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1.
Fluid/melt distribution coefficients for F have been determined in experiments conducted with peraluminous topaz rhyolite melts and fluids consisting of H 2O and H 2O+CO 2 at pressures of 0.5 to 5 kbar, temperatures of 775°–1000°C, and concentrations of F in the melt ranging from 0.5 to 6.9 wt%. The major element, F, and Cl concentrations of the starting material and run product glasses were determined by electron microprobe, and the concentration of F in the fluid was calculated by mass balance. The H 2O concentrations of some run product glasses were determined by ion microprobe (SIMS). The solubility of melt in the fluid phase increases with increasing F in the system; the solubility of H 2O in the melt is independent of the F concentration of the system with up to 6.3 wt% F in the melt. No evidence of immiscible silica- and fluoriderich liquids was detected in the hydrous but water-undersaturated starting material glasses (8.5 wt% F in melt) or in the water-saturated run product glasses. F concentrates in topaz rhyolite melts relative to coexisting fluids at most conditions studied; however, D F (wt% F in fluid/wt% F in melt) increases strongly with increasing F in the system. Maximum values of D F in this study are significantly larger than those previously reported in the literature. Linear extrapolation of the data suggests that D F is greater than one for water-saturated, peraluminous granitic melts containing 8 wt% F at 800° C and 2 kbar. D F increases as temperature and as (H 2O/H 2O+CO 2) of the fluid increase. For topaz rhyolite melts containing 1 wt% F and with H 2O-rich fluids, D F is independent of changes in pressure from 2 to 5 kbar at 800° C; for melts containing 1 wt% F and in equilibrium with CO 2-bearing fluids the concentrations of F in fluid increases with increasing pressure. F-and lithophile element-enriched granites may evolve to compositions containing extreme concentrations of F during the final stages of crystallization. If F in the melt exceeds 8 wt%, D F is greater than one and the associated magmatic-hydrothermal fluid contains >4 molal F. Such F-enriched fluids may be important in the mass transport of ore constituents, i.e., F, Mo, W, Sn, Li, Be, Rb, Cs, U, Th, Nb, Ta, and B, from the magma. 相似文献
2.
We have investigated the loss of H 2O from olivine-hosted melt inclusions (MIs) by designing an experiment using tephra samples that cooled at different rates owing to their different sizes: ash, lapilli, and bomb samples that were deposited on the same day (10/17/74) of the sub-Plinian eruption of Volcán de Fuego in Guatemala. Ion microprobe, laser ablation-ICPMS, and electron probe analyses show that MIs from ash and lapilli record the highest H 2O contents, up to 4.4 wt%. On the other hand, MIs from bombs indicate up to 30 % lower H 2O contents (loss of ~1 wt% H 2O) and 10 % post-entrapment crystallization of olivine. This evidence is consistent with the longer cooling time available for a bomb-sized clast, up to 10 min for a 3–4-cm radius bomb, assuming conductive cooling and the fastest H diffusivities measured in olivine (D~10 ?9 to 10 ?10 m 2/s). On the other hand, several lines of evidence point to some water loss prior to eruption, during magma ascent and degassing in the conduit. Thus, results point to both slower post-eruptive cooling and slower magma ascent affecting MIs from bombs, leading to H 2O loss over the timescale of minutes to hours. The important implication of this study is that a significant portion of the published data on H 2O concentrations in olivine-hosted MIs may reflect unrecognized H 2O loss via diffusion. This work highlights the importance of reporting clast and MI sizes in order to assess diffusive effects and the potential benefit of using water loss as a chronometer of magma ascent. 相似文献
3.
To investigate processes of magmatic tin enrichment and cassiterite deposition, we studied the abundances of major, trace,
and volatile elements in a large number of rehomogenized silicate melt inclusions in quartz and topaz from a pegmatite body
at the Ehrenfriedersdorf Sn–W deposit. This deposit is associated with evolved Variscan granites of the central Erzgebirge,
southeast Germany. The melt inclusions are peraluminous; the molar aluminum saturation index (ASI) ranges from 1.15 to 2.0,
and many inclusions are characterized by a very high content of fluxing components and volatiles. Some inclusions contain
more than 20 wt% of H 2O, F, Cl, and P 2O 5, plus Li as well as very high levels of Sn. Some rare, highly evolved fractions of late-stage pegmatite-forming liquid at
Ehrenfriedersdorf contained up to 7000 ppm Sn. The presence of hydrogen and methane in addition to water and carbon dioxide
in the vapor phase of the melt inclusions suggests a very low oxygen fugacity for some fractions of magma. The extreme levels
of tin, volatiles, and fluxing components in this magma had an important influence on processes of melt movement and cassiterite
precipitation. Melts, like these, that are high in volatiles and alkalis (sum of Li 2O, Na 2O, K 2O, Rb 2O, and Cs 2O is >8 wt%) have low densities (≤1.8 g/cm 3), low viscosities (<10 Pa.s at 700 °C), facilitate relatively rapid diffusion of ions through melts, and hence are excellent
solvents for extracting and transporting ore-forming elements.
