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1.
Melting experiments have been performed on a primitive, mildly alkalic glassy lava (10 wt.% MgO) from the 1965 eruption of the Surtsey volcano located at the tip of the south-eastern propagating rift zone of Iceland. At atmospheric pressure, approximately on the FMQ oxygen buffer, olivine (Fo81) crystallizes from 1240°C, followed by plagioclase (An70) from 1180°C and augite from 1140°C. The experimental glasses coexisting with olivine, plagioclase and augite are ferrobasaltic enriched in FeO (13.6–14.2 wt.%) and TiO2 (4.0–4.4 wt.%). In high pressure, piston-cylinder, graphite-controlled runs, olivine occurs as the liquidus phase until 14 kbar, above which augite is the liquidus phase. Low-Ca pyroxene is not a liquidus phase at any pressure. The high pressure liquids are, relative to the one atmosphere liquids, significantly enriched in Al2O3 and Na2O and depleted in CaO as a result of changes in the crystallizing assemblages. Furthermore, liquidus augite is dominantly subcalcic and shows significant enrichment in Al and depletion in Ti. Subliquidus plagioclase is enriched in sodium relative to low pressure phase compositions. Evaluated in normative projections, contrasting liquid lines of descent are revealed as a function of pressure. At one atmosphere, the multisaturated liquids are located close to the thermal divide defined by the plane olivine-plagioclase-augite, but appear, with advanced degrees of crystallization, to be moving away from the thermal divide toward normative quartz. The augites crystallizing in the one atmosphere experiments are calcic and slightly nepheline normative. In the 10 and 12.5 kbar experiments, the augites become subcalcic and dominantly hypersthene normative. Because of this shift in augite compositions, transitional basaltic liquids may at high pressure evolve from the tholeiitic side of the olivine-plagioclase-diopside normative divide onto the alkalic side. With increasing pressure above 15 kbar, the liquidus augite compositions move back toward the olivine-plagioclase-diopside normative divide. 相似文献
2.
The south-eastern propagating rift zone of Iceland shows a progression from tholeiitic, to transitional, and mildly alkalic basalts going toward the front of the propagator. A petrogenetic model has been formulated based on low and high pressure melting experiments. The evolution of the magmas behind the front of the propagating rift is dominated by near surface processes (e.g., Katla and Hekla volcanic systems). Compared with the one atmosphere liquid line of descent, the Vestmannaeyjar lavas, erupted at the front of the propagating rift, have systematically higher Al2O3 and Na2O contents consistent with the experimentally determined effects of high pressure, Labradoritic plagioclase megacrysts, which occur in the early phase of the Surtsey eruption at the front of the propagator, are consistent with the predicted effect of high pressure on plagioclase composition. On the other hand, augites similar to those of the high pressure experiments are unknown among the phenocryst and xenolith assemblages of the Vestmannaeyjar lavas (and other Icelandic lavas). Petrographic evidence points toward a high water content in the evolved lavas of the Vestmannaeyjar. A relatively high water activity and high pressure in the magma chambers at the front of the propagator could have caused a significant suppression of the liquids temperature, in particular for plagioclase. Seismic and magnetotelluric evidence suggest that magma chambers behind the propagating front occur at the depth equivalent to 2–3 kbar of pressure. At the front, magma chambers have been located by geophysical evidence at significantly greater depths equivalent to 3–8 kbar. The lavas erupted at the front of the propagator are located to the alkalic side of the thermal divide and, therefore, can be expected to evolve toward nepheline saturation under slightly hydrous conditions. The most evolved of these lavas are of ferrobasaltic compositions and may be saturated with augite. The lavas erupted behind the front are located to the transitional side of the divide and evolve toward quartz saturation under essentially anhydrous conditions. In contrast to the Vestmannaeyjar lavas, the lavas behind the propagator often contain augite as a phenocryst. It is concluded that the chemical variation observed along the south-eastern propagating rift of Iceland is an effect of primary chemical features and that water plays a significant role on the liquid lines of descent at the front of the propagator. 相似文献
3.
