G. Boudon1,2, B. Villemant4, J-C. Komorowski1, Ph. Ildefonse3, G. Hammouya5, M. Semet1,4
1Institut de Physique du Globe de Paris, Obs. Volcanol. et Dˇpt de Volcanologie, 4, Place Jussieu, 75252 Paris Cedex 05, France
2IPGP, Dˇpt des Gˇomatˇriaux, CNRS URA 734, 4 Place Jussieu, 75252 Paris Cedex 05, France
3Lab. de Mineralogie et Cristallographie, Univ. Paris VI et VII, CNRS UA S109, et IPGP, 4 Place Jussieu, 75252 Paris Cedex 05, France
4Lab. de Gˇochimie Comparˇe et Systˇmatique, CNRS URA 1758, IPGP et Univ. P. et M. Curie, 4 Place Jussieu, B109, 75252 Paris Cedex 05, France
5Obs. Volcanol. de la Soufriere de Guadeloupe, IPG, 97113, Gourbeyre, La Guadeloupe, F.W.I.
Determination of the physical and chemical characteristics of an active hydrothermal system are fundamental to the understanding of the nature of past and future eruptive processes. Interactions between the hydrothermal system and ascending new magma can modify the physical and chemical characteristics of the magma and host-rock thus influencing eruptive style (e.g. 650 B.P. Mt. Pele eruption; Villemant et al., 1996). Flank-collapse events occur frequently and repeatedly on volcanoes hosting a voluminous hydrothermal system weakening the edifice. Flank-collapse of Soufriere Hills Volcano (Wadge and Isaacs, 1988) produced the horseshoe-shaped English's Crater and previously undescribed debris avalanche deposits observed in the eastern seaside cliffs in 1988-89 and recently in the floor of English's Crater for brief periods (July 1996) as a result of denudation by hot pyroclastic flows and surges associated with dome growth (GVN, 1995, 1996).
Several fumarolic areas and hot-springs were active before 1995.
Hot-springs, fumarolic gases, and products of hydrothermal alteration from
the active areas of Tar River Soufriere (TRS) and Galway's Soufriere (GS)
were sampled in detail in 1988-89 (Boudon et al., 1996) and since then on
a semi-regular basis for fluids. Fluids have been analyzed on a
collaborative basis at the Soufriere of Guadeloupe Volcanological
Observatory. Rocks have been intensely hydrothermally altered (abundant
white to subordinant ochre coloration, grey to black near vents or
hot-springs) in fumarolic areas characterized by emission of gases
(96-98C) saturated in H2O, rich in CO2 and
H2S but poor in SO2 and H2, and very
acidic condensates (pH< 2). The primary anion in hot-springs (ca. 81 to
36C; 2.4
Primary host-rock minerals at TRS and GS have been transformed into
polymorphs of SiO2 (opal-A, opal CT, cristobalite, quartz), titanium oxide
(anatase), sulphates (natroalunite: (Na,K)Al2(SO4)2(OH)6, natrojarosite:
NaFe3(SO4)2(OH)6 in ochre areas, gypsum, anhydrite). Pyrite is present in
dark areas. Clay minerals are absent except as accessory phases at GS
(kaolinite, Al-smectite, mixed layer kaolinite-smectite with 75%
smectite). Alteration has led to a local reduction of the host-rock
porosity. Native sulphur (from oxidation of H2S), alunogen (Al2(SO4)318H2O), halotrichite (FeAl2(SO4)4.22H2O), opal A, and cristobalite occur
at the fumarolic vents. Such parageneses are typical of alteration
processes of unsealed acid-sulphate systems in the outgrowths of deep
hydrothermal systems. Acid-sulphate alteration implies:
Clays minerals can only form when solution pH is less acidic than at TRS
and GS. Fumarolic activity usually produces vertical and horizontal
mineral zonation, i.e. siliceous residue (opal-A + quartz) -
alunite+kaolinite - kaolinite + smectite - illite/smectite (at depth).
Thus mineralogical assemblages at TRS and GS only represent the
near-surface alteration zone of a more important hydrothermal system.
