Facies analyses and volcanic setting of the giant Neves Corvo massive sulfide deposit,Iberian Pyrite Belt,Portugal

In the Iberian Pyrite Belt, volcanic rocks are relatively scarce, accounting for approximately only 25% of the geologic record, with the remaining 75% consisting of sedimentary units. This association is very clear in the host succession to the Neves Corvo massive sulfide deposit in Portugal. The Neves Corvo host succession comprises the products of explosive and effusive rhyolitic eruptions intercalated with mudstone that records a submarine below-wave-base environment and provides precise biostratigraphic age constraints. The first and second volcanic events involved eruptions at local intrabasinal vents. The first event generated thick beds of fiamme breccia that are late Famennian in age. The fiamme were originally pumice clasts produced by explosive eruptions and were subsequently compacted. The second event was the late Strunian (latest Famennian) effusion of rhyolitic lava that was pervasively quench-fragmented. The third and final event is younger than the massive sulfide deposits poorly represented in the mine area and minor compared with the two other events. The integration of biostratigraphic data with the volcanic facies architecture indicates that the Neves Corvo ore deposits are similar in age to the late Strunian rhyolitic lava. Although regionally the Iberian Pyrite Belt is essentially a sedimentary succession, ore formation at Neves Corvo can be closely linked to discrete volcanic events that produced a relatively narrow range of volcanic facies.     

Testing spatial heterogeneity in geographically weighted principal components analysis

We propose a method to evaluate the existence of spatial variability in the covariance structure in a geographically weighted principal components analysis (GWPCA). The method, that is extensive to locally weighted principal components analysis, is based on performing a statistical hypothesis test using the eigenvectors of the PCA scores covariance matrix. The application of the method to simulated data shows that it has a greater statistical power than the current statistical test that uses the eigenvalues of the raw data covariance matrix. Finally, the method was applied to a real problem whose objective is to find spatial distribution patterns in a set of soil pollutants. The results show the utility of GWPCA versus PCA.     

Real-time Seismic Amplitude Measurement (RSAM): a volcano monitoring and prediction tool

Seismicity is one of the most commonly monitored phenomena used to determine the state of a volcano and for the prediction of volcanic eruptions. Although several real-time earthquake-detection and data acquisition systems exist, few continuously measure seismic amplitude in circumstances where individual events are difficult to recognize or where volcanic tremor is prevalent. Analog seismic records provide a quick visual overview of activity; however, continuous rapid quantitative analysis to define the intensity of seismic activity for the purpose of predicing volcanic eruptions is not always possible because of clipping that results from the limited dynamic range of analog recorders. At the Cascades Volcano Observatory, an inexpensive 8-bit analog-to-digital system controlled by a laptop computer is used to provide 1-min average-amplitude information from eight telemetered seismic stations. The absolute voltage level for each station is digitized, averaged, and appended in near real-time to a data file on a multiuser computer system. Raw realtime seismic amplitude measurement (RSAM) data or transformed RSAM data are then plotted on a common time base with other available volcano-monitoring information such as tilt. Changes in earthquake activity associated with dome-building episodes, weather, and instrumental difficulties are recognized as distinct patterns in the RSAM data set. RSAM data for domebuilding episodes gradually develop into exponential increases that terminate just before the time of magma extrusion. Mount St. Helens crater earthquakes show up as isolated spikes on amplitude plots for crater seismic stations but seldom for more distant stations. Weather-related noise shows up as low-level, long-term disturbances on all seismic stations, regardless of distance from the volcano. Implemented in mid-1985, the RSAM system has proved valuable in providing up-to-date information on seismic activity for three Mount St. Helens eruptive episodes from 1985 to 1986 (May 1985, May 1986, and October 1986). Tiltmeter data, the only other telemetered geophysical information that was available for the three dome-building episodes, is compared to RSAM data to show that the increase in RSAM data was related to the transport of magma to the surface. Thus, if tiltmeter data is not available, RSAM data can be used to predict future magmatic eruptions at Mount St. Helens. We also recognize the limitations of RSAm data. Two examples of RSAM data associated with phreatic or shallow phreatomagmatic explosions were not preceded by the same increases in RSAM data or changes in tilt associated with the three dome-building eruptions.     

