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1.
It has been an enigma that in the Tieschitz, H3, and other unequilibrated chondrites the silicates show quench textures yet their metallic minerals, according to the Wood [6] model, appear to have cooled extremely slowly.In Tieschitz, spherical metallic chondrules up to 500 μm in diameter, with textures indicating an origin as liquid droplets, consist of polycrystalline intergrowths of α(kamacite), γ(taenite) and troilite. Interface Ni compositions of contiguous α (～5 wt.%) and γ (～50 wt.%) grains define equilibrium tie-line relationships in the Fe-Ni system indicating equilibration to ～350°C (620 K). Polycrystalline γ(taenite) is multi-zoned with respect to Ni and is interpreted as the relict of a primary solidification structure. A mechanism whereby Ni compositional heterogeneities were produced in γ(taenite) by the rapid, non-equilibrium cooling of FeNiS melts during chondrule formation is discussed.Comparisons with lunar metal globules indicate solidification rates for Tieschitz metallic chondrules in the range 1–106 K/s. It is suggested that before or during aggregation, sub-solidus cooling in the temperature range ～700–1400°C with cooling times of days to weeks allowed the preservation of a relict solidification structure in metallic chondrules. At a temperature of ～700°C accretion and shallow burial (1–10 m) on the surface of the Tieschitz parent body provided insulation with slower cooling required to nucleate and grow α(kamacite) from the heterogeneous γ(taenite) under equilibrium conditions by the process of solid state diffusion proposed by Wood [6]. The cooling rate (1 K/106 yr) through 500°C derived using the Wood model is shown to be an underestimate of the real cooling rate of Tieschitz metal through that temperature, since it does not take into account Ni heterogeneities produced at higher temperatures. A rough estimate of the post-accretional cooling rate is obtained from the average size of α(kamacite) grains(<100 μm) andTeqα ～ 350°C indicating a cooling rate of the order of<1K/103yr through 500°C.  相似文献   

2.
The November 2002 eruption of Piton de la Fournaise in the Indian Ocean was typical of the activity of the volcano from 1999 to 2006 in terms of duration and volume of magma ejected. The first magma erupted was a basaltic liquid with a small proportion of olivine phenocrysts (Fo81) that contain small numbers of melt inclusions. In subsequent flows, olivine crystals were more abundant and richer in Mg (Fo83–84). These crystals contain numerous melt and fluid inclusions, healed fractures, and dislocation features such as kink bands. The major element composition of melt inclusions in this later olivine (Fo83–84) is out of equilibrium with that of its host as a result of extensive post-entrapment crystallization and Fe2+ loss by diffusion during cooling. Melt inclusions in Fo81 olivine are also chemically out of equilibrium with their hosts but to a lesser degree. Using olivine–melt geothermometry, we determined that melt inclusions in Fo81 olivine were trapped at lower temperature (1,182 ± 1°C) than inclusions in Fo83–84 olivine (1,199–1,227°C). This methodology was also used to estimate eruption temperatures. The November 2002 melt inclusion compositions suggest that they were at temperatures between 1,070°C and 1,133°C immediately before eruption and quenching. This relatively wide temperature range may reflect the fact that most of the melt inclusions were from olivine in lava samples and therefore likely underwent minor but variable amounts of post-eruptive crystallization and Fe2+ loss by diffusion due to their relatively slow cooling on the surface. In contrast, melt inclusions in tephra samples from past major eruptions yielded a narrower range of higher eruption temperatures (1,163–1,181°C). The melt inclusion data presented here and in earlier publications are consistent with a model of magma recharge from depth during major eruptions, followed by storage, cooling, and crystallization at shallow levels prior to expulsion during events similar in magnitude to the relatively small November 2002 eruption.  相似文献   

