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Acid gas and metal emission rates during long-lived
basalt degassing at Stromboli Volcano
Patrick Allard, Alessandro Aiuppa, Henri Loyer, Francine Carrot, André
Gaudry, Guy Pinte, Agnès Michel, Gaetano Dongarrà
To cite this version:
Patrick Allard, Alessandro Aiuppa, Henri Loyer, Francine Carrot, André Gaudry, et al.. Acid gas and
metal emission rates during long-lived basalt degassing at Stromboli Volcano. Geophysical Research
Letters, American Geophysical Union, 2000, 27 (8), pp.1207-1210. �10.1029/1999GL008413�.
�hal-03122698�
GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 8, PAGES 1207-1210, APRIL 15, 2000
Acid Gas and Metal Emission Rates during Long-lived Basalt
Degassing
at Stromboli Volcano
Patrick
Allard
1, Alessandro
Aiuppa
2, Henri
Loyer
3, Francine
Carrot
4,
Andr6
Gaudry
4, Guy
Pinte
4, Agngs
Michel
4 and
Gaetano
Dongarret
2
Abstract. The discharge of acid gases and metals from Stromboli is determined from airborne and ground-based filter sampling of particulate matter in the volcanic plume,
combined with COSPEC measurements of SO2 fluxes.
Smaller particle sizes and high enrichment factors distinguish the most volatile elements (by order: S, Se, Br, C1, Cd, Bi, In, As, Sb, Sn, F, Au, Pb, Cr, Cu) from those strictly (Fe, Mn, REE, Sc, Sr, Th, Ti, V) or mainly (A1, Ba, Ca, Co, K, Na, U) derived from volcanic ash. Time-averaged volatile fluxes show that Stromboli is a representative arc emittor, producing
1-2% of the global volcanic budget of sulfur, halogens and
several trace metals, while 15-25% of volcanic emissions of
Bi, Cd, Cs, Pb and Sn in southern Italy. Subaerial degassing of its S-Cl-rich shoshonitic magma over the last 2 ky of similar activity may have released as much copper and gold as is encountered in magma-derived high-sulfidation ore deposits.
1. Introduction
Quantifying the emission rates of acid gases and metals from active volcanoes is of interest to better knowledge of global geochemical cycles [e.g. Nriagu, 1989], chemical partitioning during magma degassing and the genesis of igneous ore deposits [Hedenquist and Lowenstern, 1994]. Stromboli, in the Aeolian island arc (southern Italy), is one suitable target for such studies. For over 2000 years, K-rich (shoshonitic) basalt has been continuously degassing and erupting through the summit crater of this 2.5 km high (900 m asl) strato-volcano [Barberi et al., 1993], generating an ash- laden plume that spreads over the surrounding Tyrrhenian Sea. The SO2 plume output, representative for all sulfur, has been measured several times by airborne correlation spectrometry (COSPEC) [Allard et al., 1994; Allard et al., 1998]. Yet no or few data exist for other species. He we report the first detailed study of the concentrations and fluxes of
metallic and non-metallic elements in Stromboli's emissions,
relating COSPEC data with unpublished [Allard et al., 1995] and new analyses of particulate matter in the volcanic plume. The results concern eruptive activity ranging from low (June 1997), medium (June 1993) to high (July 1994) and allow us to infer average fluxes representative for long-lived basalt degassing at this volcano.
•LSCE,
CEA-CNRS,
GiftYvette,
France
2CFTA,
Universit/•
di Palermo,
Palermo,
Italy
3MSIS, Zone de Courcelles, GiftYvette, France
4Laboratoire Pierre Stle, CEA-CNRS, CE-Saclay, GiftYvette, France
Copyright 2000 by the American Geophysical Union. Paper number 1999GL008413.
0094-8276/00/1999GL008413505.00
2. Experimental Procedures
Gaseous and particulate matters in Stromboli plume were
collected both from the crater rim and from a laboratory aircraft, by pumping at controlled rate through 47 mm wide,
0.2 gm cellulose filters. These were either dry (metals) or
impregnated with a 1 M soda solution to collect acid gases.
