首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
Melting phase relations of an augite-olivine high-magnesian andesite and an augite-olivine basalt from the Miocene Setouchi volcanic belt in southwest Japan have been studied under water-saturated, water-undersaturated and under anhydrous conditions. Both the andesite and the basalt are characterized by low FeO*/MgO ratios (0.86 and 0.76 in weight, respectively) and qualify as primary magmas derived from the upper mantle.The andesite melt coexists with olivine, orthopyroxene and clinopyroxene at 15 kbar and 1030°C under water-saturated conditions, and at 10 kbar and 1070°C under water-undersaturated conditions (7 wt.% H2O in the melt). The basalt-melt also coexists with the above three phases at 11 kbar and 1305°C under anhydrous conditions, and at 15 kbar and 1205°C in the presence of 4 wt.% water.Present studies indicate that high-magnesian andesite magmas may be produced even under water-undersaturated conditions by partial melting of mantle peridotite. It is suggested that two types of high-magnesian andesites in the Setouchi volcanic belt (augite-olivine and bronzite-olivine andesites) were produced by different degrees of partial melting; augite-olivine andesite magmas, whose mantle residual is lherzolite, were formed by lower degrees of partial melting than bronzite-olivine andesite magmas, which coexist with harzburgite. The basalt magmas, which were often extruded in close proximity to the high-magnesian andesite magmas, are not partial melting products of a mantle peridotite which had previously melted to yield high-magnesian andesite magmas.  相似文献   

2.
A high-magnesian andesite (SiO2 58.50%, MgO 9.47%) occurs at Teraga-Ike in southwest Japan. It belongs to the Setouchi volcanic rocks of middle Miocene age and carries olivine and bronzite as phenocrysts (4.2 and 1.4 modal percent, respectively). This andesite is characterized by Mg-values as high as 75, suggesting that it may be a primary andesite. Olivine phenocrysts (Fo87–91) are in equilibrium with the groundmass (= liquid) on the basis of Fe-Mg exchange partitioning between olivine and liquid, and they have high NiO contents (up to 0.45%). Chromite inclusions in olivine and rarely bronzite have high Cr2O3 contents (max. 54.87%). These features strongly suggest that the Teraga-Ike andesite keeps the chemical composition of the primary magma generated in the upper mantle, and therefore verify the existence of primary andesite magmas.  相似文献   

3.
High-magnesian andesites of middle Miocene age occur in southwest Japan, forming an obvious volcanic belt. These andesites have low FeO*/MgO ratios (0.546–0.931), and are rich in Ni (101–312 ppm), Co (30.0–45.1 ppm), and Cr (208–756 ppm). They are relatively aphyric (phenocrysts <10 vol.%), and the phenocrysts of magnesian olivine (～Fo88) are in equilibrium with the host high-magnesian andesite magmas on the basis of the Fe-Mg exchange partitioning. These features suggest that the high-magnesian andesites are not differentiated or accumulative; they appear to represent primary andesites generated in the upper mantle. These southwest Japanese high-magnesian andesites are rich in incompatible elements, and show light rare earth enrichment relative to boninites, suggesting that the former is derived from a less depleted mantle source than the latter.  相似文献   

4.
Abstract Melting experiments have been carried out on an olivine andesite of Mt Yakushi-Yama from the Miocene Setouchi volcanic belt in northeastern Shikoku, Japan. This andesite has been characterized by a low ratio of FeO*/Mg° (= 0.78). Phase relations have been determined within the pressure range of 2.8 to 19.3 kbar at 1000-1300°C under anhydrous and water-saturated conditions. At pressures less than 8.8 kbar, olivine is a liquidus phase. Orthopyroxene appears on the liquidus at 9.3 kbar under the anhydrous conditions. The multiple saturation point rises up to 17.5 kbar for water-saturated experiments. The andesite melt coexists with olivine and orthopyroxene just below the liquidus at 8.8–9.3 kbar and 1230°C for dry conditions, and at 17.5 kbar and 1060°C under water-saturated conditions. These experimental results indicate that the Yakushi-Yama olivine andesite magma could coexist with a harzburgitic mantle at depths between about 30 and 60 km, and at temperatures between 1060 and 1230°C. Experimental data also suggest a possibility that a high magnesian andesite magma would be generated by a direct partial melting of the uppermost harzburgitic mantle under anhydrous conditions.  相似文献   

