A kinetic model of metamorphism: an application to siliceous dolomites     

Andreas?LüttgeEmail author  Edward?W.?Bolton  Danny?M.?Rye 《Contributions to Mineralogy and Petrology》2004,146(5):546-565

We use a kinetic model of a metamorphic system to study the effect of competing rates of reaction, fluid injection, and heating on the evolution of the reaction pathway in temperature/composition space at constant pressure. We show that for rocks in contact with mixed volatile (e.g., CO₂-H₂O) fluids the reaction path may be quite different from what is expected from equilibrium-based petrologic models. Equilibrium-based models, used to understand the development of rock systems undergoing mineral reactions during a metamorphic event, rely on the Gibbs phase rule and only consider stable phases. For constant pressure, the temperature-composition paths follow univariant curves and significant reactions may occur at invariant points. By contrast, the more general kinetic treatment is not constrained by equilibrium, although with the proper competing rates equilibrium is a possible endmember of the kinetic approach. The deviation from equilibrium depends on the competing rates of reaction, heating, and fluid injection. A key element required by the kinetic approach is the inclusion of metastable reactions in the formulation, whereas such reactions are irrelevant for equilibrium-based models. Metastable reactions are often involved in a complex interplay with common prograde stable metamorphic reactions. We present model results for the well-studied CaO-MgO-SiO₂-CO₂-H₂O (CMS) system to show how the system evolves under kinetic control. Our simulations and discussion focus on the behavior of the CMS system under a number of closed and open system conditions. Special attention is paid to closed system behavior in the vicinity of the (first) isobaric invariant point (with Dol, Qtz, Tlc, Cal, and Tr). Also, for open systems with massive fluid infiltration we consider heating rates varying from contact to regional metamorphic conditions. For some geologically reasonable rates of reactions, heating, and fluid injection, our results demonstrate that equilibrium conditions may be significantly overstepped in metamorphic systems. We used overall mineral reactions in this model with rates based on experimental results. Future models could rely on more fundamental dissolution and precipitation reactions. Such an extension would require additional kinetic rate data, as well as mineral solubilities in mixed volatile fluids.Editorial responsibility: J. Hoefs  相似文献   

Mechanisms and rates of quartz dissolution in melts in the CMAS (CaO–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>) system     

Cliff?S.?J.?ShawEmail author 《Contributions to Mineralogy and Petrology》2004,148(2):180-200

The dissolution rate of minerals in silicate melts is generally assumed to be a function of the rate of mass transport of the released cations in the solvent. While this appears to be the case in moderately to highly viscous solvents, there is some evidence that the rate-controlling step may be different in very fluid, highly silica undersaturated melts such as basanites. In this study, convection-free experiments using solvent melts with silica activity from 0.185–0.56 and viscosity from 0.03–4.6 Pa s show that the dissolution rate is strongly dependent on the degree of superheating, silica activity and the viscosity of the solvent. Dissolution rates increase with increasing melt temperature and decreasing silica activity and viscosity. Quartz dissolution in melts with viscosity <0.59–1.9 Pa s and silica activity <0.47 is controlled by the rate of interface reaction as shown by the absence of steady state composition and silica saturation in the interface melts. Only in the most viscous melt with the highest silica activity is quartz dissolution controlled by the rate of diffusion in the melt and only after a long initiation time. The results of this study indicate that although a diffusion-based model may be applicable to dissolution in viscous magmas, a different approach that combines the interplay between the degree of undersaturation of the melt and its viscosity is required in very fluid melts.This revised version was published online September 2004 with a correction to Figure 8.  相似文献   

Melt extraction pathways and stagnation depths beneath the Madeira and Desertas rift zones (NE Atlantic) inferred from barometric studies     

Stefanie?SchwarzEmail author  Andreas?Klügel  Cora?Wohlgemuth-Ueberwasser 《Contributions to Mineralogy and Petrology》2004,147(2):228-240

