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1.
Summary Reheated silicate melt inclusions in volcanic rock samples from Mt. Somma-Vesuvius, Italy, have been analyzed for 29 constituents including H2O, S, Cl, F, B, and P2O5. This composite volcano consists of the older Mt. Somma caldera, formed between 14 and 3.55 ka before present, and the younger Vesuvius cone. The melt inclusion compositions provide important constraints on pre-eruptive magma geochemistry, identify relationships that relate to eruption behavior and magma evolution, and provide extensive evidence for magmatic fluid exsolution well before eruption. The melt inclusion data have been categorized by groups that reflect magma compositions, age, and style of eruptions. The data show distinct differences in composition for eruptive products older than 14.0 ka (pre-caldera rocks) versus eruptive products younger than 3.55 ka. Moreover, pre-caldera eruptions were associated with magmas relatively enriched in SiO2, whereas eruptions younger than 3.55 ka (i.e., the syn- and post-caldera magmas which generated the Somma caldera and the Vesuvius cone) were derived from magmas comparatively enriched in S, Cl, CaO, MgO, P2O5, F, and many lithophile trace elements. Melt inclusion data indicate that eruptive behavior at Vesuvius correlates with pre-eruptive volatile enrichments. Most magmas associated with explosive plinian and subplinian events younger than 3.55 ka contained more H2O, contained significantly more S, and exhibited higher (S/Cl) ratios than syn- and post-caldera magmas which erupted during relatively passive interplinian volcanic phenomena. Received January 10, 2000 Revised version accepted July 17, 2000  相似文献   

2.
Diffusive coarsening (Ostwald ripening) of H2O and H2O-CO2 bubbles in rhyolite and basaltic andesite melts was studied with elevated temperature–pressure experiments to investigate the rates and time spans over which vapor bubbles may enlarge and attain sufficient buoyancy to segregate in magmatic systems. Bubble growth and segregation are also considered in terms of classical steady-state and transient (non-steady-state) ripening theory. Experimental results are consistent with diffusive coarsening as the dominant mechanism of bubble growth. Ripening is faster in experiments saturated with pure H2O than in those with a CO2-rich mixed vapor probably due to faster diffusion of H2O than CO2 through the melt. None of the experimental series followed the time1/3 increase in mean bubble radius and time−1 decrease in bubble number density predicted by classical steady-state ripening theory. Instead, products are interpreted as resulting from transient regime ripening. Application of transient regime theory suggests that bubbly magmas may require from days to 100 years to reach steady-state ripening conditions. Experimental results, as well as theory for steady-state ripening of bubbles that are immobile or undergoing buoyant ascent, indicate that diffusive coarsening efficiently eliminates micron-sized bubbles and would produce mm-sized bubbles in 102–104 years in crustal magma bodies. Once bubbles attain mm-sizes, their calculated ascent rates are sufficient that they could transit multiple kilometers over hundreds to thousands of years through mafic and silicic melt, respectively. These results show that diffusive coarsening can facilitate transfer of volatiles through, and from, magmatic systems by creating bubbles sufficiently large for rapid ascent.  相似文献   

3.
Historical eruptions from Mt. Ruapehu (New Zealand) have been small (<0.001 km3 of juvenile magma) and have often occurred without significant warning. Developing better modelling tools requires an improved understanding of the magma storage and transport system beneath the volcano. Towards that end, we have analysed the volatile content and major element chemistry of groundmass glass and phenocryst-hosted melt inclusions in erupted samples from 1945 to 1996. We find that during this time period, magma has been stored at depths of ~2–9 km, consistent with inferences from geophysical data. Our data also show that Ruapehu magmas are relatively H2O-poor (<2 wt%) and CO2-rich (≤1,000 ppm) compared to typical arc andesites. Surprisingly, we find that melt inclusions are often more evolved than their transporting melt (as inferred from groundmass glass compositions). Furthermore, even eruptions that are separated by less than 2 years exhibit distinct major element chemistry, which suggests that each eruption involved magma with a unique ascent history. From these data, we infer that individual melt batches rise through, and interact with, crystal mush zones formed by antecedent magmas. From this perspective, we envision the magmatic system at Ruapehu as frequently recharged by small magma inputs that, in turn, cool and crystallise to varying degrees. Melts that are able to erupt through this network of crystal mush entrain (to a greater or lesser extent) exotic crystals. In the extreme case (such as the 1996 eruption), the resulting scoria contain melt inclusion-bearing crystals that are exotic to the transporting magma. Finally, we suggest that complex interactions between recharge and antecedent magmas are probably common, but that the small volumes and short time scales of recharge at Ruapehu provide a unique window into these processes.  相似文献   