Received: 1 February 1999 / Accepted: 19 January 2000 相似文献
4.
Basaltic fragments enclosed in andesitic dome lavas and pyroclastic flows erupted during the early stages of the 1991 eruption of Mount Pinatubo, Philippines, contain amphiboles that crystallized during the injection of mafic magma into a dacitic magma body. The amphiboles contain abundant melt inclusions, which recorded the mixing of andesitic melt in the mafic magma and rhyolitic melt in the dacitic magma. The least evolved melt inclusions have high sulfur contents (up to 1,700 ppm) mostly as SO 4
2–, which suggests an oxidized state of the magma (NNO+1.4). The intrinsically oxidized nature of the mafic magma is confirmed by spinel–olivine oxygen barometry. The value is comparable to that of the dacitic magma (NNO+1.6). Hence, models invoking mixing as a means of releasing sulfur from the melt are not applicable to Pinatubo. Instead, the oxidized state of the dacitic magma likely reflects that of parental mafic magma and the source region in the sub-arc mantle. Our results fit a model in which long-lived SO 2 discharge from underplated mafic magma accumulated in the overlying dacitic magma and immiscible aqueous fluids. The fluids were the most likely source of sulfur that was released into the atmosphere during the cataclysmic eruption. The concurrence of highly oxidized basaltic magma and disproportionate sulfur output during the 1991 Mt. Pinatubo eruption suggests that oxidized mafic melt is an efficient medium for transferring sulfur from the mantle to shallow crustal levels and the atmosphere. As it can carry large amounts of sulfur, effectively scavenge sulfides from the source mantle and discharge SO 2 during ascent, oxidized mafic magma forms arc volcanoes with high sulfur fluxes, and potentially contributes to the formation of metallic sulfide deposits.Editorial responsibility: J. Hoefs
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5.
The magmatic evolution of two eruptive episodes at Campi Flegrei (Italy) has been investigated using phase equilibria modeling (MELTS) and data from melt inclusions (MIs) in phenocrysts from the Fondo Riccio and Minopoli 1 eruptions. Assuming that isobaric fractional crystallization of a mantle-derived parental magma is the dominant petrogenetic process, major element evolution and corresponding changes in the physical and thermodynamic properties of the magma bodies from which Fondo Riccio and Minopoli1 magmas were erupted can be tracked. Fondo Riccio parental magma was trachyandesitic, approximated by the composition of FR-C1-O2-M1, which evolved mainly through fractional crystallization at low pressure ( P?≈?0.15?GPa, ≈ 7?km depth), along the QFM, QFM?+?1 oxygen buffer with an initial dissolved H 2O content of ～3?wt%. Minopoli1 parental magma was trachyandesitic, approximated by the chemistry of Mi1-C1-O5-M1, evolved through fractional crystallization at P?≈?0.3?GPa (≈ 12?km depth), with oxygen fugacity along QFM?+?1buffer and initial H 2O content of?～?2 wt%. The relationship between melt fraction and T reveals for Fondo Riccio the presence of a pseudo-invariant temperature at which the physical properties of melt change abruptly. The net effect of these changes is to drive the system towards dynamic instability, which it is suggested to be the trigger mechanism for the eruptions. 相似文献
6.