The Madeira and Desertas Islands (eastern North Atlantic) show well-developed rift zones which intersect near the eastern tip of Madeira (São Lourenço peninsula). We applied fluid inclusion barometry and clinopyroxene-melt thermobarometry to reconstruct levels of magma stagnation beneath the two adjacent rifts and to examine a possible genetic relationship during their evolution. Densities of CO₂-dominated fluid inclusions in basanitic to basaltic samples from São Lourenço yielded frequency maxima at pressures of 0.57–0.87 GPa (23–29 km depth) and 0.25–0.32 GPa (8–10 km), whereas basanites, basalts and xenoliths from the Desertas indicate 0.3–0.72 GPa (10–24 km) and 0.07–0.12 GPa (2–3 km). Clinopyroxene-melt thermobarometry applied to Ti-augite phenocryst rim and glass/groundmass compositions indicates pressures of 0.45–1.06 GPa (15–35 km; São Lourenço) and 0.53–0.89 GPa (17–28 km; Desertas Islands) which partly overlap with pressures indicated by fluid inclusions. We interpret our data to suggest a multi-stage magma ascent beneath the Madeira Archipelago: main fractionation occurs at multiple levels within the mantle (>15 km depth) and is followed by temporary stagnation within the crust prior to eruption. Depths of crustal magma stagnation beneath São Lourenço and the Desertas differ significantly, and there is no evidence for a common shallow magma reservoir feeding both rift arms. We discuss two models to explain the relations between the two adjacent rift systems: Madeira and the Desertas may represent either a two-armed rift system or two volcanic centres with separate magma supply systems. For petrological and volcanological reasons, we favour the second model and suggest that Madeira and the Desertas root in distinct regions of melt extraction. Magma focusing into the Desertas system off the hotspot axis may result from lithospheric bending caused by the load of the Madeira and Porto Santo shields, combined with regional variations in melt production due to an irregularly shaped plume.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: J. Hoefs  相似文献   

Dynamics of magma mixing and degassing recorded in plagioclase at Stromboli (Aeolian Archipelago,Italy)   总被引：12，自引：0，他引：12  

Patrizia?LandiEmail author  Nicole?Métrich  Antonella?Bertagnini  Mauro?Rosi 《Contributions to Mineralogy and Petrology》2004,147(2):213-227

Crystal-rich materials (scoriae and lava flows) emitted during the 1985–2000 activity of Stromboli were taken into consideration for systematic study of bulk rock/matrix glass chemistry and in particular for the study of chemical and textural zoning of plagioclase, the most abundant mineral phase. Over the considered time period, bulk rock composition remained fairly constant in both major (SiO₂ 49.2–50.9 wt% and K₂O 1.96–2.18 wt%) and trace elements. The quite constant chemistry of matrix glasses also indicates that the degree of crystallization of magma was maintained at around 50 vol%. Plagioclase ranges in composition between An₆₂ and An₈₈ and is characterized by alternating, <10–100 m thick, bytownitic and labradoritic concentric layers, although the dominant and representative plagioclase of scoriae is An_68. The labradoritic layers (An_62–70) show small-scale (1–5 µm), oscillatory zoning, are free of inclusions, and appear to record episodes of slow crystal growth in equilibrium with a degassed liquid having the composition of the matrix glass. In contrast the bytownitic layers (An₇₀-An₈₈) are patchy zoned, show sieve structure with abundant micrometric glass inclusions and voids, and are attributed to rapid crystal growth.A key to understanding the origin of bytownitic layers can be retrieved from the texture and composition of the coronas of plagioclase xenocrysts, inherited from crystal-rich magma, in nearly aphyric pumice which are erupted during more energetic events and represent a deep, volatile-rich, HK-basaltic magma. They show a continuum from fine-sieve to evident skeletal texture from the inner to the outer part of the corona associated with normal compositional zoning from An₉₀ to An₇₅. In the light of these observations, we propose that input of H₂O-rich melt blobs, and their mixing with the residing magma, causes partial dissolution of the labradoritic layers followed by the growth of bytownitic composition whose sieve texture attests of rapid crystallization occurring under undercooling conditions mainly induced by degassing. As a whole, the zoning of plagioclase in the scoriae records successive and discrete intrusions of volatile-rich magma blobs, its degassing and mixing with the resident degassed magma at shallow level.Editorial responsibility: T.L. GroveAn erratum to this article can be found at  相似文献   

Kokchetavite: a new potassium-feldspar polymorph from the Kokchetav ultrahigh-pressure terrane     

Shyh-Lung?HwangEmail author  Pouyan?Shen  Hao-Tsu?Chu  Tzen-Fu?Yui  Juhn?G.?Liou  Nikolay?V.?Sobolev  Ru-Yuan?Zhang  Vladislav?S.?Shatsky  Anton?A.?Zayachkovsky 《Contributions to Mineralogy and Petrology》2004,148(3):380-389