5.
Small euhedral chromite crystals are found in olivine macrophenocrysts (Fo80–84) from the basaltic andesites (150 ppm Cr) erupted in 1943–1947, and in orthopyroxene macrophenocrysts of the andesites (75 ppm Cr) erupted in 1947–1952. The majority of the chromite octahedra are 5–20 μm in diameter, and some are found in clusters and linear chains of three or more oriented chromite crystals. The composition of the majority of the chromite grains within olivine and orthopyroxene macrophenocrysts is Fe2+/(Fe2++Mg)=0.5–0.6, Cr/(Cr+Al)=0.5–0.6 and Fe3+/(Fe3++Al+Cr)=0.2–0.3. The chromite crystals in contact with the groundmass are larger, subhedral, and grade in composition from chromite cores to magnetite rims. Comparison of the composition of chromite with those of other volcanic rocks shows that the most primitive Paricutin chromite is richer in total iron and higher in Fe3+/(Fe3++Al+Cr) than primary chromite in most lavas. The linear chains of oriented chromite octahedra are found in olivine and orthopyroxene macrophenocrysts, and in the groundmass. These chromite chains are thought to result from diffusion-controlled crystallization because of the very high partition coefficient (1000) of Cr between chromite and melt. We conclude that chromite was a primary phase in the lavas at the time of extrusion and that magnetite only crystallized after extrusion during cooling of the lava flows. The presence of chromite microphenocrysts in andesitic lavas containing as little as 70 ppm Cr can be explained by dissolved H2O in the melt depressing the liquidus temperature for orthopyroxene such that chromite becomes a liquidus phase. The influence of dissolved H2O can also explain the lack of plagioclase macrophenocrysts in most of the lavas and the relatively high partition coefficient (20) of Ni between olivine and melt and the high partition coefficient (40) of Cr between orthopyroxene and melt. The liquidus temperature of the basaltic andesite is estimated to have been less than 1140°C, assuming H2O>1 wt.%, and the log fO2 to have been above that of the QFM buffer. The chromite and orthopyroxene liquidus temperature of the andesites, assuming H2O>1 wt.%, is estimated to have been 1100°C or less. The derivation of the later andesites from the earlier basaltic andesites has been explained by a combination of fractional crystallization of olivine, orthopyroxene and plagioclase, and assimilation of xenoliths. The significantly lower Cr, Ni and Mg of the andesites may have been in part due to the separation of olivine macrophenocrysts plus enclosed chromite crystals from the earlier basaltic andesites.  相似文献   