P. Thy 《Contributions to Mineralogy and Petrology》1983,83(1-2):141-149
Crystallization of spinel minerals in transitional and alkali basalts from Iceland can be related to the FeO, MgO, TiO2 and Cr contents of the coexisting melt. Chromian spinel occurs in glasses in which TiO2 is less than 2.8 wt.% and the weight ratio FeO/MgO is less than 2.0, whereas titanomagnetite occurs when the same parameters are greater than 4 wt.% and 2.7, respectively. In addition, chromian spinel only occurs in basalts with Cr greater than 200 ppm. It is suggested that chromian spinel crystallizes, together with olivine, from liquids with olivine liquidus temperatures ranging from above 1,200° C to approximately 1,150° C. A discontinuity in spinel crystallization follows until below 1,100° C, where titanomagnetite starts to crystallize. Compositional variations in chromian spinel attached to, or included, in homogeneous olivine phenocrysts, however, cannot be related to equilibrium relations. Textural relations suggest homogeneous nucleation for titanomagnetite, whereas chromian spinel nucleates heterogeneously, dependent on growth of olivine phenocrysts. The composition of chromian spinels cannot in detail be related to physical and compositional parameters of the average melt, but may be related to local compositional relations in the melt adjacent to growing crystals. Such compositional variation around growing olivine crystals may be the prime reason for the non-equilibrium precipitation of included chromian spinels. 相似文献
4.
Abstract The Ankara mélange within the Izmir–Ankara–Erzincan suture zone in north-central Turkey includes ophiolitic fragments that represent the remnants of an oceanic basement evolved between the Sakarya and Kirsehir continental blocks in the early Mesozoic. The serpentinized upper mantle peridotites and lower crustal rocks in these ophiolites are cross-cut by dolerite and plagiogranite dykes, which show mutually intrusive relations indicating their synchronous emplacement into the pre-existing oceanic lithosphere. Zircon dating (U–Pb) of a plagiogranite dyke has revealed a concordia age of ∼179 ± 15 Ma that is interpreted here as the crystallization age of this differentiated rock. A fourth fraction of the zircon separates from this rock has also shown an inherited component greater than 1.7 Ga, possibly derived from the Precambrian core of the Rhodope–Strandja Metamorphic Massif in the Balkan Peninsula. Models for plagiogranite formation were tested and it is concluded that a high extent (<70%) of anhydrous or water-undersaturated, early amphibole-free fractionation of a basaltic melt source may have readily produced the observed REE concentrations for the Ankara mélange plagiogranites. The trace element abundances and other geochemical features of the coeval dolerite dykes are similar to those of the plagiogranites, suggesting a common melt source. The Ta–Nb patterns shown by both the plagiogranite and dolerite dykes are typical of arc-related petrogenesis and can be explained by the addition of slab-derived components to a depleted mantle wedge. The Early Jurassic ophiolitic basement and the dyke intrusions were formed in a back-arc setting between the Paleo- and Neo-Tethyan domains in the eastern Mediterranean region. The Izmir–Ankara–Erzincan Sea developed in this back-arc environment and the related suture zone had a diachronous evolutionary history. 相似文献
5.
Quy Thy Truong Cyril de Runz Guillaume Touya 《International journal of geographical information science》2019,33(8):1651-1682
This paper aims to qualify the behaviour of contributors to OpenStreetMap (OSM), a volunteered geographic information (VGI) project, through a multigraph approach. The main purpose is to reproduce contributor’s interactions in a more comprehensive way. First, we define a multigraph that combines existing spatial collaboration networks from the literature with new graphs that illustrate collaboration based on specific aspects of the VGI modes of contribution through semantics, geometry and topology. Indeed, the ways that contributors interact with one another through editing, completion, or even consumption may provide additional information on each user’s operation mode and therefore, on the quality of the contributed data. Social collaborations drawn from indirect criteria – for example, comparisons between contributors’ activity areas – can also be contemplated under another network. Second, the resulting multigraph is analysed using data mining approaches to characterise individuals and identify behavioural groups. The implementation of a multiplex network based on an OSM data sample and an initial analysis make it possible to identify useful behaviours for data qualification. The initial results characterise some contributors as pioneers, moderators and truthful contributors, according to their special roles in the graphs. Mapping elements that include these contributors’ participation are likely to be reliable data 相似文献
6.
