stage of more complete crystallisation. The matrix glass, enriched in Sr and Ti, represents a re-melting event of underlying the crystal pile induced by basaltic intrusion, presumably part of the same event that erupted scoria at the start of the eruption. This recharge rhyolite melt percolated upward and hybridised with the resident melts in each of the three magma pods. The Okareka episode rhyolites contrast with other well-documented rhyolites that are either continuously or discontinuously zoned, or have been homogenised during re-activation to a uniform composition. Rapid basalt dike intrusion to shallow levels appears to have (prematurely?) triggered the Okareka rhyolites into eruption, so that their early ponding in separate melt pods has been recorded before it could be masked by mixing or stratification had amalgamation into a larger body occurred.     

Estimation of underwater eruption energy based on tsunami wave data

Empirical relations between tsunami parameters and underwater eruption energy have been improved, making use of Le Mehaute's theory of explosion-generated water waves. Formulae can be used to estimate underwater eruption characteristics by tsunami wave data. Estimates of energy for some past event have been obtained, in particular, for multiple eruptions of the 1952–1953 Myojinsho volcano, to be E 10¹⁵ – 10¹⁶ J.     

The Granada ignimbrite: A compound pyroclastic unit and its relationship with Upper Miocene caldera volcanism in the northern Puna

The Granada ignimbrite, an Upper Miocene volcanic unit from the northern Puna, previously has been interpreted as an extensive ignimbrite (>2300 km²) associated with eruptions from the Vilama caldera (trap-door event). On the basis of new data, we revise its correlation and redefine the unit as a compound, high aspect ratio ignimbrite, erupted at approximately 9.8 Ma. Calculated volumes (100 km³) are only moderate in comparison with other large volume (>1000 km³) ignimbrites that erupted approximately 2–6 m.y. later in the region (e.g. Vilama, Panizos, Atana). Six new volcanic units are recognized from sequences previously correlated with Granada (only one sourced from the same center). Consequently, the area ascribed to the Granada ignimbrite is substantially reduced (630 km²), and links to the Vilama caldera are not supported. Transport directions suggest the volcanic source for the Granada ignimbrite corresponds to vents buried under younger (7.9–5 Ma) volcanic rocks of the Abra Granada volcanic complex. Episodes of caldera collapse at some stage of eruption are likely, though their nature and timing cannot be defined from available data. The eruption of the Granada ignimbrite marks the onset of a phase of large volume (caldera-sourced) volcanism in the northern Puna.     

Insight into the upper mantle beneath an active extensional zone: the spinel-peridotite xenoliths from San Quintin (Baja California,Mexico)

Many of the peridotite xenoliths included in the San Quintin (Baja California Norte, Mexico) quaternary alkali-basalts have undergone a very intense shear deformation (deviatoric stresses up to 0.1 GPa), hence a first-order classification into coarse-grained lherzolites and deformed peridotites (porphyroclastic and mosaic textures) has been applied. All of these rocks show a very limited compositional variability in the Mg/(Mg+Fe²⁺) ratios (olivine: 0.894–0.905±0.005; orthopyroxene: 0.899–0.9105±0.005), and the observed trends in the Cr/(Cr+Al) spinel ratios (from 0.1 to 0.6) can be interpreted as resulting from gradual partial melting followed by homogenization of the bulk phases. A later and less accentuated melting event is also evidenced by internal core-rim variations in the spinels from a few samples and ascribed to the thermal effect of the host lava.Simultaneous application of exchange geothermometers which give the latest equilibrium temperatures (i.e. at the time of eruption: Fe-Mg exchange between olivine and spinel) and of pyroxene transfer thermobarometers yields two distinct behaviours: the porphyroclastic and mosaic peridotites record an event of deformation and recrystallization and were equilibrated at 800°–950° C and P-1 GPa at the time of eruption, but have also retained evidence of higher temperatures (1000°–1050° C) and pressures; the coarsegrained lherzolites, which yield conditions of 1000°–1050° C and P<-2 GPa at the time of eruption, were originally equilibrated at higher temperature and pressure conditions and were subsequently re-equilibrated to 1000°–1050° C by solid-state bulk diffusion, without exsolution.Clinopyroxenite veins provide evidence of magma injection into the host-peridotite, before deformation but after the major melting event.To explain the simultaneous sampling of both groups of peridotites by the San Quintin alkali basalts, we suggest that the ascending magma reached the critical limit for hydraulic fracturing in the coarse-grained lherzolites. At shallower depth, the magma cross-cut an active shear zone, sampling prophyroclastic and mosaic samples of the strained peridotites.Our model is consistent with the regional tectonic context: upwelling of the mantle by isostatic re-equilibration after the end of the subduction processes and subsequent opening of the California Gulf. The only questionable parameter of the model remains the geometry of the shearzone, high or low angle orientation.     

