| 1. |
- Barker, Abigail, et al.
(författare)
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A 5 million year record of compositional variations in mantle sources to magmatism on Santiago, southern Cape Verde archipelago
- 2010
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 160:1, s. 133-154
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Tidskriftsartikel (refereegranskat)abstract
- High-precision Pb isotope data and Sr–Nd–Hf isotope data are presented together with major and trace element data for samples spanning the 4.6 Ma history of volcanism at Santiago, in the southern Cape Verde islands. Pb isotope data confirm the positive Δ8/4 signature of the southern islands indicating that the north–south compositional heterogeneity in the Cape Verde archipelago has persisted for at least 4.6 Ma. The Santiago volcanics show distinct compositional differences between the old, intermediate and young volcanics, and suggest greater involvement of an enriched mantle (EM1)-like source over time. Isotopic variations in the Santiago volcanics indicate convergence towards a homogeneous EM1-like end-member and distinct temporal variations in the FOZO-like end-member. Santiago and Santo Antão (a northern island, Holm et al. 2006), show a simultaneous decrease in 208Pb/204Pb of the high 206Pb/204Pb FOZO-like source with time. Such systematic archipelago-wide variations in the FOZO-like component suggest that this component is more likely to be present as a coherent package of recycled ocean crust rather than as multiple small heterogeneities dispersed in the upwelling mantle. The temporal variations in 208Pb/204Pb reflect minor lateral variations in Th/U of this recycled ocean crust package entering the melting zone beneath the islands. The location of the EM1-like component is more equivocal. A shallow lithospheric location is possible, but this would require a coincidence between spatial compositional variations in the lithosphere (EM1 is spatially restricted to the southern islands) and flow lines in the upwelling mantle revealed by seismic anisotropy. Therefore, we favour a deeper asthenospheric mantle source for the EM1-like source.
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| 2. |
- Barker, Abigail K., et al.
(författare)
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Magmatic evolution of the Cadamosto Seamount, Cape Verde : Beyond the spatial extent of EM1
- 2012
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 163:6, s. 949-965
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Tidskriftsartikel (refereegranskat)abstract
- The Cadamosto Seamount is an unusual volcanic centre from Cape Verde, characterised by dominantly evolved volcanics, in contrast to the typically mafic volcanic centres at Cape Verde that exhibit only minor volumes of evolved volcanics. The magmatic evolution of Cadamosto Seamount is investigated to quantify the role of magma-crust interaction and thus provide a perspective on evolved end-member volcanism of Cape Verde. The preservation of mantle source signatures by Nd-Pb isotopes despite extensive magmatic differentiation provides new insights into the spatial distribution of mantle heterogeneity in the Cape Verde archipelago. Magmatic differentiation from nephelinite to phonolite involves fractional crystallisation of clinopyroxene, titanite, apatite, biotite and feldspathoids, with extensive feldspathoid accumulation being recorded in some evolved samples. Clinopyroxene crystallisation pressures of 0.38-0.17 GPa for the nephelinites constrain this extensive fractional crystallisation to the oceanic lithosphere, where no crustal assimilants or rafts of subcontinental lithospheric mantle are available. In turn, magma-crust interaction has influenced the Sr, O and S isotopes of the groundmass and late crystallising feldspathoids, which formed at shallow crustal depths reflecting the availability of oceanic sediments and anhydrite precipitated in the ocean crust. The Nd-Pb isotopes have not been affected by these processes of magma-crust interaction and hence preserve the mantle source signature. The Cadamosto Seamount samples have high Pb-206/Pb-204 (> 19.5), high epsilon Nd (+6 to +7) and negative Delta 8/4Pb, showing affinity with the northern Cape Verde islands as opposed to the adjacent southern islands. Hence, the Cadamosto Seamount in the west is located spatially beyond the EM1-like component found further east. This heterogeneity is not encountered in the oceanic lithosphere beneath the Cadamosto Seamount despite greater extents of fractional crystallisation at oceanic lithospheric depths than the islands of Fogo and Santiago. Our data provide new evidence for the complex geometry of the chemically zoned Cape Verde mantle source.
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| 3. |
- Barker, Abigail, et al.
