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1.
In this paper we describe the mineralogy and geochemistry of basanites and melt inclusions in minerals from the Tergesh pipe, northern Minusinsk Depression. The rocks are composed of olivine and clinopyroxene phenocrysts and a groundmass of olivine, clinopyroxene, titanomagnetite, plagioclase, apatite, ilmenite, and glass. Melt inclusions were found only in the olivine and clinopyroxene phenocrysts. Primary melt inclusions in olivine contain glass, rh?nite, clinopyroxene, a sulfide globule, and low-density fluid. The phase composition of melt inclusions in clinopyroxene is glass + low-density fluid ± xenogenous magnetite. According to thermometric investigations, the olivine phenocrysts began crystallizing at T = 1280–1320°C and P > 3.5 kbar, whereas groundmass minerals were formed under near-surface conditions at T ≤ 1200°C. The oxygen fugacity gradually changed during basanite crystallization from oxidizing (NNO) to more reducing conditions (QFM). The investigation of glass compositions (heated and unheated inclusions in phenocrysts and groundmass) showed that the evolution of a basanite melt during its crystallization included mainly an increase in SiO2, Al2O3, and alkalis, while a decrease in femic components, and the melt composition moved gradually toward tephriphonolite and trachyandesite. Geochemical evidence suggests that the primary basanite melt was derived from a mantle source affected by differentiation. Original Russian Text ? T.Yu. Timina, V.V. Sharygin, A.V. Golovin, 2006, published in Geokhimiya, 2006, No. 8, pp. 814–833.  相似文献   

2.
We describe and model a potential re-equilibration process that can affect compositions of melt inclusions in magnesian olivine phenocrysts. This process, referred to as “Fe-loss”, can operate during natural pre-eruptive cooling of host magma and results in lower FeOt and higher MgO contents within the initially trapped volume of inclusion. The extent of Fe-loss is enhanced by large temperature intervals of magma cooling before eruption. The compositions of homogenised melt inclusions in olivine phenocrysts from several subduction-related suites demonstrate that (1) Fe-loss is a common process, (2) the maximum observed degree of re-equilibration varies between suites, and (3) within a single sample, variable degrees of re-equilibration can be recorded by melt inclusions trapped in olivine phenocrysts of identical composition. Our modelling also demonstrates that the re-equilibration process is fast going to completion, in the largest inclusions in the most magnesian phenocrysts it is completed within 2 years. The results we obtained indicate that the possibility of Fe-loss must be considered when estimating compositions of parental subduction-related magmas from naturally quenched glassy melt inclusions in magnesian olivine phenocrysts. Compositions calculated from glassy inclusions affected by Fe-loss will inherit not only erroneously low FeOt contents, but also low MgO due to the inherited higher Mg##of the residual melt in re-equilibrated inclusions. We also demonstrate that due to the higher MgO contents of homogenised melt inclusions affected by Fe-loss, homogenisation temperatures achieved in heating experiments will be higher than original trapping temperatures. The extent of overheating will increase depending on the degree of re-equilibration, and can reach up to 50 °C in cases where complete re-equilibration occurs over a cooling interval of 200 °C. Received: 2 November 1998 / Accepted: 27 September 1999  相似文献   

3.
The compositions of approximately 70 naturally quenched melt inclusions in olivine, clinopyroxene, orthopyroxene, and plagioclase phenocrysts from tephra of the soil–pyroclastic cover of Simushir Island (Central Kuril Islands) were studied. The concentrations of the major rock-forming components, H2O, S, and Cl were analyzed in inclusions. The reconstructed melts contain 48.6–78.4 wt % SiO2, 0.3–8.26 wt % MgO, and 0.12–1.72 wt % K2O. The concentration of S and Cl in the melts changes regularly with increasing SiO2 content: from 0.14 to ~0.02 wt % S and from ~0.05 to ~0.28 wt % Cl. The content of H2O in parental melts is 4.2–4.5 wt %.  相似文献   

