Magma mixing in the acidic-basic complex of Ardnamurchan: Implications on the evolution of shallow magma chambers

The Ardnamurchan net-veined complex consists of three rock types: a porphyritic mafic rock, an aphyric intermediate rock and a silicic rock. Pillows of mafic and intermediate rock are included in the silicic rock and contain crenulated and some chilled margins. Liquid-liquid relationships are inferred for these three magmas. The trace element data, using ratio-ratio plots, are consistent with magma mixing being the dominant process and are inconsistent with any process that is dominated by crystal fractionation or melting. The major element data, using multiple linear regression techniques, are consistent with magma mixing of high-silica silicic magma and primitive mafic magma, along with about 35 percent crystal fractionation to form the intermediate rock type. All of the data taken together are consistent with a magma mixing model with some fractionation to produce the variation observed. The simplest model is that a fractionating basaltic magma comes into contact with a silicic magma and limited mixing produces the intermediate magma.This net-veined complex may be the only evidence available for interaction of mafic and silicic melts that occurred in the Ardnamurchan high-level magma chamber before the silicic magma was lost to eruptive and surface processes. In general the chemical and field relationships are consistent with Smith's model for the evolution of high-level, magma chambers.     

Equilibration Scales in Silicic to Intermediate Magmas Implications for Experimental Studies

Experimental phase equilibrium studies are increasingly beingused for the determination of intensive variables (P, T, fH₂O,fO₂ ) in silicic to intermediate magmas. In contrast, silicicigneous bodies are now perceived as open, periodically recharged,systems involving only limited chemical equilibration. Thus,the use of laboratory-determined crystal–liquid equilibriumdata needs clarification. Here we review the field, petrologicaland geochemical evidence concerning states and scales of chemicalequilibrium in silicic magma bodies. It is concluded that totalchemical equilibrium is generally not the rule. However, a subsystemin local equilibrium (the reactive magma) can be identified.Equilibration scales in silicic magmas are rate-limited eitherby diffusive flux in crystals (DICL regime) or by diffusiveflux in the melt (MD regime). The recognition that equilibriumin magmas is limited to a reactive subsystem requires phaseequilibrium studies to be chemically scaled. Experiments, eitherof total or partial equilibrium type, should aim at a closereproduction of equilibrium states specific to natural systems.The laboratory reconstruction of the natural equilibrium statesguarantees a precise determination of the pre-eruptive parametersand a reliable application of the experimental data to activevolcanic systems. KEY WORDS: silicic magmas; chemical equilibrium; timescales; experimental studies     

Chemical gradient at diffusive interfaces in magma chambers

The results of high pressure experiments on diffusion and Soret separation in natural silicate melts show that the diffusive behaviour between natural silicic and mafic magmas can be approximately modelled as if the system were a binary mixture of SiO₂ and other components such as MgO+FeO+CaO. Steady state compositional profiles across a diffusive interface between silicic and mafic magma layers are calculated on the basis of phenomenological relationships for the fluxes of chemical species and heat in the binary mixtures, using the experimental data of diffusion and Soret coefficients in natural silicate melts. The compositional profiles show a curvature with a minimum SiO₂ value within the interface due to the Soret effect and temperature dependence of diffusion coefficient. The compositional gradient at the lower half of the diffusive interface is similar to that resulting from the Soret separation of a mafic melt regardless of the composition of the silicic magmas. These results suggest that picritic magma can be formed in the interfacial region between the mafic and silicic magma layers. The compositional gradient explains chemical variation of mafic to picritic inclusions in a mixed andesite of the Abu Volcano Group, Japan.     

Magmatic processes that generate chemically distinct silicic magmas in NW Costa Rica and the evolution of juvenile continental crust in oceanic arcs

