Jean-Louis Bourdier

Effects of f O2 and H2O on andesite phase relations between 2 and 4 kbar

Experimental phase equilibria have been investigated on three medium-K silicic andesite (60–61 wt % SiO2) samples from Mount Pelee at 2–4 kbar, 850–1040°C, under both vapor-saturated CO2-free and vapor-saturated CO2-bearing conditions. Most experiments were crystallization experiments using dry glasses prepared from the natural rocks. Both normal- and rapid quench experiments were performed. Two ranges of oxygen fugacity (fO2) were investigated: NNO (Ni-NiO buffer) to NNO + 1 and NNO + 2 to NNO + 3. At 2 kbar for moderately oxidizing conditions, plagioclase (pl) and magnetite (mt) are the liquidus phases, followed by low-Ca pyroxene (opx); these three phases coexist over a large temperature (T)-H2O range (875–950°C and 5–7 wt % H2O in melt). Amphibole (am) is stable under near vapor-saturated CO2-free conditions at 876°C. At 900°C, ilmenite (ilm) is found only in experiments less than or equal to NNO. Upon increasing pressure (P) under vapor-saturated CO2-free conditions, pl + mt is replaced by am + mt on the liquidus above 3.5 kbar. For highly oxidizing conditions, mt is the sole liquidus phase at 2 kbar, followed by pl and opx, except in the most H2O-rich part of the diagram at 930°C, where opx is replaced by Ca-rich pyroxene (cpx) and am. Compositions of ferromagnesian phases systematically correlate with changingfO2 Experimental glasses range from andesitic through dacitic to rhyolitic, showing systematic compositional variations with pl + opx + mt fractionation (increase of SiO2 and K2O, decrease of Al2O3, CaO, FeOt, and MgO). FeO*/MgO moderately increases with increasing SiO2. For fO2 conditions typical of calk-alkaline magmatism (approximately NNO + 1), magnetite is either a liquidus or a near-liquidus phase in hydrous silicic andesite magmas, and this should stimulate reexamination for the mechanisms of generation of andesites by fractionation from basaltic parents.

Magma storage conditions and control of eruption regime in silicic volcanoes: experimental evidence from Mt. Pelée

Differences of eruption regimes in silicic volcanoes, e.g. effusive versus explosive, have commonly been ascribed either to stratification of volatiles in the magma storage region or to gas loss through permeable conduit walls. Recent Plinian and Pelean eruptions of silicic andesite magmas from Mt. Pelee (P1: 650 yr B.P., 1902, 1929) show no systematic variations in bulk rock and phenocryst and glass compositions. Rare coexisting Fesingle bondTi oxide pairs in Pelean products yieldT between 840 and 902°C, and ΔNNO between +0.4 and +0.8. Pre-eruptive melt H2O contents, calculated from plagioclase-melt equilibria, span values from 1.9 to 5.5 wt%. Glass inclusions from the P1 Plinian fallout have H2O contents between 4.2 and 7.1 wt%. In contrast, the Pelean inclusions have H2O contents commonly <3 wt%, due to post-entrapment modifications upon eruption. Phase equilibrium studies allow pre-eruptive conditions to be precisely determined and demonstrate that recent eruptions, either Plinian or Pelean, tapped magmas with melt H2O contents of 5.3-6.3 wt%, stored at 2 ± 0.5 kbar, 875-900°C and ΔNNO = +0.4-0.8. Differences in eruptive style at Mt. Pelee are unrelated to systematic variations in pre-eruptive magmatic H2O concentrations, but may be caused by contrasting modes of degassing in the conduit.