4.
Chlorine in submarine volcanic glasses from the eastern manus basin   总被引：4，自引：0，他引：4  
Submarine volcanic glasses from the eastern Manus Basin of Papua New Guinea, ranging from basalt to rhyodacite, clarify the geochemical behavior of Cl in arc-type magmas. For the Manus samples, Cl is well correlated with non-volatile highly incompatible trace elements, suggesting it was not highly volatile and discounting significant seawater contamination. The Cl partition coefficient is close to but slightly lower than that of Nb and K2O, a behavior similar to that in mid-ocean ridge basalts (MORB) and ocean island basalts (OIB). The similar incompatibilities of Cl and Nb imply that the Cl/Nb values of the eastern Manus Basin glasses reflect their magma source. For glasses from other west Pacific back-arc basins, Cl/Nb, Ba/Nb, and U/Nb increase towards the subduction trench, indicating increased contribution of a component enriched in Cl, Ba, and U, likely from subduction-released slab fluids. It is estimate that ∼80% of the Cl in the Manus arc-type glasses was added directly from subducted slab-derived fluids. We have also modeled Cl behavior during magma evolution in general. Our results show that the behavior of Cl in magma is strongly influenced by pressure, initial H2O content, and the degree of magmatic fractionation. At early stages of magmatic evolution, for magmas with initial H2O content of <4.0 wt%, Cl is highly incompatible under all pressures. By contrast, for more evolved magmas at moderately high pressure and high H2O contents, considerable amounts of Cl can be extracted from the magma once H2O saturation is reached. Accordingly, Cl is usually highly incompatible in MORB and OIB because of their low H2O contents and relatively low degrees of fractional crystallization. The behavior of Cl in arc magmas is more complicated, ranging from highly incompatible to compatible depending on H2O content and depth of magma chambers. The behavior of Cl in the eastern Manus Basin magmas is consistent with low H2O contents (1.1-1.7 wt%) and evolution at low pressures (<0.1 GPa). Modeling results also indicate that Cl will behave differently in intrusive rocks compared to volcanic rocks because of the different pressures involved. This may have a strong influence on the mechanisms of ore genesis in these two tectonic settings.  相似文献   

5.
Chlorine has a higher solubility in basaltic magmas than other volatiles. A theoretical degassing model predicts that less than 10% of the chlorine originally present in the magma is lost to the atmosphere during surface degassing. This prediction is born out by strong correlation between chlorine and nonvolatile elements in samples from recent volcanic eruptions in Iceland. It is concluded that the chlorine content of subaerial basaltic lavas is proportional to the chlorine content of the magmas. Assuming a roughly constant ratio between chlorine and water in the magmatic gas phase and assuming further that this ratio approximates the ClH2O ratio in sea water and sediments it is possible to assign each basaltic magma a fixed amount of water. The figures thus obtained (0.3–1.2% H2O) are identical with previously assumed water contents of basaltic magmas.The implied water contents of the melts are discussed in terms of derivation from hydrous mantle phases. It is concluded that such phases can supply only a fraction of the assumed water content, the rest is derived from an additional volatile source in the mantle. The mantle source giving rise to alkali basalts in Iceland has a lower content of volatiles than the tholeiite source.  相似文献   