(författare)
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The role of eclogite in the mantle heterogeneity at Cape Verde
- 2014
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 168:3, s. 1052-
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Tidskriftsartikel (refereegranskat)abstract
- The Cape Verde hotspot, like many other Ocean Island Basalt provinces, demonstrates isotopic heterogeneity on a 100–200 km scale. The heterogeneity is represented by the appearance of an EM1-like component at several of the southern islands and with a HIMU-like component present throughout the archipelago. Where the EM1-like component is absent, a local DMM-like component replaces the EM1-like component. Various source lithologies, including peridotite, pyroxenite and eclogite have been suggested to contribute to generation of these heterogeneities; however, attempts to quantify such contributions have been limited. We apply the minor elements in olivine approach (Sobolev et al. in Nature 434:590–597, 2005; Science, doi:10.1126/science.1138113,2007), to determine and quantify the contributions of peridotite, pyroxenite and eclogite melts to the mantle heterogeneity observed at Cape Verde. Cores of olivine phenocrysts of the Cape Verde volcanics have low Mn/FeO and low Ni*FeO/MgO that deviate from the negative trend of the global array. The global array is defined by mixing between peridotite and pyroxenite, whereas the Cape Verde volcanics indicate contribution of an additional eclogite source. Eclogite melts escape reaction with peridotite either by efficient extraction in an area of poor mantle flow or by reaction of eclogite melts with peridotite, whereby an abundance of eclogite can seal off the melt from further reaction. Temporal trends of decreasing Mn/FeO indicate that the supply of eclogite melts is increasing. Modelling suggests the local DMM-like end-member is formed from a relatively peridotite-rich melt, while the EM1-like end-member has a closer affinity to a mixed peridotite–pyroxenite–eclogite melt. Notably the HIMU-like component ranges from pyroxenite–peridotite-rich melt to one with up to 77 % eclogite melt as a function of time, implying that sealing of melt pathways is becoming more effective.
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| 4. |
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| 5. |
- Brander, Linus, 1978, et al.
(författare)
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Brittle-plastic deformation in initially dry rocks at fluid-present conditions : transient behaviour of feldspar at mid-crustal levels
- 2012
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 163:3, s. 403-425
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Tidskriftsartikel (refereegranskat)abstract
- We present detailed microstructural and chemical analyses from an initially dry anorthositic rock deformed during wet amphibolite facies conditions. Three different domains representing the microstructural variation of the deformed samples are investigated in detail in terms of fracture morphology and mode, grain characteristics and chemistry of present phases. Results show transient deformational behaviour where a close interaction between brittle, plastic and fluid-assisted deformation mechanisms can be observed. Our analysis allows us to describe the succession, interrelationships and effects of active mechanisms with progressively increasing strain in three so-called stages. In Stage 1, initial fracturing along cleavage planes promoted fluid influx that caused fragmentation and chemical reactions, producing fine-grained mineral assemblages in the fractures. Deformation twins and dislocations developed in clast pieces due to stress relaxation. Passive rotation of conjugate fracture sets and interconnection of intracrystalline fractures formed micro-shear-zones, constituting Stage 2. Microstructures and grain relationships indicate the activity and fluctuation between fracturing, dissolution-precipitation creep, grain boundary sliding and locally dislocation creep, reflecting the transient behaviour of brittle and plastic deformation mechanisms. Further rotation and widening of fractures into overall foliation parallel shear-bands (Stage 3) promoted strain partitioning into these areas through increased fluid influx, influence of fluid-assisted grain boundary sliding, phase mixing and presence of weak phases such as white mica. We suggest that local differences in fluid availability, volume fraction of weak phases produced by fluid present metamorphic reactions coupled with volume increase and local variations in stress concentration induced transient brittle-plastic behaviour. The studied shear-zone represents an example of the transformation of a rigid dry rock to a soft wet rock during deformation through syntectonic fracturing.
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| 6. |
- Chadwick, J.P., et al.
