Sulfur partitioning between apatite and melt and effect of sulfur on apatite solubility at oxidizing conditions

The effect of sulfur on phosphorus solubility in rhyolitic melt and the sulfur distribution between apatite, ±anhydrite, melt and fluid have been determined at 200 MPa and 800–1,100 °C via apatite crystallization and dissolution experiments. The presence of a small amount of sulfur in the system (0.5 wt.% S) under oxidizing conditions increases the solubility of phosphorus in the melt, probably due to changing calcium activity in the melt as a result of the formation of Ca-S complexing cations. Apatite solubility geothermometers tend to overestimate temperature in Ca-poor, S-bearing system at oxidizing conditions. In crystallization experiments, the sulfur content in apatite decreases with decreasing temperature and also with decreasing sulfur content of the melt. The sulfur partition coefficient between apatite and rhyolitic melt increases with decreasing temperature (Kd_S^apatite/melt=4.5–14.2 at T=1,100–900 °C) under sulfur-undersaturated conditions (no anhydrite). The sulfur partition coefficient is lower in anhydrite-saturated melt (~8 at 800 °C) than in anhydrite-undersaturated melt, suggesting that Kd_S^apatite/melt depends not only on the temperature but also on the sulfur content of the melt. These first results indicate that the sulfur content in apatite can be used to track the evolution of sulfur content in a magmatic system at oxidizing conditions.Editorial responsibility: J. Hoefs     

Rare earth element partition between sphene,apatite and other coexisting minerals of the Kangerdlugssuaq intrusion,E. Greenland

Cumulus apatite, sphene, feldspar, amphibole and biotite from the pulaskite of the Kangerdlugssuaq alkaline intrusion have been analysed for rare earth elements (REE) by instrumental neutron activation analysis. The apatite is particularly rich in REE, contains 3.6% Ce and shows a steep, light REE-enriched, chondrite-normalised pattern. The other minerals have light REE enrichment but with sphene showing a peak at Ce on a chondrite-normalised plot. REE partition coefficient values show that the light REE are preferentially accommodated by apatite relative to sphene. The differences in these coefficients result from differences in the co-ordination of the REE in the two minerals.     

Sulfur K-edge XANES analysis of natural and synthetic basaltic glasses: Implications for S speciation and S content as function of oxygen fugacity

XANES analyses at the sulfur K-edge were used to determine the oxidation state of S species in natural and synthetic basaltic glasses and to constrain the fO₂ conditions for the transition from sulfide (S²⁻) to sulfate (S⁶⁺) in silicate melts. XANES spectra of basaltic samples from the Galapagos spreading center, the Juan de Fuca ridge and the Lau Basin showed a dominant broad peak at 2476.8 eV, similar to the spectra obtained from synthetic sulfide-saturated basalts and pyrrhotite. An additional sharp peak at 2469.8 eV, similar to that of crystalline sulfides, was present in synthetic glasses quenched from hydrous melts but absent in anhydrous glasses and may indicate differences in sulfide species with hydration or presence of minute sulfide inclusions exsolved during quenching. The XANES spectra of a basalt from the 1991 eruption of Mount Pinatubo, Philippines, and absarokitic basalts from the Cascades Range, Oregon, USA, showed a sharp peak at 2482.8 eV, characteristic of synthetic sulfate-saturated basaltic glasses and crystalline sulfate-bearing minerals such as hauyne. Basaltic samples from the Lamont Seamount, the early submarine phase of Kilauea volcano and the Loihi Seamount showed unequivocal evidence of the coexistence of S²⁻ and S⁶⁺ species, emphasizing the relevance of S⁶⁺ to these systems. XANES spectra of basaltic glasses synthesized in internally-heated pressure vessels and equilibrated at fO₂ ranging from FMQ − 1.4 to FMQ + 2.7 showed systematic changes in the features related to S²⁻ and S⁶⁺ with changes in fO₂. No significant features related to sulfite (S⁴⁺) species were observed. These results were used to construct a function that allows estimates of S⁶⁺/ΣS from XANES data. Comparison of S⁶⁺/ΣS data obtained by S Kα shifts measured with electron probe microanalysis (EPMA), S⁶⁺/ΣS obtained from XANES spectra, and theoretical considerations show that data obtained from EPMA measurements underestimate S⁶⁺/ΣS in samples that are sulfate-dominated (most likely because of photo-reduction effects during analysis) whereas S⁶⁺/ΣS from XANES provide a close match to the expected theoretical values. The XANES-derived relationship for S⁶⁺/ΣS as a function of fO₂ indicates that the transition from S²⁻ to S⁶⁻ with increasing fO₂ occurs over a narrower interval than what is predicted by the EPMA-derived relationship. The implications for natural systems is that small variation of fO₂ above FMQ + 1 will have a large effect on S behavior in basaltic systems, in particular regarding the amount of S that can be transported by basaltic melts before sulfide saturation can occur.     