3.
For any given volcanic field the compositions of primary melts provide important constraints on models of magmatic processes and volcanic eruptions. In this paper, based on petrography, olivine and bulk rock compositions, two tholeiitic picrites (samples C122 and C123) from Haleakala Volcano, east Maui are evaluated as possible primary melts. Sample C122 (bulk rock MgO = 16.6%) has a high apparent Mg-Fe exchange coefficient, KD, between olivine phenocrysts and bulk rock (0.6). However, major-elements and Ni mass-balance calculations show that the olivines in C122 are in equilibrium with the residual melt (matrix) after closed-system equilibrium fractionation of 25 wt.% olivine. Therefore, the Mg/Fe ratio, Ca content, and Ni content of C122 are consistent with the hypothesis that the bulk composition of C122 is close to a primary melt formed by partial melting of a mantle containing olivine with composition around Fo89 to Fo91. The uniform composition and small size (mostly 0.2–0.3 mm) of the olivine, and the glass patches in the matrix suggest fast ascent, and rapid cooling at shallow depth for C122. On the contrary, sample C123, which has an apparent KD (between the most mafic olivine megacrysts and the bulk rock) close to the equilibrium value (0.27), the multiple planar subgrain boundaries in most of the olivine crystals indicate that it may not be a primary melt unless the deformed olivines are generated at magmatic condition as phenocrysts. If the deformed subgrain boundary texture in olivine could indeed be generated at magmatic condition, then the wide compositional range of olivine crystals in C123 (Fo74 to Fo91) suggests multi-stage crystallization over a wide range of cooling temperatures.The compositions of the two picrites, and a differentiated basalt which does not contain xenocrysts suggest that the Haleakala tholeiites are derived from primary melts with at least 16–17 wt.% MgO. Lavas with such high MgO content are rare in Haleakala and other Hawaiian volcanoes; therefore, most Hawaiian tholeiites must have undergone extensive fractionation histories.  相似文献   

4.
Blithfield (EL6) is one of five known enstatite chondrite breccias. It consists of troilite-rich clasts (35 ± 5vol.%) embedded in an extremely metallic Fe,Ni-rich matrix (65 ± 5 vol.%) that contains metal nodules up to 17 mm in size. Clasts and matrix agglomerated independently in the solar nebula under conditions of high and lowpS2/pO2 ratios, respectively. The matrix accreted to an EL chondrite planetesimal and was metamorphosed to～ 1000°C, above the FeNiS eutectic; chondrule textures were obliterated. The S-rich eutectic melt was lost from the matrix. The matrix material was buried to a depth of at least 3 km; accreting troilite-rich material was incorporated into the matrix as clasts. The breccia cooled through～ 500°C at 1000–10,000°C/Myr. After cooling below～ 500°C, Blithfield was quenched, possibly by impact excavation from depth and deposition onto the surface.Clasts or inclusions that are enriched in sulfide and depleted in metallic Fe,Ni are common in brecciated enstatite chondrites. Variations in thepS2/pO2 ratio in the nebular regions where these materials formed may explain many of their petrologic properties. The silica-rich clasts in Adhi Kot (EH4) formed at very highpS2/pO2ratios(> 1027); niningerite, free silica and troilite were produced from the sulfurization of enstatite and metallic Fe. The troilite-rich clasts in Blithfield and Atlanta (EL6) as well as the troilite-rich regions of the Hvittis (EL6) matrix formed at somewhat lowerpS2/pO2 ratios where sulfurization of metalic Fe produced troilite. The Ni content of the residual metal increased, forming some metal of martensitic composition. The dark inclusions in Abee (EH 4), which contain up to 9 wt.% oldhamite, formed at highpS2/pO2 ratios in the presence of an additional Ca-rich component.  相似文献   

5.
High pressure experiments using the sink/float method have bracketed the density of hydrous iron-rich ultrabasic silicate melt from 1.35 to 10.0 GPa at temperatures from 1400 to 1860 °C. The silicate melt composition was a 50–50 mixture of natural komatiite and synthetic fayalite. Water was added in the form of brucite Mg(OH)2 and was present in the experimental run products at 2 wt.% and 5 wt.% levels as confirmed by microprobe analyses of total oxygen. Buoyancy marker spheres were olivines and garnets of known composition and density. The density of the silicate melt with 5 wt.% water at 2 GPa and 1500 °C is 0.192 g cm? 3 less than the anhydrous form of this melt at the same P and T. This density difference gives a partial molar volume of water in silicate melt of ～ 7 cm3 mol? 1, which is similar to previous studies at high pressure. The komatiite–fayalite liquids with 0 and 2 wt.% H2O, have extrapolated density crossovers with equilibrium liquidus olivine at 8 and 9 GPa respectively, but there is no crossover for the liquid with 5 wt.% H2O. These results are consistent with the hypothesis that dense hydrous melts could be gravitationally stable atop the 410 km discontinuity in the Earth. The results also support the notion that equilibrium liquidus olivine could float in an FeO-rich hydrous martian magma ocean. Extrapolation of the data suggests that FeO-rich hydrous melt could be negatively buoyant in the Earth's D″-region or atop the core–mantle-boundary (CMB), although experiments at higher pressure are needed to confirm this prediction.  相似文献   

