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1.
The petrogenesis of Apollo 14 high-Al basaltic melts was studied using crystal stratigraphy, which involves textural (crystal size distributions — CSDs) and chemical analyses (electron microprobe and laser ablation inductively coupled plasma mass spectrometry). The samples studied here include pristine basalt 14072 and basaltic clasts from breccia 14321, and impact-generated crystalline samples 14073, 14276 and 14310. Plagioclase was the focus of this study because of its relatively high modal abundances and because it was on the liquidus for much of the melt cooling histories. Plagioclase crystals were analyzed (core-to-rim compositions where possible) to test and refine petrogenetic models based upon whole-rock compositions (Groups A, B, and C designations) and to investigate basalt 14072 and impact-melt crystallization. Textural studies have shown that each basalt group has distinctive plagioclase CSDs, which are in turn distinctive from those of the impact melts. Evolution of the individual basaltic melts was studied by comparing the equilibrium-melt compositions (calculated from plagioclase compositions using relevant partition coefficients) to fractional crystallization (FC) and assimilation and fractional crystallization (AFC) models. Petrogenetic modeling of trace elements in Group A basalts revealed that petrogenesis continued beyond 40% total crystallization required to model whole-rock compositions, and that there were open-system processes that affected the magma during plagioclase crystallization. Petrogenetic modeling of pristine high-Al basalts (14072 and Groups A, B and C) using trace elements shows that the equilibrium-melt compositions do not fall on a single AFC or FC trajectory. This is consistent with fluctuating degrees of assimilation (i.e., variable r-values) and/or variable assimilant compositions during petrogenesis. Petrogenetic modeling reveals that the impact melts experienced only closed-system fractional crystallization. This work demonstrates the importance of crystal stratigraphy in revealing the intricacies of lunar basalt petrogenesis.  相似文献   

2.
A systematic account of micro-textures and a few compositional profles of plagioclase from high-alumina basaltic aa lava erupted during the year 1994-1995,from Barren Island Volcano,NE India ocean,are presented for the frst time.The identifed micro-textures can be grouped into two categories:(i)Growth related textures in the form of coarse/fne-sieve morphology,fne-scale oscillatory zoning and resorption surfaces resulted when the equilibrium at the crystal-melt interface was fuctuated due to change in temperature or H2O or pressure or composition of the crystallizing melt;and(ii)morphological texture,like glomerocryst,synneusis,swallow-tailed crystal,microlite and broken crystals,formed by the infuence of dynamic behavior of the crystallizing magma(convection,turbulence,degassing,etc.).Each micro-texture has developed in a specifc magmatic environment,accordingly,a frst order magma plumbing model and crystallization dynamics are envisaged for the studied lava unit.Magma generated has undergone extensive fractional crystallization of An-rich plagioclase in stable magmatic environment at a deeper depth.Subsequently they ascend to a shallow chamber where the newly brought crystals and pre-existing crystals have undergone dynamic crystallization via dissolution-regrowth processes in a convective selfmixing environment.Such repeated recharge-recycling processes have produced various populations of plagioclase with different micro-textural stratigraphy in the studied lava unit.Intermittent degassing and eruption related decompression have also played a major role in the fnal stage of crystallization dynamics.  相似文献   