6.
Major and trace element (Rb, Sr, Ba, Zr, Y, Nb, Ni, Co, V, Cr) data are presented for 11 spinifex-textured peridotites (STP) and a number of high-magnesian and low-magnesian tholeiitic basalts. The STP, representing high-magnesian liquids, come from the Yilgarn Block of Western Australia, Munro Township in the Abitibi Belt of Canada and one sample from the Barberton area of South Africa. All of the basaltic samples come from the Yilgarn Block.The STP and high-magnesian rocks are considered to belong to the komatiite suite (1, 2) despite their low CaO/Al2O3 ratios. It is argued that the high values (about 1.5) reported for this ratio from the Barberton area can be explained by a combination of factors, viz. garnet separation, Al loss or Ca addition during metamorphism. The processes can be evaluated using CaO/TiO2, Al2O3/TiO2 ratios, the REE group and trace elements (e.g. Y, Sc). It would appear that most STP from other Archaean belts do not have abnormal CaO/Al2O3 ratios.The STP display close to chondritic ratios for Ti/Zr, Zr/Nb, Zr/Y, and TiO2/Al2O3 and are considered to represent liquids produced by large amounts of partial melting of the Archaean mantle. The data suggest that virtually all phases other than olivine were removed by melting during the production of STP liquids. In the STP, Ti/V, Ti/P ratios are non-chondritic, suggesting original depletion and/or incorporation into the core.For lower levels of partial melting, including mid-ocean ridge basalts (MORB) non-chondritic ratios are exhibited by Zr/Y, TiO2/Al2O3, TiO2/CaO, suggesting controlling phases in the residue for Y, Ca, Al. It is apparent that for STP, Cr is not being controlled, indicating the absence of chromite in the residual. However, at about 15% MgO the data suggest that chromite becomes a residual phase.The transition metals, with the exception of Mn, have higher abundances in Archaean basaltic rocks than in MORB. This is interpreted as being mainly due to more extensive partial melting of the mantle in the Archaean, as a result of higher temperatures.It is suggested that the generation of STP liquids with about 32% MgO is due to upwelling mantle diapirs which probably originated at depths greater than 400 km and at temperatures in excess of 1900°C.Modern equivalents to Archaean greenstone sequences are lacking. The closest tectonic analogue would be the development of oceanic crust within a rifted continental block.  相似文献   

7.
Cenozoic capping volcanic rocks in the Nevada portion of the Basin and Range Province of the western United States belong to the high alumina calcalkaline igneous series. Varying proportions of plagioclase (An=85 to 45 percent), pyroxene (augite, pigeonite, and hypersthene), olivine, magnetite, biotite, and oxyhornblende indicate a modal range from olivine basalt to andesite. Major element analyses made on randomly collected samples, as well as on samples from systematically measured stratigraphic sections in localities of minimum erosion show ranges in Al2O3 (from 17.5 to 22.5 percent); SiO2 (from 44.0 to 54.0 percent); MgO (from 3.47 to 8.20 percent) and CaO (from 7.19 to 11.90 percent). Na2O/K2O is always greater than 1.0. Ba++ and Sr++ abundances for some of the rocks are in agreement with those suggested, by workers in the field, for average basalt and andesite derived by melting of mantle or lower crust, but for many of the samples the values found are considerably higher. Although the presence of biotite in the earlier flows and oxyhornblende in the later ones along with the presence of much magnetite in all the rocks examined suggests that in part these rocks were derived by crystallization of a melt under conditions of high partial pressure of oxygen, the available trace element data indicates that contamination of the magma with crustal material was also a factor in their developmental history. The case for a parent magma, subsequently fractionally crystallized and contaminated in part, is strengthened by the occurrence of crystal cumulates and highly altered xenoliths in some of the flows.  相似文献   

8.
The lesser antilles — A discussion of the Island arc magmatism   总被引：1，自引：0，他引：1  
The active island arc of Lesser Antilles marks the junction between the Atlantic and Carribbean lithospheric plates. With the exception of the alkali basalts of Grenada, the volcanics of the arc can be regarded as belonging to the low-K, island arc, calc-alkaline suite. Although compositions ranging from basalt to rhyolite have been described, porphyritic andesite appears to be the dominant rock type on most volcanoes (intermediate centers). Variable amounts of basalt and basaltic andesite occur and rarely predominate over andesite (latter are basic centers), whereas the more silicic members are only occasionally found. The calc-alkaline suite is characterized by relatively high Al2O3 and CaO and low K2O, Rb and Ni. Variations, especially in the alkali elements, occur both with space and time. A characteristic feature of many of the volcanoes is the occurrence in the basalt and basaltic andesite volcanics of plutonic blocks, often showing cumulate textures. The blocks which ware composed of plagioclase — amphibole — olivine — clinopyroxene — magnetite are thought to be the products of fractionation. The differences between basic and intermediate centers is probably due to the frequency that the magma ascended to the surface or remained in high level chambers where fractionation occurred.  相似文献   