4.
Melt inclusions in olivine and pyroxene phenocrysts in kersantite and camptonite at Chhaktalao in Madhya Pradesh, India are mainly of the evolved type forming daughter minerals of olivine, pyroxene, plagioclase, spinel, mica, titanomagnetite and sulphides. Heating studies exhibit a temperature range from 1215° to 1245°C for the melt inclusions in olivine in camptonite and 1220–1245°C for olivine in kersantite. The temperature for melt inclusions in pyroxene ranged from 1000° to 1150°C in camptonite and 850–1100°C for pyroxene in kersantite. The bubble inside these melt inclusions is mainly CO2. The Th°C of CO2 into liquid phase occurred between 26° and 31°C in olivine and 25–30°C in pyroxene from kersantite and camptonite. The maximum density estimated is 0.72 g/cm3 and the minimum is 0.45 g/cm3. The depth of entrapment of the melt inclusion is estimated between 10–15 km. The pressure of entrapment of melt inclusion in olvine is 4.6 kbar where as that in pyroxene is 3.7 kbar. The lamprophyres in the Chhaktalao area are considered to be derived from low depth and low pressure region, possibly within spinel lherzolite zone.  相似文献   

5.
Melt and fluid inclusions have been studied in olivine phenocrysts (Fo 81–79) from trachybasalts of the Southern Baikal volcanic area, Dzhida field. The melt inclusions were homogenized, quenched, and analyzed on an electron and ion microprobe. The study of homogenized glasses of nine inclusions showed that basaltic melts (SiO2 = 47.1–50.3 wt %, MgO = 5.0–7.7 wt %, CaO = 7.1–11.1 wt %) have high contents of Al2O3 (17.1–19.6 wt %), Na2O (4.1–6.2 wt %), K2O (2.2–3.3 wt %), and P2O5 (0.6–1.1 wt %). The volatile contents are low (in wt %): 0.24–0.31 H2O, 0.08 F, 0.03 Cl, and 0.02 S. Primary fluid inclusions in olivines from four trachybasalt samples contain high-density CO2 (0.73–0.87 g/cm3), indicating a CO2 fluid pressure of 4.3–6.6 kbar at 1200–1300°C and olivine crystallization depths of 16–24 km. Ion microprobe analyses of 20 glasses from melt inclusions for trace elements showed that the magmas of the Baikal rift were enriched in incompatible elements, thus differing from oceanic rift basalts and resembling oceanic island basalts. A comparison of our data on melt and fluid inclusions in olivine from trachybasalts of the Dzhida field with preexisting data on the Eastern Tuva volcanic highland in the Southern Baikal volcanic area showed that they had similar contents of volatiles, major, and trace elements.  相似文献   

6.
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 SiO2 and H2O 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 H2O (3.4 wt.%) concentrations among those described in reports of previous studies. The S6+/Stotal 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 fS2) 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 H2O (0.20 wt.%) and S (< 70 ppm), suggesting that Cl was retained in the magma, in contrast to S and H2O, which degassed strongly during magma effusion.  相似文献   

7.
The results of a complex study of melt inclusions in olivine phenocrysts contained in unaltered kimberlites from the Udachnaya-East pipe indicate that the inclusions were captured late during the magmatic stage, perhaps, under a pressure of <1 kbar and a temperature of ≤800°C. The inclusions consist of fine crystalline aggregates (carbonates + sulfates + chlorides) + gas ± crystalline phases. Minerals identified among the transparent daughter phases of the inclusions are silicates (tetraferriphlogopite, olivine, humite or clinohumite, diopside, and monticellite), carbonates (calcite, dolomite, siderite, northupite, and Na-Ca carbonates), Na and K chlorides, and alkali sulfates. The ore phases are magnetite, djerfisherite, and monosulfide solid solution. The inclusions are derivatives of the kimberlite melt. The complex silicate-carbonate-salt composition of the secondary melt inclusions in olivine from the kimberlite suggests that the composition of the kimberlite melt near the surface differed from that of the initial melt composition in having higher contents of CaO, FeO, alkalis, and volatiles (CO2, H2O, F, Cl, and S) at lower concentrations of SiO2, MgO, Al2O3, Cr2O3, and TiO2. Hence, when crystallizing, the kimberlite melt evolved toward carbonatite compositions. The last derivatives of the kimberlite melt had an alkaline carbonatite composition.  相似文献   

