Alfredo Aparicio

New geodetic monitoring system in the volcanic island of Tenerife, Canaries, Spain. Combination of InSAR and GPS techniques

Even though volcanic activity in Tenerife is characterized by the fact that eruptions do not normally occur more than once in the same volcanic edifice, geodetic monitoring has mainly focused on the Las Canadas Caldera, where a geodetic micronetwork and a levelling profile are located. A sensitivity test of this geodetic network showed a clear need to extend it to cover the full island for volcano monitoring purposes. This conclusion, together with the detection of two unexpected movements on the island using InSAR that were beyond the scope of the traditional geodetic network, prompted the authors to design and observe a GPS network covering the whole of Tenerife. The network was monitored in August 2000. The results obtained were accurate to 1 cm, and confirm the deformation in the Pinar de Chio zone, but are not definitive enough to confirm the displacements detected to the south of the village of Garachico. Furthermore, new cases of possible subsidence have been detected in areas where InSAR could not be used to measure deformation due to low coherence. Future observations will be necessary for further validation and to study the time evolution of the displacements, and supplementary research must be conducted to determine the possible causes, in particular if they are connected with the water pumping operations performed on the island. One important result is that a new geodetic monitoring system based on two complementary techniques, InSAR and GPS, has been set up on Tenerife island. This is the first time that the whole surface of any of the volcanic Canary Islands has been covered with a single network. This research has displayed the need for further similar studies in the Canary Islands, at least on the islands which pose a greater risk of volcanic reactivation.

Presence of large pumice blocks on Tierra del Fuego and South Shetland Islands shorelines, from 1962 South Sandwich Islands eruption

Abstract We report on the source of pumice found on beaches of Tierra del Fuego (Argentina) and the South Shetland Islands (Antarctica), north and south of the Drake Passage. Petrological studies, chemical analyses and experiments on this pumice indicate it is most likely from a 1962 submarine eruption in the South Sandwich Islands. Previous work stated that clockwise marine currents carried such pumice as far as Tasmania. Our findings show that the currents continued to transport the pumice to Drake Passage, nearly completing a circuit around Antarctica, and up to southern Argentina showing that this material can follow complex pathways for more than 20 000 km.

Improved capillary electrophoresis method for measuring rare-earth elements in synthetic geochemical standards

An improved capillary electrophoresis (CE) method for quantifying rare-earth elements (REEs) in synthetic geochemical standards was developed. Synthetic standard solutions were obtained from high purity metal oxides. The separation of REE total group (lanthanum to lutetium) was defined as a primary objective. Special attention was also focused on the optimized separation of europium (Eu) and gadolinium (Gd) because in earlier applications they presented overlapping problems. Their separation and quantitative determinations are essential for geological applications. For the rapid separation of REEs in synthetic geochemical standards, the temperature of the separation device was optimized. An analysis temperature of 15 degrees C enabled both the rapid separation of REEs within 2 min and the overlapping problem of Eu-Gd to be resolved. The detection limits (<0.1 ng) and precision estimates (generally better than 5%) were found to be satisfactory for most geological applications.

Fast emplacement of extensive pahoehoe flow-fields: the case of the 1736 flows from Montana de las Nueces, Lanzarote

The 1730–36 Timanfaya eruption on Lanzarote, in the Canary Islands, is the second largest historical effusion on record. During its final stages, in 1736, the eruption produced the Montana de las Nueces flow-field, consisting of sheets of pahoehoe lava that, within 4 weeks, had covered 32 km2 and reached a maximum length of almost 21 km. The tholeiitic lavas have pahoehoe surface features, but internal structures that are normally associated with massive aa flows, suggesting that their fronts advanced as single units rather than as a collection of budding pahoehoe tongues. Volume conservation and a simple model of crustal failure suggest that the main flows advanced at about 0.02 ms−1 over the prevailing slopes of ∼1°. The rates of advance are (1) consistent with emplacement near the transition from pahoehoe to aa, and (2) about an order of magnitude greater than would have been expected by analogy with Hawaiian pahoehoe flow-fields of similar dimensions. Surface texture and morphology, therefore, is an insufficient guide for constraining the rate and style of pahoehoe emplacement, and a flow’s internal structure must be established before its characteristics are used to infer eruption conditions and potential hazard.

Metasedimentary xenoliths in the lavas of the Timanfaya eruption (1730- 1736, Lanzarote , Canary Islands ): metamorphism and contamination processes

We report on the investigation of contact metamorphism provoked by the emplacement of a shallow magma chamber during the Timanfaya eruption of Lanzarote from 1730 to 1736 AD. The study was carried out on metamorphic xenoliths from basaltic Timanfaya lavas, and shows how the primary basanitic magma was contaminated by sedimentary and metamorphic rocks. Mineralogical and chemical studies allowed the definition of several xenolith types. Silica xenoliths (quartz, tridymite, cristobalite or a mixture of these, constituting more than 50 % of the xenolith) and cale-silicate xenoliths (wollastonite, sometimes the 2M type, diopside, forsterite or mixture of these, constituting more than 50 % of the xenolith) are the most frequent. Other minerals recognized were calcite, dolomite, augite, enstatite, hypersthene, spinel and scapolite. The mineralogy and some textures of the metamorphic forsteritic xenoliths are identical to those found in ultrabasic xenoliths (dunites) and point to a possible metamorphic origin for some of them. Major and trace elements showed a diversity of composition, controlled by the mineralogy. The REE composition of the metamorphic xenoliths is high, compared with the sedimentary xenoliths not affected by metamorphism. The mineral assemblages define metamorphic facies of low, medium and high grade, depending on the distance of the sedimentary rocks from the magma chamber border. The IGPETWIN-MIXING program was used to verify the contamination process, taking the xenoliths as representative of the sedimentary/metamorphic rocks that were melted. The results indicated that sedimentary/metamorphic rock contamination of a basanitic magma can produce tholeiitic compositions.