Neoproterozoic crustal growth: The solid Earth system during a critical episode of Earth history

The behavior of the solid Earth system is often overlooked when the causes of major Neoproteozoic (1000–542 Ma) climate and biosphere events are discussed although  20% of the present continental crust formed or was remobilized during this time. Processes responsible for forming and deforming the continental crust during Neoproterozoic time were similar to those of the modern Earth and took place mostly but not entirely at convergent margin settings. Crustal growth and reworking occurred within the context of a supercontinent cycle, from breakup of Rodinia beginning  830 Ma to formation of a new supercontinent Greater Gondwana or Pannotia,  600 Ma. Neoproterozoic crust formation and deformation was heterogeneous in space and time, and was concentrated in Africa, Eurasia, and South America during the last 300 million years of Neoproterozoic time. In contrast, the solid Earth system was relatively quiescent during the Tonian period (1000–850 Ma). The vigor of Cryogenian and Ediacaran tectonic and magmatic processes and the similar timing of these events and development of Neoproterozoic glaciations and metazoa suggest that climate change and perhaps increasing biological complexity was strongly affected by the solid Earth system.     

Post-caldera dacites from the Santorini volcanic complex,Aegean Sea,Greece: an example of the eruption of lavas of near-constant composition over a 2,200 year period

The post-caldera Kameni islands of the Santorini volcanic complex, Aegean Sea, Greece are entirely volcanic and were formed by eleven eruptions between 197 B.C. and 1950. Petrographic, mineral chemical and whole-rock major and trace element data are presented for samples of lava collected from the products of seven eruptive cycles which span the entire period of activity. The main phenocryst phases are plagioclase, clinopyroxene, orthopyroxene and titaniferous magnetite, which are weakly zoned (e.g. plagioclase — An₅₅ to An₄₂). The lavas are typical calc-alkaline dacites and show a restricted range of composition (from 64.1 to 68.4 wt. % SiO₂). The phenocrysts were in equilibrium with the melts at temperatures of 960–1012 °C, pressures of 800–1500 bars and oxygen fugacities of 10^–9.6-10^–9.9 bars. The pre-eruptive water content of the magmas was 3–4 wt. % but since the lavas contain only 0.1–0.4 wt. % H₂O, a considerable amount (about 0.01–0.015 km³) of water was lost prior to or during eruption. This indicates that the magmas rose to the surface gradually allowing the (largely) non-explosive loss of volatiles. The lavas were probably extruded initially from more or less cylindrical conduits which developed into fissures as the eruptions proceeded. The post-caldera lavas evolved from more mafic parental magmas (basalt-andesite) via fractional crystallization. The small range of compositional variation shown by these lavas can be explained in terms of near-equilibrium crystallization. Analyses of samples of lavas belonging to single eruption cycles and to individual flows indicate that the underlying magma chamber is compositionally zoned. The average composition of erupted magma has remained approximately constant since 1570 A.D. but that fact that the 197 B.C. magma was sligthly richer in SiO₂ provides additional evidence that the magma chamber is compositionally zoned. Crystal settling has not affected the composition of the magma over a 2,200 year period of time which indicates that the melts do not behave as Newtonian fluids. Zonation was thus probably established prior to the 197 B.C. eruption though it is possible that it is developed and maintained by crystal-liquid differentiation processes other than crystal settling (e.g. boundary layer crystallization). The data indicate that there has been no significant cooling during 2,200 years; the maximum amount of cooling is <50 °C and is probably less than 30 °C. Two hypotheses are considered to explain the thermal and chemical buffering of the post-caldera magma chamber: (i) The magma chamber is large and heat losses due to conduction are largely compensated by latent heat supplied by thick, partially crystalline cumulate sequences. (ii) Periodic influx of hot mafic magma, which does not mix with the dacitic magma, inhibits cooling. The second alternative is favored because the post-caldera lavas differ geochemically from the pre-caldera lavas which signifies that a new batch of magma was formed and/or emplaced after the catastrophic eruption of 1390 B.C., and hence that mafic magmas may still be reaching upper crustal levels.     