6.
The Albany and Torbay Adamellites are composite plutons emplaced in Pre‐cambrian gneisses of the Albany‐Esperance Block in the vicinity of Albany, Western Australia. The gneissic country rocks have been metamorphosed to the lower granu‐lite facies at Albany and the upper amphibolite facies at Torbay. Granitized aureoles about 1 km wide, metasomatically enriched in SiO2, K2O, and various trace elements commonly including Rb, Ba, La, Pb, and Th, are developed in the gneisses around both plutons. Field relations suggest late‐kinematic magmatic emplacement of the Adamellites in the catazone. Both show chemical variation trends comparable with the trends normally associated with fractional crystallization of calc‐alkali magmas, and their normative compositions correspond with the thermal trough in the system An‐Ab‐Or‐Q‐H2O at 4–7 kb PH2o, suggesting an origin involving crystal‐liquid equilibria at a water vapour pressure of about this value. The initial 87Sr/86Sr ratio of 0.7118 for the Albany Adamellite is consistent with derivation of the magma from crustal rocks. The late‐kinematic field characteristics of the plutons and the limited isotopic data available are compatible with emplacement and crystallization during the closing stages of orogeny and regional metamorphism. The magmas are believed to have been generated not at their present sites of emplacement, but in a deeper, higher‐temperature zone of the crust, with magma generated during an earlier phase of the orogeny.  相似文献   

7.
The nucleation of H2O bubbles in magmas has been proposed as a trigger for volcanic eruptions. To determine how bubbles nucleate heterogeneously in silicate melts, experiments were carried out in which high-silica rhyolitic melts were hydrated at 740-800°C and 50-175 MPa, decompressed by 20-70 MPa, and held at the lower pressures for ≥10 s before being quenched. The hydration conditions were subliquidus, and all samples contain blocky magnetite + needle-shaped hematite ± plagioclase. Magnetite is abundant at 800°C and high pressures, whereas hematite becomes more abundant at lower temperatures and pressures. Bubbles nucleated in a single event in all samples, with the number density (NT) of bubbles varying between 2 × 107 and 1 × 109 cm−3. At low degrees of supersaturation, one to a few bubbles nucleate on faces of magnetite, but at medium to high degrees of supersaturation, multiple bubbles nucleate on single magnetite grains. On hematite, one to a few bubbles nucleated at the ends of the needle-shaped crystals at medium supersaturations, but formed along their entire lengths at high supersaturations. NT increases as water diffusivity decreases, indicating that the number of bubbles nucleated is influenced by their growth, which depletes the melt with respect to H2O and lowers supersaturation. If volcanic eruptions are triggered by bubble formation in magmas stored in shallow-level magma chambers, then the supersaturations needed for heterogeneous nucleation suggest that only small amounts of crystallization are needed, whereas homogeneous nucleation is unlikely to trigger eruptions.  相似文献   

8.
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 H2O 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 H2O 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   

9.
238U–230Th disequilibria and Sr and O isotope ratios have been measured in a suite of samples from most of the known prehistoric and historic eruptions of Hekla volcano, Iceland. They cover the compositional range from basaltic andesite to rhyolite. Recent basalts erupted in the vicinity of the volcano and a few Pleistocene basalts have also been studied. Geochemical data indicate that the best tracers of magmatic processes in Hekla are the (230Th/232Th) and Th/U ratios. Whereas most geochemical parameters, including Sr, Nd and O isotopes, could be compatible with crystal fractionation, (230Th/232Th) and Th/U ratios differ in the basalts and basaltic andesites (1.05 and 3.2, respectively) and in the silicic rocks, dacites and rhyolites (0.98 and 3.4–3.7, respectively). This observation precludes fractional crystallization as the main differentiation process in Hekla. On the basis of these results, the following model is proposed: basaltic magmas rise in the Icelandic crust and cause partial melting of metabasic rocks, leading to the formation of a dacitic melt. The basaltic magma itself evolves by crystal fractionation and produces a basaltic andesite magma. The latter can mix with the dacitic liquid to form andesites. At higher levels in the magma chamber, the dacitic melt sometimes undergoes further differentiation by crystal fractionation and produces subordinate volumes of rhyolites. Together all these processes lead to a zoned magma chamber. However, complete zoning is achieved only when the repose time between eruptions is long enough to allow the production of significant volumes of dacitic magma by crustal melting. This situation corresponds to the large plinian eruptions. Between these eruptions, the so-called intra-cyclic activity is characterized by the eruption of andesites and basaltic andesites, with little crustal melting. The magmatic system beneath Hekla most probably was established during the Holocene. The shape and the size of the magma chamber may be inferred from the relationships between the composition of the lavas and the location of the eruption sites. In a cross-section perpendicular to Hekla's ridge, a bell-shaped reservoir 5 km wide and 7 km deep appears the most likely; its top could be at depth of 8 km according to geophysical data.  相似文献   