3.
The 1980–1986 eruption of Mount St. Helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale. Groundmass plagioclase size measurements are coupled with measured changes in matrix glass, plagioclase and Fe–Ti oxide chemistry to document increasing groundmass crystallinity, and thus to better constrain proposed physical models of the post-May 18, 1980 magmatic reservoir. Measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth (approximately 10-9 cm/s) of groundmass plagioclase occurred immediately subsequent to May 18. Relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10-11 cm/s; plagioclase growth rates then decreased to <1x10-11 cm/s through 1986. Changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst-rim chemistry, although the matrix glass composition appears to have remained approximately constant from 1981–1986 after a rapid compositional change from May 18 until the end of 1980. Plagioclase microphenocrysts show increasingly more complex zoning patterns with time; microphenocryst-core compositions are commonly positively correlated with crystal size. Both of these observations indicate continuous groundmass plagioclase growth through 1986. Magmatic temperatures estimated from Fe–Ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930°C and at log fO2 of -10.8; by 1985–1986 oxide temperatures decreased to 870°C. Chemical and textural changes can be explained by: (1) rapid degassing and crystallization in response to the intrusion of magma into a shallow (<4.5 km) reservoir toward the end of the May 18, 1980 eruption; (2) continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir. Growth rates (and consequent chemical changes) appear to decrease at the end of 1980—this is coincident with the change in eruption style from explosive eruptions, sometimes followed by dome growth, to solely extrusive (dome-building) events, and can be explained by the expected viscosity increase of both degassing and increasing crystallinity. The model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies (Casadevall et al. 1983; Gerlach and Casadevall 1986 a, b), patterns of crater deformation (Chadwkck et al. 1988) and post-1980 seismicity (Endo et al. 1990), although it does not explain the experimental data of Hill and Rutherford (1989) on the growth rate of amphibole reaction rims. Textural measurements on Mount St. Helens dacite can also be used to evaluate crystallization kinetics in silicic magmas, systems for which experimental data is almost non-existent. Plagioclase growth rates are 5–10 times slower than estimated plagioclase growth rates in basaltic systems, a result consistent with the higher viscosity of a more silicic melt. Furthermore, patterns of textural change (both average crystal size and number density) are similar to those observed during the 1984 Mauna Loa eruption by Lipman and Banks (1987), suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates, such that the rate of crystallization scales with the melt viscosity.  相似文献   

4.
The rheology of crystal-bearing basaltic magmas from Stromboli and Etna   总被引：4，自引：0，他引：4  
Our ability to forecast volcanic hazards at active volcanoes stems from our knowledge of parameters affecting eruption dynamics. Persistent activity displayed by basaltic volcanic systems, such as Etna and Stromboli, is governed mainly by the textural evolution and rheological properties of eruptive products. Here, we investigate the high temperature (1131 °C < T < 1187 °C) rheological behavior and textural evolution of remelted natural basaltic magmas from Stromboli and Etna volcanic systems upon cooling and crystallization at atmospheric conditions. The pure liquid and subliquidus isothermal viscosities are investigated using a concentric cylinder apparatus by varying strain rate applied to the investigated materials. Detailed textural analysis has been performed in order to evaluate the effect of crystal distribution on the rheological evolution of magmas. At subliquidus conditions, the mineralogical assemblage is dominated by the presence of spinel and plagioclase with the occurrence of stable clinopyroxene only at the lowest temperatures (1157 and 1131 °C for the Stromboli and the Etna samples, respectively). The overall crystal fraction (?) varies between 0.06 and 0.27. In the high T regime the viscous response to applied stress and strain rate is strongly affected by the presence of even small amounts of crystals. Large discrepancies between measured and predicted viscosities obtained using the Einstein-Roscoe (ER) equation are also found at low crystal fractions. We find here that, at the investigated conditions, the physical effect caused by the presence of elongated crystals is orders of magnitude higher than that predicted by existing models which only account for spherical particles. It also appears that a weak shear thinning behavior occurs at the lowest temperatures investigated. Crystal shape and, with it, the strain-rate dependence of the rheological properties appear to play primary roles in influencing the transport properties of these basaltic magmas.  相似文献   

5.
An experimental study has been conducted to constrain how thermal history controls crystal size distribution (CSD) of plagioclase in cooling basalts. Data from all experiments are dominated by a log-linear segment of decreasing number density with increasing crystal size, consistent with observations in many natural rocks. The slope of the CSD is found to be a function of cooling rate, faster cooling leading to greater slopes. At constant cooling rate the CSDs flatten as temperature decreases, and are sometimes kinked, characteristics consistent with independent textural observations that crystal agglomeration contributes significantly to crystal “growth”. A downturn is observed toward small size, which suggests Ostwald ripening. Furthermore, we find that thermal history above the liquidus has a major influence on CSDs and on the temperatures of phase appearance. We conclude that near the liquidus heterogeneous nucleation typically dominates. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

