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Microlite transfer by disaggregation of mafic inclusions following magma mixing at Soufrière Hills volcano,Montserrat 总被引：1，自引：1，他引：0
Madeleine C. S. Humphreys Thomas Christopher Vicky Hards 《Contributions to Mineralogy and Petrology》2009,157(5):609-624
The Soufrière Hills volcano on Montserrat has for the past 12 years been erupting andesite with basaltic to basaltic–andesite
inclusions. The andesite contains a wide variety of phenocryst textures and strongly zoned microlites. Analysis of minor elements
in both phenocrysts and microlites allows us to put detailed constraints on their origins. Compositions of clinopyroxene,
from overgrowth rims on quartz and orthopyroxene and coarse-grained breakdown rims on hornblende, are identical to those from
the mafic inclusions, indicating that these rims form during interaction with mafic magma. In contrast, resorbed quartz and
reversely zoned orthopyroxenes form during heating. Microlites of plagioclase and orthopyroxene are chemically distinct from
the phenocrysts, being enriched in Fe and Mg, and Al and Ca respectively. However, microlites of plagioclase, orthopyroxene
and clinopyroxene are indistinguishable from the compositions of these phases in the mafic inclusions. We infer that the inclusions
disaggregated under conditions of high shear stress during ascent in the conduit, transferring mafic material into the andesite
groundmass. The mafic component of the system is therefore greater than previously thought. The presence of mafic-derived
microlites in the andesite groundmass also means that care must be taken when using this as a starting material for phase
equilibrium experiments. 相似文献
2.
To investigate the influence of microlites on lava flow rheology, the viscosity of natural microlite-bearing rhyolitic obsidians
of calc-alkaline and peralkaline compositions containing 0.1–0.4 wt.% water was measured at volcanologically relevant temperatures
(650–950 °C), stresses (103–105 Pa) and strain rates (10–5 to 10–7 s–1). The glass transition temperatures (T
g
) were determined from scanning calorimetric measurements on the melts for a range of cooling/heating rates. Based on the
equivalence of enthalpic (calorimetric) and shear (viscosity) relaxation, we calculated the viscosity of the melt in crystal-bearing
samples from the T
g
data. The difference between the calculated viscosity of the melt phase and the measured viscosity for the crystal-bearing
samples is interpreted to be the physical effect of microlites on the measured viscosity. The effect of <5 vol.% rod-like
microlites on the melt rheology is negligible. Microlite-rich and microlite-poor samples from the same lava flow and with
identical bulk chemistry show a difference of 0.6 log10 units viscosity (Pa s), interpreted to be due to differences in melt chemistry caused by the presence of microlites. The
only major differences between measured and calculated viscosities were for two samples: a calc-alkaline rhyolite with 1 vol.%
branching crystals, and a peralkaline rhyolite containing crystal-rich bands with >45 vol.% crystals. For both of these samples
a connectivity factor is apparent, with, for the latter, a close packing framework of crystals which is interpreted to influence
the apparent viscosity.
Received: 14 March 1996 / Accepted: 30 May 1996 相似文献
3.
The pyroclastic deposits of the 1300 B.P. eruption of Newberry Volcano, OR, USA, contain minor amounts of obsidian (1–6 wt.%).
The volatile (H2O and CO2) contents and textures of these clasts vary considerably. FTIR measurements of H2O in obsidian pyroclasts range from 0.1 to 1.5 wt.% indicating equilibration pressures ≤20 MPa. CO2 contents are low (<10 ppm) except in clasts that also contain xenolith powder that provided a local CO2 source. Obsidian clasts exhibit a range of color and textural types that changed in relative proportion as the eruption progressed.
Together these data indicate that there were multiple origins of obsidian and that the dominant source changed during the
eruption. Early in the eruption, obsidian was almost entirely black or grey (microlite-bearing) and probably derived from
dikes or wall rock fractures filled with vanguard magma or tuffisite that, together with wall rocks, were eroded and incorporated
into the eruption column as the vent widened. Later in the eruption, following a brief cessation of activity, the proportion
of obsidian to wallrock lithic clasts increased and new types of obsidian dominated, types that represent remnants of a shallow
conduit plug, welded fallback material from within the conduit, and sheared and degassed magma from near the conduit walls.
Analysis of bubble shapes preserved within obsidian indicates that shear stresses and shear rates varied by over two orders
of magnitude, with maxima of 88 kPa and 10−2.3 s−1, respectively, based on an assumed magma temperature of 850°C. Furthermore, the highest shear rates and stresses, and the
shortest flow times (10–20 min), are preserved in clasts that also contain wall rock. The longest deformation times (5 and
8 h) correspond to two microlite-rich clasts, suggesting that the higher microlite content results from slower ascent rates
and/or longer magma residence times at shallow levels. Differences between obsidian pyroclasts from the Newberry eruption
and those of the Mono Craters may relate to the nature of the conduit feeding the two events. From this comparison, we conclude
that obsidian can provide information on time scales and mechanisms of pre-fragmentation magma ascent. 相似文献
4.
Cynthia A. Gardner Katharine V. Cashman Christina A. Neal 《Bulletin of Volcanology》1998,59(8):537-555
The 1992 eruption of Crater Peak, Mount Spurr, Alaska, involved three subplinian tephra-producing events of similar volume
and duration. The tephra consists of two dense juvenile clast types that are identified by color, one tan and one gray, of
similar chemistry, mineral assemblage, and glass composition. In two of the eruptive events, the clast types are strongly
stratified with tan clasts dominating the basal two thirds of the deposits and gray clasts the upper one third. Tan clasts
have average densities between 1.5 and 1.7 g/cc and vesicularities (phenocryst free) of approximately 42%. Gray clasts have
average densities between 2.1 and 2.3 g/cc, and vesicularities of approximately 20%; both contain abundant microlites. Average
maximum plagioclase microlite lengths (13–15 μm) in gray clasts in the upper layer are similar regardless of eruptive event
(and therefore the repose time between them) and are larger than average maximum plagioclase microlite lengths (9–11 μm) in
the tan clasts in the lower layer. This suggests that microlite growth is a response to eruptive processes and not to magma
reservoir heterogeneity or dynamics. Furthermore, we suggest that the low vesicularities of the clasts are due to syneruptive
magmatic degassing resulting in microlitic growth prior to fragmentation and not to quenching of clasts by external groundwater.
Received: 5 September 1997 / Accepted: 1 February 1998 相似文献
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