Kokchetavite, a new polymorph of K-feldspar (KAlSi₃O₈), has been identified as micrometer-size inclusions in clinopyroxene and garnet in a garnet-pyroxene rock from the Kokchetav ultrahigh-pressure terrane, Kazakhstan. Kokchetavite has a hexagonal structure with a =5.27(1) Å, c=7.82(1) Å, V=188.09 ų, Z=1, and is found to be associated with phengite + /-cristobalite (or quartz) + siliceous glass ± phlogopite/titanite/calcite/zircon, occurring as multi-phase inclusions in clinopyroxene and garnet. It is concluded that kokchetavite could not be an exsolution phase in host minerals. Instead, it might be metastably precipitated from an infiltrated K-rich melt during rock exhumation. Alternatively, although less likely, kokchetavite might be derived from dehydration of K-cymrite, which, in turn, was formed at high pressures. In either case, kokchetavite is a metastable polymorph of K-feldspar.  相似文献   

Oxidized sulfur-rich mafic magma at Mount Pinatubo,Philippines   总被引：3，自引：2，他引：1  

J.?C.?M.?de?HoogEmail author  K.?H.?Hattori  R.?P.?Hoblitt 《Contributions to Mineralogy and Petrology》2004,146(6):750-761

Basaltic fragments enclosed in andesitic dome lavas and pyroclastic flows erupted during the early stages of the 1991 eruption of Mount Pinatubo, Philippines, contain amphiboles that crystallized during the injection of mafic magma into a dacitic magma body. The amphiboles contain abundant melt inclusions, which recorded the mixing of andesitic melt in the mafic magma and rhyolitic melt in the dacitic magma. The least evolved melt inclusions have high sulfur contents (up to 1,700 ppm) mostly as SO₄ ^2–, which suggests an oxidized state of the magma (NNO+1.4). The intrinsically oxidized nature of the mafic magma is confirmed by spinel–olivine oxygen barometry. The value is comparable to that of the dacitic magma (NNO+1.6). Hence, models invoking mixing as a means of releasing sulfur from the melt are not applicable to Pinatubo. Instead, the oxidized state of the dacitic magma likely reflects that of parental mafic magma and the source region in the sub-arc mantle. Our results fit a model in which long-lived SO₂ discharge from underplated mafic magma accumulated in the overlying dacitic magma and immiscible aqueous fluids. The fluids were the most likely source of sulfur that was released into the atmosphere during the cataclysmic eruption. The concurrence of highly oxidized basaltic magma and disproportionate sulfur output during the 1991 Mt. Pinatubo eruption suggests that oxidized mafic melt is an efficient medium for transferring sulfur from the mantle to shallow crustal levels and the atmosphere. As it can carry large amounts of sulfur, effectively scavenge sulfides from the source mantle and discharge SO₂ during ascent, oxidized mafic magma forms arc volcanoes with high sulfur fluxes, and potentially contributes to the formation of metallic sulfide deposits.Editorial responsibility: J. Hoefs

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J. C. M. de HoogEmail:

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Polycrystalline calcite to aragonite transformation kinetics: experiments in synthetic systems     

Shueh-Jung?Lin  Wuu-Liang?HuangEmail author 《Contributions to Mineralogy and Petrology》2004,147(5):604-614

The kinetics of the calcite to aragonite transformation have been investigated using synthetic polycrystalline calcite aggregates, with and without additional minerals present. The reaction progresses as a function of time were measured at four temperature/pressure conditions: (1) 550 °C/1.86 GPa; (2) 600 °C/2.11 GPa; (3) 650 °C/2.11 GPa, and (4) 700 °C/2.29 GPa. Experiments reveal that Mg-calcite and Fe-calcite transforms to aragonite at considerably slower rates than pure calcite, and that Sr-bearing calcite and calcite + quartz aggregates transform at significantly higher rates than pure calcite. The reaction progresses vs. time data for pure calcite were fitted to Cahns grain-boundary nucleation and interface-controlled growth model. Evidence for interface-controlled growth is provided by petrographic observations of grain boundaries. The activation energy for aragonite growth from the synthetic polycrystalline calcite determined in this study is significantly lower than that previously determined from a natural marble. The discrepancy in rates and activation energy may be attributed to the nature of grain boundaries, to deformational strain or the presence of impurities in the studied samples, and likely to uncertainties in experimental conditions. The results of this study imply that the variation of local petrologic conditions, in addition to temperature, pressure and grain size, may play an important role in determining the rates for the calcite to aragonite transformation in nature.Editorial responsibility: W. Schreyer  相似文献   

U–Pb and Hf-isotope analysis of zircons in mafic xenoliths from Fuxian kimberlites: evolution of the lower crust beneath the North China craton     

Jianping?Zheng  W.?L.?GriffinEmail author  Suzanne?Y.?O’Reilly  Fengxiang?Lu  Chunmei?Yu  Ming?Zhang  Huimin?Li 《Contributions to Mineralogy and Petrology》2004,148(1):79-103