9.
In order to understand the origin of iron-rich olivine in the matrices of type 3 ordinary chondrites, the reaction of metallic iron and enstatite, with and without forsterite and SiO2, has been experimentally reproduced at temperatures between 1150° and 800°C and PO2 between 10−11 and 10−16 atm (between the IQF and MW buffers). The olivine produced ranges from Fo58 to Fo34 and this composition does not change significantly with temperature and time of the runs. The magnesian olivine which forms does become more magnesian with increasing forsterite/enstatite ratio of the starting materials. Iron-rich olivine (Fo< 35) cannot be formed by the reaction of enstatite and metallic iron, with or without forsterite as starting materials but it can be formed in the presence of free silica. The composition of olivine becomes more iron-rich with increasing silica/enstatite ratio. The compositional range of olivine formed from each mixture is 25–30 mole% Fo regardless of the temperature, composition, mineral assemblage, and run duration.From these experimental results, two possibilities suggested for the origin of the iron-rich olivine in the matrices of type 3 ordinary chondrites: (1) free silica must have been present if the iron-rich olivine was formed by solid-state reactions under oxidizing condition in the solar nebula; (2) reaction of silicon-rich gas with metallic iron took place under oxidizing condition in the solar nebula. Though it is difficult to define which alternative was dominant, the formation of free silica or silicon-rich gas may be a result of fractional condensation. This is possible if there is a reaction relation between forsterite and gas to produce enstatite. The suggested fractional condensation is supported by the fact that the compositions of the fine-grained matrices of type 3 ordinary chondrites are more silica-rich than the bulk compositions of the chondrites. Though it is not known whether such conditions were established all over the nebula or locally in the nebula, both fractionation and more oxidizing conditions than the average solar nebula are required for the formation of matrix olivine.  相似文献   

10.
Origin of calc-alkalic andesite in the Japanese Islands is reviewed on the basis of the recent trace element data and new experimental results. It is suggested that calc-alkalic andesites in the Japanese Islands have at least four different origins; (1) fractional crystallization with separation of magnetite of high-alumina basalt magma, (2) partial melting of hydrous upper mantle peridotite (for magnesian andesite), (3) fractional crystallization with separation of olivine and/or orthopyroxene of magnesian andesite magma and (4) mixing of dacitic and basaltic magmas. Emphasis is placed on the possible generation of primary magnesian calc-alkalic andesite magmas by direct partial melting of the upper mantle peridotite under hydrous conditions at depths between 40 and 60 km.  相似文献   

11.
Boninite is an unusual, plagioclase-free magnesian andesite, occurring as vesicular pillow lavas and hyaloclastites, accompanied by andesites and dacites in Chichi-jima, Bonin Islands. The Bonin Islands belong to the Izu-Mariana arc and consist of dominant volcanic rocks and subordinate sedimentary rocks of late Oligocene-early Miocene age. The chemistry of boninite is characterized by high contents of MgO. Cr and Ni similar to primitive basalts, but apparently in ill accord with its relatively high SiO2 content of ? 55%. The relation of SiO2 to total FeO/MgO ratio indicates that boninite belongs to the cale-alkalic rock suite. The mineralogy of boninite consists of olivine (Fo87-90), orthopyroxene (En87-90), clinopyroxene (Wo38-35En37-44Fs25-21), hydrous glass and Cr-spinel, Experimental studies show that the magma of boninite composition could be in equilibrium with upper mantle peridotite at pressures less than 17 kb and temperatures of 1200–1050°C under high PH2O. It is suggested that boninite is a sea-floor quenched product (900°C) of a direct partial melt of the upper mantle. Related andesites and dacites are considered to be probably fractional crystallization products from the same magma.  相似文献   