8.
In this paper, we discuss the formation conditions of rhyolites and results of their interaction with later portions of basic magmas on the basis of the investigation of melt and fluid inclusions in minerals from a rhyolite xenolith and host neovolcanic basalts of the Cleft segment of the Juan de Fuca Ridge. In terms of bulk chemistry and the compositions of melt inclusions in pyroxene and olivine phenocrysts, the basic rocks of the southern part of this segment are typical MOR basalts. Their olivine, clinopyroxene, and plagioclase crystallized at temperatures of 1160–1280°C and a pressure range between 20 and 100 MPa. The xenolith is a leucocratic rock with negligible amounts of mafic minerals, which clearly distinguishes it from the known occurrences of silicic rocks in the rift valleys of MOR. The rhyolite melt crystallized at temperatures of 900–880°C. The final stages of rhyolite melt crystallization at temperatures of 780–800°C were accompanied by the release of a saline aqueous fluid with high chloride contents. Based on the geochemical characteristics of melt inclusions and melting products, it can be suggested that the magmatic melt was produced by melting of metamorphosed oceanic crust within the Cleft segment under the influence sof saline aqueous fluid trapped in the pores and interstices of the rock. The rock represented by the xenolith is a late differentiation product of such melts. The ultimate products of silicic melt fractionation show high volatile contents: H2O > 3.0 wt %, Cl ～ 2.0 wt %, and F ～ 0.1 wt %. The interaction of the xenolith with the host basaltic melt occurred at temperatures equal or slightly higher than those of ferrobasalt melts (1190–1180°C). During ascent the xenolith occurred for a few tens of hours in high-temperature basic magma, and diffusion exchange between the basaltic and silicic melts was very minor.  相似文献   

9.
There are many melt and fluid inclusions (mainly CO2-rich) in olivine and pyroxene phenocrysts in basalts from the Ross Island area. The melt inclusions can be classified as follows: (1) crystalline melt inclusions (type I), (2) fluid-melt inclusions (type II) and (3) glass inclusions (type III). The daughter minerals in type I include olivine, plagioclase, ilmenite, etc. Fluid-melt inclusions are a new type which represent the immiscibility of magma and fluid at a particular stage of evolution. Three types of fluid-melt inclusions were examined in this study: a) crystal + liquid + gas, b) inclusions coexisting with glass inclusions and fluid inclusions, and c) crystal + daughter mineral (dissolved salt) + gas. Both primary and secondary melt inclusions are recognizable in the samples. The secondary melt inclusions were formed during healing of fractures in the host minerals in the process of magma rise. The homogenization temperatures (both Leitz 1350 stage and quench method were used) of melt inclusions in basalts range from 1190 to 135°C at high pressure (about 7 kbars), indicating that the basalts may have come from the upper mantle. Melt-fluid immiscibility in basaltic magma shows that the CO2-rich fluids may be the main fluid phase in the upper mantle, which are of significance in understanding the evolution of magma and various processes in the deep levels of the earth. The homogenization temperatures of melt and aqueous fluid inclusions in granites and metamorphic rocks in this area vary from 980 to 1100°C and 279 to 350°C, respectively.  相似文献   

10.
Crystallization of spinel minerals in transitional and alkali basalts from Iceland can be related to the FeO, MgO, TiO2 and Cr contents of the coexisting melt. Chromian spinel occurs in glasses in which TiO2 is less than 2.8 wt.% and the weight ratio FeO/MgO is less than 2.0, whereas titanomagnetite occurs when the same parameters are greater than 4 wt.% and 2.7, respectively. In addition, chromian spinel only occurs in basalts with Cr greater than 200 ppm. It is suggested that chromian spinel crystallizes, together with olivine, from liquids with olivine liquidus temperatures ranging from above 1,200° C to approximately 1,150° C. A discontinuity in spinel crystallization follows until below 1,100° C, where titanomagnetite starts to crystallize. Compositional variations in chromian spinel attached to, or included, in homogeneous olivine phenocrysts, however, cannot be related to equilibrium relations. Textural relations suggest homogeneous nucleation for titanomagnetite, whereas chromian spinel nucleates heterogeneously, dependent on growth of olivine phenocrysts. The composition of chromian spinels cannot in detail be related to physical and compositional parameters of the average melt, but may be related to local compositional relations in the melt adjacent to growing crystals. Such compositional variation around growing olivine crystals may be the prime reason for the non-equilibrium precipitation of included chromian spinels.  相似文献   