(författare)
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Petrology and geochemistry of igneous inclusions in recent Merapi deposits : a window into the sub-volcanic plumbing system
- 2013
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 165:2, s. 259-282
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Tidskriftsartikel (refereegranskat)abstract
- Recent basaltic-andesite lavas from Merapi volcano contain abundant and varied igneous inclusions suggesting a complex sub-volcanic magmatic system for Merapi volcano. In order to better understand the processes occurring beneath Merapi, we have studied this suite of inclusions by petrography, geochemistry and geobarometric calculations. The inclusions may be classified into four main suites: (1) highly crystalline basaltic-andesite inclusions, (2) co-magmatic enclaves, (3) plutonic crystalline inclusions and (4) amphibole megacrysts. Highly crystalline basaltic-andesite inclusions and co-magmatic enclaves typically display liquid–liquid relationships with their host rocks, indicating mixing and mingling of distinct magmas. Co-magmatic enclaves are basaltic in composition and occasionally display chilled margins, whereas highly crystalline basaltic-andesite inclusions usually lack chilling. Plutonic inclusions have variable grain sizes and occasionally possess crystal layering with a spectrum of compositions spanning from gabbro to diorite. Plagioclase, pyroxene and amphibole are the dominant phases present in both the inclusions and the host lavas. Mineral compositions of the inclusions largely overlap with compositions of minerals in recent and historic basaltic-andesites and the enclaves they contain, indicating a cognate or ‘antelithic’ nature for most of the plutonic inclusions. Many of the plutonic inclusions plot together with the host basaltic-andesites along fractional crystallisation trends from parental basalt to andesite compositions. Results for mineral geobarometry on the inclusions suggest a crystallisation history for the plutonic inclusions and the recent and historic Merapi magmas that spans the full depth of the crust, indicating a multi-chamber magma system with high amounts of semi-molten crystalline mush. There, crystallisation, crystal accumulation, magma mixing and mafic recharge take place. Comparison of the barometric results with whole rock Sr, Nd, and Pb isotope data for the inclusions suggests input of crustal material as magma ascends from depth, with a significant late addition of sedimentary material from the uppermost crust. The type of multi-chamber plumbing system envisaged contains large portions of crystal mush and provides ample opportunity to recycle the magmatic crystalline roots as well as interact with the surrounding host lithologies.
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| 7. |
- Dahrén, Börje, et al.
(författare)
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Magma plumbing beneath Anak Krakatau volcano, Indonesia : evidence for multiple magma storage regions
- 2012
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 163:4, s. 631-651
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Tidskriftsartikel (refereegranskat)abstract
- Understanding magma plumbing is essential for predicting the behaviour of explosive volcanoes. We investigate magma plumbing at the highly active Anak Krakatau volcano (Indonesia), situated on the rim of the 1883 Krakatau caldera by employing a suite of thermobarometric models. These include clinopyroxene-melt thermobarometry, plagioclase-melt thermobarometry, clinopyroxene composition barometry and olivine-melt thermometry. Petrological studies have previously identified shallow magma storage in the region of 2–8 km beneath Krakatau, while existing seismic evidence points towards mid- to deep-crustal storage zone(s), at 9 and 22 km, respectively. Our results show that clinopyroxene in Anak Krakatau lavas crystallized at a depth of 7–12 km, while plagioclase records both shallow crustal (3–7 km) and sub-Moho (23–28 km) levels of crystallization. These magma storage regions coincide with well-constrained major lithological boundaries in the crust, implying that magma ascent and storage at Anak Krakatau is strongly controlled by crustal properties. A tandem seismic tomography survey independently identified a separate upper crustal (<7 km) and a lower to mid-crustal magma storage region (>7 km). Both petrological and seismic methods are sensitive in detecting magma bodies in the crust, but suffer from various limitations. Combined geophysical and petrological surveys, in turn, offer increased potential for a comprehensive characterization of magma plumbing at active volcanic complexes.
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| 8. |
- Högdahl, Karin, et al.
(författare)
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Reactive monazite and robust zircon growth in diatexitesand leucogranites from a hot, slowly cooled orogen : implicationsfor the Palaeoproterozoic tectonic evolution of the central Fennoscandian Shield, Sweden
- 2012
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 163:1, s. 167-188
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Tidskriftsartikel (refereegranskat)abstract
- Monazite in melt-producing, poly-metamorphic terranes can grow, dissolve or reprecipitate at different stages during orogenic evolution particularly in hot, slowly cooling orogens such as the Svecofennian. Owing to the high heat flow in such orogens, small variations in pressure, temperature or deformation intensity may promote a mineral reaction. Monazite in diatexites and leucogranites from two Svecofennian domains yields older, coeval and younger U–Pb SIMS and EMP ages than zircon from the same rock. As zircon precipitated during the melt-bearing stage, its U–Pb ages reflect the timing of peak metamorphism, which is associated with partial melting and leucogranite formation. In one of the domains, the Granite and Diatexite Belt, zircon ages range between 1.87 and 1.86 Ga, whereas monazite yields two distinct double peaks at 1.87–1.86 and 1.82–1.80 Ga. The younger double peak is related to monazite growth or reprecipitation during subsolidus conditions associated with deformation along late-orogenic shear zones. Magmatic monazite in leucogranite records systematic variations in composition and age during growth that can be directly linked to Th/U ratios and preferential growth sites of zircon, reflecting the transition from melt to melt crystallisation of the magma. In the adjacent Ljusdal Domain, peak metamorphism in amphibolite facies occurred at 1.83–1.82 Ga as given by both zircon and monazite chronology. Pre-partial melting, 1.85 Ga contact metamorphic monazite is preserved, in spite of the high-grade overprint. By combining structural analysis, petrography and monazite and zircon geochronology, a metamorphic terrane boundary has been identified. It is concluded that the boundary formed by crustal shortening accommodated by major thrusting.