10.
岩浆作用的物理过程研究进展   总被引：8，自引：0，他引：8  
概略地介绍了８０年代以来硅酸盐熔体及硅酸盐晶－液悬浮体的密度、粘度、熔体结构、流全动力学等方面的研究动向,及其对岩浆作用、岩浆运移、岩浆侵位机制的动力学约束条件,硅酸盐熔体的结构是制约熔粘度的主导因素,化学成分对熔体匠控制是通过改变熔体结构而实现的,粘度在一定程度上决定着岩浆的迁移、侵痊和喷发方式。密度和浮力是岩浆上升侵位的重要约束,地壳是岩交涉升的一个密度过滤器岩浆最终由于浮和的消失而停止上升。  相似文献   

11.
The Batur volcanic field (BVF), in Bali, Indonesia, underwenttwo successive caldera-forming eruptions that resulted in thedeposition of silicic ignimbrites. The magmas erupted duringand between these eruptions show a broad range of compositionsfrom low-SiO2 andesite to high-SiO2 dacite. On the basis oftheir geochemistry and mineralogy these magmas may be assignedto six groups: (1) homogeneous andesites with phenocryst compositionsessentially in equilibrium with the whole-rock composition;(2) remobilized crystal-rich low-SiO2 andesites with resorbedphenocrysts in equilibrium with the whole-rock composition;(3) mixed low-SiO2 dacite with a relatively large range of phenocrystcompositions, with most phenocrysts slightly too evolved tobe in equilibrium with the whole-rock; (4) extensively mixedlow-SiO2 dacites with a very large and discontinuous range ofphenocryst compositions, with most phenocrysts either more Mg-richor more evolved than the equilibrium compositions; (5) remobilizedcrystal-rich low-SiO2 dacites with resorbed and euhedral phenocrysts;(6) homogeneous high-SiO2 dacites lacking evidence for magmamixing and showing narrow ranges of phenocryst compositionsin equilibrium with the whole-rock composition. This range ofsilicic magmas is interpreted to reflect a combination of closed-and open-system fractional crystallization, magma mixing andremobilization of cumulate piles by heating. The variety ofmagmas erupted simultaneously during the caldera-forming eruptionssuggests that the magmatic system consisted of several independentreservoirs of variable composition and degree of crystallization.The magmatic evolution of individual reservoirs varied fromclosed-system fractional crystallization to fully open-systemevolution, thereby resulting in simultaneous production of magmaswith contrasted compositions and mineralogy. Extensive emptyingof the magmatic system during the caldera-forming eruptionsled to successive or simultaneous eruption of several reservoirs. KEY WORDS: caldera; ignimbrite; magmatic chambers; magma mixing; petrology; Sunda Arc  相似文献   

12.
F, Cl, S and P were determined, using electron microprobe, in magmatic inclusions trapped within minerals and glass mesostasis from Wudalianchi volcanic rocks. The initial volcanic magma from Wudalianchi corresponds to the basanitic magma crystallized near the surface ( pressure < 91 Mpa ). The potential H2O content of this magma is in the range 2 — 4 wt. %. The initial composition of volcanic magmas varies regularly from early to late volcanic events. From the Middle Pleistocene to the recent eruptions (1719 – 1721 yr.), the basicity of volcanic magma tends to increase, as reflected by an increase in MgO and CaO contents and by a progressive decrease in SiO2 and K2O contents. Meanwhile. from early (Q2 ) to late (Q3) episodic eruptions of the Middle Pleistocene, the initial concentrations of chlorine in volcanic magmas range from 1430 – 1930 ppm to 1700 ppm and decrease to 700 — 970 ppm for the first episodic eruption during the Holocene (Q 4 1 ). The chlorine concentrations of volcanic magmas of recent eruption (Q 4 2 ) are increased again to 2600 – 2870 ppm. A parallel evolution trend for phosphorus and chlorine concentrations in magmas has been certified: 1500 – 5970 ppm (Q2)→ 3500 – 4210 ppm (Q3)→ 1100– 3500 ppm (Q 4 1 )→ 6800– 7900 ppm (Q 4 2 ). The fluorine contents of volcanic magmas, from early to late volcanic events, show the same trend: 770 – 2470 ppm → 200–700 ppm → 700 – 800 ppm. During the crystallization-evolution of volcanic magmas, fluorine and phosphorus tend to be enriched in residual magmas as a result of crystal-melt differentiation. for example. the fluorine contents reach 5000– 6800 ppm and the phosphorus contents, 2.93wt.% in residual magmas. An appreciable amount of chlorine may be lost from water rich volcanic magmas prior to eruption as a result of degassing. Apparently, water serves as a gas carrier for the chlorine. The chlorine contents of residual magmas may decrease to 100 – 300 ppm. The volcanic magmas from Wudalianchi are poor in sulfur, normally ranging from 200 to 400ppm. On account of the behavior of sulfur in magmas and the strontium and oxygen isotopic analyses ((87Sr /86Sr)i=0.70503– 0.70589; δ18O = + 5.50 – + 6.89 ‰ ), it can be considered that the basanitic magmas in the Wudalianchi volcanic area came from the upper mantle and have not yet been contaminated probably by continental crust materials.  相似文献   