Capillary electrophoresis for measuring major and trace anions in thermal water and condensed-steam samples from hydrothermal springs and fumaroles.

A new application of capillary electrophoresis for measuring major and trace anions in thermal water and condensed-steam samples is presented. Ten fluid samples were collected from hydrothermal springs and fumaroles located in a volcanic zone of Deception Island, Antarctica. Anion separation was achieved in less than 6 min using indirect UV detection at 254 nm with a negative power supply (-15 kV). The electrolyte consisted of 4.7 mM sodium chromate, 4.0 mM electroosmotic flow modifier (OFM) hydroxide, 10 mM 2-(N-cyclohexylamino)ethanesulfonic acid and 0.1 mM calcium gluconate (pH 9.1). Major anions (Cl-, SO4(2), PO4H2+, and CO3H-) were measured using hydrostatic injection (10 cm for 30 s) at 25 degrees C. Trace amounts of anions (F-, Br-, and NO3-) were better determined by electromigration injection (4 kV, 10 s) at 15 degrees C. Good reproducibility of the migration times (<0.72% RSD), a satisfactory linear response and accuracy as well as acceptable detection limits were successfully obtained.

Morphologically ductile criteria for the sense of movement on slickensides from an extensional detachment fault in southern Spain

Abstract New criteria for the sense of movement on slickenside surfaces displaying evidence for mesoscopic ductility are described in the Neogene Jaloche extensional detachment fault in southern Spain. This detachment develops spectacular slickensides and fault gouges, and four of the criteria for the sense of movement observed along the highly polished slip surfaces are new to the literature. They are: (1) synthetic secondary fractures with hangingwall drag-effects in the form of roll-over microsynclines; (2) metre-scale oval-shaped asymmetric culmination and depression features; (3) microthrusts verging towards the direction of motion of the opposite block and dragging/overthrusting previous planar elements on the fault surface; and (4) flakes of fault-surface material trailed and plastered in the direction of the missing block. The geometry and microstructure of these features reveal that they deformed in a morphologically ductile manner by the coupled processes of cataclastic flow and shear localization, probably during aseismic sliding. During transient seismic pulses, deformation occurred along shiny/polished slickenside fault planes occasionally bearing jigsaw implosion breccias. Fluids under high pressure present initially may have promoted distributed cracking, whereas the process of dilatancy hardening may have served to facilitate cataclastic flow in the deformation history.

Suppressed ion chromatography for monitoring chemical impurities in steam for geothermal power plants

A suppressed ion chromatography (IC) technique has been evaluated as a chemical monitoring tool for detecting major anions (F-, Cl-, NO3- and SO4(2-)) of condensed steam in geothermal power plants. It is shown that the suppressed IC technique provides a suitable means for preventing possible damage to generating equipment in the geothermal industry. An electrical conductivity detector (0.1 microS sensitivity) with an anion-exchange column (IonPac AS4A-SC), a micro-membrane suppressor (AMMS II), and an isocratic high-pressure pump system were successfully used for detecting low concentrations of inorganic anions. Method detection limits for the anions of interest were <0.184 mg/L. Details of the IC methodology as well as some experimental results obtained during its application for the chemical monitoring of geothermal steam pipes are also described.

Chemical variations in biotites during prograde metamorphism, Sierra de Guadarrama, Sistema Central, Spain

Abstract With increasing metamorphic grade, the biotites in the metapelites of the Sierra de Guadarrama show an increase of Al IV , Ti, Mn and K, and a decrease of Si and Al VI contents. On the contrary, the biotites in meta-arkoses show increasing Si, Mn and K and decreasing Al IV and Ti contents. The increase of Al IV (and consequently the decrease of Si) of the biotites in the metapelites could have been conditioned by increase of temperature, whereas the decrease of Al IV of the biotites in the meta-arkoses could be related with modal percentage of plagioclase. The variation trends of Mn and K can be related with resorption processes of garnets and progressive breakdown of muscovite, respectively.

Cathodoluminescence Spectral Characteristics of Quartz and Feldspars in Unaltered and Hydrothermally Altered Volcanic Rocks (Almeria, Spain)

ABSTRACT The volcanic rocks of the Cabo de Gata region have been transformed by hydrothermal alteration, forming in some areas, highly silicified and feldspatized rocks. Various samples from outcrops of unaltered and hydrothermally altered volcanic rocks were studied by conventional transmitted-light microscopy to determine their genesis (magmatic, inherited or neoformed) and scanning electron microscope cathodoluminescence to define their spectral features. The magmatic quartz shows emission bands at 400, 440, and 480nm., and these bands disappear or decrease in the secondary hydrothermal quartz, characterized also by an intense emission band at around 570–580 nm, and another around 460 nm. The inherited magmatic plagioclase and neoformed anorthoclase have some similar emission bands and their genesis is not marked by cathodoluminescence features. The magmatic K-feldspars (sanidine), have emission bands at around 425, 440, and 490 nm, constituting a strong blue emission (420–500 nm). This emission is weaker in the neoformed K-feldspars that, however, have a strong emission band at around 570 nm. These data show that some cathodoluminescence spectral characteristic can be used to determine the petrogenesis of rock.