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| 9. |
- Jeffery, Adam J., et al.
(författare)
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The pre-eruptive magma plumbing system of the 2007–2008 dome-forming eruption of Kelut volcano, East Java, Indonesia
- 2013
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 166:1, s. 275-308
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Tidskriftsartikel (refereegranskat)abstract
- Kelut volcano, East Java, is an active volcanic complex hosting a summit crater lake that has been the source of some of Indonesia’s most destructive lahars. In November 2007, an effusive eruption lasting approximately 7 months led to the formation of a 260-m-high and 400-m-wide lava dome that displaced most of the crater lake. The 2007–2008 Kelut dome comprises crystal-rich basaltic andesite with a texturally complex crystal cargo of strongly zoned and in part resorbed plagioclase (An47–94), orthopyroxene (En64–72, Fs24–32, Wo2–4), clinopyroxene (En40–48, Fs14–19, Wo34–46), Ti-magnetite (Usp16–34) and trace amounts of apatite, as well as ubiquitous glomerocrysts of varying magmatic mineral assemblages. In addition, the notable occurrence of magmatic and crustal xenoliths (meta-basalts, amphibole-bearing cumulates, and skarn-type calc-silicates and meta-volcaniclastic rocks) is a distinct feature of the dome. New petrographical, whole rock major and trace element data, mineral chemistry as well as oxygen isotope data for both whole rocks and minerals indicate a complex regime of magma-mixing, decompression-driven resorption, degassing and crystallisation and crustal assimilation within the Kelut plumbing system prior to extrusion of the dome. Detailed investigation of plagioclase textures alongside crystal size distribution analyses provide evidence for magma mixing as a major pre-eruptive process that blends multiple crystal cargoes together. Distinct magma storage zones are postulated, with a deeper zone at lower crustal levels or near the crust-mantle boundary (>15 km depth), a second zone at mid-crustal levels (~10 km depth) and several magma storage zones distributed throughout the uppermost crust (<10 km depth). Plagioclase-melt and amphibole hygrometry indicate magmatic H2O contents ranging from ~8.1 to 8.6 wt.% in the lower crustal system to ~1.5 to 3.3 wt.% in the mid to upper crust. Pyroxene and plagioclase δ18O values range from 5.4 to 6.7 ‰, and 6.5 to 7.6 ‰, respectively. A single whole rock analysis of the 2007–2008 dome lava gave a δ18O value of 7.6 ‰, whereas meta-basaltic and calc-silicate xenoliths are characterised by δ18O values of 6.2 and 10.3 ‰, respectively. Magmatic δ18O values calculated from individual pyroxene and plagioclase analyses range from 5.7 to 7.0 ‰, and 6.2 to 7.4 ‰, respectively. This range in O-isotopic compositions is explained by crystallisation of pyroxenes in the lower to mid-crust, where crustal contamination is either absent or masked by assimilation of material having similar δ18O values to the ascending melts. This population is mixed with isotopically distinct plagioclase and pyroxenes that crystallised from a more contaminated magma in the upper crustal system. Binary bulk mixing models suggest that shallow-level, recycled volcaniclastic sedimentary rocks together with calc-silicates and/or limestones are the most likely contaminants of the 2007–2008 Kelut magma, with the volcaniclastic sediments being dominant.
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| 10. |
- Jolis, Ester Muñoz, et al.