12.
High-pressure polymorphs of olivine and enstatite are major constituent minerals in the mantle transition zone(MTZ).The phase transformations of olivine and enstatite at pressure and temperature conditions corresponding to the lower part of the MTZ are import for understanding the nature of the 660 km seismic discontinuity.In this study,we determine phase transformations of olivine(MgSi2O4) and enstatite(MgSiO3) systematiclly at pressures between 21.3 and 24.4 GPa and at a constant temperature of 1600℃.The most profound discrepancy between olivine and enstatite phase transformation is the occurency of perovskite.In the olivine system,the post-spinel transformation occures at 23.8 GPa,corresponding to a depth of 660 km.In contrast,perovskite appears at 23 GPa(640 km) in the enstatite system.The ~1 GPa gap could explain the uplifting and/or splitting of the 660 km seismic discountinuity under eastern China.  相似文献   

13.
Phase equilibria in a portion of the system forsterite-plagioclase (An50Ab50 by weight)-silica-H2O have been determined at 15 kbar pressure under H2O-saturated conditions. The composition of the liquid pertinent to the piercing point forsterite + enstatite solid solution + amphibole + liquid + vapor is similar to that of calc-alkaline andesite. The electron microprobe analysis of the glass coexisting with the above three crystalline phases is very close to that of the piercing point determined by phase assemblage observations; however, the glass near (< 8 μm) forsterite crystals is significantly depleted in the normative forsterite component. With the addition of 10 wt.% KAlSi3O8, the composition of this piercing point becomes even closer to the compositions of calc-alkaline andesites. It is also shown that the liquid coexisting with forsterite and enstatite solid solution remains silica-rich (60–62 wt.%) over a wide (～ 100°C) temperature range. The present experimental studies support the view that liquids similar in composition to calc-alkaline andesites can be generated by direct partial melting of hydrous upper mantle at least at or near 15 kbar.  相似文献   

14.
Data on the electrical conductivity of olivine and pyroxene obtained under redox conditions similar to those that exist in the moon indicate that the moon is at temperatures near the melting point at depths of 600–900 km. This temperature profile, combined with information on the distribution of radioactive elements and evidence of extensive differentiation of the moon, lead to the conclusion that the moon accreted at temperatures between 600–1000°C. This high accretion temperature can be reconciled with the presence of FeS and the probable FeO/MgO ratio in the lunar interior if the moon accreted from material which was depleted in H2 relative to the solar nebula.  相似文献   

15.
After a 26 years long quiescence El Reventador, an active volcano of the rear-arc zone of Ecuador, entered a new eruptive cycle which lasted from 3 November to mid December 2002. The initial sub-Plinian activity (VEI 4 with andesite pyroclastic falls and flows) shifted on 6 and 21 November to an effusive stage characterized by the emission of two lava flows (andesite to low-silica andesite Lava-1 and basaltic andesite Lava-2) containing abundant gabbro cumulates. The erupted products are medium to high-K calc-alkaline and were investigated with respect to major element oxides, mineral chemistry, texture and thermobarometry. Inferred pre-eruptive magmatic processes are dominated by the intrusion of a high-T mafic magma (possibly up to 1165 ± 15 °C) into an andesite reservoir, acting as magma mixing and trigger for the eruption. Before this refilling, the andesite magma chamber was characterized by water content of 5.3 ± 1.0%, high oxygen fugacity (> NNO + 2) and temperatures, in the upper and lower part of the reservoir, of 850 and 952 ± 65 °C respectively. Accurate amphibole-based barometry constrains the magma chamber depth between 8.2 and 11.3 km (± 2.2 km). The 6 October 2002 seismic swarm (hypocenters from 10 to 11 km) preceding El Reventador eruption, supports the intrusion of magmas at these depths. The widespread occurrence of disequilibrium features in most of the andesites (e.g. complex mineral zoning and phase overgrowths) indicates that convective self-mixing have been operating together with fractional crystallization (inferred from the cognate gabbro cumulates) before the injection of the basic magma which then gave rise to basaltic andesite and low-silica andesite hybrid layers. Magma mixing in the shallow chamber is inferred from the anomalous SiO2–Al2O3 whole-rock pattern and strong olivine disequilibria. Both lavas show three types of amphibole breakdown rims mainly due to heating (mixing processes) and/or relatively slow syn-eruptive ascent rate (decompression) of the magmas. The lack of any disequilibrium textures in the pumices of the 3 November fall deposit suggest that pre-eruptive mixing did not occur in the roof zone of the chamber. A model of the subvolcanic feeding system of El Reventador, consistent with the intrusion of a low-Al2O3 crystal-rich basic magma into an already self-mixed andesite shallow reservoir, is here proposed. It is also inferred that before entering the shallow chamber the “basaltic” magma underwent a polybaric crystallization at deeper crustal levels.  相似文献   