Northwestern Costa Rica is built upon an oceanic plateau that has developed chemical and geophysical characteristics of the upper continental crust. A major factor in converting the oceanic plateau to continental crust is the production, evolution, and emplacement of silicic magmas. In Costa Rica, the Caribbean Large Igneous Province (CLIP) forms the overriding plate in the subduction of the Cocos Plate—a process that has occurred for at least the last 25 my. Igneous rocks in Costa Rica older than about 8 Ma have chemical compositions typical of ocean island basalts and intra-oceanic arcs. In contrast, younger igneous deposits contain abundant silicic rocks, which are significantly enriched in SiO₂, alkalis, and light rare-earth elements and are geochemically similar to the average upper continental crust. Geophysical evidence (high Vp seismic velocities) also indicates a relatively thick (~40 km), addition of evolved igneous rocks to the CLIP. The silicic deposits of NW Costa Rica occur in two major compositional groups: a high-Ti and a low-Ti group with no overlap between the two. The major and trace element characteristics of these groups are consistent with these magmas being derived from liquids that were extracted from crystal mushes—either produced by crystallization or by partial melting of plutons near their solidi. In relative terms, the high-Ti silicic liquids were extracted from a hot, dry crystal mush with low oxygen fugacity, where plagioclase and pyroxene were the dominant phases crystallizing, along with lesser amounts of hornblende. In contrast, the low-Ti silicic liquids were extracted from a cool, wet crystal mush with high oxygen fugacity, where plagioclase and amphibole were the dominant phases crystallizing. The hot-dry-reducing magmas dominate the older sequence, but the youngest sequence contains only magmas from the cold-wet-oxidized group. Silicic volcanic deposits from other oceanic arcs (e.g., Izu-Bonin, Marianas) have chemical characteristics distinctly different from continental crust, whereas the NW Costa Rican silicic deposits have chemical characteristics nearly identical to the upper continental crust. The transition in NW Costa Rica from mafic oceanic arc and intra-oceanic magma to felsic, upper continental crust-type magma is governed by a combination of several important factors that may be absent in other arc settings: (1) thermal maturation of the thick Caribbean plateau, (2) regional or local crustal extension, and (3) establishment of an upper crustal reservoir.     

Calculation of Si NMR shifts of silicate complexes with carbohydrates, amino acids, and muhicarboxylic acids: potential role in biological silica utilization

The existence of ether or ester-like complexes of silicate with organic compounds has long been debated in the literature on biological utilization of silicon. Comparison of theoretically calculated ²⁹Si NMR chemical shifts for such complexes with experimentally measured values in biological systems could provide a diagnostic tool for identifying which, if any of these molecules exist under physiological conditions. Results are presented here for ab initio molecular orbital calculations of ²⁹Si NMR shifts and formation energies of silicate complexes with polyalcohols, sugar-acids, pyranose sugars, amino acids and multicarboxylic acids. The effects of functional group and molecular structure including ligand size, denticity, ring size, silicon polymerization and coordination number on calculated ²⁹Si shifts were considered. The potential role of such compounds in biological silica utilization pathways is discussed.²⁹Si NMR shifts and energies were calculated at the HF/6-311+G(2d,p)//HF/6-31G* level. The main result is that only five-membered rings containing penta- and hexa-coordinated Si can explain experimentally observed resonances at ∼ −101 and −141 ppm. Further, the heptet observed in ¹H-²⁹Si coupled spectra can only be explained by structures where Si bonds to oxygens atoms in H-C-O-Si linkages with six symmetrically equivalent H atoms.While compounds containing quadra-coordinated silicon may exist in intracellular silicon storage pools within diatoms, calculated reaction energies suggest that the organism has no thermodynamic advantage in taking up extracellular organ-silicate compounds, instead of silicic acid, from the ambient aqueous environment. Hyper-coordinated complexes are deemed unlikely for transport and storage, though they may exist as transient reactive intermediates or activated complexes during enzymatically- catalyzed silica polymerization, as known previously from sol-gel silica synthesis studies.     

北京沿河城地区东岭台组酸性火山碎屑流的碎屑分形特征及其成因

火山碎屑岩中的碎屑颗粒形态、分布和显微结构保存着岩浆房内岩浆的结晶状态、火山爆炸过程中的岩浆气泡化和碎裂作用以及火山碎屑堆积和变形过程的大量物理信息。为揭示北京西山沿河城地区东岭台组酸性火山碎屑流形成的物理过程,本文以东岭台组第三岩性段的酸性熔结火山碎屑岩的火山地质、岩相学研究为基础,应用分形理论和方法对酸性熔结火山碎屑岩中的碎屑形态的分形特征开展了定量研究。不同类型碎屑的表面和边界盒维数的数值范围大小具有一致性。玻屑变幅较大,石英晶屑和岩屑变幅较小,这说明碎屑的塑性和流变性对碎屑形态多样性的影响较大。在几类碎屑中,熔蚀石英的边界盒维数和周长-面积法获得的分形维数最大,说明熔蚀作用对石英斑晶边界形态复杂性的影响超过了其它机制对火山碎屑形态复杂性的影响。东岭台组酸性火山碎屑岩中的熔蚀石英、石英碎屑、长石碎屑、玻屑和岩屑的分形特征有明显差异,反映了火山喷发过程中围岩和岩浆性质、物理化学条件以及火山作用机制的差异。火山碎屑形态的分形特征和幂率规律,证明火山爆炸过程是一种自组织临界条件下发生的。     