Determination of Sr and Ba partition coefficients between apatite from fish (Sparus aurata) and seawater: The influence of temperature

The Sr/Ca and Ba/Ca ratios in inorganic apatite are strongly dependent on the temperature of the aqueous medium during precipitation. If valid in biogenic apatite, these thermometers would offer the advantage of being more resistant to diagenesis than those calibrated on biogenic calcite and aragonite. We have reared seabreams (Sparus aurata) in tanks with controlled conditions during experiments lasting for more than 2 years at 13, 17, 23 and 27 °C, in order to determine the variations in Sr and Ba partitioning relative to Ca (D^Sr and D^Ba, respectively) between seawater and fish apatitic hard tissues (i.e. teeth and bones), as a function of temperature. The sensitivity of the Sr and Ba thermometers (i.e. ∂D^Sr/∂T and ∂D^Ba/∂T, respectively), are similar in bone (/∂T = 0.0036 ± 0.0003 and /∂T = 0.0134 ± 0.0026, respectively) and enamel (/∂T = 0.0037 ± 0.0005 and /∂T = 0.0107 ± 0.0026, respectively). The positive values of ∂D^Sr/∂T and ∂D^Ba/∂T in bone and enamel indicate that D^Sr and D^Ba increase with increasing temperature, a pattern opposite to that observed for inorganic apatite. This distinct thermodependent trace element partitioning between inorganic and organic apatite and water highlights the contradictory effects of the crystal-chemical and biological controls on the partitioning of Ca, Sr and Ba in vertebrate organisms. Taking into account the diet Sr/Ca and Ba/Ca values, it is shown that the bone Ba/Ca signature of fish can be explained by Ca-biopurification and inorganic apatite precipitation, whereas both of these processes fail to predict the bone Sr/Ca values. Therefore, the metabolism of Ca as a function of temperature still needs to be fully understood. However, the biogenic Sr thermometer is used to calculate an average seawater temperature of 30.6 °C using the Sr/Ca compositions of fossil shark teeth at the Cretaceous/Tertiary boundary, and a typical seawater Sr/Ca ratio of 0.02. Finally, while the present work should be completed with data obtained in natural contexts, it is clear that Sr/Ca and Ba/Ca ratios in fossil biogenic apatite already constitute attractive thermometers for marine paleoenvironments.     

Study on the compositional dependence of the apparent partition coefficient of iron and magnesium between coexisting garnet and clinopyroxene solid solution

Compositional dependence of apparent partition coefficient of iron and magnesium between coexisting garnet and clinopyroxene from Mt. Higasiakaisi is studied by means of a multicomponent regular solution model. It is shown that garnet and clinopyroxene solid solutions are positively non-ideal, and the non-ideal parameters according to the symmetric regular solution model are 2.58 kcal and 2.39 kcal, respectively, assuming the equilibration temperature of the mass to be 550° C.Notations a _i ^h activity of component i in phase h - _ij interaction parameter of component i and j in a solid solution - _i activity coefficient of component i - X _i mole fraction of component i - K partition coefficient of Fe and Mg between coexisting garnet and clinopyroxene - K apparent partition coefficient of Fe and Mg between coexisting garnet and clinopyroxene - G ⁰ difference in free energy of the partition reaction - H ⁰ difference in enthalpy of the partition reaction - S ⁰ difference in entropy of the partition reaction - R gas constant - G garnet - Alm almandine component - Py pyrope component - Gr grossular component - Sp spessartine component - CPx clinopyroxene - Hd hedenbergite component - Di diopside component - Jd jadeite component - Ts Tschermac's molecule component Deceased on April 17, 1974.     