13.
An experimental study of H2O exsolution, bubble growth and microlite crystallisation during ascent (decompression) of silicic magmas in the volcanic conduit is presented. Isobaric and decompression experiments were performed on a rhyolitic melt at 860 °C, NNO+1, H2O saturation, and pressures between 15 and 170 MPa. Two sets of decompression experiments were performed, with decompression rates varying between 0.001 and 960 MPa/min: (1) from 150 to 50 MPa (high-pressure decompression), and (2) from 50 to 15 MPa (low-pressure decompression). The experiments highlight incomplete H2O exsolution for decompression rates>100 MPa/min, incomplete bubble growth for decompression rates>0.1 MPa/min, crystal nucleation time lags, and incomplete chemical re-equilibration to final pressures. The observed crystallisation process, i.e. growth versus nucleation, depends on the decompression range. Indeed, decompression-induced crystallisation during high-pressure decompressions is dominated by growth of existing crystals, whereas during low-pressure decompressions crystal nucleation is the dominating process. This study provides a means to infer magma ascent rates in eruptions of silicic magmas through a combined petrologic and experimental approach.  相似文献   

14.
Multiple eruptions of silicic magma (dacite and rhyolites) occurred over the last ~3 My in the Kos-Nisyros volcanic center (eastern Aegean sea). During this period, magmas have changed from hornblende-biotite-rich units with low eruption temperatures (≤750–800°C; Kefalos and Kos dacites and rhyolites) to hotter, pyroxene-bearing units (>800–850°C; Nisyros rhyodacites) and are transitioning back to cooler magmas (Yali rhyolites). New whole-rock compositions, mineral chemistry, and zircon Hf isotopes show that these three types of silicic magmas followed the same differentiation trend: they all evolved by crystal fractionation and minor crustal assimilation (AFC) from parents with intermediate compositions characterized by high Sr/Y and low Nb content, following a wet, high oxygen fugacity liquid line of descent typical of subduction zones. As the transition between the Kos-Kefalos and Nisyros-type magmas occurred immediately and abruptly after the major caldera collapse in the area (the 161 ka Kos Plateau Tuff; KPT), we suggest that the efficient emptying of the magma chamber during the KPT drew out most of the eruptible, volatile-charged magma and partly solidified the unerupted mush zone in the upper crust due to rapid unloading, decompression, and coincident crystallization. Subsequently, the system reestablished a shallow silicic production zone from more mafic parents, recharged from the mid to lower crust. The first silicic eruptions evolving from these parents after the caldera collapse (Nisyros units) were hotter (up to >100°C) than the caldera-forming event and erupted from reservoirs characterized by different mineral proportions (more plagioclase and less amphibole). We interpret such a change as a reflection of slightly drier conditions in the magmatic column after the caldera collapse due to the decompression event. With time, the upper crustal intermediate mush progressively transitioned into the cold-wet state that prevailed during the Kefalos-Kos stage. The recent eruptions of the high-SiO2 rhyolite on Yali Island, which are low temperature and hydrous phases (sanidine, quartz, biotite), suggest that another large, potentially explosive magma chamber is presently building under the Kos-Nisyros volcanic center.  相似文献   