Crustal velocity inhomogeneities along the Hirapur–Mandla profile, central India and its tectonic implications

Wide-angle seismic and gravity data across the Narmada-Son lineament (NSL) in central India are analyzed to determine crustal structure, velocity inhomogeneities and hence constrain the tectonics of the lineament. We present the 2-D crustal velocity structure from deep wide-angle reflection data by using a ray-trace inverse approach. The main result of the study is the delineation of fault-bounded horst raised to a subsurface depth (1.5 km) and the Moho upwarp beneath the NSL. The crust below the basement consists of three layers with velocities of 6.45–6.7, 6.2–6.5 and 6.7–6.95 km/s and interface depths of about 5.5–8.7, 14–17 and 18–23 km along the profile. The low-velocity (6.2–6.5 km/s) layer goes up to a depth of 5 km and becomes the thickest part (13 km), while the overlying high-velocity (6.45–6.7 km/s) layer becomes the thinnest (3 km) and upper boundary lies at a depth of 1.5 km beneath the NSL. The overall uncertainties of various velocity and boundary nodes are of the order of ±0.12 km/s and ±1.40 km, respectively. The up-lifted crustal block and the up-warping Moho beneath the NSL indicate that the north and south faults bounding the NSL are deeply penetrated through which mafic materials from upper mantle have been intruded into the upper crust. Gravity modeling was also undertaken to assess the seismically derived crustal features and to fill the seismic data gap. The lateral and vertical heterogeneous nature of the structure and velocity inhomogeneities in the crust cause instability to the crustal blocks and played an important role in reactivation of the Narmada south fault during the 1997 Jabalpur earthquake.     

Structural and stratigraphical constraints on the kinematics history of the Southern Tan–Lu Fault Zone during the Mesozoic Anhui Province, China

The Tan–Lu Fault Zone (TLFZ) extends in a NNE–SSW direction for more than 2000 km in Eastern China. It has been considered either as a major sinistral strike-slip fault, as a suture zone or as a normal fault. We have conducted a structural analysis of the southern segment of this fault zone (STLFZ) in the Anhui Province. The ages (Triassic to Palaeocene) of the formations affected by the faults have been re-appraised taking into account recent stratigraphical studies to better constraint the ages of the successive stages of the kinematics of the STLFZ. Subsequently, the kinematics of the faults is presented in terms of strain/stress fields by inversion of the striated fault set data. Finally, the data are discussed in the light of the results obtained by previous workers.We propose the following history of the STLFZ kinematics during the Mesozoic. At the time of collision, a  NNE orientated Tan–Lu margin probably connected two margins located north of the Dabie and Sulu collision belts. During the Middle–Late Triassic, the SCB has been obliquely subducted below the NCB along this margin which has acted as a compressional transfer zone between the Dabie and Sulu continental subduction zones. The STLFZ has been initiated during the Early Jurassic and has acted as a sinistral transform fault during the Jurassic, following which the NCB/SCB collision stopped. A  NW-trending extension related to metamorphic domes was active during the basal Early Cretaceous ( 135–130 Ma); it has been followed by a NW–SE compression and a NE–SW tension during the middle–late Early Cretaceous ( 127 to  105 Ma, possibly  95 Ma); at that time the TLFZ was a sinistral transcurrent fault within the eastern part of the Asian continent. During the Late Cretaceous–Palaeocene, the STLFZ was a normal fault zone under a WNW–ESE tension.     