Primary phases (in wt % from quantitative XRD analysis) in the < 63
micrometers size-fraction of phreatic ashes (Jul. 27-vent 1, Aug. 21-vent
3, Sep. 7, Sep. 17 1995) are: plagioclase (77-82), quartz (17-8),
cristobalite (8-2), gypsum (4-0), and pyrite (2-1). Clay minerals are
absent. Total silica polymorphs (quartz+cristobalite) contents are high
and constant (17- 19 wt %) in all samples. Quantitative XRD determination
of crystalline silica content in separates of the respirable fraction (< 5
micrometers) for accurate assessment of human toxicity of the airborne
phreatic ash failed on our limited samples.
Castle Peak dome, 1995-96 domes, and 1995 phreatic tephra are chemically
similar except for highly mobile elements (Au, As, Br, Mo, W, Ag). Tephra
show systematic As, Br, Sb, Mo, W enrichment. Castle Peak dome and tephra
have extreme Au contents. Thus tephra represent mixtures of dome magma
and material strongly enriched in mobile and volatile elements.
Hydrothermal material from TRS show a quasi-systematic strong depletion in
all measured elements except highly incompatible Ta, Hf, to a lesser
degree Th and U, which are little or unmodified. The enriched component
of tephra is clearly distinct from the signature of hydrothermalized
material and was likely inherited from deep hydrothermal fluids.
Under the binocular microscope, phreatic tephra (Jul. to Sep. 1995)
consist of a heterogeneous mixture of porphyritic strongly to weakly
altered (grey, reddish brown, greenish grey) vitreous and crystalline
fragments in varying proportions. Minerals are magmatic (plagioclase,
quartz, pyroxene, amphibole) and hydrothermal (sulphates, opal,
cristobalite, quartz, pyrite) in origin. The most conspicuous
characteristic of this tephra is the ubiquitous presence of irridescent to
bright metallic-yellow pyrite as isolated subhedral crystals up to 250
micrometers in size or irregular specks on the surface of vitreous
fragments. This is the distinctive criterion together with the abundance
of hydrothermal phases that fingerprints the non-juvenile origin of this
tephra. Vitreous grains in tephra from ash-clouds associated with
gravitational destruction of the growing dome are texturally similar to
vitreous grains from phreatic tephra (chiefly comminuted Castle Peak dome)
but lack hydrothermal alteration phases.
Microtextural and microchemical analysis of polished resin-impregnated
grains (1000-125 micrometers) from these samples using the SEM's
Backscattered Electron Imaging mode confirm the presence of pyrite as the
only abundant sulphide. Porphyritic vitreous grains show pervasive
microcrystallization (plagioclase), alteration of the former vitreous
matrix, and precipitation of silica (opal, cristobalite, quartz)
associated with pyrite and other hydrothermal phases (gypsum, alunite) in
most available relict vesicles, cavities, and fractures. Porosity of such
clasts is almost nil. Micro-breccia fragments consisting of altered
vitric clasts (100-300 micrometers) in a matrix of angular microclasts
(10-50 micrometers) cemented by pyrite, banded and hydrated vuggy silica
and other alteration phases are common. Similar textures and mineralogy
have been described in the 1976 phreatic ashes from Soufriere of
Guadeloupe (Heiken et al., 1980), in non-juvenile 1994-95 tephra from
Popocatepetl (Siebe et al., in prep), and to a lesser extent in
hydrothermally-altered material that surrounded the Mt. St. Helens 1980
dacite cryptodome (Komorowski, 1991; Komorowski et al., 1997). The
well-developed hydrothermal textures and mineralization in the 1995
phreatic tephra indicate the presence of an active acid-sulphate
hydrothermal system below Soufriere Hills Volcano whose alteration
products were well and repeatedly sampled by the 1995 phreatic explosions
and mixed with material from the Castle Peak dome complex. SEM studies of
samples from Castle Peak dome and 1995-96 dome rocks show similar
extensive microcrystallization and alteration textures in the glass matrix
with abundant cracked cristobalite infilling voids and former irregular
vesicles. However hydrothermal minerals such as sulphates and
particularly pyrite are absent from fresh Castle Peaks dome rocks sampled
in 1989 and "fresh" 1995-96 dome rocks.