吉林省辉南县饮用天然矿泉水资源浅析

介绍了吉林省辉南县饮用天然矿泉水资分布,其中已完成勘查评价6处,大椅山镇13处、抚民镇12处、金川镇18处,总计49处。根据化学成分确定了天然矿泉水类型,在已发现的泉点中,偏硅酸质量浓度大于30 mg/l的泉（井）点40余处,占总数95%以上,个别为偏硅酸—锶复合型。分析了矿泉水形成的地球化学条件和地质构造条件及其水源补给径流条件。最后对矿泉水进行了评价并展望了其开发利用前景。     

Geochemical stability of fine-grained silicic Holocene tephra in Iceland and Scotland

This paper tests two assumptions fundamental to the use of fine-grained silicic tephras from Iceland for creating chronological frameworks in northwest Europe. It is shown firstly, that glass shards can retain their overall chemical integrity on at least a four millennial time-scale in contrasting depositional environments in Iceland and Scotland, and secondly, that an acid digestion process, the most practical method for extracting tephra from peat, does not significantly bias the results of major element analysis by electron microprobe. The implication is that there is great potential and an appropriate method for developing both the resolution and the spatial coverage of chronologies based on silicic Icelandic tephras in northwest Europe.     

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     

Geothermometry, geochronology, and mass transfer associated with hydrothermal alteration of a rhyolitic hyaloclastite from Ponza Island, Italy

A rhyolitic hyaloclastite from Ponza Island, Italy, was hydrothermally altered, producing four distinct alteration zones based on X-ray diffraction mineralogy and field textures: (1) nonpervasive argillic zone; (2) propylitic zone; (3) silicic zone; and (4) sericitic zone. The unaltered hyaloclastite is volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli. Incomplete alteration of the hyaloclastite resulted in the nonpervasive argillic zone, characterized by smectite and disordered opal-CT. The other three zones exhibit more complete alteration of the hyaloclastite. The propylitic zone is characterized by mixed-layer illite-smectite (I-S) with 10 to 85% I, mordenite, opal-C, and authigenic K-feldspar (akspar). The silicic zone is characterized by I-S with ≥90% I, pure illite, quartz, akspar, and occasional albite. The sericitic zone consists primarily of I-S with ≥66% I, pure illite, quartz, and minor akspar and pyrite. K/Ar dates of I-S indicate hydrothermal alteration occurred at 3.38 ± 0.08 Ma.Oxygen isotope compositions of I-S systematically decrease from zones 1 to 4. In the argillic zone, smectite has δ¹⁸O values of 21.7 to 22.0‰ and I-S from the propylitic, silicic, and sericitic zones ranges from 14.5 to 16.3‰, 12.5 to 14.0‰, and 8.6 to 11.9‰, respectively. δ¹⁸O values for quartz from the silicic and sericitic zones range from 12.6 to 15.9‰. By use of isotope fractionation equations and data from authigenic quartz-hosted primary fluid inclusions, alteration temperatures ranged from 50 to 65°C for the argillic zone, 85 to 125°C for the propylitic zone, 110 to 210°C for the silicic zone, and 145 to 225°C for the sericitic zone. Fluid inclusion data and calculated δ¹⁸O_water values indicate that hydrothermal fluids were seawater dominated.Mass-transfer calculations indicate that hydrothermal alteration proceeded in a relatively open chemical system and alteration in the sericitic zone involved the most extensive loss of chemical species, especially Si. Systematic gains in Mg occur in all alteration zones as a result of I-S clay mineral formation, and systematic losses of Na, Ca, and K occur in most zones. With the exception of Ca, calculations of mass transfer associated with hydrothermal alteration on Ponza agree with chemical fluxes observed in laboratory experiments involving hydrothermal reactions of rhyolite and seawater. The anomalous Ca loss at Ponza may be due to hydrothermal formation of anhydrite and later low-temperature dissolution. On the basis of Mg enrichments derived from circulating seawater, we estimate the following minimum water/rock ratios: 9, 3, 6, and 9 for the argillic, propylitic, silicic, and sericitic zones, respectively. Hydrothermal fluid pH for the propylitic and silicic zones was neutral to slightly basic and relatively acidic for the sericitic zone as a result of condensation of carbonic and perhaps other acids.     