15.
Two distinctive magmatic fluids were recognized in the Tatun volcanic group (TVG), Taiwan. One is a relatively reduced fluid represented by the fumarolic gases at Hsiao-you-ken (HYK) geothermal field. Another is an oxidized fluid containing high concentrations of HCl represented by the fumarolic gases at Da-you-ken (DYK). An intermediate gas was recognized at Gung-tze-ping (GTP) and She-hung-ping (SHP). The fumarolic gases at HYK and GTP possess the features of so-called primary steam generated on mixing of magmatic gas and meteoric groundwater. The fumarolic gases at DYK are a simple mixture between magmatic gas and water vapor of meteoric origin. The CO2/H2O molar ratio of the magmatic component in the fumarolic gases at DYK was estimated to be 0.018, meanwhile it was estimated to be 0.027 for the fumarolic gases at HYK and GTP, suggesting the magma beneath DYK is depleted in volatiles relative to the magma beneath HYK and GTP. The estimated CO2/H2O ratio for the magmatic component is comparable to that of some active volcanoes in Japan, suggesting the enrichment of volatiles in the magmas beneath TVG.  相似文献   

16.
In volatile-saturated magmas, degassing and crystallisation are interrelated processes which influence the eruption style. Melt inclusions provide critical information on volatile and melt evolution, but this information can be compromised significantly by post-entrapment modification of the inclusions. We assess the reliability and significance of pyroxene-hosted melt inclusion analyses to document the volatile contents (particularly H2O) and evolution of intermediate arc magmas at Volcán de Colima, Mexico. The melt inclusions have maximal H2O contents (≤4 wt%) consistent with petrological estimates and the constraint that the magmas crystallised outside the amphibole stability field, demonstrating that pyroxene-hosted melt inclusions can preserve H2O contents close to their entrapment values even in effusive eruptions with low effusion rates (0.6 m3 s?1). The absence of noticeable H2O loss in some of the inclusions requires post-entrapment diffusion coefficients (≤1 × 10?13 m2 s?1) at least several order of magnitude smaller than experimentally determined H+ diffusion coefficient in pyroxenes. The H2O content distribution is, however, not uniform, and several peaks in the data, interpreted to result from diffusive H2O reequilibration, are observed around 1 and 0.2 wt%. H2O diffusive loss is also consistent with the manifest lack of correlations between H2O and CO2 or S contents. The absence of textural evidence supporting post-entrapment H2O loss suggests that diffusion most likely occurred via melt channels prior to sealing of the inclusions, rather than through the host crystals. Good correlation between the melt inclusion sealing and volcano-tectonic seismic swarm depths further indicate that, taken as a whole, the melt inclusion population accurately records the pre-eruptive conditions of the magmatic system. Our data demonstrate that H2O diffusive loss is a second-order process and that pyroxene-hosted melt inclusions can effectively record the volatile contents and decompression-induced crystallisation paths of vapour-saturated magmas.  相似文献   

17.
The sequence of crystallization in a biotite-granite from the Bohus batholith of Norway and Sweden, deduced from its texture, was magnetite, plagioclase, microcline, quartz, and finally biotite. Several sequences of crystallization were determined experimentally at 2 kb in the presence of varying only for H2O contents below 1.2% by weight. The rock was fused to a homogeneous glass, and each experiment included samples of finely crushed rock and glass. The samples were reacted in Ag-Pd capsules with measured H2O content in coldseal pressure vessels with NNO buffer. With excess H2O (more than 6.5%) the crystallization interval extends from 865° C to 705° C. In the H2O-deficient region, the solidus temperature remains unchanged as long as a trace of vapor is present, but the liquidus temperature increases as H2O content decreases; with 0.8 % H2O the liquidus temperature is 1125° C, the crystallization interval is 420° C, and a separate aqueous vapor phase is evolved only a few degrees above the solidus at 705° C. The biotite phase boundary increases slightly from 845° C with excess H2O to 875° C with 1% H2O, and it intersects the steep phase boundaries for quartz and feldspars; the sequence of crystallization changes at each intersection point. Similar diagrams at various pressures for related rock compositions involving muscovite, biotite and amphibole will provide grids useful in defining limits for the water content of granitic and dioritic magmas. Applications are considered for the Bohus batholith, other granitic rocks, and rhyolites. The Bohus magma could have been formed by crustal anatexis as a mobile assemblage of H2O-undersaturated liquid and residual crystals with initial total H2O content less than 1.2%, or it could have been derived by fractionation of a more basic parent with low H2O content from mantle or subduction zone, but it could not have been derived from a primary andesite generated from mantle peridotite. We consider it unlikely that the H2O content of large granitic magma bodies exceeds about 1.5% H2O; these magmas are H2O-undersaturated through most of their histories. Uprise and progressive crystallization of magma bodies produces H2O-saturation around margins and in the upper regions of magma chambers. H2O-saturated rhyolitic and dacitic magmas with phenocrysts can be tapped from the upper parts of the magma chambers.  相似文献   