Phosphorus and the rare earth elements in felsic magmas: an assessment of the role of apatite

The solubility of fluorapatite in 17 silica-rich melts in the system Na₂O-K₂O-Al₂O₃-SiO₂ (with and without CaO or CaF₂) was determined at 1 kbar water pressure and 750 900°C. Apatite saturation occurs at levels of dissolved P₂O₅ ranging between 0.04 (± 0.02) and 0.28 (± 0.13) wt%. with only 4 values outside the 0.09–0.20 wt% range.The results demonstrate not only that apatite is a common liquidus phase in felsic melts, but also that, under most circumstances, it remains in the residue during episodes of partial fusion of the crust. Given a solubility limit of 0.14 wt% dissolved P₂O₅ (the mean of the experimental values) a source containing as little as 0.05% P₂O₅ must be 35% melted before apatite is lost from the residue and no longer buffers the melt P₂O₅ concentration at the saturation value. Higher abundances of P₂O₅ in the source postpone the loss of residual apatite to still higher degrees of melting, and if the source P₂O₅ content exceeds 0.14 wt%, apatite must be residual for all degrees of melting, increasing in abundance as melting proceeds.The generally secondary influence of apatite on the rare earth element (REE) patterns of melt and residue is most apparent when garnet and/or amphibole is minor or lacking in the residue. Fractional crystallization of intermediate (e.g. andesitic) magmas toward felsic compositions invariably results in saturation in apatite and some consequent depletion of REE in the melt.     

Experimentally determined biomediated Sr partition coefficient for dolomite: Significance and implication for natural dolomite

Two strains of moderately halophilic bacteria were grown in aerobic culture experiments containing gel medium to determine the Sr partition coefficient between dolomite and the medium from which it precipitates at 15 to 45 °C. The results demonstrate that Sr incorporation in dolomite does occur not by the substitution of Ca, but rather by Mg. They also suggest that Sr partitioning between the culture medium and the minerals is better described by the Nernst equation (D_Sr^dol = Sr_dol/Sr_bmi), instead of the Henderson and Kracek equation (D_Sr^dol = (Sr/Ca)_dol/(Sr/Ca)_solution. The maximum value for D_Sr^dol occurs at 15 °C in cultures with and without sulfate, while the minimum values occur at 35 °C, where the bacteria exhibit optimal growth. For experiments at 25, 35 and 45 °C, we observed that D_Sr^dol values are greater in cultures with sulfate than in cultures without sulfate, whereas D_Sr^dol values are smaller in cultures with sulfate than in cultures without sulfate at 15 °C.Together, our observations suggest that D_Sr^dol is apparently related to microbial activity, temperature and sulfate concentration, regardless of the convention used to assess the D_Sr^dol. These results have implications for the interpretation of depositional environments of ancient dolomite. The results of our culture experiments show that higher Sr concentrations in ancient dolomite could reflect microbial mediated primary precipitation. In contrast, previous interpretations concluded that high Sr concentrations in ancient dolomites are an indication of secondary replacement of aragonite, which incorporates high Sr concentrations in its crystal lattice, reflecting a diagenetic process.     

泥石流堆积物中细颗粒含量与渗透系数关系试验研究

泥石流拦砂坝通常建造在沟床堆积物上。泥石流堆积物的渗透性是影响坝底扬压力的关键因素。目前关于土体渗透性的研究主要集中于无黏性粗粒土,对泥石流堆积物这种宽级配土体渗透性的研究比较缺乏。选择北川县泥石流沟床堆积物作为试验材料,通过渗透试验,确定了影响宽级配土渗透性的细颗粒上限粒径;在此基础上,通过试验研究了细颗粒含量与渗透系数的关系。结果表明,泥石流堆积物中粗颗粒仅起骨架作用,细颗粒才是决定其渗透性能的关键;显著影响宽级配土渗透性的细颗粒上限粒径为0.1 mm;细颗粒（<0.1 mm）含量与渗透系数呈负指数关系,并且当细颗粒含量超过20%以后,泥石流堆积土的渗透性趋于稳定。     

Experimental determination of the temperature and pressure dependence of the Fe-Mg partition coefficient for coexisting garnet and clinopyroxene