11.
<正>GRV 020175 is an Antarctic mesosiderite,containing about 43 vol%silicates and 57 vol% metal.Metal occurs in a variety of textures from irregular large masses,to veins penetrating silicates, and to matrix fine grains.The metallic portion contains kamacite,troilite and minor taenite.Terrestrial weathering is evident as partial replacement of the metal and troilite veins by Fe oxides.Silicate phases exhibit a porphyritic texture with pyroxene,plagioclase,minor silica and rare olivine phenocrysts embedded in a fine-grained groundmass.The matrix is ophitic and consists mainly of pyroxene and plagioclase grains.Some orthopyroxene phenocrysts occur as euhedral crystals with chemical zoning from a magnesian core to a ferroan overgrowth;others are characterized by many fine inclusions of plagioclase composition.Pigeonite has almost inverted to its orthopyroxene host with augite lamellae, enclosed by more magnesian rims.Olivine occurs as subhedral crystals,surrounded by a necklace of tiny chromite grains(about 2-3μm).Plagioclase has a heterogeneous composition without zoning. Pyroxene geothermometry of GRV 020175 gives a peak metamorphic temperature(～1000℃) and a closure temperature(～875℃).Molar Fe/Mn ratios(19-32) of pyroxenes are consistent with mesosiderite pyroxenes(16-35) and most plagioclase compositions(An_(87.5_96.6)) are within the range of mesosiderite plagioclase grains(An_(88-95)).Olivine composition(Fo_(53.8)) is only slightly lower than the range of olivine compositions in mesosiderites(Fo_(55-90)).All petrographic characteristics and chemical compositions of GRV 020175 are consistent with those of mesosiderite and based on its matrix texture and relatively abundant plagioclase,it can be further classified as a type 3A mesosiderite.Mineralogical, penological,and geochemical studies of GRV 020175 imply a complex formation history starting as rapid crystallization from a magma in a lava flow on the surface or as a shallow intrusion.Following primary igneous crystallization,the silicate underwent varying degrees of reheating.It was reheated to 1000℃,followed by rapid cooling to 875℃.Subsequently,metal mixed with silicate,during or after which,reduction of silicates occurred;the reducing agent is likely to have been sulfur.After redox reaction,the sample underwent thermal metamorphism,which produced the corona on the olivine, rims on the inverted pigeonite phenocrysts and overgrowths on the orthopyroxene phenocrysts,and homogenized matrix pyroxenes.Nevertheless,metamorphism was not extensive enough to completely reequilibrate the GRV 020175 materials.  相似文献   

12.
Based on the investigation of melt inclusions using electron and ion microprobe analysis, we estimated the composition, evolution, and formation conditions of magmas responsible for the calcite-bearing ijolites and carbonatites of the Belaya Zima alkaline carbonatite complex (eastern Sayan, Russia). Primary melt and coexisting crystalline inclusions were found in the nepheline and calcite of these rocks. Diopside, amphibole (?), perovskite, potassium feldspar, apatite, calcite, pyrrhotite, and titanomagnetite were identified among the crystalline inclusions. The melt inclusions in nepheline from the ijolites are completely crystallized. The crystalline daughter phases of these inclusions are diopside, phlogopite, apatite, calcite, magnetite, and cuspidine. During thermometric experiments with melt inclusions in nepheline, the complete homogenization of the inclusions was attained through the dissolution of a gas bubble at temperatures of 1120–1130°C. The chemical analysis of glasses from the homogenized melt inclusions in nepheline of the ijolites revealed significant variations in the content of components: from 36 to 48 wt % SiO2, from 9 to 21 wt % Al2O3, from 8 to 25 wt % CaO, and from 0.6 to 7 wt % MgO. All the melts show very high contents of alkalis, especially sodium. According to the results of ion microprobe analysis, the average content of water in the melts is no higher than a few tenths of a percent. The most salient feature of the melt inclusions is the extremely high content of Nb and Zr. The glasses of melt inclusions are also enriched in Ta, Th, and light rare earth elements but depleted in Ti and Hf. Primary melt inclusions in calcite from the carbonatites contain a colorless glass and daughter phlogopite, garnet, and diopside. The silicate glass from the melt inclusions in calcite of the carbonatite is chemically similar to the glasses of homogenized melt inclusions in nepheline from the ijolites. An important feature of melt inclusions in calcite of the carbonatites is the presence in the glass of carbonate globules corresponding to calcite in composition. The investigation of melt inclusions in minerals of the ijolites and carbonatites and the analysis of the alkaline and ore-bearing rocks of the Belaya Zima Massif provided evidence for the contribution of crystallization differentiation and silicate-carbonate liquid immiscibility to the formation of these rocks. Using the obtained trace-element compositions of glasses of homogenized melt inclusions and various alkaline rocks and carbonatites, we determined to a first approximation the compositions of mantle sources responsible for the formation of the rock association of the Belaya Zima alkaline-carbonatite complex. The alkaline rocks and carbonatites were derived from the depleted mantle affected by extensive metasomatism. It is supposed that carbonate melts enriched in sodium and calcium were the main agents of mantle metasomatism.  相似文献   