Sulphur dioxide,particle and elemental emissions from Mount Etna,Italy during July 1987

Sulphur dioxide emission rates were measured at Mount Etna, Italy during July 1987 while the volcano was undergoing relatively quiet activity. The SO₂ flux averaged 930 ± 587 (1) Mg/d, excluding 19 July when the flux was 3200 ± 1730 (1) Mg/d. Rising magma and/or an influx of less degassed magma could explain the increased SO₂ flux. The high SO₂ flux did not correlate with changes in observed volcanic activity. This suggests that SO₂ monitoring may be useful as an indicator of shallow magmatic activity, but not as a predictor of future eruptions of Mount Etna.Particles emitted from the two active craters, Bocca Nuova and Southeast Crater (SE), were composed of silicates, sulphates and dithionites. Chloride species were only observed in particles from SE. Different eruptive styles probably produced the differences in particle compositions emitted from each crater.Vapour — magma enrichment factors were calculated for many elements from both craters. Cl, Br and S were the most enriched elements in the sampled fumes. Similar enrichment factors at both craters suggest a common magma supply.Cl, S and F have the largest elemental fluxes emitted from Mount Etna. During quiet activity, the Cl flux represents 27% of the global anthropogenic emissions, but its effect is limited to the local region due to atmospheric removal processes. Mount Etna also exhaled significant amounts of Zn, Br, Mo, F and Cu compared with regional anthropogenic emissions.     

Anhydrous P-T phase relations of near-primary high-alumina basalt from the South Sandwich Islands

Anhydrous P-T phase relations, including phase compositions and modes, are reported from 10–31 kbar for a near-primary high-alumina basalt from the South Sandwich Islands in the Scotia Arc. The water content of natural subduction-related basalt is probably <0.5 wt.% and thus, these results are relevant to the generation of primary basaltic magmas in subduction zones. At high pressures (>27 kbar) garnet is the liquidus phase followed by clinopyroxene, then quartz/coesite at lower temperatures. At intermediate pressures (17–27 kbar), clinopyroxene is the liquidus phase followed by either garnet, quartz, plagioclase, then orthopyroxene or plagioclase, quartz, garnet, then orthopyroxene depending on the pressure within this interval. At all lower pressures, plagioclase is the liquidus phase followed at much lower temperatures (100° C at 5 kbar) by clinopyroxene. The absence of olivine from the liquidus suggests that the composition studied here could not have been derived from a more mafic parent by olivine fractionation at any pressure investigated, and supports the interpretation that it is primary. If so, these results also preclude an origin for this melt by partial melting of olivine-rich mantle periddotite and suggest instead that it was generated by partial melting of the descending slab (quartz eclogite) leaving clinopyroxene, garnet, or both in the residue. The generally flat REE patterns for low-K series subduction related basalts argue against any significant role for garnet, however, and it is thus concluded that the composition studied here was extracted at 20–27 kbar after sufficiently high degrees of partial melting (50%) to totally consume garnet in the eclogite source. Melting experiments on three MORB composition, although not conclusive, are in agreement with this mechanism. Results at 30 kbar support an origin for tonalite/trondhjemite series rocks by lower degrees of melting (15–30%), leaving both garnet and clinopyroxene in the residue.     