An experimental study initiated to calibrate the distribution coefficient \(K_D = \frac{{({\text{FeO}}/{\text{MgO}})_{{\text{ga}}} }}{{{\text{(FeO}}/{\text{MgO)}}_{{\text{cpx}}} }}\) in eclogites as a geothermometer has been done on (a) a mineral mis, (b) a glass of the typical tholeiite composition and (c) a series of glasses of tholeiite compositions with \(6.2 < \frac{{100{\text{Mg}}}}{{{\text{Mg}} + {\text{Fe}}^{ + + } }} < 93.\) The mineral mix was found to be unsuitable as reactant due to incomplete equilibration but the minimum K _D of the mineral mix and the K _D from glass of tholeiite composition are identical within experimental uncertainty. These data constitute a reversal of the garnet/clinopyroxene partition relationship and provide justification of the use of glass as a reactant for the project. To eliminate any uncertainty in interpretation of mineral analyses due to possible variation in Fe⁺⁺⁺/Fe⁺⁺ between runs, experiments were carried out in iron capsules on the nine tholeiite glasses, thus maintaining iron as Fe⁺⁺. Microprobe analytical techniques yielded mineral analyses of comparable accuracy to analyses of natural phases for experiments within the temperature range from 600° C to 1500° C and a pressure range from 20 kb to 40 kb. It has been shown that for \(6.2 < \frac{{100{\text{Mg}}}}{{{\text{Mg}} + {\text{Fe}}^{ + + } }} < 85\) , the bulk chemical composition does not perceptibly affect the K _D value. At 30 kb the K _D value ranges from 18.0 at 600° C to 1.45 at 1400° C, defining the linear relationship in a ln K _D vs 1/T(°K) plot. The pressure dependence of the K _D -value has been shown to be greater than previously predicted. There is a straight line relationship in ln K _D vs Pressure (Kb) between 20 and 40 kb at constant temperature (1100°C). This enables us to determine K _D =fn (T, P) and \(T(^\circ {\text{K}}) = \frac{{3686 + 28.35 \times P({\text{Kb}})}}{{\ln K_D + 2.33}}\) . This expression uniquely determines the temperature of equilibration of natural eclogites of basaltic bulk composition when the K _D ^ga,cpx is known and a pressure estimate can be given.     

Olivine/silicate melt partitioning of germanium: an example of a nearly constant partition coefficient

The partitioning of germanium between forsterite (Fo) and liquids in the diopside-anorthiteforsterite join was investigated by electron microprobe analysis of Ge-doped samples equilibrated at 1300°–1450°C. Germanium is somewhat incompatible in Fo relative to the haplobasaltic melts, with a grand mean for all simple partition coefficients (D_Fo-l^Ge) of 0.68 ± 0.06. For the melt composition range studied, D_Fo-l^Ge is virtually constant in isothermal series of experiments, and shows only minor overall temperature dependence. The exchange reaction partition coefficient $\text{K}{}_{\mathrm{D}}\text{= (}\text{Mg}{}_2\text{GeO}{}_4\text{)}{}_{\mathrm{Fo}}\text{(SiO}{}_2\text{)}{}_{\mathrm{l}}\text{(Mg}{}_2\text{SiO}{}_4\text{)}{}_{\mathrm{Fo}}\text{(GeO}{}_2\text{)}{}_{\mathrm{l}}$] is near unity in all cases, with a grand mean of 0.93 ± 0.11. One exploratory run at 20 kbar yielded a distinctly lower partition coefficient (D_Fo-l^Ge = 0.54 ± 0.04), which confirms the negative pressure dependence predicted by the thermodynamics of Ge ai Si exchange.These new data indicate that absolute Ge enrichment must occur in terrestrial magmas undergoing olivine fractionation, while $\text{Ge}\text{Si}$ remains nearly constant.     

Partitioning of F between H2O and CO2 fluids and topaz rhyolite melt

Fluid/melt distribution coefficients for F have been determined in experiments conducted with peraluminous topaz rhyolite melts and fluids consisting of H₂O and H₂O+CO₂ at pressures of 0.5 to 5 kbar, temperatures of 775°–1000°C, and concentrations of F in the melt ranging from 0.5 to 6.9 wt%. The major element, F, and Cl concentrations of the starting material and run product glasses were determined by electron microprobe, and the concentration of F in the fluid was calculated by mass balance. The H₂O concentrations of some run product glasses were determined by ion microprobe (SIMS). The solubility of melt in the fluid phase increases with increasing F in the system; the solubility of H₂O in the melt is independent of the F concentration of the system with up to 6.3 wt% F in the melt. No evidence of immiscible silica- and fluoriderich liquids was detected in the hydrous but water-undersaturated starting material glasses (8.5 wt% F in melt) or in the water-saturated run product glasses. F concentrates in topaz rhyolite melts relative to coexisting fluids at most conditions studied; however, D_F (wt% F in fluid/wt% F in melt) increases strongly with increasing F in the system. Maximum values of D_F in this study are significantly larger than those previously reported in the literature. Linear extrapolation of the data suggests that D_F is greater than one for water-saturated, peraluminous granitic melts containing 8 wt% F at 800° C and 2 kbar. D_F increases as temperature and as (H₂O/H₂O+CO₂) of the fluid increase. For topaz rhyolite melts containing 1 wt% F and with H₂O-rich fluids, D_F is independent of changes in pressure from 2 to 5 kbar at 800° C; for melts containing 1 wt% F and in equilibrium with CO₂-bearing fluids the concentrations of F in fluid increases with increasing pressure. F-and lithophile element-enriched granites may evolve to compositions containing extreme concentrations of F during the final stages of crystallization. If F in the melt exceeds 8 wt%, D_F is greater than one and the associated magmatic-hydrothermal fluid contains >4 molal F. Such F-enriched fluids may be important in the mass transport of ore constituents, i.e., F, Mo, W, Sn, Li, Be, Rb, Cs, U, Th, Nb, Ta, and B, from the magma.     