6.
Fine textures of clinopyroxene in an excentroradial pyroxene chondrule (EPC) and a comb-like pyroxene chondrule (CPC) in the Yamato-74191 chondrite (L3) have been studied by analytical electron microscopy. Both pyroxenes consist of three regions different in composition and texture; core, mantle and marginal regions, though the pyroxenes of the CPC are more Fe-rich than those of the EPC. The core region is the most Mg-rich with no Ca component and commonly shows polysynthetic (100) twins. The mantle region is slightly calcic, and the marginal region shows a rapid increase of Ca outward.The polysynthetic twins, cracks and subgrain boundaries in the core in the EPC and CPC must have formed during the transition from proto-type to clino-type pyroxenes. The exsolution textures in the mantle and marginal regions indicate initial crystallization of pigeonite-C followed by decomposition into pigeonite-P and augite. The decomposition must have taken place by nucleation growth in the mantle region and by spinodal decomposition in the marginal region. The periodicity of 15–20 nm in the spinodal decomposition textures indicates that the cooling rate of the pyroxenes, when passing through about 1000°C, was of the order of a few tens to several degrees centigrade per hour. The cooling history of the chondrules has been explained by a monotonous cooling controlled by the cooling rate of the surrounding medium.  相似文献   

7.
Sixteen sets of apatite/liquid partition coefficients (Dap/liq) for the rare earth elements (REE; La, Sm, Dy, Lu) and six values for Sr were experimentally determined in natural systems ranging from basanite to granite. The apatite + melt (glass) assemblages were obtained from starting glasses artificially enriched in REE, Sr and fluorapatite components; these were run under dry and hydrous conditions of 7.5–20 kbar and 950–1120°C in a solid-media, piston-cylinder apparatus. An SEM-equipped electron microprobe was used for subsequent measurement of REE and Sr concentrations in coexisting apatites and quenched glasses. The resulting partition coefficient patterns resemble previously determined apatite phenocryst/groundmass concentration ratios in the following respects: (1) the rare earth patterns are uniformly concave downward (i.e., the middle REE are more compatible in apatite than the light and heavy REE); (2) DREEap/liq is much higher for silicic melts than for basic ones; and (3) strontium (and therefore Eu2+) is less concentrated by apatite than are the trivalent REE. The effects of both temperature and melt composition on DREEap/liq are systematic and pronounced. At 950°C, for example, a change in melt SiO2 content from 50 to 68 wt.% causes the average REE partition coefficient to increase from ～7 to ～30. A 130°C increase in temperature, on the other hand, results in a two-fold decrease in DREEap/liq. Partitioning of Sr is insenstitive to changes in melt composition and temperature, and neither the Sr nor the REE partition coefficients appear to be affected by variations in pressure or H2O content of the melt.The experimentally determined partition coefficients can be used not only in trace element modelling, but also to distinguish apatite phenocrysts from xenocrysts in rocks. Reported apatite megacryst/host basalt REE concentration ratios [12], for example, are considerably higher than the equilibrium partition coefficients, which suggest that in this particular case the apatite is actually xenocrystic.A reversal experiment incorporated in our study yielded diffusion profiles of REE in apatite, from which we extracted a REEαCa interdiffusion coefficient of 2–4×10?14 cm2/s at 1120°C. Extrapolated downward to crustal temperatures, this low value suggests that complete REE equilibrium between felsic partial melts and residual apatite is rarely established.  相似文献   