Super-silicic garnet microstructures from an orogenic garnet peridotite, evidence for an ultra-deep (>6 GPa) origin

We report the field, petrographic and mineral chemical characteristics of relict super‐silicic (=majoritic) garnet microstructures from the Otrøy peridotites in the Western Gneiss Region, Norway. The evidence for the former existence of super‐silicic garnet consists of two‐pyroxene exsolution microstructures from garnet. Estimates of the initial composition of the super‐silicic garnet imply pressures of 6–6.5 GPa, indicating that the Otrøy garnet peridotites were derived from depths >185 km. The garnet peridotites consist of inter‐banded variable compositions with c. 50% garnet peridotite and 50% garnet‐free peridotite. Two distinct garnet types were identified: (a) normal matrix garnet, grain‐size ≤4 mm, and (b) large isolated single garnet crystals and/or (polycrystalline) garnet nodules up to 10 cm in size. Large garnet nodules occur only within limited bands within the garnet peridotites. The relicts of super‐silicic garnet were exclusively found in some (not all) of the larger garnet nodules. Petrographic observations revealed that the microstructure of nodular garnet consists of the following four characteristic elements. (1) Individual garnet nodules are polycrystalline, with grain sizes of 2–8 mm. Garnet grain boundaries are straight with well‐defined triple junctions. (2) Some garnet triple junctions and garnet grain boundaries are decorated by interstitial orthopyroxene. (3) Cores of larger polycrystalline garnet contain two‐pyroxene exsolution microstructures. (4) Precipitation‐free rims (2 mm thick) surround garnet cores containing the exsolved pyroxene microstructure. Pyroxene exsolution from super‐silicic garnet was subsequently followed by brittle–ductile deformation of garnet. Both exsolved pyroxene needles and laths become undulous or truncated by fractures. Simultaneous garnet plasticity is indicated by the occurrence of high densities of naturally decorated dislocations. Transmission electron microscopy observations indicate that decoration is due to Ti‐oxide precipitation. Estimates of the P–T conditions for mineral chemical equilibration were obtained from geothermobarometry. The mineral compositions equilibrated at mantle conditions around 805±40 °C and 3.2±0.2 GPa. These P–T estimates correspond to cold continental lithosphere conditions at depths of around 105 km. From a combination of both depth estimates it can be concluded that the microstructural memory of the rock extends backwards to twice as great a depth range as obtained by thermobarometric methods. Available geochronological and geochemical data of Norwegian garnet peridotites suggest a multi‐stage, multi‐orogenic exhumation history.     

Origin of silicic rocks of the Deccan Traps continental flood basalt province: Inferences from field observations,petrography, and geochemistry

The origin of silicic rocks (SiO₂ > 65 wt%) in Continental Flood Basalt (CFB) provinces could be attributed to complex petrogenetic processes. The 65.5–66 Ma old Deccan Traps CFB contains eight sporadic but significant silicic rock exposures that are studied here in a comprehensive framework using field observations, petrography, major oxides (n = 56), and trace element chemistry. Rhyolite and granophyre, as well as subordinate felsite, ignimbrite, trachyte, pitchstone, and microgranite coexist with volcanic and plutonic mafic rocks such as basalt, basaltic andesite, and gabbro. Multiple isolated and circular/semi-circular hills and linear dykes of silicic rocks are present in the form of lavas with prominent flow folding, rheomorphic ignimbrite, and tuffs. The ‘Rheological Agpaitic Index’ (RAI) indicates that most of the silicic rocks in the Deccan Traps are effusive in nature, except for Rajpipla, Alech, Bombay, and Osham silicic rocks, which are marked by explosive volcanism. Thermodynamic-based Rhyolite-MELTS modelling suggests that the major oxide composition of Pavagadh and Barda basalt is a likely candidate for the parental melt composition of the silicic rocks of the Deccan Traps. Ba, Sr, P, Zr, and Ti anomalies are consistent with the fractionation of K-feldspar, plagioclase, apatite, zircon, and Fe-Ti oxides, respectively. Two broad REE patterns are noticed in the Deccan Traps silicic rocks: a flat pattern for Barda, Alech, and Chogat-Chamardi silicic rocks, and a steep REE pattern for Osham, Rajula, Pavagadh, Rajpipla, and Bombay silicic rocks. Trace element modelling reveals that 5–10 % partial melting of a spinel peridotite source could produce an REE pattern and abundances similar to the associated basalts. Further extensive fractional crystallization (60–90 %) of the parental mafic melt at a deeper depth (where spinel is stable) could produce the REE composition and pattern observed in most silicic rocks except for those of Barda, Alech, and Chogat-Chamardi, which require fractional crystallization of the same parental melt at a shallower depth (where spinel is not stable). The geochemical variability of Deccan Traps silicic rocks reveals an origin from a mantle-derived parental mafic melt that evolved via the assimilation and fractional crystallization (AFC) process to form the silicic exposures, which is typical of silicic volcanism in other global CFBs.     