(författare)
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Experimental simulation of magma-carbonate interaction beneath Mt. Vesuvius, Italy
- 2013
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 166:5, s. 1335-1353
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Tidskriftsartikel (refereegranskat)abstract
- We simulated the process of magma-carbonate interaction beneath Mt. Vesuvius in short duration piston-cylinder experiments under controlled magmatic conditions (from 0 to 300 s at 0.5 GPa and 1,200 A degrees C), using a Vesuvius shoshonite composition and upper crustal limestone and dolostone as starting materials. Backscattered electron images and chemical analysis (major and trace elements and Sr isotopes) of sequential experimental products allow us to identify the textural and chemical evolution of carbonated products during the assimilation process. We demonstrate that melt-carbonate interaction can be extremely fast (minutes), and results in dynamic contamination of the host melt with respect to Ca, Mg and Sr-87/Sr-86, coupled with intense CO2 vesiculation at the melt-carbonate interface. Binary mixing between carbonate and uncontaminated melt cannot explain the geochemical variations of the experimental charges in full and convection and diffusion likely also operated in the charges. Physical mixing and mingling driven by exsolving volatiles seems to be a key process to promote melt homogenisation. Our results reinforce hypotheses that magma-carbonate interaction is a relevant and ongoing process at Mt. Vesuvius and one that may operate not only on a geological, but on a human timescale.
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| 11. |
- Menegon, Luca, et al.
(författare)
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The effect of Dauphin, twinning on plastic strain in quartz
- 2011
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 161:4, s. 635-652
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Tidskriftsartikel (refereegranskat)abstract
- We present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies (T = 300-400A degrees C) granitoid protomylonite from the Arolla unit of the NW Alps. Mechanical Dauphin, twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane {z} almost orthogonal to the compression direction (z-twin orientation). Twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb {r} nearly parallel to the compression direction (r-twin orientation). In contrast, we report the lack of twinning in two porphyroclasts already oriented with one of the {r} planes orthogonal to the compression direction. One twinned porphyroclast has been investigated with more detail. It shows the localization of much of the plastic strain into discrete r-twins as a consequence of the higher amount of elastic strain energy stored by r-twins in comparison to z-twins. The presence of Dauphin, twins induced a switch in the dominant active slip systems during plastic deformation, from basal < a > (regions without twinning) to {pi} and {pi'} < a > (pervasively twinned regions). Dynamic recrystallization is localized along an r-twin and occurred dominantly by progressive subgrain rotation, with a local component of bulging recrystallization. Part of the recrystallized grains underwent rigid-body rotation, approximately about the bulk vorticity axis, which accounts for the development of large misorientation angles. The recrystallized grain size piezometer for quartz yields differential stress of 100 MPa. The comparison of this palaeostress estimate with literature data suggests that mechanical Dauphin, twinning could have a potential use as palaeopiezometer in quartz-bearing rocks.
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| 12. |
- Muir, Duncan D., et al.
(författare)
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Petrological imaging of an active pluton beneath Cerro Uturuncu, Bolivia
- 2014
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 167:3, s. 980-
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Tidskriftsartikel (refereegranskat)abstract
- Uturuncu is a dormant volcano in the Altiplano of SW Bolivia. A present day similar to 70 km diameter interferometric synthetic aperture radar (InSAR) anomaly roughly centred on Uturuncu's edifice is believed to be a result of magma intrusion into an active crustal pluton. Past activity at the volcano, spanning 0.89 to 0.27 Ma, is exclusively effusive and almost all lavas and domes are dacitic with phenocrysts of plagioclase, orthopyroxene, biotite, ilmenite and Ti-magnetite plus or minus quartz, and microlites of plagioclase and orthopyroxene set in rhyolitic groundmass glass. Plagioclasehosted melt inclusions (MI) are rhyolitic with major element compositions that are similar to groundmass glasses. H2O concentrations plotted versus incompatible elements for individual samples describe a trend typical of near-isobaric, volatile-saturated crystallisation. At 870 degrees C, the average magma temperature calculated from Fe-Ti oxides, the average H2O of 3.2 +/- 0.7 wt% and CO2 typically <160 ppm equate to MI trapping pressures of 50-120 MPa, approximately 2-4.5 km below surface. Such shallow storage precludes the role of dacite magma emplacement into preeruptive storage regions as being the cause of the observed InSAR anomaly. Storage pressures, whole-rock (WR) chemistry and phase assemblage are remarkably consistent across the eruptive history of the volcano, although magmatic temperatures calculated from Fe-Ti oxide geothermometry, zircon saturation thermometry using MI and orthopyroxenemelt thermometry range from 760 to 925 degrees C at NNO +/- 1 log. This large temperature range is similar to that of saturation temperatures of observed phases in experimental data on Uturuncu dacites. The variation in calculated temperatures is attributed to piecemeal construction of the active pluton by successive inputs of new magma into a growing volume of plutonic mush. Fluctuating temperatures within the mush can account for sieve-textured cores and complex zoning in plagioclase phenocrysts, resorption of quartz and biotite phenocrysts and apatite microlites. That Fe-Ti oxide temperatures vary by similar to 50-100 degrees C in a single thin section indicates that magmas were not homogenised effectively prior to eruption. Phenocryst contents do not correlate with calculated magmatic temperatures, consistent with crystal entrainment from the mush during magma ascent and eruption. Microlites grew during ascent from the magma storage region. Variability in the proportion of microlites is attributed to differing ascent and effusion rates with faster rates in general for lavas >0.5 Ma compared to those <0.5 Ma. High microlite contents of domes indicate that effusion rates were probably slowest in dome-forming eruptions. Linear trends in WR major and trace element chemistries, highly variable, bimodal mineral compositions, and the presence of mafic enclaves in lavas demonstrate that intrusion of more mafic magmas into the evolving, shallow plutonic mush also occurred further amplifying local temperature fluctuations. Crystallisation and resorption of accessory phases, particularly ilmenite and apatite, can be detected in MI and groundmass glass trace element covariation trends, which are oblique to WRs. Marked variability of Ba, Sr and La in MI can be attributed to temperature-controlled, localised crystallisation of plagioclase, orthopyroxene and biotite within the evolving mush.