6.
Giant plagioclase basalts (GPBs) reflect the storage of flood basalt magma in subvolcanic magma chambers at crustal depths. In this study of the Late Permian Emeishan large igneous province in southwest China, we focus on understanding the plumbing system and ascent of large-volume basaltic magma. We report a quantitative textural analysis and bulk-rock geochemical composition of clustered touching crystals (CT-type) and single isolated crystal (SI-type) GPB samples from 5- to 240-m-thick flows in the Daqiao section. Both types of GPBs are evolved (<6 MgO wt%), but have high Ti/Y ratios (>500) and high total FeO content (11.5–15.2 wt%). The mineral chemistry of the two types of plagioclase displays a small range of anorthite content (<5 mol%), which is consistent with their unzoned characteristics. The two types of GPBs have S-type crystal size distributions but have quite different slopes, intercepts, and characteristic lengths. The characteristic lengths of the five flows are 1.54, 2.99, 1.70, 3.22, and 1.86 mm, respectively. For plagioclase growth rates of 10?11 to 10?10 mm/s, steady-state magma chamber models with simple continuous crystal growth suggest that CT-type plagioclase megacrysts have the residence time of about 500–6,000 years, whereas the residence time for SI-type plagioclase is significantly longer, about 1,000–10,000 years. By combining field geology, quantitative textural data with geochemistry, we suggest that CT- and SI-type crystals grew and were coarsened in the outer part and inner part of a magma chamber, respectively. Magma evolution during storage is controlled by crystallization, crystal growth, and magma mixing, and pulsating eruptions occur in response to the continuous supply of hot magma.  相似文献   

7.
The basaltic Martian meteorite Yamato 980459 consists of large olivine phenocrysts and often prismatic pyroxenes set into a fine-grained groundmass of smaller more Fe-rich olivine, chromite, and an interstitial residual material displaying quenching textures of dendritic olivine, chain-like augite and sulfide droplets in a glassy matrix. Yamato 980459 is, thus, the only Martian meteorite without plagioclase/maskelynite. Olivine is compositionally zoned from a Mg-rich core to a Fe-rich rim with the outer few micrometers being especially rich in iron. With Fo84 the cores are the most magnesian olivines found in Martian meteorites so far. Pyroxenes are also mostly composite crystals of large orthopyroxene cores and thin Ca-rich overgrowths. Separate pigeonite and augites are rare. On basis of the mineral compositions, the cooling rates determined from crystal morphologies, and crystal grain size distributions it is deduced that the parent magma of Yamato 980459 initially cooled under near equilibrium conditions e.g., in a magma chamber allowing chromite and the Mg-rich silicates to form as cumulus phases. Fractional crystallization at higher cooling rates and a low degree of undercooling let to the formation of the Ca-, Al-, and Fe-rich overgrowths on olivine and orthopyroxene while the magma was ascending towards the Martian surface. Finally and before plagioclase and also phosphates could precipitate, the magma was very quickly erupted quenching the remaining melt to glass, dendritic silicates and sulfide droplets. The shape preferred orientation of olivine and pyroxene suggests a quick, thin outflow of lava. According to the shock effects found in the minerals of Yamato 980459, the meteorite experienced an equilibration shock pressure of about 20-25 GPa. Its near surface position allowed the ejection from the planet’s surface already by a single impact event and at relatively low shock pressures.  相似文献   

8.
Isothermal single-step decompression experiments (at temperature of 1075 °C and pressure between 5 and 50 MPa) were used to study the crystallization kinetics of plagioclase in hydrous high-K basaltic melts as a function of pressure, effective undercooling (ΔT eff) and time. Single-step decompression causes water exsolution and a consequent increase in the plagioclase liquidus, thus imposing an effective undercooling (?T eff), accompanied by increased melt viscosity. Here, we show that the decompression process acts directly on viscosity and thermodynamic energy barriers (such as interfacial-free energy), controlling the nucleation process and favoring the formation of homogeneous nuclei also at high pressure (low effective undercoolings). In fact, this study shows that similar crystal number densities (N a) can be obtained both at low and high pressure (between 5 and 50 MPa), whereas crystal growth processes are favored at low pressures (5–10 MPa). The main evidence of this study is that the crystallization of plagioclase in decompressed high-K basalts is more rapid than that in rhyolitic melts on similar timescales. The onset of the crystallization process during experiments was characterized by an initial nucleation event within the first hour of the experiment, which produced the largest amount of plagioclase. This nucleation event, at short experimental duration, can produce a dramatic change in crystal number density (N a) and crystal fraction (?), triggering a significant textural evolution in only 1 h. In natural systems, this may affect the magma rheology and eruptive dynamics on very short time scales.  相似文献   