Mafic xenoliths from the Paleozoic Fuxian kimberlites in the North China craton include garnet granulite, and minor pyroxene amphibolite, metagabbro, anorthosite and pyroxenite. The formation conditions of the amphibolites are estimated at 745–820 °C and 7.6–8.8 Kb (25–30 km); the granulites probably are derived from greater depths in the lower crust. LAM-ICPMS U–Pb dating of zircons from four granulites reveals multiple age populations, recording episodes of magmatic intrusion and metamorphic recrystallisation. Concordant ages and upper intercept ages, interpreted as minimum estimates for the time of magmatic crystallisation, range from 2,620 to 2,430 Ma in three granulites, two amphibolites and two metagabbros. Lower intercept ages, represented by near-concordant zircons, are interpreted as reflecting metamorphic recrystallisation, and range from 1,927 to 1,852 Ma. One granulite contains two metamorphic zircon populations, dated at 1,927±55 Ma and 600–700 Ma. Separated minerals from one granulite and one amphibolite yield Sm–Nd isochron ages of 1,619±48 Ma (¹⁴³Nd/¹⁴⁴Nd)i=0.51078), and 1,716±120 Ma (¹⁴³Nd/¹⁴⁴Nd)i=0.51006), respectively. These ages are interpreted as recording cooling following metamorphic resetting; model ages for both samples are in the range 2.40–2.66 Ga. LAM-MC-ICPMS analyses of zircon show a range in ¹⁷⁶Hf/¹⁷⁷Hf from 0.28116 to 0.28214, corresponding to a range of _Hf from –34 to +12. The relationships between ²⁰⁷Pb/²⁰⁶Pb age and _Hf show that: (1) the granulites, amphibolites and metagabbro were derived from a depleted mantle source at 2.6–2.75 Ga; (2) zircons in most samples underwent recrystallisation and Pb loss for 100–200 Ma after magmatic crystallisation, consistent with a residence in the lower crust; (3) metamorphic zircons in several samples represent new zircon growth, incorporating Hf liberated from breakdown of silicates with high Lu/Hf; (4) in other samples metamorphic and magmatic zircons have identical ¹⁷⁶Hf/¹⁷⁷Hf, and the younger ages reflect complete resetting of U–Pb systems in older zircons. The Fuxian mafic xenoliths are interpreted as the products of basaltic underplating, derived from a depleted mantle source in Neoarchean time, an important period of continental growth in the North China craton. Paleoproterozoic metamorphic ages indicate an important tectonic thermal event in the lower crust at 1.8–1.9 Ga, corresponding to the timing of collision between the Eastern and Western Blocks that led to the final assembly of the North China craton. The growth of metamorphic zircon at 600–700 Ma may record an asthenospheric upwelling in Neoproterozoic time, related to uplift and a regional disconformity in the North China craton.  相似文献   

Metasomatism and melting history of a Variscan lithospheric mantle domain: evidence from the Puy Beaunit xenoliths (French Massif Central)     

Olivier?FéméniasEmail author  Nicolas?Coussaert  Julien?Berger  Jean-Claude?C.?Mercier  Daniel?Demaiffe 《Contributions to Mineralogy and Petrology》2004,148(1):13-28

Mantle xenoliths from Puy Beaunit (French Massif Central) are compositionally varied, ranging from relatively fertile spinel lherzolites to refractory spinel dunites. Fertile peridotites have registered a modal (amphibole-bearing lherzolites) and cryptic metasomatic event that took place before the last Permian (257 Ma) melting episode. Depletion processes have been constrained by chemical modelling: the depletion is related to different degrees of partial melting, but two major melt extraction episodes are needed to explain the range of major element composition. The second event was responsible for the local large-scale dunitification of former residues. The first melting event (F25%) and metasomatic enrichment are attributed to an ancient fluid and/or liquid infiltration that could be related to a pre-Variscan regional subduction (located to the north of the Beaunit area). Texture acquisition and major deformation of the mantle xenoliths were sub-contemporaneous of the subduction and would result from lithospheric delamination. The second melting event (F17%) produced high-Mg basalts with calc-alkaline trace element signature that gave rise to the Permian underplating episode recognised in western Europe.  相似文献   

Intra-oceanic production of continental crust in a Th-depleted ca. 3.0 Ga arc complex,western Superior Province,Canada     

Joseph?B.?WhalenEmail author  John?A.?Percival  Vicki?J.?McNicoll  Frederick?J.?Longstaffe 《Contributions to Mineralogy and Petrology》2004,146(5):654-655

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