Filtered
volumes
ranged
from
1 to 8 m
3. Crater
sampling
was
done with a battery-powered portable pump, in series with filter packs and a flow meter. Airborne sampling was made
during flights along the horizontal plume axis (from above the
crater to 5-6 km distance), through an isokinetic probe
preventing any size-fractionation of aerosols and fixed ahead
of any possible engine contamination. The samples were first examined by energy dispersive X-ray (EDXR) and, after preparation, were analysed by ion chromatography (S, CI, F), instrumental neutron activation (INA) and ICP-MS (in Pierre S0e Laboratory, CE-Saclay). The analytical conditions and the
results, corrected for blanks, are given in Table 1. As a
reference for Stromboli basalt, we also analysed by INA and ICP-MS a bulk lava flow sample from the 1985-86 eruption
(Table
1), by far the largest
effusion
in the 20
th
century
[De
Fino et al., 1988].
3. Particle Sizes
Two types of particles were observed on the filters: i) large (-5-40 gm), angular shaped fragments of silicate composition (volcanic glass, crystals), derived from the ash blasted during the recurrent explosions and/or entrained by the wind; and, coating these former, ii) sub-micron sized grains (0.2-1 gm) and larger clusters (1-5 gm) of metal-rich sulfate incrustations, sulfate minerals (alunite, anhydrite) and halides (sylvite, halite) which were clearly derived by condensation of volcanic vapours. Compared to crater filters, airborne filters contained less abundant particles, relatively enriched in the finest size fractions. Airborne sampling with a multistage cascade impactor on June 14, 1997 (Fig. 1) shows that 66% of the plume aerosol load was due to sub-micron sized particles (0.013-1.2 gm), while only 5% to particles larger than 5.75 gm (in full agreement with the proportion of silicate material inferred from chemical results; see below and Table 1).
4. Concentration and Volatility of Elements
The absolute concentrations of elements detected on crater filters broadly correlate with the intensity of volcanic activity on the respective dates (Table 1). Elements identified on
airborne filters are fewer and much less concentrated, due to
enhanced plume dilution (x10-40). Nevertheless, on both filter types most elements are highly enriched compared to their concentration in the Mediterranean marine atmosphere at the
1208 ALLARD ET AL.: ACID GAS AND METAL EMISSIONS RATES 20.
DIAMETER CLASS (I.l,m)
Figure 1. Mass distribution
of particles
with size ranging
from 0.013 to 7.75 pm in Stromboli plume (June 14, 1997).
Airborne sampling with a multistage quartz cascade
impactor
combined
with a 6-stages
SDI 2000 diffusional
spectrometer
[Allard et al., 1998]. See text.
altitude of Stromboli crater [Seinfeld and Pandis, 1998]. Only
for sodium
was the air background
(1.53 I.
tg/m
3) either
prevalent (airborne samples) or important (1.5-36% of Na on
crater
filters;
Table
1). Its influence
upon
chlorine
(2.6 •tg/m
3)
was small in airborne samples (4-12%) and negligible in crater ones. All other elements are entirely volcanic in origin.
In order to eliminate the effect of variable plume dilution and to discriminate the volatile contribution from that of solid
particles, we computed the enrichment factor (EF) of each element in the plume (p) with respect to the erupted degassed
basalt (b), in reference to the concentration of bromine:
EF•
= 105
(X/Br)•/(X/Br)•
Br is one most volatile element under magmatic conditions,
accurately determined by INA, and is a trace constituent of
volcanic
ash
(its content
is multiplied
by 105
to account
for
this feature [Crowe et al., 1987]). The basalt composition is
assumed representative for the ash component, as was verified
by De Fino et al. (1988) during the 1985-86 eruption. The mean EF values (Table 1) allow to distinguish three categories of elements (listed in alphabetic order): a) highly enriched, purely volatile elements (LogEF >3 to 6: As, Au, Bi, Br, Cd, CI, F, In, Pb, S, Se, Sb and Sn), likely occurring in the finest
particles formed by gas condensation. S, Se, Br, CI, Cd, Bi
and In are the most enriched ones; b) non-enriched, typically lithophile metals (LogEF <1.0-1.5: Fe, Mn, REE, Sc, Sr, Th, Ti and V), which were thus carried by the silicate particles.