9.
K-feldspar megacrysts in granite and granodiorite plutons are generally inferred to be early crystallizing phases (grown to large sizes when the magma was mostly liquid) owing to their large size, euhedral form, and features that suggest deposition by magmatic sedimentation. However, phase equilibrium experiments and natural examples of crystallization and partial melting demonstrate that K-feldspar is one of the last phases to nucleate and that most crystal growth must occur after the magma has exceeded 50% crystallization and is thus largely incapable of flow and sedimentation. Megacryst size distributions, compositions, and textural relationships from the Cretaceous Tuolumne Intrusive Suite, California, reveal that the gradational transition from equigranular to megacrystic granodiorite likely occurred via textural coarsening caused by thermal cycling. Experimental and theoretical studies demonstrate that rising temperature induces relatively more melting in small crystals than in large ones, whereas linear growth rates during cooling are similar. Thus, during thermal cycling material is transferred from small crystals to larger ones. Megacryst growth via thermal cycling during incremental emplacement is consistent with the required late growth of K-feldspar, explains the presence of megacrysts in the inner parts of theTuolumne Intrusive Suite and elsewhere, and may be a common process in formation of megacrystic granitic rocks.  相似文献   

10.
Nucleation and growth of crystals, and the resulting crystal size distribution, play a fundamental role in controlling the physical properties of magmas and consequently the dynamics of the eruptions. In the past decades, laboratory experiments demonstrated that size and shape of crystals strongly control the physical properties of magma and lava. Additionally, natural and experimental samples are usually characterized in terms of their crystal size distribution to link it with physical processes that are not directly observable, such as cooling or decompression mechanisms. In this paper, we present CrystalMoM, a new predictive model, based on the quadrature-based method of moments, developed for studying the kinetic of crystallization in volcanic systems. The quadrature-based method of moments, well established in the field of chemical engineering, represents a mesoscale modelling approach that rigorously simulates the space–time evolution of a distribution of particles, by considering its moments. The method is applied here, for the first time, for studying the equilibrium/disequilibrium crystallization in magma, modelling the temporal evolution of the moments of a crystal size distribution. The model, verified against numerical and experimental data, represents a valuable tool to infer the cooling and decompression rates from the crystal size distribution observed in natural samples.  相似文献   

11.
基于控制降温速率的动力结晶实验,对五大连池老黑山钾质玄武质熔岩的冷却结晶过程中橄榄石晶体形态随降温速率变化动力学演变进行了研究。实验研究表明,老黑山钾质玄武岩的熔融温度发生在1 022 ℃～1 260 ℃之间,峰值为1 166.3 ℃～1 186.5 ℃;实验产物主要由橄榄石和玻璃质组成,新生的橄榄石晶体形态差异较大,有针状骸晶、铁轨状骸晶、羽毛状雏晶、细长条状和粒状微晶。橄榄石晶体的生长、尺寸和形态、分布和实验产物的结晶程度与降温速率之间存在着密切关联。随着降温速率的变慢,橄榄石从针状（0.075 ℃ /min～0.5 ℃ /min）、铁轨状（0.075 ℃ /min～0.5 ℃ /min）、放射状与羽毛状（0.046 8 ℃ /min～0.075 ℃ /min）、细长条状（0.062 5 ℃ /min）等晶体演变为粒状微晶（0.03 ℃ /min）;同时,岩石的结晶程度逐渐增强,橄榄石晶体尺寸逐渐变大、自形程度逐渐变高,玻璃相和气孔逐渐减少。这种新生橄榄石形态的变化与降温速率的内在关联性,对认识老黑山熔岩流固结的动力结晶机制、熔岩流的运动机制有一定科学意义。  相似文献   