Quantitative field constraints on the dynamics of silicic magma chamber rejuvenation and overturn

The catastrophic eruption of large-volume, crystal-rich silicic magmas is often proposed to be a consequence of reheating, melting and overturn of partially molten, buoyant silicic material following repeated injection of dense, hot mafic magma. To test this “rejuvenation hypothesis”, we analyze at high spatial resolution 33 examples of deformed interfaces between intrusive mafic and silicic layers in two plutons of the Coastal Maine Magmatic Province, USA. These deformed interfaces are thought to record the buoyant overturn of silicic crystal mush layers, apparently in response to the injection and cooling of hot, dense mafic magmas. We use spectral analysis and scaling theory along with petrologic and textural data to identify, characterize, and understand periodic deformations from the scale of individual crystals (≈1 cm) to the thicknesses of mafic and silicic layers. Deformations at the largest scale lengths (>100 m) are at wavelengths comparable to, or greater than, silicic layer thicknesses and support a conjecture that mafic recharge can cause large-scale Rayleigh–Taylor-type overturning of silicic mushy layers. By contrast, the smallest scales of individual crystals probably record effects related to production and buoyancy-driven rise of melt from the tops of silicic mushes in contact with overlying hot basalt, whereas intermediate scales are explained by compaction. Our results constrain the evolution of a thermal rejuvenation event and potentially identify a condition for a large-scale overturn of the magma chamber that may lead to eruption. This work provides the first quantitative field-based constraints on some of the key physical processes inherent to the rejuvenation hypothesis.     

Possible correlation of Oligocene climate changes with Ethiopian Oligocene ignimbrite eruptions

The Oligocene Ethiopian continental flood basalt province (ca. 29–31 Ma) contains significant silicic pyroclastic rocks (>60,000 km³ constituting up to 20% of the volcanic stratigraphy). Rhyolitic tephras, synchronous with the Ethiopian silicic pyroclastic rocks, are found in Indian Ocean ODP holes 711A. They are geochemically akin to the Ethiopian silicic pyroclastic rocks. This suggests that the Indian Ocean tephras originated from Ethiopian silicic eruptions and represents more distal fallout of this volcanism. The temporal coincidence of the Ethiopian flood volcanism with the Oligocene global cooling event (Oi2?～?30.3 Ma) and the emplacement of the Ethiopian silicic pyroclastic eruptions on a near-global scale strongly suggest that the Ethiopian continental flood basalt province may have contributed or at least accelerated the climate change that was already underway.     

地壳硅质岩浆演化的动力学机制SCIEI北大核心CSCD

硅质岩浆的成因及演化机制研究是认识大陆地壳成分结构演化机制的关键途径。地壳硅质岩浆储库的基本性质及其演化的动力学过程是制约硅质岩浆演化及其多样性的根本原因,也是受到广泛关注的前沿和热点问题。本文总结了硅质岩浆储库的基本性质及其演化机制研究的相关进展,重点探讨了岩浆体系演化的动力学机制,即其物质成分及温度压力条件的动态演化过程,亦即岩浆储库在补给驱动下的存留、活化、分异、喷发过程。经由累积生长的方式形于地壳浅部的硅质岩浆储库以晶粥为主要赋存形式,具有较低的温度压力范围和高硅高结晶度高粘度的特征。岩浆储库的基本性质导致其自身不具有持续演化的能力,只有在基性岩浆补给的驱动下才能长时间存留分异或快速活化喷发。基性岩浆的补给及其携带的热及流体/挥发份深刻地改变了岩浆储库的物质成分及物理条件,是导致硅质岩浆分异演化或喷发的根本原因。本文还结合研究进展探讨了东南沿海地区中生代火山-侵入岩的成因演化机制及相关问题。     