This also applies to a main fraction of AI, Ba, Ca, Co, K, Na
and U (LogEF <2). The amount of collected ash (given in Table 1) and its contribution to each element were computed by cross-correlating the concentrations of the best lithophile markers (Sc, La, Sm, Th and Ce) with the basalt composition; and c) a group of mildly enriched, moderately volatile elements of possible 'mixed' origin (Ag, Cr, Cs, Cu, Li, Ni, Rb, Te, W and Zn; LogEF >2 to <3). We infer a minor silicate contribution to Cr, Cs, Cu, Rb and W (0-5%), but a significant one to Li, Te and Zn (10%), Ag (15%) and Ni (21%).
The volatile elements unaffected by ash loading display consistent EFs in airborne and crater samples obtained on the same day. This implies a uniform chemistry of gases emitted by the separate vents feeding the plume (integrated in airborne
samples) and, so, a common source for their conduits. The pulsated but variable explosive activity of Stromboli is reflected in the quite large EF range of several elements (Table 1). However, apart from a relative increase of As, Cd and Se during lava fountaining in July 1994, we find no systematic chemical trends with volcanic activity in the period investigated. Rather, ratios such as Bi/Pb, Cu/Pb, Sb/Pb, Cu/Br but especially S/C1 (1.2-1.8) and F/Br (18-24) varied limitedly. We note that S/CI was lower than previously measured (2.5-2.9; Allard et al., 1994) and was closer to the ratios for intermediate (1.3) to extensive (1.8) degassing of the shoshonitic basalt, as constrained by its original and post- eruptive contents in sulfur and chlorine [Allard et al., 1994; Allard and M•trich, 1999 and in prep.].
The Br-normalized enrichment factors of volatile trace elements in Stromboli plume are for a peculiar S-Cl-rich arc basalt but are comparable, within an order of magnitude, to those determined at other degassing volcanoes [e.g. Crowe et al., 1987; Andres et al., 1993]. The emanation coefficient (œ)
of these elements from the magma can be assessed from their
mean EF and the emanation coefficient of Pb from molten basalt (0.01; Pennisi et al., 1988), as:
(œ,•)4
= 1 + [(EFWEF,•)(1-œv•)/œv•]
The results (Table 1) are coherent with estimates at other basaltic volcanoes [Pennisi et al., 1988; Rubin, 1997;
Gauthier and Le Cloarec, 1998], even though numerically
different. They outline a greatest volatilization of Se (41%),
followed by Br (33%), Cd (17%), Bi (11%), In (9%), As
(5%), Sb (4%), Sn (2%) and Au (1.5%). Among alkalis, the
greater volatility of heaviest Cs and Rb (Cs>Rb>>K>Na) is
consistent with a lower boiling temperature of their respective halides, as also observed at Mt. Etna [Gauthier and Le
Cloarec, 1998]. Finally, our 1997 data confirm a slight but significant degree of volatilization of uranium (0.06%) from
Stromboli magma [Allard et al., 1995].
5. Volatile Fluxes
Mean SO2 fluxes of 340, 820 and 170 Mg/d were measured during medium, high and low eruptive activity in June 1993, July 1994 and June 1997, respectively (Table 2). These and
previous
data
provide
an upgraded
estimate
of (1.1+0.3)
105
Mg/yr for Stromboli's time-averaged SO2 discharge over two decades [Allard et al., 1998]. Based on the elemental/S ratios on crater filters, corrected for ash particles and considering all sulfur derived from SO2, we can derive both the single-date and time-averaged output of volatile metals and other elements (Table 2). The metal flux variations correlate with the intensity of eruptive activity, in agreement with a key role of the gas phase in the volcano dynamics [Allard et al., 1994]. We find that, on average, Stromboli produces 1-2% of the global volcanic yield of SO2, As, AI, Bi, Br, Cd, Cs, Cu, Ni, Pb, Rb, Se, Sn and Zn, which is a proportion quite typical for steadily degassing are volcanoes [e.g. Symonds et al., 1987; Le Cloarec et al., 1992; Symonds et al., 1992]. Its contribution to volcanic emissions in southern Italy is broadly smaller than that of Mt. Etna, the greatest volcano emittot on Earth [e.g.
Allard et al., 1991; Andres et al., 1993; Gauthier and Le
Cloarec, 1998], but is significant (15-25%) for a few metals
such as Bi, Cd, Cr, Cs, Pb and Sn.