12.
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13.
The crystal size distributions (CSDs) of plagioclase and amphibolewere determined from andesites of the Soufrière Hillsvolcano, Montserrat. Plagioclase occurs as separate crystalsand as chadocrysts in large amphibole oikocrysts. The chadocrystsrepresent an earlier stage of textural development, preservedby growth of the oikocryst. Seventeen rock and eight chadocrystplagioclase CSDs are considered together as a series of samplesof textural development. All are curved, concave up, and coincident,differing only in their maximum crystal size. Three amphiboleCSDs have a similar shape and behaviour, but at a differentposition from the plagioclase CSDs. A dynamic model is proposedfor the origin of textures in these rocks. Crystallization ofplagioclase started following emplacement of andesite magmaat a depth of at least 5 km. A steep, straight CSD developedby nucleation and growth. This process was interrupted by theinjection of mafic magma into the chamber, or convective overturnof hotter magma. The magma temperature rose until it was buffered,initially by plagioclase solution and later by crystallization.During this period textural coarsening (Ostwald ripening) ofplagioclase and amphibole occurred: small crystals dissolvedsimultaneously with the growth of large crystals. The CSD becameless steep and extended to larger crystal sizes. Early stagesof this process are preserved in coarsened amphibole oikocrysts.Repetitions of this cycle generated the observed family of CSDs.Textural coarsening followed the ‘Communicating Neighbours’model. Hence, each crystal has its own, unique growth–solutionhistory, without appealing to mixing of magmas that crystallizedin different environments. KEY WORDS: Ostwald ripening; textural coarsening; oikocryst; CSD; texture  相似文献   

14.
Three genetically unrelated magma suites are found in the extrusivesequences of the Troodos ophiolite, Cyprus. A stratigraphicallylower pillow lava suite contains andesite and dacite glassesand shows the crystallization order plagioclase; augite, orthopyroxene;titanomagnetite (with the pyroxenes appearing almost simultaneously).These lavas can in part be correlated chemically and mineralogicallywith the sheeted dikes and the upper part of the gabbro complexof the ophiolite. The second magma suite is represented in astratigraphically upper extrusive suite and contains basalticandesite and andesite glasses with the crystallizaton orderchromite; olivine; Ca-rich pyroxene; plagioclase. This magmasuite can be correlated chemically and mineralogically withparts of the ophiolitic ultramafic and mafic cumulate sequence,which has the crystallization order olivine; Ca-rich pyroxene;orthopyroxene; plagioclase. The third magma suite is representedby basaltic andesite lavas along the Arakapas fault zone andshows a boninitic crystallization order olivine; orthopyroxene;Ca-rich pyroxene; plagioclase. One-atmosphere, anhydrous phaseequilibria experiments on a lava from the second suite indicateplagioclase crystallization from 1225?C, pigeonite from 1200?C,and augite from 1165?C. These experimental data contrast withthe crystallization order suggested by the lavas and the associatedcumulates. The observed crystallization orders and the presenceof magmatic water in the fresh glasses of all suites are consistentwith evolution under relatively high partial water pressures.In particular, high PH2O (1–3 kb) can explain the lateappearances of plagioclase and Ca-poor pyroxene in the majorityof the basaltic andesite lavas as the effects of suppressedcrystallization temperatures and shifting of cotectic relations.The detailed crystallization orders are probably controlledby relatively minor differences in the normative compositionsof the parental magmas. The basaltic andesite lavas are likelyto reach augite saturation before Ca-poor pyroxene saturation,whereas the Arakapas fault zone lavas, which have relativelyless normative diopside and more quartz, reached the Ca-poorpyroxene-olivine reaction surface and crystallized Ca-poor pyroxeneafter olivine.  相似文献   

15.
The Lower Zone–Critical Zone boundary of the BushveldComplex is an intrusion-wide, major stratigraphic transitionfrom ultramafic harzburgite and pyroxenite in the Lower Zoneto increasingly plagioclase-rich pyroxenites and norites inthe Critical Zone. Quantitative textural and compositional datafor 29 samples through this transition show the following: LowerZone orthopyroxene grains are larger, have higher aspect ratios,are better foliated and have a lower trapped liquid componentthan those of the Critical Zone. The larger grain size of theLower Zone results in crystal size distribution plots that arerotated to lower slopes and intercepts relative to those inthe Critical Zone. Although all rocks show differing amountsof foliation, mineral lineations are weak to absent. These dataare consistent with significant compaction-driven recrystallizationin the study section. Numerical modeling of concurrent compactionand crystallization provides a quantitative model of how theLower Zone–Critical Zone transition may have formed: plagioclaseis rare in the Lower Zone because compaction removes interstitialliquid before it reaches plagioclase saturation. However, asthe crystal pile grows, plagioclase saturation is reached inthe interstitial liquid before compaction is complete in moreevolved pyroxenites, producing more abundant but still modestamounts of plagioclase characteristic of the Lower CriticalZone. It is concluded that both the textures and the modal mineralogyare largely controlled by compaction and compaction-driven recrystallization;primary magmatic textures are not preserved. KEY WORDS: Bushveld Complex; compaction; crystal size distributions; crystal aging; igneous textures  相似文献   