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| 13. |
- Svahnberg, Henrik, et al.
(författare)
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Interaction of chemical and physical processes during deformation at fluid-present conditions : a case study from an anorthosite leucogabbro deformed at amphibolite facies conditions
- 2013
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 165:3, s. 543-562
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Tidskriftsartikel (refereegranskat)abstract
- We present microstructural and chemical analyses of chemically zoned and recrystallized plagioclase grains in variably strained samples of a naturally deformed anorthosite-leucogabbro, southern West Greenland. The recorded microstructures formed in the presence of fluids at mid-crustal conditions (620-640 A degrees C, 7.4-8.6 kbar). Recrystallized plagioclase grains (average grain size 342 mu m) with a random crystallographic orientation are volumetrically dominant in high-strain areas. They are characterized by asymmetric chemical zoning (An(80) cores and An(64) rims) that are directly associated with areas exhibiting high amphibole content and phase mixing. Analyses of zoning indicate anisotropic behaviour of bytownite plagioclase with a preferred replacement in the direction and along the (001) plane. In areas of high finite strain, recrystallization of plagioclase dominantly occurred by bulging recrystallization and is intimately linked to the chemical zoning. The lack of CPO as well as the developed asymmetric zoning can be explained by the activity of grain boundary sliding accommodated by dissolution and precipitation creep (DPC). In low-strain domains, grain size is on average larger and the rim distribution is not related to the inferred stress axes indicating chemically induced grain replacement instead of stress-related DPC. We suggest that during deformation, in high-strain areas, pre-existing phase mixture and stress induced DPC-caused grain rotations that allowed a deformation-enhanced heterogeneous fluid influx. This resulted in local plagioclase replacement through interface-coupled dissolution and precipitation and chemically induced grain boundary migration, accompanied by bulging recrystallization, along with neocrystallization of other phases. This study illustrates a strong interaction and feedback between physical and chemical processes where the amount of stress and fluids dictates the dominant active process. The interaction is a cause of deformation and external fluid infiltration with a result of strain localization and chemical re-equilibration at amphibolite facies conditions.
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| 14. |
- van der Zwan, Froukje M., et al.
(författare)
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Textural history of recent basaltic-andesites and plutonic inclusions from Merapi volcano
- 2013
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 166:1, s. 43-63
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Tidskriftsartikel (refereegranskat)abstract
- Mt. Merapi in Central Java is one of the most active stratovolcanoes on Earth and is underlain by a multistage plumbing system. Crystal size distribution analyses (CSD) were carried out on recent Merapi basaltic-andesites and co-eruptive magmatic and plutonic inclusions to characterise the crystallisation processes that operate during storage and ascent and to obtain information on respective time scales. The basaltic-andesites exhibit log-linear, kinked-upwards CSD curves for plagioclase and clinopyroxene that can be separated into two main textural populations. Large plagioclase phenocrysts (a parts per thousand yen1.6 mm) make up one population, but correspond to crystals with variable geochemical composition and reflect a period of crystal growth at deep to mid-crustal levels. This population was subsequently influenced by crystal accumulation and the onset of crustal assimilation, including the incorporation of high-Ca skarn-derived xenocrysts. Textural re-equilibration is required for these crystals to form a single population in CSD. A second episode of crystal growth at shallower levels is represented by chemically homogenous plagioclase crystals < 1.6 mm in size. Crustal assimilation is indicated by, for example, oxygen isotopes and based on the CSD data, crystallisation combined with contamination is likely semi-continuous in these upper crustal storage chambers. The CSD data observed in the basaltic-andesite samples are remarkably consistent and require a large-volume steady state magmatic system beneath Merapi in which late textural equilibration plays a significant role. Plagioclase CSDs of co-eruptive magmatic and plutonic inclusions may contain a third crystal population (< 1 mm) not found in the lavas. This third population has probably formed from enhanced degassing of portions of basaltic-andesite magma at shallow crustal levels which resulted in increased crystallinity and basaltic-andesite mush inclusions. A suite of coarse plutonic inclusions is also present that reflects crystallisation and accumulation of crystals in the deep Merapi plumbing system, as deduced from CSD patterns and mineral assemblages.