Tectonic controls on the genesis of ignimbrites from the Campanian Volcanic Zone, southern Italy

Summary ¶The Campanian Plain is an 80×30km region of southern Italy, bordered by the Apennine Chain, that has experienced subsidence during the Quaternary. This region, volcanologically active in the last 600ka, has been identified as the Campanian Volcanic Zone (CVZ). The products of three periods of trachytic ignimbrite volcanism (289–246ka, 157ka and 106ka) have been identified in the Apennine area in the last 300ka. These deposits probably represent distal ash flow units of ignimbrite eruptions which occurred throughout the CVZ. The resulting deposits are interstratified with marine sediments indicating that periods of repeated volcano-tectonic emergence and subsidence may have occurred in the past. The eruption, defined as the Campanian Ignimbrite (CI), with the largest volume (310km³), occurred in the CVZ 39ka ago. The products of the CI eruption consist of two units (unit-1 and unit-2) formed from a single compositionally zoned magma body. Slightly different in composition, three trachytic melts constitute the two units. Unit-1 type A is an acid trachyte, type B is a trachyte and type C of unit-2 is a mafic trachyte.The CI, vented from pre-existing neotectonic faults, formed during the Apennine uplift. Initially the venting of volatile-rich type A magma deposited the products to the N–NE of the CVZ. During the eruption, the Acerra graben already affected by a NE–SW fault system, was transected by E–W faults, forming a cross-graben that extended to the gulf of Naples. E–W faults were then further dislocated by NE–SW transcurrent movements. This additional collapse significantly influenced the deposition of the B-type magma of unit-1, and the C-type magma of unit-2 toward the E–SE and S, in the Bay of Naples. The pumice fall deposit underlying the CI deposits, until now thought to be associated with the CI eruption, is not a strict transition from plinian to CI-forming activity. It is derived instead from an independent source probably located near the Naples area. This initial volcanic activity is assumed to be a precursor to the CI trachytic eruptions, which vented along regional faults.Received October 23, 2002; revised version accepted July 29, 2003     

The Baffin Bay lavas and the value of picrites as analogues of primary magmas

The Baffin Bay picrites have been the focal point of a controversy concerning the MgO content of primary magmas derived from the upper mantle. A sample population of 48 lava chilled margins collected across the Baffin Bay volcanic succession at the northeastern tip of Padloping Island exhibits a prominent compositional mode between 14 and 16 weight percent MgO (19–22 Mg, cation units = Mg/100 cations). The petrography of these samples, however, requires that the Padloping magmas were mixtures of olivine crystals and liquid at their eruption. Olivine phenocrysts constituted 15 to 30 volume percent of these magmas and retain compositions requiring coexisting liquid compositions with only 10 to 13.5 weight percent MgO (14–18.5 Mg). However, highly magnesian, olivine xenocrysts (up to Fo 93) found in the most magnesian lavas require the former existence of liquids with at least 18 weight percent MgO (24 Mg). If these xenocrysts represent early cumulates, then the primary liquids of the Padloping suite must have been at least this MgO rich with temperatures greater than 1,425° C. Such primary liquids could have evolved by olivine crystallization to a steady state, equilibrated crystal — liquid mixtures in a shallow reservoir system prior to eruption. The compositions of the liquids of these mixtures appear to have been perched at the point of plagioclase saturation at approximately 1,275° C.Despite the complications of mechanical sorting of olivine crystals, the virtual compositional reciprocity of olivine addition and olivine fractionation requires that the bulk compositions of picritic lavas provide compositional analogues of their primary magmas. A comparison of Phanerozoic picrite suites indicates that the Fe contents of terrestrial primary magmas of tholeiitic affinity have a restricted range from 6–9 Fe. Primary magmas associated with intra-plate volcanism appear to be distinctly more Fe-rich than those associated with inter-plate volcanism. The Al/Si ratios of Phanerozoic picrite suites could suggest that the primary magmas of MORB volcanism have equilibrated with relatively Fe-poor source regions at deeper levels in the Earth's mantle than those of other tholeiitic primary magmas.     