Water diffusion in silicate melts is important for understanding bubble growth in magma, magma degassing and eruption dynamics of volcanos. Previous studies have made significant progress on water diffusion in silicate melts, especially rhyolitic melt. However, the pressure dependence of H 2O diffusion is not constrained satisfactorily. We investigated H 2O diffusion in rhyolitic melt at 0.95–1.9 GPa and 407–1629 °C, and 0.2–5.2 wt.% total water (H 2O t) content with the diffusion-couple method in a piston-cylinder apparatus. Compared to previous data at 0.1–500 MPa, H 2O diffusivity is smaller at higher pressures, indicating a negative pressure effect. This pressure effect is more pronounced at low temperatures. Assuming H 2O diffusion in rhyolitic melt is controlled by the mobility of molecular H 2O (H 2O m), the diffusivity of H 2O m ( DH2Om) at H 2O t ≤ 7.7 wt.%, 403–1629 °C, and ≤ 1.9 GPa is given by | where D0 is in µm2/s, X is mole fraction of H2Ot on a single oxygen basis, T is temperature in K, and P is pressure in GPa.H2Ot diffusivities (DH2Ot, in µm2/s) can be calculated from H2Om diffusivity, or directly from the following expression:At low H2Ot content (up to 2 wt.% if an error of a factor of 2 is allowed), H2Ot diffusivity is approximately proportional to H2Ot content:where C is H2Ot content in wt.% and C0 is 1 wt.%. The new expressions for H2O diffusion not only reproduce our own data, but also match data in literature from different laboratories and using different methods, indicating good inter-laboratory and multi-method consistency. The new expressions cover a wide range of geological conditions, and can be applied to H2O diffusion in rhyolitic melts in various volcanic and magmatic processes. 相似文献
7.
The >60 km
3 rhyolitic Kos Plateau Tuff provides an exceptional probe into the behavior of volatile components in highly evolved arc magmas:
it is crystal-rich (30–40 vol% crystals), was rapidly quenched by the explosive eruptive process, and contains abundant homogeneous
melt inclusions in large quartz crystals. Several methods for measuring major, trace and volatile element concentrations (SIMS,
FTIR, Raman spectroscopy, electron microprobe, LA–ICPMS) were applied to these melt inclusions. We found a ~2 wt% range of
H
2O contents (4.5–6.5 wt% H
2O, measured independently by SIMS, FTIR, and Raman spectroscopy) and relatively low CO
2 concentrations (15–140 ppm measured by FTIR, with most analyses <100 ppm). No obvious correlations between H
2O, CO
2, major and trace elements are observed. These observations require a complex, protracted magma evolution in the upper crust
that included: (1) vapor-saturated crystallization in a chamber located between 1.5 and 2.5 kb pressure, (2) closed-system
degassing (with up to 10 vol% exsolved gas) as melts percolated upwards through a vertically extensive mush zone (2–4 km thick),
and (3) periodic gas fluxing from subjacent, more mafic and more CO
2-rich magma, which is preserved as andesite bands in pumices. These processes can account for the range of observed H
2O and CO
2 values and the lack of correlation between volatiles and trace elements in the melt inclusions.
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8.
Constraining the composition of primitive kimberlite magma is not trivial. This study reconstructs a kimberlite melt composition using vesicular, quenched kimberlite found at the contact of a thin hypabyssal dyke. We examined the 4 mm selvage of the dyke where the most elongate shapes of the smallest calcite laths suggest the strongest undercooling. The analyzed bulk compositions of several 0.09-1.1 mm
2 areas of the kimberlite free from macrocrysts were considered to be representative of the melt. The bulk analyses conducted with a new “chemical point-counting” technique were supplemented by modal estimates, studies of mineral compositions, and FTIR analysis of olivine phenocrysts. The melt was estimated to contain 26-29.5 wt% SiO
2, ∼7 wt% of FeO
T, 25.7-28.7 wt% MgO, 11.3-15 wt% CaO, 8.3-11.3 wt% CO
2, and 7.6-9.4 wt% H
2O. Like many other estimates of primitive kimberlite magma, the melt is too magnesian (Mg# = 0.87) to be in equilibrium with the mantle and thus cannot be primary. The observed dyke contact and the chemistry of the melt implies it is highly fluid (
η = 10
1-10
3 Pa s at 1100-1000 °C) and depolymerized (NBO/T = 2.3-3.2), but entrains with 40-50% of olivine crystals increasing its viscosity. The olivine phenocrysts contain 190-350 ppm of water suggesting crystallization from a low SiO
2 magma (
aSiO2 below the olivine-orthopyroxene equilibrium) at 30-50 kb. Crystallization continued until the final emplacement at depths of few hundred meters which led to progressively more Ca- and CO
2-rich residual liquids. The melt crystallised phlogopite (6-10%), monticellite (replaced by serpentine, ∼10%), calcite rich in Sr, Mg and Fe (19-27%), serpentine (29-31%) and minor amounts of apatite, ulvöspinel-magnetite, picroilmenite and perovskite. The observed content of H
2O can be fully dissolved in the primitive melt at pressures greater than 0.8-1.2 kbar, whereas the amount of primary CO
2 in the kimberlite exceeds CO
2 soluble in the primitive kimberlite melt. A mechanism for retaining CO
2 in the melt may require a separate fluid phase accompanying kimberlite ascent and later dissolution in residual carbonatitic melt. Deep fragmentation of the melt as a result of volatile supersaturation is not inevitable if kimberlite magma has an opportunity to evolve.
