Plagioclase-melt partition coefficients (D) for 34 trace elements at natural concentration levels were determined experimentally in a natural MORB composition at atmospheric
pressure using thin Pt-wire loops. Experiments were carried out at three temperatures (1,220, 1,200, and 1,180°C), and at
three different oxygen fugacities (fO2 = IW, QFM, air) in order to assess the effect of fO2 on the partitioning of elements with multiple valence (Fe, Eu, Cr). Run products were analyzed by laser-ablation ICP-MS.
Most trace element Ds increase slightly as temperature decreases, except for DZr, DFe, DEu and DCr that vary systematically with fO2. Applying the Lattice Strain Model to our data suggests the presence of Fe2+ entirely in the octahedral site at highly to moderate reducing conditions, while Fe3+ was assigned wholly to the tetrahedral site of the plagioclase structure. Furthermore, we provide a new quantitative framework
for understanding the partitioning behaviour of Eu, which occurs as both 2+ and 3+ cations, depending on fO2and confirm the greater compatibility of Eu2+, which has an ionic radius similar to Sr, relative to Eu3+ in plagioclase and the higher Eu2+/ Eu3+ under reducing conditions. For petrogenetic basaltic processes, a combined fractionation of Eu2+–Sr and Fe–Mg by plagioclase has considerable potential as an oxybarometer for natural magmatic rocks. 相似文献
Dissolution rates were calculated for a range of grain sizes of anorthite and biotite dissolved under far from equilibrium conditions at pH 3, T = 20 °C. Dissolution rates were normalized to initial and final BET surface area, geometric surface area, mass and (for biotite only) geometric edge surface area. Constant (within error) dissolution rates were only obtained by normalizing to initial BET surface area for biotite. The normalizing term that gave the smallest variation about the mean for anorthite was initial BET surface area. In field studies, only current (final) surface area is measurable. In this study, final geometric surface area gave the smallest variation for anorthite dissolution rates and final geometric edge surface area for biotite dissolution rates. 相似文献
The localisation of strain into natural ductile shear zones is often associated with the occurrence of metamorphic reactions. In order to study the effects of solid–solid mineral reactions on plastic deformation of rocks, we have investigated the shear deformation of plagioclase–olivine composites during the reaction plagioclase + olivine → orthopyroxene + clinopyroxene + spinel (± garnet). Microstructures of plagioclase–olivine composites were studied after shear deformation experiments in a Griggs apparatus. Experiments were performed on anorthite–forsterite (An–Fo) and labradorite–forsterite (Lab–Fo) composites at 900 °C, confining pressures between 1000–1600 MPa and with constant shear strain rates of 5 × 10−5 s−1.In absence of reaction, Lab–Fo composites are stronger than pure olivine and labradorite end-members that deform with a high temperature plasticity mechanism. Lab–Fo composites strain–harden due to the inhibition of extensive recrystallisation by interphase boundaries.In An–Fo composites, the reaction induces strain weakening by a switch from dislocation creep to grain size sensitive deformation mechanisms through the development of fine-grained (size < 0.5 μm) polyphase reaction products. Interconnecting layers of reaction products accommodate most of the applied strain by grain size sensitive creep. Recovery processes are pronounced during syndeformational reaction: original anorthite and olivine dynamically recrystallise by subgrain rotation and bulging recrystallisation. Presumably, the dynamic recrystallisation is caused by reduced stress conditions and partitioning of strain and strain rates between the new reaction products and the relict An–Fo grains. The results of our experiments are in good agreement with natural observations of shear localisation in the lower crust and upper mantle, and imply that anhydrous mineral reactions can be important causes for localisation of deformation. 相似文献
The present study aimed to test reliability of luminescence and electron spin resonance (ESR) methods to date tephra. We investigated on three Japanese marker tephras, Ikeda-ko (6.4 ka), Aira-Tn (30 ka) and Aira-Iwato (45–50 ka). A systematic studies were performed using different minerals (quartz and feldspar), different grain fractions (75–250 and 250–500 μm), different luminescence and ESR signals, like optically stimulated luminescence (OSL) of quartz, infrared stimulated luminescence (IRSL) of feldspar, including recently developed least faded post infrared IRSL (pIR-IRSL), and ESR signals from paramagnetic centers Al and Ti–Li of quartz. Ages obtained using pIR-IRSL signal of plagioclase with preheat of 320 °C, 60 s and stimulation at 300 °C are consistent with the reference ages. High dose detection range (up to ∼600 Gy) and accurate age estimation enable pIR-IRSL of feldspar a promising methodology to date quaternary tephra. ESR ages from quartz are grossly correlated with the reference ages but large deviation and large associated errors are observed, possibly due to either low signal to noise ratio or heterogenous dose response of different aliquot in multiple aliquot additive dose (MAAD) approach. 相似文献