Cataclasis and processes of particle size reduction

The particle size distribution (P.S.D.) of fragmented geological materials is affected by the fragmentation process, initial size distribution, number of fracturing events, energy input, strain, and confining pressure. A summary of literature shows that the fractal dimension (D) of the P.S.D. is increased by the number of fracturing events, energy input, strain, and confining pressure. Cenozoic cataclasis of granite, granodiorites, gneisses and arkose seen in cores from the Cajon Pass drillhole, southern California, produced P.S.D.s with values ofD that varied from 1.88 to 3.08. Each rock type has a characteristic and more limited range ofD. Areas of dilatant texture and modeI fracture-fillings have low average values (2.32 and 2.37) compared to an average value of 2.67 in shear fracture-fillingsD has a good inverse correlation with average particle size. Data from fault rocks in the San Gabriel fault zone, southern California (Anderson et al., 1983) have been reanalyzed to show that values ofD are higher (2.10–5.52) and average particle size is lower than the Cajon Pass samples, but the ranges of values overlap, and the inverse correlation betweenD and average particle size is extended. Microstructural observations combined with these results suggest that three processes contributed to particle size reduction during cataclasis. The first process of feldspar alteration, which leads to low values ofD, has not been previously recognized. The second process is probably constrained comminution (Sammis et al., 1987), since the averageD in shear fracture-fillings is close to the value of 2.58 predicted by this theory. A further stage of particle size reduction is demonstrated by an increase ofD with cataclasis. This third process is selective fracture of larger particles, which may also operate during localization and the cataclastic flow-to-faulting transition as observed in experiments. A transition from constrained comminution to selective fracture of large particles, and increasingD values with cataclastic evolution and grain size reduction, may be general features of experimental and natural cataclasis.     

Fluid and silicate glass inclusions in ultramafic and mafic xenoliths from Hierro,Canary Islands: implications for mantle metasomatism

Fluid and solid inclusions have been studied in selected samples from a series of spinel-bearing Crdiopside-and Al-augite-series ultramafic (harzburgites, lherzolites, and olivine-clinopyroxene-rich rocks), and gabbroic xenoliths from Hierro, Canary Islands. In these samples several generations of fluid inclusions and ultramafic-and mafic-glass inclusions may be texturally related to different stages of crystal growth. The fluid inclusions consist of pure, or almost pure, CO₂. The solid inclusions in the ultramafic xenoliths comprise early inclusions of devitrified ultramafic glass, sulphide inclusions, as well as polyphase inclusions (spinel+clinopyroxene±glass±other silicates) believed to have formed from trapped basaltic melts. Vitreous basaltic glass±CO₂±sulphide±silicates are common as secondary inclusions in the ultramafic xenoliths, and as primary inclusions in the gabbroic xenoliths. Microthermometry gives minimum trapping temperatures of 1110° C for the early ultramafic-and mafic-glass inclusions, and a maximum of 1260–1280° C for late inclusions of host basaltic glass. In most samples the CO₂ inclusions show a wide range in homogenization temperatures (-40 to +31° C) as a result of decrepitation during ascent. The lowest homogenization temperatures of about-40° C, recorded in some of the smallest CO₂ inclusions, indicate a minimum depth of origin of 35 km (12 kbar) for both the Cr-diopside-and Al-augite-series xenoliths. The gabbroic xenoliths originate from a former magma chamber at a depth of 6–12 km.Contribution no. 100 of the Norwegian programme of the International Lithosphere Project     

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The National Center for Atmospheric Research is sponsored by the National Science Foundation.