基于元素含量和空间变异系数的化探异常信息识别

化探异常信息识别是化探数据分析最重要的任务之一, 也是化探数据在资源勘查领域受到广泛关注的最重要原因, 前人对化探异常信息识别做过大量研究, 这些研究中的大多数主要关注化探示踪元素的含量, 近而根据含量指标计算异常阈值, 而对示踪元素在空间中的分布特征关注较少。本文选择 1: 20万比例尺的克拉玛依幅为研究区, 根据区内金矿的矿床地球化学特征选择Ag、As、Au和Sb等4种元素为本区内金矿的示踪元素, 以地球化学元素分散晕形成理论为依据, 使用GIS技术和Matlab软件绘制研究区内4种金矿示踪元素的综合地球化学异常图。结果表明, 与传统阈值方法得到的化探异常图相比, 本文得到的化探异常图能够更好地指示研究区内已知金矿。     

Hawaiian basalt and Icelandic rhyolite: Indicators of differentiation and partial melting

In spite of the voluminous basaltic volcanism on the island of Hawaii, rhyolite is not produced. Iceland, on the other hand, exhibits common rhyolitic volcanism amounting to some 10–12% of its surface rocks. This contrast is investigated using the fundamental igneous processes exhibited by sheet-like Hawaiian lava lakes and Shonkin Sag laccolith in Montana. Highly differentiated, residual melts normally reside within inwardly advancing solidification fronts and are generally inaccessible to eruptive processes. Only when a large initial phenocryst population is present, from which a thick basal cumulate can rapidly form, is it possible to supply highly differentiated melt into the active (i.e., eruptable) portion of the magma chamber. Although there is protracted control of differentiation at Hawaii by settling of olivine, further differentiation occurs within the solidification fronts. Only by repeated transport and holding is it possible to differentiate beyond the critical composition of the leading edge of the solidification front ( 7% MgO and 51.5% SiO₂). Crystal size distributions (CSDs) for Hawaii and Shonkin Sag are used to demonstrate the inferred physical and chemical processes of solidification, including the kinetics of crystallization.A ubiquitous feature of these basaltic bodies is the formation of coarse veins and segregations of refined melt and granophyres within the upper solidification front. It is this fundamental bimodal feature which is the key to understanding Icelandic silicic volcanism.Rhyolites in Iceland occur mainly as a bimodal population with basalts associated with central volcanoes. Rhyolites, granophyres, and felsites are common, with the intrusions often being layered. Ash flows and true granite-like intrusions are rare. The voluminous silicic lavas at Torfajokull central volcano contain disequilibrium phenocryst assemblages. This, and the disagreement in oxygen isotopic values between rhyolites and basalts, reflects extensive partial melting of the heterogeneous basaltic crust of Iceland to produce these rhyolites. Relatively small, chemically distinct, and spatially intimate silicic bodies are formed by concentrating granophyric segregations from earlier cycles of solidification. This process is also reflected in the layered granophyric instrusion of Slaufrudalur in eastern Iceland. Slaufrudalur is an unvented subterranean caldera, equivalent in igneous processes and style to the subaerial Torfajokull caldera.Hawaii is dominated by fractional crystallization due to crystal settling and does not produce rhyolite. Iceland's tectonics allow continual and extensive reprocessing of thin, hot basaltic crust which produces rhyolite by concentrating original silicic segregations and veins and by partially melting intermediate extrusives, which have subsided deep into the crust.