相似文献
9.
Volatiles contribute to magma ascent through the sub-volcanic plumbing system. Here, we investigate melt inclusion compositions in terms of major and trace elements, as well as volatiles (H
2O, CO
2, SO
2, F, Cl, Br, S) for Quaternary Plinian and dome-forming dacite and andesite eruptions in the central and the northern part of Dominica (Lesser Antilles arc). Melt inclusions, hosted in orthopyroxene, clinopyroxene and plagioclase are consistently rhyolitic. Post-entrapment crystallisation effects are limited, and negligible in orthopyroxene-hosted inclusions. Melt inclusions are among the most water-rich yet recorded (≤?8 wt% H
2O). CO
2 contents are generally low (<?650 ppm), although in general the highest pressure melt inclusion contain the highest CO
2. Some low-pressure (<?3 kbars) inclusions have elevated CO
2 (up to 1100–1150 ppm), suggestive of fluxing of shallow magmas with CO
2-rich fluids. CO
2-trace element systematics indicate that melts were volatile-saturated at the time of entrapment and can be used for volatile-saturation barometry. The calculated pressure range (0.8–7.5 kbars) indicates that magmas originate from a vertically-extensive (3–27 km depth) storage zone within the crust that may extend to the sub-Dominica Moho (28 km). The vertically-extensive crustal system is consistent with mush models for sub-volcanic arc crust wherein mantle-derived mafic magmas undergo differentiation over a range of crustal depths. The other volatile range of composition for melt inclusions from the central part is F (75–557 ppm), Cl (1525–3137 ppm), Br (6.1–15.4 ppm) and SO
2 (<?140 ppm), and for the northern part it’s F (92–798 ppm), Cl (1506–4428 ppm), Br (not determined) and SO
2 (<?569; one value at 1015 ppm). All MIs, regardless of provenance, describe the same Cl/F correlation (8.3?±?2.7), indicating that the magma source at depth is similar. The high H
2O content of Dominica magmas has implications for hazard assessment.
相似文献
10.
Halogens show a range from moderate (F) to highly (Cl, Br, I) volatile and incompatible behavior, which makes them excellent tracers for volatile transport processes in the Earth’s mantle. Experimentally determined fluorine and chlorine partitioning data between mantle minerals and silicate melt enable us to estimate Mid Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB) source region concentrations for these elements. This study investigates the effect of varying small amounts of water on the fluorine and chlorine partitioning behavior at 1280?°C and 0.3 GPa between olivine and silicate melt in the Fe-free CMAS+F–Cl–Br–I–H
2O model system. Results show that, within the uncertainty of the analyses, water has no effect on the chlorine partitioning behavior for bulk water contents ranging from 0.03 (2) wt% H
2O (D
Cl ol/melt = 1.6?±?0.9 × 10
?4) to 0.33 (6) wt% H
2O (D
Cl ol/melt = 2.2?±?1.1 × 10
?4). Consequently, with the effect of pressure being negligible in the uppermost mantle (Joachim et al. Chem Geol 416:65–78,
2015), temperature is the only parameter that needs to be considered for the determination of chlorine partition coefficients between olivine and melt at least in the simplified iron-free CMAS+F–Cl–Br–I–H
2O system. In contrast, the fluorine partition coefficient increases linearly in this range and may be described at 1280?°C and 0.3 GPa with (
R 2?=?0.99):
\(D_{F}^{\text{ol/melt}}\ =\ 3.6\pm 0.4\ \times \ {{10}^{-3}}\ \times \ {{X}_{{{\text{H}}_{\text{2}}}\text{O}}}\left( \text{wt }\!\!\%\!\!\text{ } \right)\ +\ 6\ \pm \ 0.4\times \,{{10}^{-4}}\). The observed fluorine partitioning behavior supports the theory suggested by Crépisson et al. (Earth Planet Sci Lett 390:287–295,
2014) that fluorine and water are incorporated as clumped OH/F defects in the olivine structure. Results of this study further suggest that fluorine concentration estimates in OIB source regions are at least 10% lower than previously expected (Joachim et al. Chem Geol 416:65–78,
2015), implying that consideration of the effect of water on the fluorine partitioning behavior between Earth’s mantle minerals and silicate melt is vital for a correct estimation of fluorine abundances in OIB source regions. Estimates for MORB source fluorine concentrations as well as chlorine abundances in both mantle source regions are within uncertainty not affected by the presence of water.