16.
Reaction coronas of pyroxene ± ilmenite occur around clasts of olivine in Apollo 14 high-grade metamorphic breccias. In experiments of several months duration, there was no evidence of corona formation at 1000°C, but at 1050°, withfO2 at or above Ilm-Ru-Fe and below Fe-Fe1?x O, incipient coronas formed around Fo50–70 in synthetic 14311 matrix. In addition, withfO2 controlled by Ilm-Ru-Fe at 1050°C, the olivines reduced to Fo68, En69 + Fe. Reduction of olivine under these conditions is inconsistent with the calculated stability relations and is attributed to uncertainties in the activity coefficient for olivine or pyroxene. The experiments also suggest that vesicularity in the Apollo 14 high-grade breccias may correlate with the amount of glassy material in their unmetamorphosed precursors. The metamorphic event is attributed to burial in a hot ejecta blanket, such as that of the Imbrium event.  相似文献   

17.
To calculate accurately the pressure interval and mineral proportions (i.e. yields) across the olivine to wadsleyite and wadsleyite to ringwoodite transformations requires a detailed knowledge of the non-ideality of Fe-Mg mixing in these (Mg,Fe)2SiO4 solid solutions. In order to constrain the activity-composition relations that describe non-ideal mixing, Fe-Mg partitioning experiments have been conducted between magnesiowüstite and (Mg,Fe)2SiO4 olivine, wadsleyite and ringwoodite as a function of pressure at 1400°C. Using known activity-composition relations for magnesiowüstite the corresponding relations for the three polymorphs were determined from the partitioning data. In all experiments the presence of metallic iron ensured redox conditions compatible with the Earth’s transition zone. The non-ideality of the (Mg,Fe)2SiO4 solid solutions was found to decrease in the order WwadsleyiteFeMg>WringwooditeFeMg>WolivineFeMg. These partitioning data were used, along with published phase equilibria measurements for the Mg2SiO4 and Fe2SiO4 end-member transformations, to produce an internally consistent thermodynamic model for the Mg2SiO4-Fe2SiO4 system at 1400°C. Using this model the pressure interval of the olivine to wadsleyite transformation is calculated to be significantly smaller than previous determinations. By combining these results with Fe-Mg partitioning data for garnet, the widths of transition zone phase transformations in a peridotite composition were calculated. The olivine to wadsleyite transformation at 1400°C in dry peridotite was found to occur over a pressure interval equivalent to approximately 6 km depth and the mineral yields were found to vary almost linearly with depth across the transformation. This transformation is likely to be even sharper at higher temperatures or could be significantly broader in wet mantle or in regions with a significant vertical component of mantle flow. The entire range of estimated widths for the 410 km discontinuity (4-35 km) could, therefore, be explained by the olivine to wadsleyite transformation in a peridotite composition over a range of quite plausible mantle temperatures and H2O contents. The wadsleyite to ringwoodite transformation in peridotite mantle was calculated to take place over an interval of 20 km at 1400°C. This transformation yield was also found to be near linear.  相似文献   