Neogene ignimbrites of the Nevsehir plateau (Central Turkey): stratigraphy, distribution and source constraints

Abstract In Anatolia (Turkey), extensive calc-alkaline volcanism has developed along discontinuous provinces from Neogene to Quaternary times as a consequence of plate convergence and continental collision. In the Nevsehir plateau, which is located in the Central Anatolian Volcanic Province, volcanism consists of numerous monogenetic centres, several large stratovolcanoes and an extensive, mainly Neogene, rhyolitic ignimbrite field. Vent and caldera locations for the Neogene ignimbrites were not well known based on previous studies. In the Neogene ignimbrite sequence of the Nevsehir plateau, we have identified an old group of ignimbrites (Kavak ignimbrites) followed by five major ignimbrite units (Zelve, Sarimaden Tepe, Cemilkoy, Gordeles, Kizilkaya) and two smaller, less extensive ones (Tahar, Sofular). Other ignimbrite units at the margin of the plateau occur as outliers of larger ignimbrites whose main distributions are beyond the plateau. Excellent exposure and physical continuity of the units over large areas have allowed establishment of the stratigraphic succession of the ignimbrites as, from bottom to top: Kavak, Zelve, Sarimaden Tepe, Cemilkoy, Tahar, Gordeles, Sofular, Kizilkaya. Our stratigraphic scheme refines previous ones by the identification of the Zelve ignimbrite and the correlation of the previously defined ‘Akkoy’ ignimbrite with the Sarimaden Tepe ignimbrite. Correlations of distant ignimbrite remnants have been achieved by using a combination a field criteria: (1) sedimentological characterisitics; (2) phenocryst assemblage; (3) pumice vesiculation texture; (4) presence and characteristics of associated plinian fallout deposits; and (5) lithic types. The correlations significantly enlarge the estimates of the original extent and volume of most ignimbrites: volumes range between 80 km 3 and 300 km 3 for the major ignimbrites, corresponding to 2500–10,000 km 3 in areal extent. The major ignimbrites of the Nevsehir plateau have an inferred source area in the Derinkuyu tectonic basin which extends mainly between Nevsehir and the Melendiz Dag volcanic complex. The Kavak ignimbrites and the Zelve ignimbrite have inferred sources located between Nevsehir and Derinkuyu, coincident with a negative gravity anomaly. The younger ignimbrites (Sarimaden Tepe, Cemilkoy, Gordeles, Kizilkaya) have inferred sources clustered to the south between the Erdas Dag and the Melendiz Dag volcanic complex. We found evidence of collapse structures on the northern and southern flanks of the Erdas Dag volcanic massif, and of a large updoming structure in the Sahinkalesi Tepe massif. The present-day Derinkuyu tectonic basin is mostly covered with Quaternary sediments and volcanics. The fault system which bounds the basin to the east provides evidence that the ignimbrite volcanism and inferred caldera formation took place in a locally extensional environment while the basin was already subsiding. Drilling and geophysical prospecting are necessary to decipher in detail the presently unknown internal structure of the basin and the inferred, probably coalesced or nested, calderas within it.

Post-collision neogene volcanism of the Eastern Rif (Morocco): magmatic evolution through time

Abstract Neogene volcanism in the Eastern Rif (Morocco) comprises a series of calc-alkaline, potassic calc-alkaline, shoshonitic and alkaline volcanic rocks. According to new stratigraphical, along with new and previous chronological and geochemical data, the orogenic volcanism was successively (1) calc-alkaline (basaltic andesites and andesites: 13.1 to 12.5 Ma, rhyolites: 9.8 Ma), (2) K-calc-alkaline (basaltic andesitic to rhyolitic lavas and granodiorites: 9.0 to 6.6 Ma), and (3) shoshonitic (absarokites, shoshonites, latites, trachytes: ∼7.0 to 5.4 Ma). The later Pliocene volcanism was basaltic and alkaline (5.6 to 1.5 Ma). The calc-alkaline and K-calc-alkaline series exhibit lower K2O (0.7–5.3 wt.%), Nb (8–19 ppm) contents and higher 87 Sr / 86 Sr (0.70773–0.71016) than the shoshonitic series (K2O: 2.4–7.2 wt.%, Nb: 21–38 ppm, 87 Sr / 86 Sr : 0.70404–0.70778). Pliocene alkaline basalts have a sodic tendency (Na2O/K2O: 1.7–3.5), high Nb content (up to 52 ppm), and low 87 Sr / 86 Sr ratio (0.70360–0.70413). The variations through time of K2O, Nb and Sr isotopic ratio reflect different mantle sources: (i) calc-alkaline, potassic calc-alkaline and shoshonitic series are derived from a mantle source modified by older subduction, (ii) alkaline basalts are derived mainly from an enriched mantle source. Through time, incompatible elements such as Nb increased while 87 Sr / 86 Sr decreased, suggesting a decreasing influence of metasomatized mantle (inherited subduction). Such evolution is related to the post-collision regimes operating in this area, and could be linked to the succession of extensional, compressional and strike-slip fault tectonics.