Kinematics of the Iberia–Maghreb plate contact from seismic moment tensors and GPS observations

The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique,  NW–SE convergence between Africa and Eurasia at a rate of  5 mm/yr. Coeval extension in the Alboran Basin and a N35°E trending band of active, left-lateral shear deformation in the Alboran–Betic region are not straightforward to understand in the context of regional shortening, and evidence complexity of deformation at the plate contact. We estimate 86 seismic moment tensors (M_W 3.3 to 6.9) from time domain inversion of near-regional waveforms in an intermediate period band. Those and previous moment tensors are used to describe regional faulting style and calculate average stress tensors. The solutions associated to the Trans-Alboran shear zone show predominantly strike-slip faulting, and indicate a clockwise rotation of the largest principal stress orientation compared to the regional convergence direction (σ₁ at N350°E). At the N-Algerian and SW-Iberian margins, reverse faulting solutions dominate, corresponding to N350°E and N310°E compression, respectively. Over most of the Betic range and intraplate Iberia, we observe predominately normal faulting, and WSW–ENE extension (σ₃ at N240°E). From GPS observations we estimate that more than 3 mm/yr of African (Nubian)–Eurasian plate convergence are currently accommodated at the N-Algerian margin,  2 mm/yr in the Moroccan Atlas, and  2 mm/yr at the SW-Iberian margin. 2 mm/yr is a reasonable estimate for convergence within the Alboran region, while Alboran extension can be quantified as  2.5 mm/yr along the stretching direction (N240°E). Superposition of both motions explains the observed left-lateral transtensional regime in the Trans-Alboran shear zone. Two potential driving mechanisms of differential motion of the Alboran–Betic–Gibraltar domain may coexist in the region: a secondary stress source other than plate convergence, related to regional-scale dynamic processes in the upper mantle of the Alboran region, as well as drag from the continental-scale motion of the Nubian plate along the southern limit of the region. In the Atlantic Ocean, the  3.5 mm/yr, westward motion of the Gibraltar Arc relative to intraplate Iberia can be accommodated at the transpressive SW-Iberian margin, while available GPS observations do not support an active subduction process in this area.     

Rapid transport of anthropogenic lead through soils in southeast Missouri

To investigate Pb transport and cycling, soils from the forest floor and cores from White Oaks (Quercus alba L.) were collected near a Pb smelter in SE Missouri at varying depths from the surface and varying distances. Lead concentrations in soil samples at the surface drop dramatically with distance from approximately 1500 mg/kg at less than 2 km from the smelter to around 100 mg/kg at localities greater than 2 km from the smelter. Lead contents in tree rings are below 0.5 mg/kg in samples dated prior to 1970, and rapidly increase in 1975–1990 samples. Isotopic compositions of soils and tree rings exhibit systematic variations of Pb isotopic compositions with depth and tree ring age. Distinguishable isotopic signatures for Pb sources allowed quantification of the contribution of smelter Pb to the soils. At depths where Pb concentrations decreased and approached constant values (10–25 cm, 10–30 mg/kg), 50–90%, 40–50% and 10–50% of the Pb could be derived from the smelter for the samples at locations less than 2, 2–4 and over 4 km from the smelter, respectively. The remaining portion was attributable to automobile emission and bedrock sources. Because the smelter operated from 1963 to 2003 and samples were collected in 1999, it is estimated that smelter Pb infiltrates at rates of 1 cm/yr (30 cm in 30 yr). At distances less than 1.5 km from the smelter, even though Pb concentrations become asymptotic at a depth of 30 cm, isotopic evidence suggests that Pb has migrated below this depth, presumably through exchange with naturally occurring Pb in the soil matrix. This implies that soils heavily polluted by Pb can exceed their Pb carrying capacity, which could have potential impacts on shallow groundwater systems and risk further exposure to human and ecological receptors.     