相似文献
11.
Single-crystal polarized Raman spectra (3,000–4,000 cm
−1 at 3 ≤
T ≤ 300 K) were measured for synthetic alkali-free and natural beryl, Be
2Al
3Si
6O
18·
xH
2O, to determine the behavior of H
2O molecules of both Type I and Type II in the cavities. At low temperature, the H
2O molecules of Type I displace from the center of cavity and give rise to very weak hydrogen bonding with the host lattice.
The H
2O Type I translational motion is characterized by substantial anharmonicity and looks like a motion of “a particle in the
box” with a frequency of 6.3 cm
−1. Water Type II is characterized by a free rotation with respect to the
C
2 molecule axis, and it makes possible the water nuclear isomers (i.e.
ortho- and
para-) to be observed at low temperature.
相似文献
12.
Detailed analyses of melt and fluid inclusions combined with an electron-microprobe survey of boron-bearing minerals reveal the evolution of boron in a highly evolved peraluminous granite-pegmatite complex and the associated high- and medium-temperature ore-forming hydrothermal fluids (Ehrenfriedersdorf, Erzgebirge, Germany). Melt inclusions in granite represent embryonic pegmatite-forming melts containing about 10 wt% H
2O and 1.8 wt% B
2O
3. These melts are also enriched in F, P, and other incompatible elements such as Be, Sn, Rb, and Cs. Ongoing differentiation and volatile enrichment drove the system into a solvus, where two pegmatite-forming melts coexisted. The critical point is at about 712 °C, 100 MPa, 20 wt% H
2O and 4.1 wt% B
2O
3. Cooling and concomitant fractional crystallisation from 700 to 500 °C induced development of two conjugate melts, an H
2O-poor (A-melt) and an H
2O-rich melt (B-melt) along the opening solvus. Boron is a major element in both melts and is preferentially partitioned into the H
2O-rich melt. Temperature-dependent distribution coefficients
DboronB - melt/A - melt D_{{\rm{boron}}}^{{\rm{B - melt/A - melt}}} are 1.3 at 650 °C, 1.5 at 600 °C, and 1.8 at 500 °C. In both melts, boron concentrations decreased during cooling because of exsolution of a boron-rich hypersaline brine throughout the pegmatitic stage. Boromuscovite containing up to 8.5 wt% was another sink for boron at this stage. The end of the melt-dominated pegmatitic stage was attained at a solidus temperature of around 490 °C. Fluid inclusions of the hydrothermal stage reveal trapping temperatures of 480 to 370 °C, along with varying densities and highly variable B
2O
3 contents ranging from 0.20 to 2.94 wt%. A boiling system evolved, indicating a complex interplay between closed- and open-system behaviour. Pressure switched from lithostatic to hydrostatic and back, generating hydrothermal convection cells where meteoric waters were introduced and mixed with magmatic fluids. Boron-rich solutions originated from magmatic fluids, whereas boron-depleted fluids were mainly of meteoric origin. This highlights the potential of boron for discriminating fluids of different origin. Tin is continuously enriched during the evolution because tin and boron are cross-linked by formation of boron-, fluorine- and tin-fluorine-bearing complexes and is finally deposited within quartz-cassiterite veins during the transition from closed- to open-system behaviour. Boron does not only trace the complex evolution of the Ehrenfriedersdorf complex but exerts, together with H
2O, F and P, an important control on the physical and chemical properties of pegmatite-forming melts, and particularly on the formation of a two-melt solvus at low pressure. We discuss this with respect to experimental results on H
2O solubility and the critical behaviour of the haplogranite-water system which contained variable concentrations of volatiles.
相似文献
13.