Metallogenesis of Porphyry Cu Deposits of the Western Luzon Arc, Philippines: K-Ar ages, SO3 Contents of Microphenocrystic Apatite and Significance of Intrusive Rocks

Abstract: K–Ar ages of the following porphyry Cu deposits in the western Luzon arc are determined: Lobo-Boneng (10.50.4 Ma), Santo Niño (9.50.3 Ma), Black Mountain (2.10.1 Ma), Dizon (2.50.2 Ma) and Taysan (7.30.2 Ma). Microphenocrys-tic apatite in the late Cenozoic intermediate to silicic intrusions associated with porphyry Cu deposits in the western Luzon arc contains sulfur as SO₃ detectable by electron probe microanalyzer. Sulfur is supposed to have been accommodated dominantly as oxidized species in oxidizing hydrous magmas that generated porphyry Cu deposits. Likewise, such high SO₃ contents in microphenocrystic apatite are common characteristics of the intermediate to silicic magmatism of the western Luzon arc, from tonalitic rocks of the Luzon Central Cordillera of about 15 Ma to an active magmatism at Mount Pinatubo. Thus, the western Luzon arc has been generating porphyry Cu mineralization associated with oxidizing hydrous intermediate to silicic magmatism related to eastward subduction, since Miocene to the present day. Intermediate to silicic rocks since 15 Ma to present-day western Luzon arc generally show high whole-rock Sr/Y ratio ranging from 20 to 184. However, porphyry Cu deposit is not necessarily related to the rocks that show higher Sr/Y ratios compared to the other barren rocks in the western Luzon arc. The characteristics of the intermediate to silicic magma associated with porphyry Cu deposit are not attributed to the composition of the source material of the magma, but to the properties defined by the high activity of oxidized species of sulfur in the fluid phase that is encountered during the generation of intermediate to silicic magmas.     

Simple mixing as the major control of the evolution of volcanic suites in the Ecuadorian Andes

Examination of an extensive major and trace element database for about 700 whole rocks from the Ecuadorian Andes reveals series of local trends typified by three volcanoes: Iliniza and Pichincha from the Western Cordillera and Tungurahua from the Eastern Cordillera. These local trends are included in a more scattered global trend that reflects typical across-arc chemical variations. The scatter of the global trend is attributed to greater crustal contributions or decreasing melt fractions. Trace element modelling shows that the local trends are consistent with mixing, and not with any fractional crystallization or progressive melting dominated processes. These local trends are extendable to include samples from other Ecuadorian volcanoes, suggesting that mixing processes are dominant throughout the region. Mixing model using trace and major element analyses identifies two end-members: low-silica, basaltic and high-silica, dacitic magmas. It also shows that mixing occurred between magmas after their segregation, rather than earlier mixing between the solid sources prior to melting. As a consequence, there must exist efficient magma-mixing processes that can overcome the obstacles to mixing magmas with contrasting physical properties, and can produce series of hybrid liquids over regional-scale. Model calculations show that estimated silicic end-members are primary magmas and are not co-magmatic derivatives of the corresponding mafic end-members. Lavas of Ecuadorian volcanoes are likely originated from magmas of contrasting origins, such as basaltic magmas generated by fluxed melting of peridotites in the mantle wedge and dacitic, adakite-type magmas originating from the slab or the mafic lower crust.     