8.
The electrical conductivity of a single crystal of San Carlos olivine (Fo92, 0.16 wt.% Fe2O3) has been measured as a function of temperature and oxygen fugacity (fO2). After heating to 1338°C at fO2 = 10?12 atm., the conductivity at 950°C was 10?5 (ohm-m)?1, almost 3 orders of magnitude less than that measured in air. This decrease is due to the reduction of Fe3+ to Fe2+. Further heating to 1500°C at fO2 = 10?14 atm., decreased the electrical conductivity at 950°C to 10?6 (ohm-m)?1. When recovered at room temperature, the speciment had Fo96 composition and contained small, opaque blebs distributed throughout the crystal. Derivations of temperature profiles for the earth's mantle from conductivity-depth models must take account of the important role played by iron oxidation state in the electrical conductivity of olivine.  相似文献   

9.
High-temperature experiments on ferromagnesian compositions have been hampered by the rapid absorption of up to 95% of the original iron by platinum and 40% by silver-palladium capsules. Molybdenum or iron capsule materials can decrease or alleviate iron loss, but restrict oxygen fugacities to values near the iron-wustite buffer. Because Co2+ is stable at fO2 =HM and because the solubility of Co in platinum in this range of fO2 is ～0.05% at temperatures to 1350°C, its use as an analogue for Fe2+ is possible. In addition, experiments simulating various Fe2+ ratios can be easily performed by choosing appropriate Co2+/Fe3+ ratios. The cobalt phases produced possess brilliant and distinctive colors which are valuable aids in optical identification of minute phases. The cobalt analogue hypothesis was tested with atmospheric pressure experiments in air on the cobalt analogue of the 1921 Kilauea basalt at three simulated Fe2+/Fe3+ ratios. The results were compared with those of R.E.T. Hill (1969) for the natural 1921 basalt. The phase relations were the same, with the cobalt system stability fields systematically shifted by about +50°C. Microprobe analysis of olivines and the coexisting glasses indicate that the distribution of Co2+ between olivine and melt is independent of temperature and liquid composition. Although the analogue liquid composition differs from the equilibrium composition of the natural system, it may be corrected be employing distribution coefficients (KD = 0.61 for the Co system; KD = 0.33 for the Fe system) to closely approximate what the natural system would yield if iron loss did not occur.  相似文献   

10.
Major-element, Cl, S, F analyses have been performed on a wide selection of melt inclusions trapped in olivine (Fo81–87) from scoria and crystal-rich lapilli samples of Piton de la Fournaise volcano. As a whole, they display a transitional basaltic composition. The melt inclusions (8–9 wt.% MgO, 0.62–0.73 wt.% K2O) are in equilibrium with olivines (Fo81–85) in the samples from the Central Feeding Zone and the South-East Feeding Zone and show a slight alkaline affinity. The melt inclusions in olivines (Fo85–87) from the North-West Rift (NWR) contain 9.3–9.7 wt.% MgO and 0.54–0.58 wt.% K2O, with a more tholeiitic tendency. In oceanitic lavas and crystal-rich lapilli, the olivine xenocrysts are recognisable by the presence of one or more secondary shear plane fracture(s) filled up with CO2 and alkali-rich basaltic melt inclusions. In dunite nodules, olivines present also contain several secondary shear plane fracture(s) filled up with CO2 and high-SiO2 melt inclusions. Secondary CO2-rich fluid inclusions in olivine (Fo85–87) from the NWR samples indicate PCO2 up to 500 MPa whereas, PCO2 ranges from 95 MPa to few tenths of bars in the other samples. Both the primary melt inclusions and the secondary fluid inclusions strongly suggest that the olivine crystallises and accumulates over a wide depth range (15 km). It is envisioned that cumulative pockets with low residual porosity are repeatedly percolated with a CO2-rich fluid phase, possibly associated with basaltic to SiO2-rich melts, and are finally disrupted and entrained to the surface when vigorous magma transfer occurs. The SiO2-rich residual melts in early-formed dunitic or gabbroic bodies may have acted as contaminant agents for the more alkali character of magmas vented through the central feeding system, where a well-developed cumulative system is thought to exist. Finally, the existence of secondary fluid and melt inclusions in olivines implies that the dunitic bodies are weakened on the micrometric scale.  相似文献   