Heterogenous basalt degassing at Stromboli results in the average emission of more gas than lava by mass [Allard et al.,
ALLARD ET AL.: ACID GAS AND METAL EMISSIONS RATES 1209
Table 1. Concentration,
enrichment
factor
and
emanation
coefficient
of elements
in Stromboli
volcanic
plume
Volcanic
plume
Q.t•/m
3)
Erupted
Enrichment
Emanation
STR 93 STR 94 STR 97-1 STR 97-2 STR 97-3 GPF F 4 F 6 basalt' factor b coeff.'
06/06/93 01/07/94 13/06/97 14/07/97 28/09/97 06/06/93 13/06/97 14/06/97 1986 Log EFBr %
crater crater crater crater crater aircraft aircraft aircraft gg/g mean s.d. mean
S* 1060 4540 963 na na 166 41 66 40 ø 6.06 0.41 98 • CI* 870 2910 523 105' 243* 63(60) 22(19) 54(51) 550* 4.63 0.37 74 • F* 57 132 14 na na bd bd bd 600* 3.55 0.07 12 • Ag# <0,004 <0.006 0.0015 <0.009 0.0008 bd bd bd 0.51 2.53 0.07 0.14 AI 75 na 52 bd 125 bd 2.7 2.7 102600 ø 1.61 0.42 0.016 As 0.39 3.0 0.20 0.96 0.63 <0.015 0.008 0.019 4.3 3.73 0.63 5 Au 0.0026 <0.002 0.0039 0.0007 0.0002 0.00075 <0.0001 <0.0001 0.07 3.36 0.51 1.5 Ba# <1.5 8.9 1.3 1.4 0.15 bd bd bd 1073 ø 1.97 0.35 0.05 Bi# na 0.17 0.042 0.026 0.005 na na na 0.14 4.26 0.29 11 Br 2.3 6.3 0.8 0.9 0.5 0.12 0.3 0.4 1.0 5.00 0.00 33 Ca <15 na 93 89 11 bd bd bd 65900 1.98 0.39 0.03 Cd# na 0.48 0.022 0.017 0.0013 na na na 0.07 4.41 0.61 17 Ce 0.045 na 0.053 0.021 0.015 bd bd bd 100 1.48 0.24 - Co 0.057 0.037 0.016 0.030 0.0015 0.0026 0.0029 0.0015 32 1.54 0.40 0.017 Cr 1.9 na 0.47 3.5 na 0.09 0.15 0.083 65 3.05 0.41 0.71 Cs 0.026 0.18 0.021 0.012 0.003 0.0029 0.032 0.019 3.8 2.79 0.40 0.44 Cu# 2.2 3.0 0.87 1.1 0.14 na na na 87 2.90 0.28 0.39 Eu 0.0005 0.0008 0.0008 0.0012 <0.0017 bd bd bd 2.1 1.30 0.49 - Fe 38 37 41 29 1.8 1.9 2.2 2.9 53000 1.36 0.39 0.008 In 0.083 na 0.0039 0.0020 0.0032 bd bd bd 0.05 4.15 0.51 9 K 30 na 25 9.2 5.9 bd 3.3 2.9 17300 1.86 0.21 0.026 La 0.026 0.018 0.024 0.006 0.005 <0.003 0.0016 0.0004 50 1.09 0.48 - Li# na na 0.081 0.009 0.032 na na na 15.9 2.39 0.55 0.15 Mn 1.5 na 0.52 0.33 0.04 <0.5 na na 1320 ø 1.40 0.43 0.0089 Na 30(28.5) 100 17(15.5) 4.8(3.3) 4.2(2.7) <3 1.2(0) 1.5(0) 18570 1.65 0.29 0.017 Ni# 0.9 <0.4 0.004 2.2 na 0.26 bd bd 43.9 2.86 1.25 1.3 Pb# 0.9 1.3 0.51 0.36 0.12 na na na 16.2 3.33 0.20 1.0 Rb# 0.15 0.63 0.33 0.25 , 0.47 <0.06 na na 68.9 2.53 0.47 0.27 Sb 0.010 0.033 0.009 0.015 0.027 0.0015 bd bd 0.25 3.68 0.40 4 Sc 0.005 0.0048 0.0092 0.0023 0.00028 0.00022 0.00036 0.00036 29 0.76 0.41 - Se 0.06 0.74 0.081 0.069 0.036 0.0052 0.19 bd 0.05 5.27 0.46 41 Sm 0.0045 0.0016 0.0033 0.0012 bd 0.0001 bd bd 8.2 1.16 0.45 - Sn# 0.35 0.11 bd 0.042 bd <0.75 bd bd 0.7 3.69 0.65 2 Sr# 1.9 0.56 0.46 0.12 0.025 bd bd bd 714 ø 1.43 0.53 0.013 Te# na 0.02 0.014 0.006 0.005 na na na 0.7 3.05 0.31 0.52 Th 0.0049 <0.0018 0.0069 0.0015 bd <0.001 0.0003 na 14.2 1.26 0.35 - Ti# 4.1 2.6 na na na <2 na na 5760 1.17 0.45 - U# 0.02 0.024 0.0064 0.0004 0.002 <0.009 na na 4.0 1.93 0.53 0.06 V 0.075 na 0.14 0.051 bd <0.04 <0.033 na 250 ø 1.43 0.37 0.0026 W <2.5 0.1 <0.01 <0.008 bd bd bd bd 2.5 2.83 0.00 0.52 Zn 5.9 <0.4 1.2 8.7 0.04 0.28 0.048 0.046 112 2.82 0.80 0.83 Total 2181 7747 1735 255 392 232 72 153 Ash œ 531 363 480 126 100 8 33 9
Elements
(but
S, C1
and
F) ranked
in alphabetic
order.