13.
Using various methods of melt inclusion investigation, including electron and ion microprobe techniques, we estimated the composition, evolution, and formation conditions of melts producing the trachydacites and pantellerites of the Late Paleozoic bimodal volcanic association of Dzarta-Khuduk, Central Mongolia. Primary crystalline and melt inclusions were detected in anorthoclase from trachydacites and quartz from pantellerites and pantelleritic tuffs. Among the crystalline inclusions, we identified hedenbergite, fluorapatite, and pyrrhotite in the trachydacites and F-arfvedsonite, fluorite, ilmenite, and the rare REE diorthosilicate chevkinite in the pantellerites. Melt inclusions in anorthoclase from the trachydacites are composed of glass, a gas phase, and daughter minerals (F-arfvedsonite, fluorite, villiaumite, and anorthoclase rim on the inclusion wall). Melt inclusions in quartz from the pantellerites are composed of glass, a gas phase, and a fine-grained salt aggregate consisting of Li, Na, and Ca fluorides (griceite, villiaumite, and fluorite). Melt inclusions in quartz crystalloclasts from the pantelleritic tuffs are composed of homogeneous silicate glasses. The phenocrysts of the trachydacites and pantellerites crystallized at temperatures of 1060–1000°C. During thermometric experiments with quartz-hosted melt inclusions from the pantellerites, the formation of immiscible silicate and salt (fluoride) melts was observed at a temperature of 800°C. Homogeneous melt inclusions in anorthoclase from the trachydacites have both trachydacite and rhyolite compositions (wt %): 68–70 SiO2, 12–13 Al2O3, 0.34–0.74 TiO2, 5–7 FeO, 0.4–0.9 CaO, and 9–12 Na2O + K2O. The agpaitic index ranges from 0.92 to 1.24. The glasses of homogenized melt inclusions in quartz from the pantellerites and pantelleritic tuffs have rhyolitic compositions. Compared with the homogeneous glasses trapped in anorthoclase of the trachydacites, quartz-hosted inclusions from the pantellerites show higher SiO2 (72–78 wt %) and lower Al2O3 contents (7.8–10.0 wt %). They also contain 0.14–0.26 wt % TiO2, 2.5–4.9 wt % FeO, 9–11 wt % Na2O + K2O, and 0.9–0.15 wt % CaO and show an agpaitic index of 1.2–2.05. Homogeneous melt inclusions in quartz from the pantelleritic tuffs contain 69–72 wt % SiO2. The contents of other major components, including TiO2, Al2O3, FeO, and CaO, are close to those in the homogeneous glasses of quartzhosted melt inclusions in the pantellerites. The contents of Na2O + K2O are 4–10 wt %, and the agpaitic index is 1.0–1.6. The glasses of melt inclusions from each rock group show distinctive volatile compositions. The H2O content is up to 0.08 wt % in anorthoclase of the trachydacites, 0.4–1.4 wt % in quartz of the pantellerites, and up to 5 wt % in quartz of the pantelleritic tuffs. The content of F in the glasses of melt inclusions in the phenocrysts of the trachydacites is no higher than 0.67 wt %, and up to 1.4–2.8 wt % in quartz from the pantellerites. The Cl content is up to 0.2 wt % in the glasses of melt inclusions in the minerals of the trachydacites and up to 0.5 wt % in the glasses of quartz-hosted melt inclusions from the pantellerites. The investigation of trace elements in the homogenized glasses of melt inclusions in minerals showed that the trachydacites and pantellerites were formed from strongly evolved rare-metal alkaline silicate melts with high contents of Li, Zr, Rb, Y, Hf, Th, U, and REE. The analysis of the composition of homogeneous melt inclusions in the minerals of the above rocks allowed us to distinguish magmatic processes resulting in the enrichment of these rocks in trace and rare earth elements. The most important processes are the crystallization differentiation and immiscible separation of silicate and fluoride salt melts. It was also shown that all the melts studied evolved in spatially separated magma chambers. This caused the differences in the character of melt evolution between the trachydacites and pantellerites. During the final stages of differentiation, when the magmatic system was saturated with respect to ore elements, Na-Ca fluoride melts were separated and extracted considerable amounts of Li.  相似文献   