11.
Tholeiitic basalt glasses from the FAMOUS area of the Mid-Atlantic Ridge are among the most primitive basaltic liquids reported from the ocean basins. One of the more primitive of these[Mg/(Mg+Fe2+) = 0.68;Ni= 232ppm;TiO2 = 0.61] glasses (572-1-1) was selected for an experimental investigation. This study found olivine to be the liquidus phase from 1 atm to 10.5 kbar where it is replaced by clinopyroxene. The sequence of appearance of phases at 1 atm pressure is olivine (1268°C), plagioclase (1235°C) and clinopyroxene (1135°C). The sample is multiply saturated at 10.5 kbar with olivine (Fo88), clinopyroxene (Wo32En60Fs9), and orthopyroxene (Wo5En83Fs12). From the 1-atm data we have measured (FeO/MgO) olivine/(FeO*/MgO) liquid (K′D) for olivine-melt pairs equilibrated at 12 temperatures in the range 1268–1205°C.K′D varies from 0.30 at 1205°C to 0.27 at 1268°C. Analysis of high-pressure olivine melt pairs indicates a systematic increase inK′D with pressure.Evaluation of the 1-atm experiments reveals that fractionation of olivine followed by olivine + plagioclase can generate much of the variation in major element chemistry observed in the FAMOUS basalt glasses. However, it cannot account for the entire spectrum of glass compositions — particularly with respect to TiO2 and Na2O. The variations in these components are such as to require different primary liquids.Comparison of clinopyroxene microphenocrysts/xenocrysts found in oceanic tholeiites with experimental clinopyroxenes reveal that the majority of those in the tholeiites may have crystallized from the magma at pressures greater than ～ 10 kbar and are not accidental xenocrysts. Clinopyroxene fractionation at high pressures may be a viable mechanism for fractionating basaltic magmas.The major and minor element mineral/meltK′d's from our experiments have been used to model the source region residual mineralogy for given percentages of partial melting. These data suggest that ～20% partial melting of a lherzolite source containing 0–10% clinopyroxene can generate the major and minor element concentrations in the parental magmas of the Project FAMOUS basalt glasses.  相似文献   

12.
The majority (26/37) of the largest chondrules (d ≥ 1400 μm) exposed in a thin section of the Manych chondrite are more or less rounded fragments of microporphyry, most of which contain from 50 to 80 vol.% olivine. Modal and phase analyses were used to calculate the approximate bulk compositions of nine such chondrules. Six vary modestly around the mean composition of L-group chondrites less most of their metal and troilite and are thought to have formed by bulk melting of L-group material with loss of an immiscible Fe-Ni-S liquid. Two other chondrules, which are olivine-rich and Na- and Si-poor, formed in the same way but with some loss of volatile constituents to a vapor phase. The ninth chondrule, an olivine-poor microporphyry, may be a non-representative sample of a coarser microporphyritic rock.Comparison of these microporphyritic chondrules with the products of controlled cooling experiments and with chemically similar olivine microporphyry in the St. Mesmin chondrite (LL-breccia) suggests that the microporphyritic chondrules are fragments of magmatic rocks which crystallized from masses of liquid no less than 10 cm across.  相似文献   

13.
Abundant fluid inclusions in olivine of dunite xenoliths (～1–3 cm) in basalt dredged from the young Loihi Seamount, 30 km southeast of Hawaii, are evidence for three coexisting immiscible fluid phases—silicate melt (now glass), sulfide melt (now solid), and dense supercritical CO2 (now liquid + gas)—during growth and later fracturing of some of these olivine crystals. Some olivine xenocrysts, probably from disaggregation of xenoliths, contain similar inclusions.Most of the inclusions (2–10 μm) are on secondary planes, trapped during healing of fractures after the original crystal growth. Some such planes end abruptly within single crystals and are termed pseudosecondary, because they formed during the growth of the host olivine crystals. The “vapor” bubble in a few large (20–60 μm), isolated, and hence primary, silicate melt inclusions is too large to be the result of simple differential shrinkage. Under correct viewing conditions, these bubbles are seen to consist of CO2 liquid and gas, with an aggregate ? = ～ 0.5–0.75 g cm?3, and represent trapped globules of dense supercritical CO2 (i.e., incipient “vesiculation” at depth). Some spinel crystals enclosed within olivine have attached CO2 blebs. Spherical sulfide blebs having widely variable volume ratios to CO2 and silicate glass are found in both primary and pseudosecondary inclusions, demonstrating that an immiscible sulfide melt was also present.Assuming olivine growth at ～ 1200°C and hydrostatic pressure from a liquid lava column, extrapolation of CO2P-V-T data indicates that the primary inclusions were trapped at ～ 220–470 MPa (2200–4700 bars), or ～ 8–17 km depth in basalt magma of ? = 2.7 g cm?3. Because the temperature cannot change much during the rise to eruption, the range of CO2 densities reveals the change in pressure from that during original olivine growth to later deformation and rise to eruption on the sea floor. The presence of numerous decrepitated inclusions indicates that the inclusion sample studied is biased by the loss of higher-density inclusions and suggests that some part of these olivine xenoliths formed at greater depths.  相似文献   