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| 15. |
- Whitehouse, Martin, 1962-, et al.
(författare)
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Behaviour of radiogenic Pb in zircon during ultrahigh-temperature metamorphism: an ion imaging and ion tomography case study from the Kerala Khondalite Belt, southern India
- 2014
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 168:2, s. 1042-1042
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Tidskriftsartikel (refereegranskat)abstract
- Zircon crystals from a locally charnockitized Paleoproterozoic high-K metagranite from the Kerala Khondalite Belt (KKB) of southern India have been investigated by high-spatial resolution secondary ion mass spectrometry analysis of U-Th-Pb and rare earth elements (RE), together with scanning ion imaging and scanning ion tomography (depth-profiled ion imaging). The spot analyses constrain the magmatic crystallization age of the metagranite to ca. 1,850 Ma, with ultrahigh-temperature (UHT) metamorphism occurring at ca. 570 Ma and superimposed charnockite formation at ca. 520-510 Ma, while the ion imaging reveals a patchy distribution of radiogenic Pb throughout the zircon cores. Middle-to heavy-RE depletion in ca. 570 Ma zircon rims suggests that these grew in equilibrium with garnet and therefore date the UHT metamorphism in the KKB. The maximum apparent Pb-207/Pb-206 age obtained from the unsupported radiogenic Pb concentrations is also consistent with formation of the Pb patches during this event. The superimposed charnockitization event appears to have caused additional Pb-loss in the cores and recrystallization of the rims. The results of depth-profiling of the scanning ion tomography image stack show that the Pb-rich domains range in size from <5 nm to several 10 nm (diameter if assumed to be spherical). The occurrence of such patchy Pb has previously been documented only from UHT metamorphic zircon, where it likely results from annealing of radiation-damaged zircon. The formation of a discrete, heterogeneously distributed and subsequently immobile Pb phase effectively arrests the normal Pb-loss process seen at lower grades of metamorphism.
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| 16. |
- Wiesmaier, Sebastian, et al.
(författare)
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Magma mixing in the 1100 AD Montaña Reventada composite lava flow, Tenerife, Canary Islands : Interaction between rift zone and central volcano plumbing systems
- 2011
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer. - 0010-7999 .- 1432-0967. ; 162:3, s. 651-669
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Tidskriftsartikel (refereegranskat)abstract
- Zoned eruption deposits commonly show a lower felsic and an upper mafic member, thought to reflect eruption from large, stratified magma chambers. In contrast, the Montaña Reventada composite flow (Tenerife) consists of a lower basanite and a much thicker upper phonolite. A sharp interface separates basanite and phonolite, and chilled margins at this contact indicate the basanite was still hot upon emplacement of the phonolite, i.e. the two magmas erupted in quick succession. Four types of mafic to intermediate inclusions are found in the phonolite. Inclusion textures comprise foamy quenched ones, others with chilled margins and yet others that are physically mingled, reflecting progressive mixing with a decreasing temperature contrast between the end-members. Analysis of basanite, phonolite and inclusions for majors, traces and Sr, Nd and Pb isotopes show the inclusions to be derived from binary mixing of basanite and phonolite end-members in ratios of 2:1 to 4:1. Although, basanite and phonolite magmas were in direct contact, contrasting 206Pb/204Pb ratios show that they are genetically distinct (19.7193(21)–19.7418(31) vs. 19.7671(18)–19.7807(23), respectively). We argue that the Montaña Reventada basanite and phonolite first met just prior to eruption and had limited interaction time only. Montaña Reventada erupted from the transition zone between two plumbing systems, the phonolitic Teide-Pico Viejo complex and the basanitic Northwest rift zone. A rift zone basanite dyke most likely intersected the previously emplaced phonolite magma chamber. This led to eruption of geochemically and texturally unaffected basanite, with the inclusion-rich phonolite subsequently following into the established conduit.