14.
Based on the analysis of data on the composition of melt inclusions in minerals and quenched glasses of igneous rocks, we considered the problems of the formation of peralkaline silicic magmas (i.e., whose agpaitic index, the molar ratio AI = (Na2O + K2O)/Al2O3, is higher than one). The mean compositions of peralkaline silicic melts are reported for island arcs and active continental margins and compared with the compositions of melts from other settings, primarily, intraplate continental areas. Peralkaline silicic rocks are rather common in the latter. Such rocks are rare in island arcs and active continental margins, but agpaitic melts were observed in inclusions in phenocrysts of plagioclase, quartz, pyroxene, and other minerals. Plagioclase fractionation from an alkali-rich melt with AI < 1 is considered as a possible mechanism for the formation of peralkaline silicic melts (Bowen’s plagioclase effect). However, the analysis of available experimental data on plagioclase-melt equilibria showed that natural peralkaline melts are almost never in equilibrium with plagioclase. For the same reason, the melting of the majority of crustal rocks, which usually contain plagioclase, does not produce peralkaline melts. The existence of peralkaline silicic melt inclusions in plagioclase phenocrysts suggests that plagioclase can crystallize from peralkaline melts, and the plagioclase effect may play a certain role. Another mechanism for the formation of peralkaline silicic magmas is the melting of alkali-rich basic and intermediate rocks, including the spilitized varieties of subalkali basalts.  相似文献   

15.
A technique is described for determining the cooling historyof olivine phenocrysts. The technique is based on the analysisof the diffusive re-equilibration of melt inclusions trappedby olivine phenocrysts during crystallization. The mechanismof re-equilibration involves diffusion of Fe from and Mg intothe initial volume of the inclusion. The technique applies toa single crystal, and thus the cooling history of differentphenocrysts in a single erupted magma can be established. Weshow that melt inclusions in high-Fo olivine phenocrysts frommantle-derived magmas are typically partially re-equilibratedwith their hosts at temperatures below trapping. Our analysisdemonstrates that at a reasonable combination of factors suchas (1) cooling interval before eruption (<350°C), (2)eruption temperatures (>1000°C), and (3) inclusion size(<70 µm in radius), partial re-equilibration of upto 85% occurs within 3–5 months, corresponding to coolingrates faster than 1–2°/day. Short residence timesof high-Fo phenocrysts suggest that if eruption does not happenwithin a few months after a primitive magma begins cooling andcrystallization, olivines that crystallize from it are unlikelyto be erupted as phenocrysts. This can be explained by efficientseparation of olivine crystals from the melt, and their rapidincorporation into the cumulate layer of the chamber. Theseresults also suggest that in most cases erupted high-Fo olivinephenocrysts retain their original composition, and thus compositionsof melt inclusions in erupted high-Fo olivine phenocrysts donot suffer changes that cannot be reversed. Short residencetimes also imply that large unzoned cores of high-Fo phenocrystscannot reflect diffusive re-equilibration of originally zonedphenocrysts. The unzoned cores are a result of fast efficientaccumulation of olivines from the crystallizing magma, i.e.olivines are separated from the magma faster than melt changesits composition. Thus, the main source of high-Fo crystals inthe erupted magmas is the cumulate layers of the magmatic system.In other words, olivine-phyric rocks represent mixtures of anevolved transporting magma (which forms the groundmass of therock) with crystals that were formed during crystallizationof more primitive melt(s). Unlike high-Fo olivine phenocrysts,the evolved magma may reside in the magmatic system for a longtime. This reconciles long magma residence times estimated fromthe compositions of rocks with short residence times of high-Foolivine phenocrysts. KEY WORDS: melt inclusions; olivine; picrites; residence time; diffusion  相似文献   

16.
The products of the 1974 eruption of Fuego, a subduction zone volcano in Guatemala, have been investigated through study of silicate melt inclusions in olivine. The melt inclusions sampled liquids in regions where olivine, plagioclase, magnetite, and augite were precipitating. Comparisons of the erupted ash, groundmass, and melt inclusion compositions suggest that the inclusions represent samples of liquids present in a thermal boundary layer of the magma body. The concentrations of H2O and CO2 in glass inclusions were determined by a vacuum fusion manometric technique using individual olivine crystals (Fo77 to Fo71) with glass inclusion compositions that ranged from high-alumina basalt to basaltic andesite. Water, Cl, and K2O concentrations increased by a factor of two as the olivine crystals became more iron-rich (Fo77 to Fo71) and as the glass inclusions increased in SiO2 from 51 to 54 wt.% SiO2. The concentration of H2O in the melt increased from 1.6 wt.% in the least differentiated liquid to about 3.5% in a more differentiated liquid. Carbon dioxide is about an order of magnitude less abundant than H2O in these inclusions. The gas saturation pressures for pure H2O in equilibrium with the melt inclusions, which were calculated from the glass inclusion compositions using the solubility model of Burnham (1979), are given approximately by P(H2O)(Pa)=(SiO2−48.5 wt.%) × 1.45 × 107. The concentrations of water in the melt and the gas saturation pressures increased from about 1.5% to 3.5% and from 300 to 850 bars, respectively, during pre-eruption crystallization.  相似文献   