Concentrations
are
corrected
for filter
blanks
determined
in identical
experimental
conditions and for air background in case of Na and C1 (in brackets). ha: not analysed. bd: below detection (detection limit indicated for
elements
of specific
interest).
*NaOH-impregnated
œfiters,
analysis
by ion
chromatography
(+_5%);
+INA
analysis
of dry
filters
(particulate
CI only).
#Analysis
by ICP-MS
(Plasmaquad
Fison
PQ2+),
with
In and
Rh internal
standards
(accuracy:
+_3-5%),
after
filter
attack
with
ultra-pure
HF-•O3-HCI at 110øC.
Other
elements
analysed
by INA (Osiris_
reactor,
CE-Saclay;
accuracy:
+5-10%,
depending
on
elements):
filter
irradiation
in quartz
container
under
neutron
flux
of 10
TM
n cm
'2 s
4, gamma-ray
spectra
acquired
with
ultra-pure
Ge and
Ge(Li)
detectors
and
processed
with
the
k0
method
[.Piccot
et al., 1993].
aAnalysed
by INA and
ICP-MS,
using
internal
calibration
with
GS-
N and BE-N rock standards. øDe Fino et al., 1988. *Bulk concentration for a 45% crystal content [Allard et al., 1994; Allard and M•trich,
1999].
bBr-normalized
mean
enrichment
factor
in both
crater
and
airborne
samples
(see
text).
tin reference
to œv•
= 0.01 [Pennisi
et al.,
1988],
at•er
correction
for ash
particles
(see
text);
õFrom
pre-eruptive
and
residual
contents
in Stromboll
basalt
[Allard
et al., 1994;
Allard
and
M•trich,
1999
and
in prep.];
blanks
are
for
strictly
lithophile
elements.
œBulk
ash
amount
collected.