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| 17. |
- Triebold, S., et al.
(författare)
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Discrimination of TiO2 polymorphs in sedimentary and metamorphic rocks
- 2011
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Ingår i: Contributions to Mineralogy and Petrology. - 0010-7999. ; 161:4, s. 581-596
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Tidskriftsartikel (refereegranskat)abstract
- Investigation by Raman spectroscopy of samples from different geological settings shows that the occurrence of TiO2 polymorphs other than rutile can hardly be predicted, and furthermore, the occurrence of anatase is more widespread than previously thought. Metamorphic pressure and temperature, together with whole rock chemistry, control the occurrence of anatase, whereas variation of mineral assemblage characteristics and/or fluid occurrence or composition takes influence on anatase trace element characteristics and re-equilibration of relict rutiles. Evaluation of trace element contents obtained by electron microprobe in anatase, brookite, and rutile shows that these vary significantly between the three TiO2 phases. Therefore, on the one hand, an appropriation to source rock type according to Nb and Cr contents, but as well application of thermometry on the basis of Zr contents, would lead to erroneous results if no phase specification is done beforehand. For the elements Cr, V, Fe, and Nb, variation between the polymorphs is systematic and can be used for discrimination on the basis of a linear discriminant analysis. Using phase group means and coefficients of linear discriminants obtained from a compilation of analyses from samples with well-defined phase information together with prior probabilities of groupings from a natural sample compilation, one is able to calculate phase grouping probabilities of any TiO2 analysis containing at least the critical elements Cr, V, Fe, and Nb. An application of this calculation shows that for the appropriation to the phase rutile, a correct-classification rate of 99.5% is obtained. Hence, phase specification by trace elements proves to be a valuable tool besides Raman spectroscopy.
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| 18. |
- Zack, Thomas, 1968, et al.
(författare)
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In situ U-Pb rutile dating by LA-ICP-MS: Pb-208 correction and prospects for geological applications
- 2011
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Ingår i: Contributions to Mineralogy and Petrology. - 0010-7999. ; 162:3, s. 515-530
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Tidskriftsartikel (refereegranskat)abstract
- Rutile is a common accessory mineral that occurs in a wide spectrum of metamorphic rocks, such as in blueschists, eclogites, and granulites and as one of the most stable detrital heavy minerals in sedimentary rocks. The advent of rutile trace element thermometry has generated increased interest in a better understanding of rutile formation. This study documents important analytical advances in in situ LA-ICP-MS U/Pb geochronology of rutile: (1) Matrix matching, necessary for robust in situ dating is fulfilled by calibrating and testing several rutile standards (R10, R19, WH-1), including the presentation of new TIMS ages for the rutile standard R19 (489.5 +/- A 0.9 Ma; errors always stated as 2 s). (2) Initial common lead correction is routinely applied via Pb-208, which is possible due to extremely low Th/U ratios (usually < 0.003) in most rutiles. Employing a 213 nm Nd:YAG laser coupled to a quadrupole ICP-MS and using R10 as a primary standard, rutile U/Pb concordia ages for the two other rutile standards (493 +/- A 10 Ma for R19; 2640 +/- A 50 Ma for WH-1) and four rutile-bearing metamorphic rocks (181 +/- A 4 Ma for Ivrea metapelitic granulite; 339 +/- A 7 Ma for Saidenbach coesite eclogite; 386 +/- A 8 Ma for Fjortoft UHP metapelite; 606 +/- A 12 Ma for Andrelandia metepelitic granulite) always agree within 2% with the reported TIMS ages and other dating studies from the same localities. The power of in situ U/Pb rutile dating is illustrated by comparing ages of detrital rutile and zircon from a recent sediment from the Christie Domain of the Gawler Craton, Australia. While the U/Pb age spectrum from zircons show several pronounced peaks that are correlated with magmatic episodes, rutile U/Pb ages are marked by only one pronounced peak (at ca 1,675 Ma) interpreted to represent cooling ages of this part of the craton. Rutile thermometry of the same detrital grains indicates former granulite-facies conditions. The methods outlined in this paper should find wide application in studies that require age information of single spots, e.g., provenance studies, single-crystal zoning and texturally controlled dating.
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| 19. |
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