18.
Ultrasonic compressional wave velocity Vp and quality factor Qp have been measured in alkali basalt, olivine basalt and basic andesite melts in the frequency range of 3.4–22 MHz and in the temperature range of 1100–1400°C. Velocity and attenuation of the melts depend on frequency and temperature, showing that there are relaxation mechanisms in the melts. Complex moduli are calculated from the ultrasonic data. The results fit well a complex modulus of Arrhenius temperature dependence with log-normal Gaussian distribution in relaxation times of attenuation. The analysis yields average relaxation time, its activation energy, relaxed modulus, unrelaxed modulus and width of Gaussian distribution in relaxation times. Relaxed modulus is smaller (17.5 GPa) for basic andesite melt of high silica and high alumina contents than for the other two basalt melts (18.1–18.4 GPa). The most probable relaxation times decrease from ～ 3 × 10?10 s for basic andesite to ～ 10?11 s for alkali basalt at 1400°C. Activation energies of attenuation, ranging from 270 to 340 kJ mol?1 in the three melts, are highest in basic andesite. Longitudinal viscosity values and their temperature dependences are also calculated from Vp and Qp data. The volume viscosity values are estimated from the data using the shear viscosity values. Longitudinal, volume and shear viscosities and their activation energies are highest in the basic andesite melt of the most polymerized structure.  相似文献   

19.
Thermal grooving of low angle tilt boundary of San Carlos olivine in the albite melt were experimentally investigated at 1200–1300°C in mixed CO2 and H2 gases for 1–20 h. The depth, d, of the thermal groove on (010) of olivine along the (100) sub-boundaries is in the function of time and temperatures as follows; d4 = ko · t · exp(− 190 000/RT), in which R is the gas constant, and ko is the material constant.The melt shape changes due to the thermal grooving driven by surface tension and deformation of the upper mantle. Compared with the time scales of these two counteracting mechanisms, it is inferred that the melt shape is unstable in the high temperature and low stress conditions, and that the melt shape takes a stable form during progressive deformation in the low temperature and high stress conditions.  相似文献   

20.
Volcanoes of the East Japan volcanic arc are divided into two groups on the basis of their phenocryst assemblages; volcanoes with lavas or pyroclastic rocks containing quartz phenocrysts and no hornblende phenocrysts (type A), and those with rocks containing hornblende phenocrysts and no quartz phenocrysts (type B). Type A volcanoes occur only in the narrow region along the volcanic front, whereas type B volcanoes are distributed in the area closer to the Sea of Japan.Recent experimental studies on calc-alkaline andesite-dacite under H2O-saturated and -undersaturated conditions indicate that the liquidus temperature (maximum thermal stability limit) of quartz decreases drastically with increasing H2O content in magma, whereas the liquidus temperatures of hornblende and biotite are relatively constant with variations in the H2O content and bulk chemical composition of the magma.It is suggested from the lateral variation of mafic phenocryst assemblages [1] and from the above result that the temperature of the parental magmas of these volcanoes increases, and their H2O contents decrease, towards the volcanic front in the East Japan volcanic arc.Such lateral variations in the H2O contents of magmas under the East Japan volcanic arc are in agreement with those of other incompatible elements (K, Rb, REE, etc.). If H2O-undersaturated partial melting of upper mantle peridotite can be represented by the univariant line (olivine, Ca-rich clinopyroxene, orthopyroxene and liquid coexist) in the system H2OMg2SiO4z.sbnd;CaMgSi2O6z.sbnd;SiO2, the decrease of H2O content in the magma suggests that the melting temperature of the peridotitic mantle may gradually increase, and so the degree of partial melting may increase, towards the volcanic front. The lateral variation of other incompatible elements can also be explained by this model.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号