Geochemical and Sr-Nd-Pb isotopic characteristics of Late Cenozoic leucite lamproites from the East European Alpine belt (Macedonia and Yugoslavia)

In the East European Alpine belt, leucite-sanidine-phlogopite-olivine-bearing volcanic rocks of Late Cenozoic age occur at eight localities within the Vardar suture zone and at one locality in the Southern Carpathian fold-and-thrust belt. Most of these volcanics are characterized by high Mg# (66.6–78.6), high abundances of Ni (117–373 ppm) and Cr (144–445 ppm) as well as high primary K₂O contents (5.63–7.01 %) and K₂O/Na₂O values (1.93–4.91). Rocks with more differentiated compositions are rare. A lamproite affinity of these rocks is apparent from their relatively low contents of Al₂O₃ (9.9–14.3 wt%) and CaO (6.2–8.3 wt%) in combination with high abundances of Rb (85–967 ppm), Ba (1,027–4,189 ppm), Th (18.9–76.5 ppm), Pb (19–54 ppm), Sr (774–1,712 ppm) and F (0.16–0.52 wt%), and the general lack of plagioclase. Although eruption of the magmas took place in post-collisional extensional settings, significant depletions of Nb and Ta relative to Th and La, low TiO₂ contents (0.92–2.17 %), low ratios of Rb/Cs, K/Rb and Ce/Pb as well as high ratios of Ba/La and Ba/Th suggest close genetic relationships to subduction zone processes. Whereas Sr and Nd isotope ratios show relatively large variations (⁸⁷Sr/⁸⁶Sr = 0.7078–0.7105, ¹⁴³Nd/¹⁴⁴Nd = 0.51242–0.51215), Pb isotope ratios display a very restricted range with ²⁰⁶Pb/²⁰⁴Pb = 18.68–18.88 and variable but generally high 7/4 (11–18) and 8/4 (65–95) values. The observed petrographic, geochemical and isotopic characteristics are best explained by a genetic model involving preferential melting of phlogopite-rich veins in an originally depleted lithospheric mantle source, whereby the metasomatic enrichment of the mantle source is tentatively related to the addition of components from subducted sediments during consumption of Tethyan oceanic lithosphere.Editorial responsibility: J. Hoefs     

Natural deformation related to serpentinisation of an ultramafic inclusion within a continental shear zone: The key role of fluids

The role of fluids in the deformation of continental serpentinites is investigated from structural, microstructural and petrographic analyses applied to a km-scale porphyroclast mantled in a viscous matrix of amphibolites. The clast is sited within a shear zone of the Palaeozoic Maures massif, France. Syntectonic fluid–rock interactions occurred from km to mm scales, at first on the clast borders (along the main rheological boundaries) then within the clast. They are accommodated macroscopically by slickenfibers faults and microscopically by shear microcracks within crack-seal veins, typifying an intermediate, brittle–ductile behaviour of serpentinites. Three main stages of deformation–serpentinisation processes occurred in relation with the left-lateral movement of the hosted shear zone. They developed under metamorphic conditions evolving from amphibolites to green-schists facies conditions ( 400 MPa/550 °C to  200 MPa/< 300 °C), as inferred from the surrounding sheared amphibolites. Deformation and serpentinisation increase through time although fluid pressure decreases. If the shape of the inclusion and its orientation relative to the shear zone mainly controlled the deformation pattern though time (P then R' shears), fluid pressure is required for starting deformation–serpentinisation processes along inherited anisotropy planes. Whatever the origin of fluids, they play a key role all along the deformation processes by influencing stress states within the shear zone at the onset of deformation and by changing at various scales and through time behaviour of the rock, depending of the intensity of serpentinisation and the rate of deformation.     