Fine distal ashes (fractions =BE 125 micrometers) from the 17 Sept. 1996
dome explosion collected on Guadeloupe island (85 km) consist of a
heterogeneous mixture of vitric porphyritic light grey to whitish
fragments variably vesiculated (poorly to reticulitic) with numerous
minute black oxide inclusions. They lack surficial pyrite in contrast
with 1995 phreatic tephra. Angular fragments of hornblende, plagioclase
and or quartz predominate. White hydrothermal minerals (gypsum, silica),
and brown to orange clasts with a wavy appearance (natrojarosite ?) are
relatively abundant. Pyrite is present in minor amounts and typically
with white fragments (silica, opalescent lustre). Leachate analysis show
an important sulphate and calcium component and a non-hazardous but marked
fluorine enrichment. Distal tephra only provide a limited analysis of the
products of the Sept. 17 1996 dome explosion. However these observations
clearly indicate that this explosive event involved gas-rich parts of the
interior of the dome and the hydrothermal system in the roots of Castle
Peak dome and/or Castle Peak-conduit interface. Pervasive silicification
(as described at Mount St Helens by Komorowski, 1991, and Komorowski et
al., 1997, and Popocatepetl by Siebe et al., in prep.) has drastically
reduced the porosity of older dome rock hosting the hydrothermal system
and within which magma extrusion has taken place since 1995. This might
be the explanation for the apparent lack of a significant leakage of
magmatic gases (CO, SO2, H2) into the hydrothermal
system as seen in long term monitoring of gas compositions from TRS
(within Englishs Crater) and GS (outside Englishs Crater). An alternative
hypothesis would see the aquifer of the hydrothermal system as an
efficient chemical buffer for magmatic gases released during slow
degassing of the rising magma (Hammouya et al., 1996; Young, 1996). The
dome explosion of Sept 17-18 1996 (McGuire et al., 1996) and associated
pumiceous products indicate that pressurization was maintained locally in
the dome/conduit despite fracturing and degassing related to continued
growth. Correlation of U-Th disequilibrium data (underway) with mobile
element chemistry and microtextural data will better constrain the nature
and extent of interaction at the hydrothermal-magma interface and their
influence on eruptive style. Studies of the similar well-developed active
hydrothermal system of Soufriere of Guadeloupe volcano through systematic
long term sampling of the fluids and altered edifice (in debris avalanche
deposits) will provide an interesting comparative database to understand
the relationships between the hydrothermal system and eruptive processes
at Soufriere Hills Volcano.
We thank the staff of the Montserrat Volc. Obs. for their hospitality and
sharing information on the ongoing eruption. Logistics in 1995-96 were
provided through joint MVO-SRU-BGS monitoring efforts. Supporting
research funds came from the French DBT, PNRN and DIPCN programs, the Obs.
Volcanol. of IPGP. Samples were kindly provided by MVO staff, W. Ambeh,
S. Young, R. Robertson, G. Norton, P. Allard, G. Heiken , J.P. Viode, X.
Sole, M. Feuillard, G. Hammouya, Meteo France Guadeloupe.
References:
Boudon, G., Ildefonse, Ph., Komorowski J-C., Semet, M., Villemant, B. (1996) 2nd Caribbean Conference on Natural Hazards and Disasters, University of the West Indies, Jamaica, October 9-12 1996, abstract-poster
Hammouya, G., Allard, P., Clochiatti, R., Jean-Baptiste P., Parello, F., Semet, M. (1996) 2nd Caribbean Conference on Natural Hazards and Disasters,University of the West Indies, Jamaica, October 9-12 1996, abstract
Heiken, G., Crowe, B., McGetchin, T., West, F., Eichelberger, J., Bartram, D., Peterson, R., and Wohletz, K., 1980. Bull. Volcanol. 43:383-395
Komorowski J-C (1991) PhD, Ariz. State Univ., Univ. Microfilms, Ann Harbor, Michigan, 317 pp
Komorowski J-C, Hoblitt, RP, Sheridan MF. (1997) IAVCEI General Assembly, Abstract submitted
McGuire, W.J., Norton, G.E., Sparks, R.S.J., Robertson, R., Young, S.R., Miller, A.D. (1996) MVO, Special Report 01, URL site http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/
Villemant, B., Boudon, G., Komorowski J-C. (1996) EPSL, 140/1-4:259-267
Wadge, G., Isaacs, M.C. (1988) Journal of the Geological Society of London 145:541-551
Young, S. (1996) IAVCEI News 1/2, 2-4.
Montserrat Volcano Observatory