Primitive chemical characteristics of high-Mg andesites (HMA) suggest equilibration with mantle wedge peridotite, and they
may form through either shallow, wet partial melting of the mantle or re-equilibration of slab melts migrating through the
wedge. We have re-examined a well-studied example of HMA from near Mt. Shasta, CA, because petrographic evidence for magma
mixing has stimulated a recent debate over whether HMA magmas have a mantle origin. We examined naturally quenched, glassy,
olivine-hosted (Fo
87–94) melt inclusions from this locality and analyzed the samples by FTIR, LA-ICPMS, and electron probe. Compositions (uncorrected
for post-entrapment modification) are highly variable and can be divided into high-CaO (>10 wt%) melts only found in Fo > 91
olivines and low-CaO (<10 wt%) melts in Fo 87–94 olivine hosts. There is evidence for extensive post-entrapment modification
in many inclusions. High-CaO inclusions experienced 1.4–3.5 wt% FeO
T loss through diffusive re-equilibration with the host olivine and 13–28 wt% post-entrapment olivine crystallization. Low-CaO
inclusions experienced 1–16 wt% olivine crystallization with <2 wt% FeO
T loss experienced by inclusions in Fo > 90 olivines. Restored low-CaO melt inclusions are HMAs (57–61 wt% SiO
2; 4.9–10.9 wt% MgO), whereas high-CaO inclusions are primitive basaltic andesites (PBA) (51–56 wt% SiO
2; 9.8–15.1 wt% MgO). HMA and PBA inclusions have distinct trace element characteristics. Importantly, both types of inclusions
are volatile-rich, with maximum values in HMA and PBA melt inclusions of 3.5 and 5.6 wt% H
2O, 830 and 2,900 ppm S, 1,590 and 2,580 ppm Cl, and 500 and 820 ppm CO
2, respectively. PBA melts are comparable to experimental hydrous melts in equilibrium with harzburgite. Two-component mixing
between PBA and dacitic magma (59:41) is able to produce a primitive HMA composition, but the predicted mixture shows some
small but significant major and trace element discrepancies from published whole-rock analyses from the Shasta locality. An
alternative model that involves incorporation of xenocrysts (high-Mg olivine from PBA and pyroxenes from dacite) into a primary
(mantle-derived) HMA magma can explain the phenocryst and melt inclusion compositions but is difficult to evaluate quantitatively
because of the complex crystal populations. Our results suggest that a spectrum of mantle-derived melts, including both PBA
and HMA, may be produced beneath the Shasta region. Compositional similarities between Shasta parental melts and boninites
imply similar magma generation processes related to the presence of refractory harzburgite in the shallow mantle.
相似文献
14.
H
2O diffusion in dacitic melt was investigated at 0.48-0.95 GPa and 786-893 K in a piston-cylinder apparatus. The diffusion couple design was used, in which a nominally dry dacitic glass makes one half and is juxtaposed with a hydrous dacitic glass containing up to ∼8 wt.% total water (H
2O
t). H
2O concentration profiles were measured on quenched glasses with infrared microspectroscopy. The H
2O diffusivity in dacite increases rapidly with water content under experimental conditions, similar to previous measurements at the same temperature but at pressure <0.15 GPa. However, compared with the low-pressure data, H
2O diffusion at high pressure is systematically slower. H
2O diffusion profiles in dacite can be modeled by assuming molecular H
2O (H
2O
m) is the diffusing species. Total H
2O diffusivity
DH2Ot within 786-1798 K, 0-1 GPa, and 0-8 wt.% H
2O
t can be expressed as: where
DH2Ot is in m
2/s,
T is temperature in K,
P is pressure in GPa,
K = exp(1.49 − 2634/
T) is the equilibrium constant of speciation reaction (H
2O
m+O?2OH) in the melt,
X =
C/18.015/[
C/18.015 + (100 −
C)/33.82],
C is wt.% of H
2O
t, and 18.015 and 33.82 g/mol correspond to the molar masses of H
2O and anhydrous dacite on a single oxygen basis. Compared to H
2O diffusion in rhyolite, diffusivity in dacite is lower at intermediate temperatures but higher at superliquidus temperatures. This general H
2O diffusivity expression can be applied to a broad range of geological conditions, including both magma chamber processes and volcanic eruption dynamics from conduit to the surface.
相似文献
15.