1994, 1995]. This degassing also produces greater quantities
of several
metal,
by a factor
10 (Au,
Pb,
Zn) to 102
(Bi, Cd),
than lava extrusion
(Table 2). At the present-day
rates
estimated
here,
(1-2)x105
Mg of Cu, Cr and
Zn, 7x10•Mg of
Pb and 3x102
Mg of Au could
have been
released
in the
atmosphere over the last 2 ky of similar activity. The figures
for Cu and Au are of the same order as the amounts
accumulated
in magma-derived
high-sulfidation
ore deposits
[Hedenquist
and Lowenstern,
1994]. Despite obvious
differences in the fate of elements, we thus outline that long-
lived subaerial
degassing
of Stromboli-like
S-Cl-rich
basaltic
magma
may
be as productive
in Au, Cu and
other
trace
metals
as underground
degassing
of a felsic pluton
with a few ky
1210 ALLARD ET AL.: ACID GAS AND METAL EMISSIONS RATES
Table 2. Volatile and solid fluxes of elements at Stromboli
6/06/93 1/07/94 13/06/97 Average Solid % of
medium high low activ. flux' output b global
Mg/d Mg/d Mg/d Mg/yr Mg/yr volc. c SO2'
st. dv. 8.0E+01 2.1E+02 3.0E+01
3.4E+02 8.2E+02 1.7E+02 1.1E+05 2.8E+01 0.9
3.0E+04 - 0.2
CI 1.4E+02 2.6E+02 5.0E+01 3.7E+04 1.2E+02 0.03-9 F 8.6E+00 1.2E+01 1.6E+00 1.8E+03 1.1E+02 0.07-7 K 3.4E+00 nd 1.SE+00 1.0E+03 3.2E+03 0.4 AI 3.8E+00 nd 2.1E-01 6.8E+02 1.9E+04 0.8 Na 1.0E+00 8.0E+00 5.8E-01 5.9E+02 3.4E+03 0.1 Zn 9.3E-01 nd 9.8E-02 1.3E+02 2.0E+01 2.0 Br 3.7E-01 5.7E-01 7.1E-02 9.8E+01 1.BE-01 0.1 Cu 3.4E-01 2.7E-01 7.3E-02 7.5E+01 1.6E+01 1.7 Cr 2.9E-01 nd 3.9E-02 6.0E+01 1.2E+01 0.8
Ni 1.4E-01 nd nd 4.5E+01 8.0E+00 0.7
Pb 1.4E-01 1.2E-01 4.4E-02 3.5E+01 3.0E+00 1.4 As 6.2E-02 2.7E-01 1.7E-02 2.2E+01 7.8E-01 1.2 Rb 1.9E-02 5.5E-02 2.6E-02 1.0E+01 1.3E+01 1.0 Se 9.6E-03 6.7E-02 7.2E-03 5.5E+00 9.1E-03 1.2 Cd nd 4.3E-02 1.9E-03 5.6E+00 1.3E-02 1.4 Li nd nd 6.5E-03 4.2E+00 2.9E+00 -
Bi nd 1.5E-02 3.7E-03 3.8E+00 2.6E-02 1.3
Sn 5.6E-02 1.0E-02 nd 3.7E+00 1.3E-01 1.5 In 1.3E-02 nd 3.5E-04 2.4E+00 9.1E-03 - Cs 3.8E-03 1.6E-02 1.7E-03 1.5E+00 6.9E-01 2.5
W nd 9.3E-03 nd 1.2E+00 4.6E-01 -
Co 6.6E-03 2.3E-03 3.5E-05 8.2E-01 5.8E+00 0.2 Te nd 1.8E-03 1.2E-03 5.0E-01 1.3E-01 -
U 2.9E-03 2.0E-03 4.0E-04 4.9E-01 7.3E-01 - Sb 1.5E-03 3.0E-03 8.0E-04 4.7E-01 4.6E-02 0.1
Au 4.2E-04 nd 3.4E-04 1.8E-01 1.3E-02 -
Ag nd 5.3E-04 1.1E-04 7.2E-02 9.3E-02 - *Daily mean SO2 plume fluxes measured by COSPEC from an aircra• (+15%) in 1993 (n=14) and 1997 (n=19) and from a boat (+25%) in 1994 (n=34) (Allard et al., 1994; Allard et al., 1998 and
unpub. data). 113 standard deviation includes true volcanic variations and propagates to flux estimates for other species (_+30-50%). nd: not determined. aFrom time-averaged SO2 flux and average X/S ratios on
crater
filters,
corrected
for ash particles
(see
text).
bFrom
basalt
composition
in Table
1 and a mean
solid
output
of 1.8 105
Mg/yr
during explosions and lava flows (Allard et al., 1994 and referencestherein). CStromboli's contribution to global volcanic emissions (data
source: B luth et al., 1993; S ymonds et al., 1 988; Cadle, 1 980; Nriagu, 1989, but with revised S data from B luth et al., 1993; Gauthier & Le Cloarec, 1998). Blanks: no reliable data available.
Acknowledgments.
This
work
benefited
from
European
Community
supports
(contracts
EV5V-CT92-177
and
ENV4-CT96-288)
and
from
helpful
local
assistance
of Catania's
aeroclub.
We are
grateful
to M.
Beaulieu
(our
acrobatic
pilot)
and
E. Robin
for EDXR
analysis.
J.W.
Hedenquist
and
P.R. Kyle's
reviews
helped
us to improve
the
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(Received July 23, 1999; revised January 7, 2000; accepted January 24, 2000.)