Anatolian surface wave evaluated at GEOFON Station ISP Isparta, Turkey

We have studied seismic surface waves of 255 shallow regional earthquakes recently recorded at GEOFON station ISP (Isparta, Turkey) and have selected these 52 recordings with high signal-to-noise ratio for further analysis. An attempt was made by the simultaneous use of the Rayleigh and Love surface wave data to interpret the planar crust and uppermost mantle velocity structure beneath the Anatolian plate using a differential least-square inversion technique. The shear-wave velocities near the surface show a gradational change from approximately 2.2 to 3.6 km s^− 1 in the depth range 0–10 km. The mid-crustal depth range indicating a weakly developed low velocity zone has shear-wave velocities around 3.55 km s^− 1. The Moho discontinuity characterizing the crust–mantle velocity transition appears somewhat gradual between the depth range  25–45 km. The surface waves approaching from the northern Anatolia are estimated to travel a crustal thickness of  33 km whilst those from the southwestern Anatolia and part of east Mediterranean Sea indicate a thicker crust at  37 km. The eastern Anatolia events traveled even thicker crust at  41 km. A low sub-Moho velocity is estimated at  4.27 km s^− 1, although consistent with other similar studies in the region. The current velocities are considerably slower than indicated by the Preliminary Reference Earth Model (PREM) in almost all depth ranges.     

Diffusion gradients in an eclogite xenolith from the Roberts Victor kimberlite pipe: (2) kinetics and implications for petrogenesis

In a bimineralic eclogite xenolith (sample JJG41) from the Roberts Victor kimberlite, compositional gradients in clinopyroxene are related to garnet exsolution. Two principal reactions involving clinopyroxene and garnet occur: (i) The net-transfer Al₂Si^-1Mg^-1 which is responsible for garnet growth according to the equation 2Di+Al₂Si^-1Mg^-1=Grossular+MgCa^-1 (reaction 1). This has created substantial compositional gradients in Al, Si and Mg within clinopyroxene. (ii) The exchange of Fe–Mg between garnet and clinopyroxene (reaction 2). During the stage of garnet growth (reaction 1) the lamellae crystallized sequentially as a result of a temperature decrease from around 1400 to 1200° C. This exsolution growth-stage was under the control of Al diffusion in clinopyroxene and at around 1200° C further growth of garnet lamellae became impeded by the sluggishness of Al diffusion in the clinopyroxene host. However, reaction 2 continued during further cooling down to about 1000° C; this temperature being inferred from the constant Fe–Mg partitioning at clinopyroxene-garnet interfaces for the whole set of lamellae. The initial clinopyroxene in JJG41 was probably formed by crystallization from a melt in Archaean time. The cessation of Fe–Mg exchange between garnet and clinopyroxene at about 1000° C may well predate the eruption of the eclogite in kimberlite at around 100 Ma. Kinetic models of reaction are examined for both reactions. Modelling of reaction 1, involving both diffusion and interface migration, allows several means of estimating the diffusion coefficient of Al in clinopyroxene; the estimates are in the range 10^-16-10^-20 cm²/s at 1200° C. These estimates bracket the experimentally determined data for Al diffusion in clinopyroxene, and from these experimental data a preferred cooling rate of about 300° C/Ma is obtained for the period of growth of garnet exsolution lamellae. A geospeedometry approach (Lasaga 1983) suitable for a pure-exchange process (reaction 2) is used to estimate the cooling rate in the later stages of the thermal history (after garnet growth); values 4–40° C/Ma are consistent with the shape of the Fe-diffusion gradients in the clinopyroxene. The extensive thermal history recorded by JJG41, including probable melt involvement at ca. 1400° C, demonstrates the complex evolution of rocks within the mantle. Whilst the notion of formation of mantle eclogites from subducted oceanic crust has become fashionable, it is clear that tracing eclogite geochemical and P-T characteristics backwards from their nature at the time of xenolith eruption, through high-temperature mantle events to the characteristics of the original subducted oceanic crust, will be very complex.