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Zusammenfassung Auf Hawaii treten, trotz intensiven Basalt-Vulkanismusses, keine Rhyolithe auf. Auf Island dagegen ist Rhyolith, mit 10–12% des anstehenden Gesteins, verbreitet. Dieser Kontrast wurde anhand grundlegender magmatischer Prozesse untersucht, wie sie in flachen Lava-Seen Hawaiis und im Shonkin Sag Laccolith Montanas auftreten. Hochdifferenzierte Restschmelzen verbleiben innerhalb langsam nach innen vorrückender Erstarrungsfronten und sind meist unerreichbar für eruptive Prozesse. Nur wenn anfänglich bereits große Mengen von Einsprenglingen vorhanden sind, die rasch am Boden der Magmenkammer akkumulieren, kann eine hochdifferenzierte Schmelze in den aktiven (d.h. eruptiven) Teil der Magmenkammer gelangen. Obwohl auf Hawaii die Differentiation durch die Kristallisation von Olivin anhaltend kontrolliert wird, findet an der Erstarrungsfront weitere Differentiation statt. Nur durch wiederholten Transport und zeitweiliges Verharren ist es möglich, über die kritische Zusammensetzung der vordersten Erstarrungsfront hinaus zu differenzieren (ca. 7% MgO und 51,5% SiO₂). An Kristallgrö-ßenverteilungen (CDS) von Hawaii und Shonkin Sag können die angenommenen physikalischen und chemischen Prozesse der Kristallisation und die Kristallisationskinetik gezeigt werden. Ein weit verbreitetes Merkmal dieser Basaltkörper ist die Bildung grobkristalliner Gänge und Absonderung von stark differenzierten Schmelzen und Granophyren innerhalb der oberen Erstarrungsfront. Diese ausgeprägt bimodale Charakteristik ist der Schlüssel zum Verständnis des sauren isländischen Vulkanismus.Isländische Rhyolithe treten meist in bimodaler Verbreitung mit Basalten in Zusammenhang mit zentralen Vulkanen auf. Rhyolithe, Granophyre und Feisite sind häufig, in oft geschichteten Intrusionen. Ignimbrite und echte Granitintrusionen sind selten. Die großen Mengen SiO₂-reicher Laven am Torfajokull-Zentralvulkan enthalten Ein-sprenglinge, die sich nicht im Gleichgewicht mit der Matrix befinden. Dies, und die unterschiedlichen delta-¹⁸O-Werte von Rhyolithen und Basalten, zeigen, daß ausgeprägtes teilweises Aufschmelzen der heterogenen Basaltkruste von Island zur Produktion dieser Rhyolithe führte. Relativ kleine, nahe benachbarte saure Körper, die aber deutliche Unterschiede in ihrem Chemismus aufweisen, werden gebildet durch die Konzentration granophyrischer Teilschmelzen aus früheren Kristallisationszyklen. Dieser Vorgang wird auch widergespiegelt in der »layered intrusions« von Slaufrudalur in Ostisland. Slaufrudalur ist eine geschlossene unterirdische Kaldera, deren magmatische Prozesse und Baustil der subaerischen Torfajokull-Kaldera entsprechen.Die Prozesse in Hawaii sind dominiert von gravitativer Kristallisationsdifferentiation und es werden keine Rhyolithe produziert. Die isländische Tektonik führt zu kontinuierlicher starker Wiederaufarbeitung von dünner, heißer basaltischer Kruste. Dabei wird, durch die Konzentration ursprünglicher saurer Teilschmelzen und Gänge und durch die teilweise Aufschmelzung intermediärer Intrusiva, die tief in die Kruste abgesunken sind, Rhyolith produziert.

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Résumé En dépit du volcanisme basaltique volumineux des îles Hawaï, il n'y existe pas de rhyolite. En Islande, par contre, le volcanisme rhyolitique est commun et représente 10 à 12% des roches de la surface. Ce contraste est examiné sur la base des processus ignés fondamentaux présentés par les lacs de lave d'Hawaï et le laccolite de Shonkin Sag au Montana. Normalement, les liquides résiduels hautement différenciés résident à l'intérieur des fronts de solidification qui progressent vers l'arrière et sont généralement à l'abri des processus éruptifs. Ce n'est que dans le cas d'une population initiale abondante de phénocristaux, qui se rassemblent dans un cumulat basai épais, que des liquides hautement différenciés peuvent être fournis à la portion active (c'est-à-dire »éruptible«) de la chambre magmatique. A Hawaï, bien que la différenciation soit continuellement régie par la cristallisation d'olivine, la poursuite du processus a lieu à l'intérieur des fronts de solidification. Ce n'est que par la répétition d'actions de transport et de stagnation qu'il est possible de différencier audelà de la composition critique du front de solidification (±7% MgO et 51,5% SiO₂). A partir de la distribution de la taille des cristaux à Hawaï et à Shonkin Sag, on peut déduire les processus physique et chimique de la solidification, y compris la cinétique de la cristallisation.Une particularité courante de ces corps basaltiques est la formation de veines grenues et de ségrégations de liquides très différenciés et de granophyres à l'intérieur du front supérieur de solidification. Cette manifestation bimodale est la clé qui permet de comprendre le volcanisme siliceux islandais.En Islande, les rhyolites constituent d'ordinaire une population bimodale avec les basaltes centraux. Les rhyolites, les granophyres et les felsites sont fréquents, et souvent sous forme d'intrusions litées. Les coulées ardentes et les vraies intrusions de type granitique sont rares. Les volumineuses laves siliceuses du volcan central de Torfajokull contiennent des assemblages de phénocristaux en déséquilibre. Ce fait, ainsi que la non concordance des isotopes de l'oxygène entre rhyolites et basaltes, traduisent, à l'origine de ces rhyolites, une fusion partielle extensive de la croûte basaltique hétérogène d'Islande. Des corps siliceux relativement petits et chimiquement distincts bien que d'emplacements très voisins se sont formés par concentration de fusions partielles granophyriques lors des premiers cycles de solidification. Ce processus s'exprime également dans l'intrusion granophyrique litée de Slaufrudalur, en Islande orientale. Slaufrudalur est une caldeira souterraine fermée, équivalente par son style et son processus igné à la caldeira subaérienne de Torfajokull.A Hawaï, le phénomène dominant est la cristallisation fractionnée gravitative, sans production de rhyolite. La tectonique de l'Islande permet la régénération continue et extensive d'une mince croûte basaltique chaude. Les rhyolites y sont engendrées par la concentration des veines et ségrégations siliceuses originelles et par la fusion partielle de masses extrusives intermédiaires descendues profondément dans la croûte.