16.
A. nal 《Geological Journal》2008,43(1):95-116
The Middle Miocene Orduzu volcanic suite, which is a part of the widespread Neogene Yamadağ volcanism of Eastern Anatolia, consists of a rhyolitic lava flow, rhyolitic dykes, a trachyandesitic lava flow and basaltic trachyandesitic dykes. Existence of mafic enclaves and globules in some of the volcanic rocks, and microtextures in phenocrysts indicate that magma mingling and mixing between andesitic and basaltic melts played an important role in the evolution of the volcanic suite. Major and trace element characteristics of the volcanic rocks are similar to those formed in convergent margin settings. In particular, incompatible trace element patterns exhibit large depletions in high field strength elements (Nb and Ta) and strong enrichments in both large ion lithofile elements (Ba, Th and U) and light rare earth elements, indicating a strong subduction signature in the source of the volcanic rocks. Furthermore, petrochemical data obtained suggest that parental magmas of rhyolite lava and dykes, and trachyandesite lava and basaltic trachyandesite dykes were derived from subduction‐related enriched lithospheric mantle and metasomatized mantle (± asthenosphere), respectively. A detailed mineralogical study of the volcanic suite shows that plagioclase is the principal phenocryst phase in all of the rock units from the Orduzu volcano. The plagioclase phenocrysts are accompanied by quartz in the rhyolitic lava flows and by two pyroxenes in the trachyandesitic lava flows and basaltic trachyandesitic dykes. Oxide phases in all rocks are magnetite and ilmenite. Calculated crystallization temperatures range from 650°C to 800°C for plagioclase, 745°C–1054°C for biotite, 888°C–915°C for pyroxene and 736°C–841°C for magnetite–ilmenite pairs. Calculated crystallization pressures of pyroxenes vary between 1.24–5.81 kb, and oxygen fugacity range from −14.47 to −12.39. The estimates of magmatic intensive parameters indicate that the initial magma forming the Orduzu volcanic unit began to crystallize in a high‐level magma chamber and then was stored in a shallow reservoir where it underwent intermediate‐mafic mixing. The rhyolitic lava flow and dykes evolved in relatively shallower crustal magma chambers. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

17.
Crystal size distribution (CSD) theory has been applied to drill core samples from Makaopuhi lava lake, Kilauea Volcano, Hawaii. Plagioclase and Fe-Ti oxide size distribution spectra were measured and population densities (n)were calculated and analyzed using a steady state crystal population balance equation: n=n 0 exp(-L/G). Slopes on ln(n) versus crystal size (L) plots determine the parameter G, a. product of average crystal growth rate (G) and average crystal growth time (). The intercept is J/G where J is nucleation rate. Known temperature-depth distributions for the lava lake provide an estimate of effective growth time (), allowing nucleation and growth rates to be determined that are independent of any kinetic model. Plagioclase growth rates decrease with increasing crystallinity (9.9–5.4×10–11 cm/s), as do plagioclase nucleation rates (33.9–1.6×10–3/cm3 s). Ilmenite growth and nucleation rates also decrease with increasing crystallinity (4.9–3.4 ×10–10 cm/s and 15–2.2×10–3/cm3 s, respectively). Magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus (G =2.9×10–10 cm/s, J=7.6×10–2/cm3 s). Moments of the population density function were used to examine the change in crystallization rates with time. Preliminary results suggest that total crystal volume increases approximately linearly with time after 50% crystallization; a more complete set of samples is needed for material with <50% crystals to define the entire crystallization history. Comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling. This interpretation is consistent with proposed thermal models of magmatic cooling, where heat loss is balanced by latent heat production to maintain equilibrium cooling.  相似文献   