Summary ¶The Campanian Ignimbrite rock samples include two compositionally distinct populations of clinopyroxene phenocrysts, and the entrapped MI (melt inclusions) are also different in composition. The cores of the more MgO-enriched phenocrysts carry basaltic trachyandesite MI that contain >6wt.% MgO, whereas other phenocrysts contain MI with <4wt.% MgO. The MgO-enriched MI also contain comparatively greater abundances of F, CaO, TiO
2, P
2O
5, SO
2, and Sr and show marginally higher ratios of (CaO/Al
2O
3) than the low-MgO MI. Most of the high-MgO MI also contain comparatively more H
2O. The MgO-enriched MI are restricted to diopsidic clinopyroxenes and show minimal compositional variability, demonstrating that they were derived from a common magmatic source or sources. We interpret these MI to represent primary, mafic magma. In contrast, the more evolved, low-MgO melt inclusions, which are restricted to salitic clinopyroxenes, span the compositional range of trachyandesite to trachyte. The low-MgO fractions of Campanian Ignimbrite magma evolved via fractional crystallization with or without mingling or mixing with more primitive, high-MgO magma.Interestingly, the MI from the Giugliano sample also cluster into low-MgO and high-MgO fractions, and the evolutionary trends for major, minor, and trace elements mirror those exhibited by the Campanian Ignimbrite MI, suggesting that both magmas were derived from similar or the same source(s) and that the processes of magma evolution were equivalent for both magmas.The MI also indicate that the Campanian Ignimbrite and Giugliano magmas did not form by evolution of Taurano magma, because the geochemical trends expressing melt evolution of the former and latter magmas are too dissimilar. Most Taurano MI show higher (CaO/Al
2O
3) and contain less SiO
2, (Na
2O+K
2O), Cl, Li, Rb, Cs, Sr, Nb, Th, and U than the high-MgO and low-MgO Campanian Ignimbrite and Giugliano MI, indicating that the Taurano MI represent magmas which were much more primitive.Received July 15, 2002; revised version accepted March 27, 2003
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16.
The origin and the relationships between the high potassic (HKS) and potassic (KS) suites of the Roman Comagmatic Province and the nature of their primary magmas have been intensively debated over the past 35 years. We have addressed these problems by a study of mineralogy (olivine Fo
92-87, Cr-spinel and diopside) and melt inclusions in olivine phenocrysts from a scoria sample of Montefiascone (Vulsini area). This rock is considered as one of the most primitive (MgO=13.5 wt%, NiO=340 ppm; Cr=1275 ppm) in the northern part of the Roman Comagmatic Province. The compositions of both the olivine and their melt inclusions are controlled by two main processes. In the case of the olivine Fo<90.5, fractional crystallization (olivine + diopside + minor spinel) was the principal mechanism of the magma evolution. The olivine (Fo
92-90.5) and the Cr-spinel (Cr#=100. Cr/(Cr+Al)=63-73) represent a near-primary liquidus assemblage and indicate the mantle origin of their parental magmas. The compositions of melt inclusions in these olivine phenocrysts correspond to those of poorly fractionated H
2O-rich ( 1 wt%) primary melts (MgO=8.4-9.7 wt%,FeO
total=6-7.5 wt%). They evidence a wide compositional range (in wt%: SiO
2=46.5-50, K
2O=5.3-2.8, P
2O
5=0.4-0.2, S=0.26-0.12; Cl=0.05-0.03, and CaO/Al
2O
3= 0.8-1.15), with negative correlations between SiO
2 and K
2O, Al
2O
3 and CaO, as well as positive correlations between K
2O, and P
2O
5, S, Cl, with nearly constant ratios between these elements. These results are discussed in terms of segregation of various mantle-derived melts. The high and constant Mg# [100.Mg/(Mg+Fe
2+)] 73-75 of studied melts and their variable Si, K, P, Ca, Al, S contents could be explained by the melting of a refractory lithospheric mantle source, heterogeneously enriched in phlogopite and clinopyroxene (veined mantle source).
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17.
Experiments have been carried out on the separation of H
2O-rich vapor phase from a hydrous andesite melt at pressures between 5 and 15 kbar at 1,150 ° C. The pressure at which the vapor phase separates from the melt by isothermal decompression depends on the H
2O content in the melt; for example, 14 kbar for 12wt.% and 8 kbar for 9wt.% H
2O. These values are lower than the solubility of H
2O in andesite melt previously estimated. Extensive decompression to near atmospheric pressure resulted in the formation of pumiceous glass.Vapor phase separated from the melt moves upward and transports significant amounts of alkalies (Na
2O and K
2O), resulting in the depletion of alkalies near the bottom and concentration of alkalies near the top of the container. The maximum concentration observed is 5.0 wt.% for Na
2O and 1.7% for K
2O, compared to the initial contents 3.3 and 1.3 wt.% respectively. The approximate viscosity of hydrous andesitic melt with 7.5–12 wt.% H
2O was roughly estimated to be less than 10 poise. The results of the present experiments imply that when H
2O-rich vapor separates from magma in a magma chamber (or in a conduit) and moves upward, the top of the chamber would be enriched in alkalies while the bottom would be depleted.