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, . , 10–12% . , Shonkin Sag Laccolith Montanas. . , , . , . . ( 7% MgO 51,5% SiO₂). (CDS) Shonkin Sag , , . . . . , , . . , Torfajokull , ., ¹⁸O , . , , , , « » («layered intrusions») Slaufrudalur, . , , Torfajokull. . . , , , .

     

An experimental investigation of high-temperature interactions between seawater and rhyolite,andesite, basalt and peridotite

Natural seawater was allowed to react with rhyolite, andesite, basalt, and peridotite at 200°–500° C, and 1,000 bars at water/rock mass ratios of 5 and 50 in order to investigate the effects of rock type, water/rock ratio, and temperature on solution chemistry and alteration mineralogy. The results indicate that interactions of seawater with various igneous rocks are similar in the production of a hydrous Mg-silicate and anhydrite as major alteration products. Fluids involved in the interactions lose Mg to alteration phases while leaching Fe, Mn, and Si from the rocks. The pH of the solutions is primarily controlled by Mg-OH-silicate formation and therefore varies with Mg and Si concentration of the system. Other reactions which involve Mg (such as Mg-Ca exchange) or which produce free H⁺, cause major differences in fluid chemistry between different seawater/ rock systems. High water/rock ratio systems (50/1) are generally more acidic and more efficient in leaching than low ratio systems (5/1), due to relatively more seawater Mg available for Mgsilicate production. The experiments show that large-scale seawater/rock interaction could exert considerable control on the chemistry of seawater, as well as producing large bodies of altered rock with associated ore-deposits.Active plate margins of convergence or divergence are suitable environments for hydrothermal systems due to the concurrence of igneous activity, tectonism, and a nearby water reservoir (seawater or connate water). The experimental data indicate that seawater interactions with igneous host rocks could generate many of the features of ore-deposits such as the Kuroko deposits of Japan, the Raul Mine of Peru, the Bleida deposit of Morocco, and deposits associated with ophiolites. Serpentinization of peridotite and alteration of igneous complexes associated with plate margins can also be explained by seawater interaction with the cooling rock. Geothermal energy production could benefit from experimental investigations of hot water/rock systems by development of chemical, temperature, and pressure control systems to maximize the lifetime of hydrothermal flow.     

Petrochemistry of the Fish Cove rhyolite,Keweenaw peninsula,Michigan, U.S.A.

A small (360 × 180 m) rhyolitic intrusive body in the lower portion of the Portage Lake Lava Series of Michigan's Keweenaw peninsula was mapped and sampled in detail. The rhyolite is one of a number of similar bodies which make up less than 1% of the total volume of this thick Late-Precambrian plateau basalt pile. The rock is a low-calcium rhyolite with fine-grained homogeneous texture and sparse phenocrysts of plagioclase and quartz. Analyses of selected trace and major elements for 21 samples taken from the body reveal a chemical zonation consisting of a core zone enriched in K, Rb and Ba, and a border zone relatively poor in these elements. Little areal difference is found with respect to other elements tested (Mn, Sr, Zr, Ca, Ti, and Fe). This apparently primary zonation seems to result from the migration of K, Rb and Ba during crystallization of the shallow intrusive. Though zoned, the trace-element chemistry of the Fish Cove body is distinct from that of eight other rhyolites in the Portage Lake Lava Series, and suggests that fingerprinting by trace elements might be a fruitful method for identifying and correlating the sources of numerous rhyolitic pebbles in conglomerates interbedded with the basaltlava flows of the Portage Lake Series.     