18.
Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO2. The range of major and trace element compositions of the Shasta lavas can be explained through fractional crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H2O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of fractional crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H2O) to lower crustal depths with magmatic H2O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H2O-rich magmas containing 10–15 wt.% H2O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H2O-rich melts could also play an important role in mixing and eruption.  相似文献   

19.
周金胜  王强 《岩石学报》2022,38(5):1399-1418
岩浆是将地球内部物质传送到表层系统的主要载体,并造成显著的资源聚集和环境效应。岩浆动力学是研究岩浆的迁移、储存、演化、就位以及喷发过程,侧重物理机制。这些岩浆过程主要发生在岩浆通道系统中,包括岩浆储库和岩浆管道。本文对目前国际岩浆动力学领域一些热点和前沿进行了介绍,这包括从岩浆房到岩浆储库概念的转变、岩浆储库的生长和动力学演化过程、岩浆过程的时间尺度以及岩浆中晶体的生长。然后阐述了岩浆中挥发分的种类和溶解度、获取天然岩浆挥发分含量的方法、一些典型镁铁质岩浆中的挥发分含量、岩浆去气的化学和物理机制,并简要梳理了热液金属矿床的形成过程和岩浆挥发分进入地表圈层系统引发的环境气候效应。最后列举了一些岩浆动力学有关的重要科学问题并建议了进一步的研究方向。  相似文献   

20.
Recent work on the Panzhihua intrusion has produced two separate models for the crystallisation of the intrusion:（1） low-Ti,high CaO and low H2O（0.5 wt.%） parent magma（equivalent to Emeishan low-Ti basalt） at FMQ;and（2） high-Ti,low CaO and higher H2O（>1.5 wt.%） parent magma（equivalent to Emeishan high-Ti basalt） at FMQ + 1.5.Modelling of these parent magma compositions produces significantly different results. We present here detailed f（O2） and H2O modelling for average compositions of both Emeishan high-Ti and low-Ti ferrobasalts in order to constrain the effects on crystallisation sequences for Emeishan ultra-mafic -mafic layered intrusions.Modelling is consistent with numerous experimental studies on ferro-basaltic magmas from other localities（e.g.Skaergaard intrusion）.Modelling is compared with the geology of the Panzhihua intrusion in order to constrain the crystallisation of the gabbroic rocks and the Fe-Ti oxides ore layers.We suggest that the gabbroic rocks at the Panzhihua intrusion can be best explained by crystallisation from a parent magma similar to that of the high-Ti Emeishan basalt at moderate H2O contents（0.5-1 wt.%） but at the lower end of TiO2 content for typical high-Ti basalts（2.5 wt.%TiO2）. Distinct silicate disequilibrium textures in the Fe-Ti oxide ore layers suggest that an influx of H2O may be responsible for changing the crystallisation path.An increase in H2O during crystallisation of gabbroic rocks will result in the depression of silicate liquidus temperatures and resultant disequilibrium with the liquid.Continued cooling of the magma with high H2O then results in precipitation of Mt-Uv alone. The H2O content of parent magmas for mafic layered intrusions associated with the ELIP is an important variable.H2O alters the crystallisation sequence of the basaltic magmas so that at high H2O and f（O2） Mt -Uv crystallises earlier than plagioclase and clinopyroxene.Furthermore,the addition of H2O to an anhydrous magma can explain silicate disequilibrium texture observed in the Fe-Ti oxide ore layers.  相似文献   

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