Rais Latypov Tony Morse Brian Robins Richard Wilson Grant Cawthorn Christian Tegner Marian Holness Charles Lesher Steve Barnes Brian O’Driscoll Ilya Veksler Michael Higgins Allan Wilson Olivier Namur Sofya Chistyakova Richard Naslund Peter Thy 《Contributions to Mineralogy and Petrology》2015,169(2):1-10
7.
The Skaergaard liquid line of descent revisited 总被引：1，自引：0，他引：1
Peter Thy Charles E. Lesher Christian Tegner 《Contributions to Mineralogy and Petrology》2009,157(6):735-747
There is a fundamental conflict between the suggestion that the iron content of Skaergaard liquids increases during Fe–Ti
oxide fractionation and the observation that at the same time oxygen fugacity () drops by two log-units below the fayalite-magnetite-quartz oxygen buffer (FMQ). A new petrographic study of average Skaergaard
gabbros shows that the total modal content of Fe–Ti oxides is about 22% in the early LZc and markedly decreases to below 5%
in the UZc. Forward modeling based on these modal constraints, as well as experimental results on Skaergaard-related dikes,
predicts that fractionation of troctolitic LZa gabbros drives the derivative liquid towards a high-iron content. Strong iron
enrichment continues, together with a small decline in silica, during LZb crystallization due to the appearance of augite
as a fractionating phase. The fractionation of Fe–Ti oxides in the LZc initially suppresses iron enrichment and reverses the
silica trend to one of slight enrichment. However, continued evolution into the UZ produces liquids with maximum UZc FeO*
content of 23–25 wt.% and SiO2 content of 53 wt.% (FeO* is total iron as FeO). The maximum in FeO* is dependent on several factors of which the Fe–Ti oxide
mode has the strongest effect. The during crystallization of the LZc is widely thought to have been at, or slightly below, the fayalite-magnetite-quartz oxygen
buffer (FMQ). Under closed system evolution, incorporation of ferric iron into augite during formation of the LZb restricts
the increase in to about 0.1 log-units above FMQ (=0.1 ΔFMQ). Likewise, crystallization of the LZc through the UZa, involving Fe–Ti oxide
minerals, leads to a decline in of less than 0.1 ΔFMQ. Crystallization of the UZb-c gabbros results in oxidation to a maximum of 0.5 ΔFMQ. This behavior
can account for the iron-rich character of the UZ gabbros, as well as, the low modal content of Fe–Ti oxides. Thus, evolved
Skaergaard liquids are high in iron and contain a modest amount of SiO2. Our modeling result do not account for a strong drop in through the layered series. Such a drop would require an unacceptably high proportion of Fe–Ti oxides and high-magnetite
content in the fractionating assemblage.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
8.
P. Thy 《Mineralogy and Petrology》1983,32(1):1-24
Summary The Ruten Sequence of the basic Fongen-Hyllingen complex is composed of rhythmically layered olivine-plagioclase-clinopyroxene-orthopyroxene-Fe–Ti oxide cumulates. Phase layering is defined by the entry of orthopyroxene and Fe–Ti oxides. Only limited cryptic variation occurs through the c. 1800 m thick cumulate sequence. Rhythmically repeated units of interlayered dunite and troctolite are common throughout most of the sequence. Dunite units range from laminae a few centimeters thick to massive units 2–3 m thick. The modal abundance of olivine in the cumulates varies from 7 to 100 wt.%. In cumulates with MgO above 15 wt.% plagioclase and clinopyroxene occur in relatively fixed ratios between 4:1 and 3:2. Clinopyroxene-rich layers less than a centimeter thick are present only in olivine gabbros. In these, with MgO below 15 wt.%, modal variation of clinopyroxene in part controls the chemical variation of the cumulates. The modal and normative proportions of cumulus phases in the olivine gabbros can be related to equilibrium cotectics in the natural basalt system at approximately 5 kbar. It is suggested that the olivine-rich layers originated by supersaturated nucleation and growth of olivine.