Origin of rhyolite and rhyodacite lavas and associated mafic inclusions of Cape Akrotiri,Santorini: the role of wet basalt in generating calcalkaline silicic magmas

Amphibole-bearing mafic inclusions (low to medium-K high-alumina basalt to basaltic andesite) comprise 4.1 vol% of calc-alkaline rhyolite and rhyodacite lavas on Akrotiri Peninsula, Santorini, Greece. Physical features indicate a magmatic origin for the inclusions, involving mingling with the host silicic magma and quenching. Water contents of the mafic magmas are estimated to have been above 4% at water pressures of 1.8 kbars or more at temperatures of approximately 950–1,000 °C. Three evolutionary stages are inferred in their petrogenesis. In the first stage infiltration of slab fluids promotes partial melting in the mantle to generate primitive wet basaltic magmas enriched in LREE, LILE, Th and U in comparison to N-type MORB. In the second stage storage and crystal differentiation of primitive magmas occurred in the lithospheric mantle or deep crust, involving olivine, spinel and clinopyroxene followed by amphibole and plagioclase. In the third stage differentiated mafic magma intrudes into porphyritic silicic magma at shallower crustal levels (estimated at 7–10 km). Mingling and quenching of the mafic magmas within the silicic host causes chemical or physical interactions between the inclusions and the host prior to and during eruption. The silicic lavas have geochemical affinities with the mafic inclusions, but are relatively depleted in MREE, HREE and Y and enriched in Rb relative to Ba and K. These observations are consistent with involvement of amphibole in magma genesis due either to crystal differentiation from wet basalt or to partial melting of mafic rocks with residual amphibole. Crystallization of wet basalt in the deep crust is preferred on the basis of physical considerations.Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.Editorial responsibility: I. Parsons     

The role of apatite in mantle enrichment processes and in the petrogenesis of some alkali basalt suites

Analyses of Sr and REE in apatites from a variety of mantle-derived parageneses are used in conjunction with trace element data from the literature to investigate relationships between alkali basalts and apatite-rich materials in upper-mantle source regions. Despite difficulties in interpretation, positive P-anomalies in the hygromagmatophile element abundance patterns of some continental primary alkali basalts suggest either P-enrichment of their source or assimilation of P-rich material, or both. Amphibole- and apatite-rich xenoliths occur in several alkali-basalt provinces, and by virtue of the P and LREE enrichment represent a probable source of the P anomalies and part of the other trace element enrichments of these magmas. Incorporation of such apatite-rich materials by later primary magmas would be enhanced by the high P₂O₅ concentrations required to achieve apatite saturation in basaltic liquids.In the early stages of mantle diapirism an undersaturated magma, produced by slight partial melting of garnet peridotite, might fractionate as it rises to the range of amphibole stability. Hygromagmatophile element patterns of clinopyroxenite xenoliths indicate that clinopyroxene fractionation could produce P-enriched liquids which might subsequently crystallize amphibole- and apatite-rich materials now represented by xenoliths. During generation of later primary magma, apatite-rich materials might preferentially contaminate the liquids, to yield positive P-anomalies. This model requires that magmas undergo prolonged fractionation at considerable depth (~ 100 km), a process which is apparently most probable in subcontinental environments.An apatite- and zircon-bearing mica-clinopyroxenite xenolith from Matsoku provides a link between the S. African MARID suite and amphibole and apatite-rich xenoliths from various alkali basalt provinces. Unusual REE patterns ($\text{La}{}_{\mathrm{N}}\text{<}\text{Ce}{}_{\mathrm{N}}\text{<}\text{Nd}{}_{\mathrm{N}}\text{,}\text{Ce}{}_{\mathrm{N}}\text{/Y}{}_{\mathrm{N}}\text{?10}$) of apatites in this xenolith suggest a link between the MARID suite xenoliths and postulated pre-Karroo mantle metasomatism.     

多孔介质孔隙率与体积模量的关系

用两相等效体的概念,根据Walsh公式和球形孔隙的弹性公式导出了多孔介质的孔隙率和体积模量之间的近似公式和精确公式,近似关系式不适用于较大的孔隙率。精确公式可以较好的适用于较大孔隙率。试验结果表明,精确公式能够很好地表达多孔介质孔隙率与体积模量之间的关系。