Textures, water content and degassing of silicic andesites from recent plinian and dome-forming eruptions at Mount Pelée volcano (Martinique, Lesser Antilles arc)

Previous petrological and phase-equilibrium experimental studies on recent silicic andesites from Mount Pelee volcano have evidenced comparable pre-eruptive conditions for plinian and dome-forming (pelean herein) eruptions, implying that differences in eruptive style must be primarily controlled by differences in degassing behaviour of the Mount Pelee magmas during eruption. To further investigate the degassing conditions of plinian and pelean magmas of Mount Pelee, we study here the most recent Mount Pelees products (P1 at 650 years B.P., 1902, and 1929 eruptions, which cover a range of plinian and pelean lithologies) for bulk-rock vesicularities, glass water contents (glass inclusions in phenocrysts and matrix glasses) and microtextures. Water contents of glass inclusions are scattered in the plinian pumices but on average compare with the experimentally-deduced pre-eruptive melt water content (i.e., 5.3-6.3 wt.%), whereas they are much lower in the dominant pelean lithologies (crystalline, poorly vesicular lithics and dome samples). This indicates that the glass inclusions of the pelean products have undergone strong leakage and do not represent pre-eruptive water contents. The water content of the pyroclast matrix glasses are thought to closely represent the residual water content in the melt at the time of fragmentation. Determination of the water contents of both the pre-eruptive melt and matrix glasses allows the estimation of the amount of water exsolved upon syn-eruptive degassing. We find the amount of water exsolved during the eruptive process to be higher in the pelean products than in the plinian ones, typically 90-100 and 65-70% of the initial water content, respectively. The vesicularities calculated from the amount of exsolved water compare with the measured vesicularities for the plinian pumices, consistent with a closed-system, near-equilibrium degassing up to fragmentation. By contrast, the low residual water contents, low groundmass vesicularities and extensive groundmass crystallization of the pelean products are direct evidence of open-system degassing. Microtextural features, including silica-bearing and silica-free voids in the pelean lithologies may represent a two-stage vesiculation.

Hidden calderas evidenced by multisource geophysical data; example of Cappadocian Calderas, Central Anatolia

Abstract The Cappadocian volcanic field in central Anatolia (Turkey) is characterised by a sequence of 10 Neogene ignimbrites. The associated calderas have been partly dismantled and buried by subsequent tectonic and sedimentary processes and, therefore, cannot be readily recognized in the field. Recent progress in the understanding of the stratigraphic correlations and flow patterns has identified two main probable source areas for the ignimbrites. Detailed study of these areas, based on gravity surveys, remote sensing data (SPOT and ERS1 images) and digital elevation models (DEM), has provided evidence for two major caldera complexes and their relationship to old stratovolcanoes and Neogene tectonics. The older Nevsehir–Acigol caldera complex, located between the towns of Acigol, Nevsehir and Cardak, is inferred to be the source of the Kavak and Zelve ignimbrites. The Nevsehir–Acigol caldera complex is defined mainly by a −35 mGal circular gravimetry anomaly about 15 km in diameter. The boundaries of this, now buried, caldera complex are shown by high gradients on the Bouguer gravity anomaly map. The younger Derinkuyu caldera complex, located between the Erdas stratovolcano and the Ciftlik basin, is inferred to be the source of the Sarimaden, Cemilkoy, Gordeles and Kizilkaya ignimbrites. It is well-defined by a rectangular (35×23 km) gravity low (−30 mGal) with a positive high (+20 mGal) in the center. Gravity, remote sensing data and the DEM provide evidence that the Erdas stratovolcano, on the northern margin of the Derinkuyu caldera complex, represents the remnants of a large stratovolcano partly cut by one or more caldera collapses. The positive anomaly within the Derinkuyu caldera complex is centered on the 15-km-wide Sahin Kalesi volcanic massif. Field evidence and structural features inferred from the DEM and remote sensing data strongly suggest that this massif is a resurgent doming associated with the Gordeles ignimbrite eruption. High-resolution ERS1, SPOT and DEM images reveal that the transtensive regime, active at least since the Miocene, influenced the location of eruptive centers and caldera complexes in Cappadocia. The two caldera complexes are located in transtensive grabens. The subsidence of these grabens, continuing after the caldera collapse events, most likely resulted in the burying of the calderas and could explain the difficulties in identifying them in the field.