With 13 Figures 相似文献
Chemie der Kumulate und ihre Bedeutung für die Entstehung der Schichtung, dargestellt am Beispiel des basischen Komplexes von Fongen-Hyllingen (Norwegen)
Zusammenfassung Die Ruten-Sequenz des basischen Fongen-Hyllingen-Komplexes setzt sich zusammen aus rhythmisch abgelagerten Kumulaten aus Olivin, Plagioklas, Klinopyroxen, Orthopyroxen und Eisentitanoxid. Phasenschichtung/Ablagerung wird durch das erste Auftreten von Orthopyroxen und Eisentitanoxiden charakterisiert. In der ca. 1800 m mächtigen Sequenz tritt kryptische Variation nur begrenzt auf; rhythmische Wiederholung eingeschalteter Dunit- und Troktolithorizon te dagegen häufig. Die Mächtigkeit der Dunite reicht von wenige Zentimeter dünnen Schichten bis zu zwei bis drei Meter dicken Bänken. Der Modalbestand an Olivin in den Kumulaten variiert zwischen 7 und 100 Gew.%. In Kumulaten mit einem MgO-Gehalt von über 15 Gew.% liegt das Mengenverhältnis von Plagioklas zu Klinopyroxen relativ konstant bei 4:1 bis 3:2. Klinopyroxen-reiche, weniger als zentimeterdicke Lagen finden sich nur in Olivingabbros. In diesen ist der MgO-Gehalt weniger als 15 Gew.% und die modale Variation des Klinopyroxens steuert teilweise die chemische Variation der Kumulate. Die modalen und normativen Verhältnisse der Kumulusphasen im Olivingabbro lassen sich in Beziehung setzen zu dem kotektischen Gleichgewicht natürlicher Basaltsysteme bei 5 kbar. Die olivinreichen Lagen suggerieren Bildung durch übersättigte Nukleation und Kristallisation.
With 13 Figures 相似文献
9.
10.
N. Abrahamsen J. R. Wilson P. Thy N. Ø. Olesen K. H. Esbensen 《Geophysical Journal International》1979,59(2):231-248
Summary. Palaeomagnetic results are presented from the c . 160 km2 Caledonian synorogenic layered Fongen-Hyllingen gabbro complex (of probable late Silurian age) located about 75 km SE of Trondheim, Norway, in the allochthonous Seve-Kdli Nappe Complex. A total of 80 oriented samples from eight sites in the northern part of the gabbro were investigated. After detailed af demagnetization two stable high coercivity components emerge: one with a well defined NW direction with D =325°, I =−21° (α95 =8°, N =8), and another, less well defined, probably younger, SW direction with D = 237°, I = 6° (α95 = 9°, N = 8). Correction for dip of these two directions gives D = 329°, I =−7° (α95 = 10°) and D = 238°, I =−11° (α95 = 12°), respectively. The corresponding pole positions are P 1 : 19° N, 225° E and P 2 : 19° S, 308° E, respectively. The reversed pole -P 2 of the SW direction lies close to other NW European palaeomagnetic poles of Caledonian, Upper Silurian-Lower Devonian age. However, the dominant pole PI is far away from these, and could be due to a late Caledonian geomagnetic excursion of considerable duration; or it could record a c . 90° rotation around a vertical axis of a crustal block within the Scandinavian Caledonides. Block rotation could have been related to nappe translation, although geological observations do not at present appear to support the occurrence of such an event. 相似文献