18.
 The use of ocean floor basalt chemistry as a tool to investigate mantle composition and processes requires that we work with basalts that have been modified little since leaving the mantle. One source of such basalts is melt inclusions trapped in primitive crystals. However, obtaining information from these melt inclusions is complicated by the fact that melt inclusions in natural basalts are essentially always altered by post-entrapment crystallization. This requires that we develop techniques for reconstructing the original trapped liquid compositions. We conducted a series of experiments to reverse the effects of post-entrapment crystallization by re-heating the host crystals to their crystallization temperature. For these experiments we used plagioclase crystals separated from a single Gorda Ridge lava. The crystallization temperature for these crystals was determined by a set of incremental re-heating experiments to be ∼1240–1260° C. The inclusions are primitive, high Ca-Al basaltic melts, saturated with plagioclase, olivine and Al-rich chromite at low pressure. The inclusion analyses can be linked to the host lava composition by low pressure fractionation. The major element composition of the re-homogenized melt inclusions within each crystal is relatively constant. However, the incompatible element analyses have extremely wide ranges. The range of La/Sm and Ti/Zr from inclusions analyzed from a single sample from the Gorda Ridge exceeds the range reported for lavas sampled from the entire ridge. The pyroxene compositions predicted to be in equilibrium with the melt inclusion trace element signature cover much of the range represented by pyroxenes from abyssal peridotites. The volumetric proportions of the magmas entering the base of the crust can be evaluated using frequency distribution of melt inclusion compositions. This distribution suggests that the array of magmas was skewed towards the more depleted compositions, with little evidence for an enriched component in this system. This pattern is more consistent with a dynamic flow model of the mantle or of a passive flow model where the melts produced in the peripheral areas of the melting regime were not focused to the ridge. Received: 5 January 1995 / Accepted: 13 June 1995  相似文献   

19.
Intermediate-composition plagioclase (An40–60) is typicallyless dense than the relatively evolved basaltic magmas fromwhich it crystallizes and the crystallization of plagioclaseproduces a dense residual liquid, thus plagioclase should havea tendency to float in these magmatic systems. There is, however,little direct evidence for plagioclase flotation cumulates eitherin layered intrusions or in Proterozoic anorthosite complexes.The layered series of the Poe Mountain anorthosite, southeastWyoming, contains numerous anorthosite–leucogabbro blocksthat constrain density relations during differentiation. Allblocks are more mafic than their hosting anorthositic cumulates,their plagioclase compositions are more calcic, and each blockis in strong Sr isotopic disequilibrium with its host cumulate.Associated structures—disrupted and deformed layering—indicatethat (1) a floor was present during crystallization and thatplagioclase was accumulating and/or crystallizing on the floor,(2) compositional layering and plagioclase lamination formeddirectly at the magma–crystal pile interface, and (3)the upper portions of the crystal pile contained significantamounts of interstitial melt. Liquid densities are calculatedfor proposed high-Al olivine gabbroic parental magmas and Fe-enrichedferrodioritic and monzodioritic residual magmas of the anorthositestaking into account pressure, oxygen fugacity, P2O5, estimatedvolatile contents, and variable temperatures of crystallization.For all reasonable conditions, calculated block densities aregreater than those of the associated melt. The liquid densities,however, are greater than those for An40–60 plagioclase,which cannot have settled to the floor. Plagioclase must eitherhave been carried to the floor in relatively dense packets ofcooled liquid plus crystals or have crystallized in situ. Asloping floor, possibly produced by diapiric ascent of relativelylight plagioclase-rich cumulates, is required to allow for drainingand removal of the dense interstitial liquid produced in thecrystal pile and may be a characteristic feature during thecrystallization of many Proterozoic anorthosites and layeredintrusions. KEY WORDS: magma; density; Proterozoic anorthosites; blocks; plagioclase  相似文献   

20.
The size distributions of crystals of olivine, plagioclase and oxides of the 1991/93 eruption at Mt. Etna (Italy) are analyzed. The simultaneous collection of this information for different minerals gives precious insight into the cooling history of lavas. Three distinct episodes are detectable: a storage of the magma in a deep reservoir, characterized by nearly constant and low nucleation and growth rates (near to equilibrium); an ascent phase, with an ever increasing nucleation rate related to volatile exsolution; and finally a quenching phase. In addition to geochemical and geophysical evidence, the similarity of the crystal size distributions of the present eruption with those of previous ones of this century makes it possible to exclude that crystal size distributions of Etnean lavas are due to mixing of different populations. This strongly suggests that the main features of the volcano feeding system have not changed despite observed variations in the magma output rates.  相似文献   

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