The primitive volcano of Mount Pelée: its construction and partial destruction by flank collapse

Abstract The geology of Mount Pelee, the active volcano of Martinique (Lesser Antilles), has been actively studied during the past decade. This paper reviews the current knowledge about the oldest, least well-known part of the volcano (here called Paleo-Pelee) and the relations between Paleo-Pelee and the Mont Conil volcanic complex. Paleo-Pelee consists mostly of volcanic breccias with a few lava flows. It differs in this respect from neighbouring active volcanoes such as the Grande Decouverte (Soufriere) in Guadeloupe and the Soufriere in St. Vincent, the oldest products of which are mainly lava flows. Paleo-Pelee initially formed a regular cone which was comparable in size with the present edifice. It is now largely eroded and its products form prominent radial ridges on the western flank of Mount Pelee. These are buried under younger deposits on the eastern flank. The uniformly andesitic breccias of Paleo-Pelee are, for the most part, brecciated lava flows. It is inferred that the activity of Mount Pelee was less explosive at this stage than during following periods (Neo-Pelee). The Paleo-Pelee formations overlap those of Mont Conil. The geology of Mont Conil is poorly known. However, its lowest exposed products are submarine andesitic breccias cut by many dikes. This contrasts with Paleo-Pelee where all breccias are of subaerial origin. Pleistocene limestones are interbedded within the Mont Conil breccias between 120 and 150 m above sea level. Though Mont Conil seems to be closely linked to Mount Pelee (on the basis of available petrological data), a major uplift must have occurred sometime during or after the construction of Mont Conil and forms a recognizable geological boundary between the two volcanic systems. The Paleo-Pelee edifice was partially destroyed by a flank-collapse event that occurred on the southwest flank during the early stage (> 40,000-20,000 y.B.P.) of Neo-Pelee. This event is evidenced by a large (6 × 2.5 km), horseshoe-shaped structure which is limited by well-preserved morphological scarps on land and by a 300-m offshore scarp, as shown by Seabeam bathymetry. Paleo-Pelee breccias and lava flows are truncated by the boundary of the collapse structure. Lava domes occupied the upslope part of the structure at a later time. These domes are intersected by the present crater (“Etang Sec”), which has probably been the active vent for at least 2000 years. The active crater is located at the intersection between the flank-failure depression and an older caldera formed at the Paleo-Pelee stage but probably enlarged later. The flank-failure structure appears as a major structure of the volcano and creates a strong structural asymmetry in the edifice. This fact should be considered when dealing with such various aspects as the hydrogeology of the volcano, the structural control of past and future eruptions, and the volcanic hazards.