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18.
The solubility of sulphur in sulphide-saturated, H
2O-bearing basaltic–andesitic and basaltic melts from Hekla volcano (Iceland) has been determined experimentally at 1,050°C,
300 and 200 MPa, and redox conditions with oxygen fugacity (logfO
2) between QFM−1.2 and QFM+1.1 (QFM is a quartz–fayalite–magnetite oxygen buffer) in the systems containing various amounts
of S and H
2O. The S content of the H
2O-rich glasses saturated with pyrrhotite decreases from 2,500 ppm in basalt to 1,500 ppm in basaltic andesite at the investigated
conditions. Furthermore, the reduction of water content in the melt at pyrrhotite saturation and fixed T, P and redox conditions
leads to a decrease in S concentration from 2,500 to 1,400 ppm for basaltic experiments (for H
2O decrease from 7.8 to 1.4 wt%) and from 1,500 to 900 ppm (for H
2O decrease from 6.7 to 1.7 wt%) for basaltic andesitic experiments. Our experimental data, combined with silicate melt inclusion
investigations and the available models on sulphide saturation in mafic magmas, indicate that the parental basaltic melts
of Hekla were not saturated with respect to sulphide. During magmatic differentiation, the S content in the residual melts
increased and might have reached sulphide saturation with 2,500 ppm dissolved S. With further magma crystallization, the S
concentration in the melt was controlled by the sulphide saturation of the magma, decreasing from ~2,500 to 900 ppm S.
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19.
Olivine-hosted melt inclusions in the O95 pyroclastic layer of Izu-Oshima volcano, Japan are basaltic to basaltic-andesitic in composition. The negative correlation between SiO
2 and H
2O in melt inclusions and reverse compositional zoning observed in olivine and other mineral phenocrysts is inferred to arise from mixing between a highly evolved and a less evolved magma. The latter is characterized by the highest S (0.15 wt.%) and H
2O (3.4 wt.%) concentrations among those described in reports of previous studies. The S
6+/S
total ratios in melt inclusions were 0.64?–?0.73, suggesting a relatively high oxidation state (NNO + 0.87 at 1150°C). The presence of pyrrhotites, which are found only in titanomagnetite microlites, suggests that sulfide saturation occurred during microlite growth under at a sulfur fugacity (log fS
2) value of around + 0.5 for T = 1060°C. The groundmass glass compositions are more evolved (andesitic composition) than any melt inclusions containing high amounts of Cl (0.13 wt.%) but negligible H
2O (0.20 wt.%) and S (< 70 ppm), suggesting that Cl was retained in the magma, in contrast to S and H
2O, which degassed strongly during magma effusion.
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20.
Evaluating the magma depth and its physical properties is critical to conduct a better geophysical assessment of magma chambers of caldera volcanoes that may potentially cause future volcanic hazards. To understand pre-eruptive conditions of a magma chamber before its first appearance at the surface, this paper describes the case of Hijiori caldera volcano in northeastern Japan, which emerged approximately 12,000 years ago at a place where no volcano ever existed. We estimated the depth, density, bulk modulus, vesicularity, crystal content, and bulk H
\(_2\)O content of the magma chamber using petrographic interpretations, bulk and microchemical compositions, and thermodynamic calculations. The chemical mass balance calculations and thermodynamic modeling of the erupted magmas indicate that the upper portion of the Hijiori magmatic plumbing system was located at depths between 2 and 4 km, and had the following characteristics: (1) pre-eruptive temperature: about 780
\(^{\circ }\)C; (2) bulk magma composition: 66 ± 1.5 wt% SiO
\(_{2}\); (3) bulk magmatic H
\(_2\)O: approximately 2.5 wt%, and variable characteristics that depend on depth; (4) crystal content:
\(\le\)57 vol%; (5) bulk modulus of magma: 0.1–0.8 GPa; (6) magma density: 1.8–2.3 g/cm
3; and (7) amount of excess magmatic H
\(_2\)O: 11–32 vol% or 48–81 mol%. The range of melt water contents found in quartz-hosted melt inclusions (2–9 wt%) suggests the range of depth phenocrysts growth to be wide (2
\(\sim\)13 km). Our data suggest the presence of a vertically elongated magma chamber whose top is nearly solidified but highly vesiculated; this chamber has probably grown and re-mobilized by repeated injections of a small amount of hot dacitic magma originated from the depth.
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