Despite the fact that porphyry Cu deposits contain large amounts of Cu in one or more small stocks, few studies have discussed evidence for significant magma recharge in porphyry Cu deposits. A systematic elemental and Sr isotopic study of plagioclase crystals from mineralized diorite and granodiorite porphyry constrains the processes of crystallization and magma recharge at the Baogutu reduced porphyry copper deposit, western Junggar, NW-China. Large compositional changes in An (12–24 mol%) are observed along with strong positive correlations between An and FeO. Significant resorption textures are also preserved in plagioclase crystals as well as repeated oscillatory zoning in An and FeO, and complex Sr isotope variations. Three types of crystals with different core-to-rim Sr isotope variations are recognized. Type I crystals have core-to-rim increases in (87Sr/86Sr)i that could be explained by diffusion. For example, Sr isotope variations recorded in BCK2-1-2 crystal could be generated by diffusion with simulated maximum crystal residence times of 100–500 years with the proximate value of ∼300 years. Type II crystals with different variation trends in (87Sr/86Sr)i on opposite sides of the core could be produced either by multi-stage crystallization or by analytical uncertainty. Whereas, type III crystals with complex core-to-rim variations in (87Sr/86Sr)i, may record repeated magma recharge events. All these results suggest repeated recharging of the magma chamber by hotter, more mafic and less radiogenic Sr isotope melts. Therefore, more mafic melt injection recorded in plagioclase profiles may provide significant metal contributions to the magma chamber, which ultimately results in Cu mineralization at the Baogutu reduced porphyry copper deposit. 相似文献
The Cemilköy ignimbrite is one of the voluminous ignimbrite deposits in Cappadocia. The Cemilköy ignimbrite contains pumice and lithic clasts of volcanic and ophiolitic origin in an ash matrix. The unwelded Cemilköy ignimbrite is distinguished from other deposits in Cappadocia by flattened pumices, elongate vesicles and a slaty fabric. The mineral assemblage of the Cemilköy ignimbrite is plagioclase, biotite, quartz and oxides (magnetite and Ti-magnetite) and the matrix is glassy. Eutaxitic texture is dominant and all pumice clasts have a vitrophyric-porphyritic texture. The same textural properties were observed throughout the spatial distribution of Cemilköy ignimbrite. Microprobe studies reveal that plagioclase compositions range from albite through oligoclase-andesine. Estimated plagioclase-liquid temperatures (T) and pressures (P) are varying between 806 and 847?°C and 4.2–7.1 (kbar), and the H2O content of the melt is estimated to have been 5?wt.% from the pumice clasts. Based on geochemical data, the Cemilköy ignimbrite is rhyolitic and calc-alkaline in character, and all pumice clasts are enriched in LIL and LRE elements relative to HFS elements. Negative Nb, Ta and Ti anomalies, ratios of Ba/Nb >28 (56–77), Ba/Ta ?450 (590–700) and Th/Yb vs. Ta/Yb are consistent with a subduction-related origin. According to the geochemical and mineralogical-petrographical data, the Cemilköy ignimbrite originated from partial melting of a mantle source which was enriched during previous subduction processes with variable degrees of assimilation fractional crystallization (AFC) through time and Cemilköy ignimbrite erupted from a crustal magma chamber at shallow to intermediate depth. 相似文献
Granodiorite from the Gęsiniec Intrusion, Strzelin Crystalline Massif, SW Poland contains complexly zoned plagioclases. Five chemically and structurally distinct zones can be correlated among crystals: ‘cores’ (25–35% An), inner mantles (40–45% An), outer mantles (40–25% An), resorption zones (35–50% An) and rims (35–30% An). Good structural and chemical (major and trace elements) correlation of zones between crystals indicates that zonation was produced by changes in conditions of crystallization on a magma chamber scale. Plagioclase, being the liquidus phase, records a time span from the beginning of crystallization to emplacement and rapid cooling of granodiorite as thin dykes.
Crystallization began with the formation of inner mantles. The paucity and different sizes of inner mantles suggests slow crystallization in high temperature magma. Normally zoned inner mantles were formed under increasing undercooling. Compositional trends in mantles suggest closed system crystallization.
The major resorption zones were caused by injection of less evolved magma as indicated by the strontium increase in plagioclase. The injection triggered a rapid rise of magma and plagioclase crystals facilitating mixing but also inducing fast, kinetically controlled growth of complex multiple, oscillatory zonation within resorption zones. The ascent of magma caused decompression melting of plagioclase and produced melt inclusions within inner mantles—the ‘cores’. The decompression range is estimated at a minimum of 2 kbar. Emplacement of granodiorite as thin dykes allow rapid cooling and preservation of magmatic zonation in plagioclases. Melt inclusions crystallized completely during post-magmatic cooling.
The zonation styles of plutonic plagioclase differ markedly from volcanic ones suggesting different magma evolution. Zones in plutonic plagioclase are well correlated indicating crystallization in quiescent magma where crystals accumulation and compositional magma stratification may occur. Crystals probably did not travel between different regimes. Resorption occurred but as single albeit complex episodes. Good correlation of zones in plutonic plagioclases allows a distinction between the main processes controlling zonation and superimposed kinetic effects. 相似文献