Stratigraphy of the 1902 and 1929 nuée-ardente deposits, Mt. Pelée, Martinique

Mild nuee ardentes in 1902–1903 and 1929–1930 at Mt. Pelee formed high-aspect ratio (H.A.R.) deposits (i.e. deposits with a high thickness/extent ratio) in the Riviere Blanche channel. Several violent nuees ardentes from May to August, 1902, yielded low aspect ratio (L.A.R.) deposits unevenly blanketing an area of 58 km2 on the southwestern flank of the volcano. Three closely spaced nuees ardentes on August 30 additionally affected 56 km2 on the southeastern flank. The L.A.R. deposits are divided into eight stratigraphic units (U1-U8), either compound or consisting of a single flow unit. The deposits of the largest nuees ardentes, on May 8, 20 and August 30, 1902 are identified as U1, U3 and U8, respectively, and can be traced over most of the area. Individual L.A.R. flow units are lenticular and show large, erratic lateral variations of thickness and grain-size. Most flow units consist of three distinct beds: Bed 1 is a few centimeters thick, lenticular, fines-depleted layer. Bed 2 is the thickest layer and contains more silt-sized ash than bed 1. It is normally graded as a whole, though reverse grading may occur in the lower part. Bed 2 generally consists of two parts: bed 2a, nonbedded, and bed 2b, finer-grained and laminated with cross-bedding and duned structures. Bed 3 is a thin, silty, capping layer which contains accretionary lapilli. The distribution of the L.A.R. flow units, especially of the coarse-grained facies, suggests that they were formed from overpressured blast-flows expanding radially from the vent at a high rate, in the same manner as the laterally directed blast of Mount St. Helens on May 18, 1980. The transport system of the major 1902 nuees ardentes was an inflated, relatively low-concentration, turbulent cloud as suggested by several lines of evidence: (1) the transported material was able to cross ridges up to 300 m high; (2) many scour-and-fill and other erosional features can be observed in the deposits at any distance from the vent; (3) survivors within the devastated area on May 8 did not experience a dense, low-profile cloud but instead a choking, hot ash-laden cloud; (4) many human corpses in St. Pierre remained where they fell instead of having floated away. If the current flow-surge terminology were to be used, the 1902 blast-flows would be better considered as pyroclastic surges. The normally graded, multi-layered structure of the flow units is also consistent with deposits primarily formed from relatively low-concentration flows.

Observations, stratigraphy and eruptive processes of the 1990 eruption of Kelut volcano, Indonesia

Abstract The February 10, 1990 eruption of Kelut volcano (eastern Java) reportedly began with seven discrete, short-lived explosions between 11.41 and 12.35 local times. Deposits of this initial, phreatomagmatic stage include a basal ash-fall layer (unit A1), widespread pumice surge deposits (unit S) and related pisolitic ash layer (unit A2). The main, plinian phase of the eruption lasted about 4 hours from 12.35 and produced pumice-flow deposits (unit PF) overlain by a pumice fallout layer distributed mainly to the southwest (unit P), and intra-plinian scoria-flow deposits (unit SF). Uppermost scoria-rich ash fall layers (unit A3) likely relate to late, discrete eruptive pulses. A few small explosions resumed on February 11 and 12 leaving no recognizable deposit. An embryonic lava dome had formed in the crater bottom by April, then was submerged by the new crater lake. Destruction of the summit area resulted from emplacement of the pre-plinian pumice surge up to 4–5 km on the south and west flanks, and of the early plinian pumice flows up to 1–2 km radially from the crater, before these were channelized in the main valleys to further travel 3 km. Most of the 32 human deaths resulted from roof collapse under the load of fallout tephra beyond the devastated area, which had been evacuated before the eruption began. The eruption produced 0.13 km 3 of tephra, of which 0.12 km 3 represent the products of the plinian phase. The average eruptive rate of the plinian phase is estimated to have been ∼7.5×10 6 kg/s magma DRE. The pumice flows are interpreted to have been formed due to unsteadiness and low velocity of the eruptive column at the beginning of the plinian phase. The intra-plinian scoria flows incorporate either more degassed or colder juvenile magma; they were presumably erupted at the edge of the column, due to fluctuations in the mass flux and in pressure in the conduit.

Caractérisation et stratigraphie de dépôts volcanoclastiques marqueurs dans le Miocène supérieur du bassin de Melilla-bas Kert (Rif oriental, Maroc)

Abstract Pumice-rich volcaniclastic layers of primary (eruptive) origin belonging to the Gourougou volcanic complex are interbedded in the Tortonian-Messinian sediments of the Melilla-lower Kert Basin. Ten tuffs have been identified by combining field criteria and petrographical and mineralogical fingerprinting. The extents of the tuffs have been mapped and their relative stratigraphy has been established. This tephrostratigraphy provides a potentia tool for stratigraphic correlations in the Gourougou volcanic complex and in the basin.