14.
Plagioclase in cataclastic anorthosite 67075 occurs as angular matrix grains and as recrystallized clasts of micro-anorthosite. Olivines are Fe-rich and fall into two compositional groupings. Large grains of pyroxene show exceptionally well-developed exsolution lamellae analogous to those observed in pyroxenes from layered complexes. The low-Ca component in both pigeonites and augites shows varying degrees of inversion to orthopyroxene. The lattices of host and lamellae may deviate slightly (up to 2°) from the ideal orientation. Very slow cooling from magmatic temperatures is required to produce the coarse exsolution textures and inversion features. Augite macrocrystals are distinctly subcalcic indicating crystallization at temperatures around1100 ± 50°C while host-lamellae pairs and small grains in lithic clasts and matrix indicate reequilibration on a micron scale to temperatures less than 800°C. Pyroxene compositions tend to cluster into two groups both of which are among the most Fe-rich reported for highland pyroxenes. Ti and Al contents of pyroxenes are very low and Ti, Cr, and Mn follow well-established magmatic differentiation trends. The high Cr content may reflect low?O2 conditions and/or early crystallization of olivine and plagioclase.The87Sr/86Sr ratios in lunar anorthosites are the lowest reported for any lunar rock. It is likely that anorthosites formed as cumulates during the major differentiation episode which occurred prior to～4.3AE. Recrystallization features are common and39Ar/40Ar ages cluster around 4.0 AE. This may be the result of the intense bombardment prior to 4.0 AE which caused repeated cycles of in-situ fracturing and granulation followed by recrystallization. The low siderophile element content and the inferred slow cooling indicate a plutonic source region (10km) not frequently plumbed by impact events. The Fe-rich silicates indicate crystallization from a melt at an advanced stage of fractionation. However, the low REE abundances are not consistent with late-stage crystallization. Plagioclase apparently crystallized relatively early and was concentrated by flotation and/or convection currents while the mafic minerals crystallized from a fractionated trapped liquid. The chemical, isotopic, and mineralogical data place stringent constraints on the nature of genetically related rocks and the relationship of anorthosites to other members of the ANT suite does not appear to be one ofsimple fractionation. The data presented in this paper are consistent with the Taylor-Jake?model of lunar evolution.  相似文献   

15.
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   

16.
The solubility of fluorapatite in a wide variety of basic magmatic liquids was experimentally determined over a range of upper mantle P-T conditions (8–25 kbar, 1275–1350°C). Fluorapatite is stable over the entire range of conditions investigated, but its solubility in melts is variable, depending negatively on SiO2 content of the melt and positively upon temperature, with relatively little sensitivity to pressure above 8 kbar. At upper mantle pressures and a temperature of 1250°C, molten basalt (50% SiO2) will dissolve 3–4 wt.% P2O5 before saturation in apatite is reached. For a magma 100°C cooler or containing 10 wt.% more SiO2, apatite saturation occurs at less than 2 wt.% dissolved P2O5. The observed high solubility of apatite in basic magmas at their normal near-liquidus temperatures virtually precludes the occurrence of residual apatite in mantle source regions. If relatively low-temperature melting conditions prevail (e.g., 1100°C), as might be possible in H2O-bearing regions of the upper mantle, apatite could remain in the residue, but only in amounts too small to have significant effects on the rare earth patterns of the liquids.Because of the high solubility of apatite in basic magmas, phosphorus can be confidently treated as an incompatible element in peridotite melting models. Such models, in combination with observed characteristics of basic lavas, indicate that the upper mantle contains ～200 ppm of phosphorus, much less than the chondritic abundance of ～900 ppm.  相似文献   