17.
Compositional relations among natural glasses in basalts recovered by Legs 45 and 46 (DSDP) provide powerful constraints on their differentiation histories. Residual glass compositions in the moderately evolved aphyric and abundantly phyric basalts within each site demonstrate that none of the units is mutually related to any other or to a common parent by simple fractional crystallization. At Site 396, where clinopyroxene phenocrysts are absent, progressively more evolved liquids (lower Mg/ (Mg+Fe) and higher TiO2) are characterized by lower calcium-aluminum ratios, which can only be generated by clinopyroxene fractionation. This paradox is amplified by some melt inclusions in olivine phenocrysts that have higher CaO/Al2O3 and lower TiO2 than any residual glasses. The occurrences of these distinctive compositions are correlated with the highly magnesian character of the host olivines (Fo90–89), and the melts are interpreted as trapped primitive liquids, parental to the more fractionated derivatives.Melt inclusions intermediate in composition between the residual glasses and the most primitive olivine melt inclusions are present in the cores of some plagioclase phenocrysts that have had a history of resorption. On the basis of a petrographic and microprobe analysis of the zoning relations in these phenocrysts, the inclusions are inferred to be melts entrapped at the time of extensive corrosion of the host crystals.Interpreted in conjunction with other mineral and geochemical data, the compositional trends in the glasses indicate that magma mixing has played a major role in the genesis of the Leg 45 and 46 basalts. The reality of mixing is demonstrated by extensive disequilibrium textures in the plagioclase phenocrysts and the presence in evolved lavas of refractory plagioclase and olivine phenocrysts bearing primitive melt inclusions. The chemical imprint of clinopyroxene fractionation despite the absence of clinopyroxene phenocrysts is believed to be accomplished by plating of gabbro on to the upper walls of the subvolcanic magma chamber as it evolves between mixing events. Repeated influxes of primitive magma batches will move the resultant hybrids alway from clinopyroxene saturation and generate olivine-plagioclase cotectic magmas. This model provides a physical buffering mechanism that accounts for the volumetric dominance of moderately evolved basalts among ocean floor tholeiites. Major and trace element models based on the combination of mixing and fractional crystallization also explain heretofore enigmatic geochemical characteristics of MORB.Lunar and Planetary Institute Contribution no. 326After August 1, 1978: Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, USAThe Lunar and Planetary Institute is operated by the Universities Space Research Association under Contract No. NSR 09-051-001 with the National Aeronautics and Space Administration  相似文献   

18.
ABSTRACT

Silicate melt inclusions (SMIs) are small droplets of magma that become trapped in minerals during crystal growth. SMIs in olivine crystals can provide critical information on the range of melt compositions and processes that occur during melt generation, evolution, transport, and eruption. The Pliocene–Quaternary volcanic rocks in the Qorveh–Bijar volcanic belt of western Iran show porphyritic and microlithic textures, with olivine and clinopyroxene being the dominant minerals. Magnesian olivines in these volcanic rocks contain primary SMIs. The composition and characteristic of olivine-hosted SMI of these rocks are investigated to constrain the source lithology for mafic volcanism. Bulk compositions of the SMIs overlap those of their host rocks and extend to higher CaO/Al2O3 values. The estimated entrapment pressures and temperatures of the studied SMIs are 9.1–10.3 kbar and 1220–1355°C. The calculated mafic parental melt contains 42.36 wt.% SiO2, low total alkalis (3.22 wt.%), and high MgO (16.1 wt.%). Exploratory calculations using pMELTS show that this parental composition underwent variable degrees of fractional crystallization, as reflected by the variable compositions of the SMIs. Several lines of evidence including pyroxene xenocrysts and high FeO/MnO, FC3MS (FeO/CaO – 3*MgO/SiO2), and Zn/Fe ratios (14–21), suggest that a metasomatized pyroxenitic source contributed to the genesis of the parental melt. Amphibole in the SMIs indicates a high volatile content in the parental melt, which we conclude was generated from a metasomatized lithospheric mantle source. The pyroxenite source also contained garnet. Our geochemical results lead us to propose a new petrogenetic model. Specifically, we infer that a dense and unstable portion of the lithosphere underwent localized laminar detachment and downward flow, i.e. lithospheric drip. This drip underwent volatile-enhanced partial melting during descent through the underlying hot asthenosphere and generated the studied volcanic rocks.  相似文献   