The Cold Bay Volcanic Center,Aleutian Volcanic Arc

The Cold Bay Volcanic Center has experienced two major stages of eruptive activity. Early (M-Series) acitivity produced bimodal Hi-Alumina basalt and calc-alkaline andesite lavas while later (FPK-Series) activity produced only calc-alkaline andesite. The spectrum of basalt compositions is believed to be due to high pressure (8 kb) fractionation at or near the base of the crust. Abundant mineralogical and geochemical evidence support a lower pressure mixing origin for all andesites. Inspection of the mineralogical data has shown that the earliest (M-Series) andesites were produced by mixing of basalt (<53 wt% SiO₂) and silicic andesite (60.5 to 62.5 wt%) while later (FPK-Series) andesites resulted from the mixing of basaltic-andesite (53 to 56 wt%) and less silicic andesite (58.5 to 60.0 wt%). The major element and trace element geochemical data are consistent with a low pressure fractionation origin for the silicic endmember magmas and support the temporal variations in both mafic and silicic endmember compositions. The complete lack of crustal inclusions in all lavas is taken as evidence for a minimal crustal melting and/or assimilation role in the origin of the silicic endmembers. Many of the features of all andesites, including the important long term convergence of endmember magma compositions, are consistent with the process of liquid fractionation, accompanied by large scale magma mixing. A deduced upper limit of 62.5 wt% SiO₂ for the silicic endmember magmas suggests that liquid fractionation, in the absence of major crustal melting, cannot produce more silicic magmas. A possible explanation is the presence of a rheological barrier, based on the concept of critical crystallinity (Marsh 1981), which prohibits more silicic liquids from being extracted from a crystal-liquid suspension.     

Generation of Porphyritic and Equigranular Mafic Enclaves During Magma Recharge Events at Unzen Volcano, Japan

Mafic to intermediate enclaves are evenly distributed throughoutthe dacitic 1991–1995 lava sequence of Unzen volcano,Japan, representing hundreds of mafic recharge events over thelife of the volcano. This study documents the morphological,textural, chemical, and petrological characteristics of theenclaves and coexisting silicic host lavas. The eruptive productsdescribed in this study appear to be general products of magmamingling, as the same textural types are seen at many othervolcanoes. Two types of magmatic enclaves, referred to as Porphyriticand Equigranular, are easily distinguished texturally. Porphyriticenclaves display a wide range in composition from basalt toandesite, are glass-rich, spherical and porphyritic, and containlarge, resorbed, plagioclase phenocrysts in a matrix of acicularcrystals and glass. Equigranular enclaves are andesitic, non-porphyritic,and consist of tabular, medium-grained microphenocrysts in amatrix glass that is in equilibrium with the host dacite magma.Porphyritic enclaves are produced when intruding basaltic magmaengulfs melt and phenocrysts of resident silicic magma at theirmutual interface. Equigranular enclaves are a product of a moreprolonged mixing and gradual crystallization at a slower coolingrate within the interior of the mafic intrusion. KEY WORDS: mafic enclaves; quenched mafic inclusions; magma mingling; Unzen volcano; Unzen Scientific Drilling Project; resorbed plagioclase     

Origins of cryptic variation in the Ediacaran–Fortunian rhyolitic ignimbrites of the Saldanha Bay Volcanic Complex,Western Cape,South Africa

The Saldanha eruption centre, on the West Coast of South Africa, consists of 542 Ma, intracaldera, S-type, rhyolite ignimbrites divided into the basal Saldanha Ignimbrite and the partly overlying Jacob’s Bay Ignimbrite. Depleted-mantle Nd model ages suggest magma sources younger than the Early Mesoproterozoic, and located within the Neoproterozoic Malmesbury Group and Swartland complex metasedimentary and metavolcanic rocks that form the regional basement. The Sr isotope systematics suggest that the dominant source rocks were metavolcaniclastic rocks and metagreywackes, and that the magmas formed from separate batches extracted from the same heterogeneous source. No apparent magma mixing trends relate the Saldanha to the Jacob’s Bay Ignimbrites, or either of these to the magmas that formed the Plankiesbaai or Tsaarsbank Ignimbrites in the neighbouring Postberg eruption centre. The magmas were extracted from their source rocks carrying small but significant proportions of peritectic and restitic accessory minerals. Variations in the content of this entrained crystal cargo were responsible for most of the chemical variations in the magmas. Although we cannot construct a cogent crystal fractionation model to relate these groups of magmas, at least some crystal fractionation occurred, as an overlay on the primary signal due to peritectic assemblage entrainment (PAE). Thus, the causes of the cryptic chemical variation among the ignimbrite magmas of the Saldanha centre are variable, but dominated by the compositions of the parent melts and PAE. The preservation of clear, source-inherited chemical signatures, in individual samples, calls into question the common interpretation of silicic calderas as having been formed in large magma reservoirs, with magma compositions shaped by magma mingling, mixing, and fractional crystallization. The Saldanha rocks suggest a more intimate connection between source and erupted magma, and perhaps indicate that silicic magmas are too viscous to be significantly modified by magma-chamber processes.