17.
Relaxation experiments on vitreous SiO2, densified at 900°C, 20 kbar to a permanent density of 2.36 g cm?3 yield an activation energy of69 ± 5kcal mol?1 in the temperature range 700–800°C. The data can be used to estimate minimum cooling rates, maximum depositional temperatures or maximum thicknesses of shock-metamorphosed rocks containing diaplectic glasses, if relaxation of a glass densified under dynamic conditions is assumed to be similar to that of statically densified glass.  相似文献   

18.
The temperature dependence of water diffusivity in rhyolite melts over the range 650–950°C and [PT(H2O] = 700 bars is evaluated from water concentration-distance profiles measured in glass with an ion microprobe. Diffusivities are exponentially dependent on concentration over this temperature range and vary from about 10?8 cm2/s at 650°C to about 10?7 cm2/s at 950°C at 2 wt.% water. Water solubility also varies with temperature at a rate of ?0.14 wt. per 100°C increase. The avtivation energy (Ea) appears to be constant at 19 ± 1kal/mole for 1, 2,and 3 wt.% H2O. Comparison of these data with results for cation diffusion indicates that this value is a minimum Ea for diffusion of any species in a rhyolite melt.Compensation plots of log10D0 (the frequency factor) versus Ea indicate that hydrous rhyolite melts follow the same trend as anhydrous basalts. D0 increases for H2O and Ca2+ [1] as Ea decreases. This suggests that these molecules may diffuse by different mechanisms than do monovalent cations, and that hydration of the melt affects diffusion of Ca2+ and H2O differently than it does monovalent cation diffusion. The results imply that dramatic increases in cation diffusivities by hydration [1] may occur with additions of less than 1 wt.% H2O.  相似文献   

19.
Individual chondrules have been separated from the H5 chondrite Richardton and subjected to a detailed chemical-petrological study. A portion of each chondrule has been examined petrographically and phase chemistry determined by electron microprobe analysis. Of the remaining portion an aliquot was taken for measurement of major element abundances by microprobe using a microfusion technique. Rb, Sr,87Sr/86Sr and REE were determined by mass spectrometric isotope dilution.The chondrules define a Rb-Sr isochron age of 4.39 ± 0.03Ga(λ = 1.42 × 10?11 a?1) and an initial ratio of 0.7003 ± 7. The age is interpreted as a metamorphic age and indicates that Sr isotope equilibration occurred in the Richardton parent body for some 100 Ma or more after condensation of the solar system. Metamorphism had little effect on chondrule textures but effected Fe/Mg exchange to produce highly uniform olivine and pyroxene compositions, and may have caused some redistribution of REE.The major element compositions of Richardton chondrules are mostly constant and close to reported averages for Tieschitz, Bishunpur and Chainpur. They contain high-temperature condensate elements in close to cosmic proportions, but are deficient in Fe. Theories of chondrule origin are briefly reviewed, and while it is difficult to distinguish between direct condensation and dust fusion by impacting, it is postulated that iron was fractionated from silicate prior to or during chondrule formation.  相似文献   

20.
Samples of Ni2SiO4 in both olivine and spinel phases have been compressed to pressures above 140 kbar in a diamond-anvil cell and heated to temperatures of 1400–1800°C using a continuous YAG laser. After quenching and releasing pressure, X-ray diffraction examination indicates that the samples disproportionate to a mixture of stishovite (SiO2) and bunsenite (NiO) at pressures between 140 and 190 kbar. The exact disproportionation pressure is not certain due to transient increases in pressure during the local and rapid heating. However, thermodynamic calculations suggest that the transition pressure is about 192 ± 4 kbar at 1545°C and that the equation of the spinel-mixed oxides phase boundary isP(kbar) = 121 + (0.046 ± 0.020) T (°C).  相似文献   

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