19.
On eruption, the tholeiitic basalt lava of the prehistoric Makaopuhi lake contained nearly seven percent euhedral olivine phenocrysts of approximately Fa14 composition. In the center of the 225 foot vertical section of the lake, the lava became more than 90 percent solid at 1000° C after about 30 years. At the surface the lava was quenched to air temperature, whereas, at the bottom, quenching to 800° C was followed by a 40 year period before the temperature reached 700° C. The olivine phenocrysts settled at an average rate of about 4 × 10–6 cm Sec–1 to form a zone that contains 21 percent olivine 75 feet above the base. Sinking of olivines continued until some time after the beginning of the crystallization of augite and plagioclase. Thin rims of iron-rich olivine (up to Fa55) surrounding the phenocrysts, and a second generation of fine-grained olivines (Fa20 Fa48) restricted to the uppermost 20 feet indicate local extensions of the period of crystallization of olivine. During crystallization of the groundmass and later subsolidus cooling in the range 1000° C to at least as low as 800° C, the olivine phenocrysts were converted to Fa30–40 by interdiffusion of Fe, Mg, Ni, and Mn. Homogenization of Mg-rich cores and Fe-rich margins and equilibration of olivine composition with the groundmass phases was progressively less well achieved toward the top of the lake. Reaction rims around the olivines are composed primarily of Ca-rich pyroxene. Pigeonite crystallized alongside augite except in the uppermost 5 feet where there is abundant ground mass olivine. Poikilitic hypersthene grew at the expense of pre-existing ferromagnesian minerals in the cumulate zone.Publication authorized by the Director, U.S. Geological Survey.  相似文献   

20.
Basaltic glasses from the three alkalic areas of Iceland (Snaefellsnes Volcanic Zone, Sudurland Volcanic Zone and Vestmannaeyjar Volcanic Area) contain plagioclase, olivine, clinopyroxene, chromian spinel and titanomagnetite as phenocryst phases. The glasses are hypersthene to nepheline normative alkali basaltic with FeO/ MgO ratios between 1.4–4.7. Olivine ranges in composition from Fo90 to Fo55, plagioclase from An90 to An50 and clinopyroxene from En45Fs10Wo45 to En40Fs17Wo43. Clinopyroxene reveals systematic Ti:Al metastable crystallization trends related to the composition of the enclosing glass. Two types of phenocryst are present in most glasses and show a bimodality in size and composition. Microphenocryst phases are those most likely to have crystallized from the enclosing glass, while macrophenocrysts may have crystallized from a liquid of slightly less evolved composition. The glasses show complex phenocryst-glass relations which can be related to a polybaric effect. The normative glass compositions are related to 2-phase cotectic surfaces in the basalt tetrahedron and define the position of the 3-phase cotectic line. In general with increasing FeO/MgO in the glass the phenocryst assemblages vary from clinopyroxene, olivine and plagioclase along a clinopyroxene-olivine surface to olivine and plagioclase along an olivine-plagioclase surface. The normative glass compositions show a deflection from clinopyroxene-bearing to clinopyroxene-free glasses. The appearance of plagioclase together with clinopyroxene and olivine can be explained in the light of experimental investigations of the effect of pressure on phase relations. The major element variation of the glasses is interpreted as representing mantle derived magma batches of primary liquids, modified to some degree by high (6 kbar) and intermediate to low pressure (below 3 kbar) crystal fractionation towards equilibrium phase relations during ascent and residence in crustal magma chambers. The observed deflection in normative compositions of the glasses marks the position of the high pressure 3-phase cotectic line. The bimodality in size and composition of plagioclase and olivine phenocrysts can be related to high pressure crystal fractionation in the melt. The Fe-Ti basalt glasses from Sudurland are believed to be quenched high pressure compositions.  相似文献   

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