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Preview: Journal of Petrology - current issue

Journal of Petrology Current Issue

Published: Fri, 25 Aug 2017 00:00:00 GMT

Last Build Date: Sat, 23 Sep 2017 03:45:55 GMT


Experimental Crystallization of the Macusani Obsidian, with Applications to Lithium-rich Granitic Pegmatites


Experiments at 200 MPa with the peraluminous (S-type) rhyolite obsidian from Macusani, Peru, asses the dynamics of crystallization starting with non-vesicular solid glass cores as compared to earlier experiments starting with powders of the same composition. Textures, spatial zonation of feldspars and quartz, and their compositional relations are substantively different and more clearly revealed in the solid-core experiments. The new experiments with solid cores possess more sharply bounded segregation of feldspathic and quartz-rich domains of crystallization, a shift from a predominance of feldspars to increasing mica with initial H2O content >6 wt %, and the simultaneous crystallization of solvus pairs of plagioclase and alkali feldspar at opposite ends of the melt volume. As in other similar studies, the maximum in the rate of crystallization, a function principally of crystal growth rate, occurs at a liquidus undercooling of ∼200 ± 50°C. Both experimental studies with the Macusani obsidian apply to the chemical, textural, and spatial zonation of minerals within granitic pegmatites, particularly the Li-rich peraluminous pegmatites of S-type granite affinity. The new results have now reproduced and can account for the following features of pegmatites: (1) feldspathic outer zones and quartz-rich to pure quartz cores; (2) massive fine-grained border zones, followed by coarsening wall zones with unidirectional solidification texture, culminating in central domains of more isotropic fabric and coarse grain size; (3) locally, alternating laminations of mineral assemblages as in layered pegmatites and layered aplites; (4) a steady decrease in the An content of plagioclase from margin to core via subsolidus isothermal fractional crystallization; (5) spatial segregation of plagioclase and alkali feldspar along opposite margins of the melt body via far-field chemical diffusion; (6) an inward increase in the size of individual crystals by ∼102; (7) albite + lepidolite bodies as the latest primary assemblage, and following the crystallization of pure quartz bodies. All of these experimental results followed from the appreciable undercooling of the melt prior to the onset of crystallization. All of the features cited, except for the formation of miarolitic cavities, are entirely igneous in origin, owing nothing to the simultaneous occurrence of an aqueous solution.

Reaction Infiltration Instabilities in Mantle Rocks: an Experimental Investigation


Melt extraction from partially molten regions of the mantle occurs along high-permeability pathways. Melt–rock reactions can lead to the formation of high-permeability channels due to a positive feedback between melt flow and reaction. To study this process, we performed a series of Darcy-type experiments in which a cylinder of partially molten rock sandwiched between a melt source and a porous sink was annealed at high pressures (P = 300 MPa) and high temperatures (T = 1200° or 1250°C) under a controlled pressure gradient (∂P/∂x = 0–100 MPa mm–1) for up to 5 h. The partially molten rock was formed from 50:50 mixtures of olivine (Ol) and clinopyroxene (Cpx) plus 4, 10 or 20 vol. % of alkali basalt. The melt source was a disk of alkali basalt undersaturated in silica with respect to the partially molten rock, and the sink was a disk of porous alumina. During an experiment, melt from the source dissolved Cpx in the partially molten rock and precipitated Ol, thereby forming a Cpx-free reaction layer at the interface between the melt source and the partially molten rock. The melt fraction as well as the grain size in the reaction layer increased significantly compared with that present in the starting material, confirming that the reaction increased the local permeability of the partially molten rock, one of the prerequisites for the reaction infiltration instability process to operate. In experiments carried out under a small pressure gradient (and hence slow melt flow velocity), the reaction layer remained roughly planar and no channels developed. However, if the melt flow velocity by porous flow exceeded ∼0·1 µm s–1, the reaction layer locally protruded into the partially molten rock forming finger-like, melt-rich channels. The morphology and spacing of the channels depended on the initial melt fraction. In a partially molten rock with 20 vol. % melt, multiple, voluminous channels with an elliptical core of pure melt developed. At lower melt contents, fewer and thinner channels formed. Our experiments demonstrate that melt–rock reactions can lead to melt channelization in mantle lithologies, consistent with general predictions of the reaction infiltration instability theory.

The Thickness of the Mushy Layer on the Floor of the Skaergaard Magma Chamber at Apatite Saturation


We present a novel way of constraining the thickness of the crystal mush in fractionated layered intrusions using detailed microstructural analysis. The results are combined with geochemical data to create a snapshot of the crystal mush on the floor of the Skaergaard magma chamber in the period immediately before and after the saturation of the bulk liquid in apatite (the UZa–b boundary). The step-change in the fractional latent heat (that part of the total enthalpy budget associated with crystallization) accompanying the arrival of a new liquidus phase is recorded by a step-change in the median clinopyroxene–plagioclase–plagioclase dihedral angle, Θcpp, in fully solidified cumulates. Dihedral angles are formed during the last stages of solidification and hence the change of Θcpp associated with apatite-in marks a point close to the base of the mushy layer at the moment the bulk liquid became saturated in apatite, whereas the first appearance of abundant, homogeneously scattered, cumulus apatite crystals in the stratigraphy marks the top of the mushy layer at this moment. Comparison of the offset between these two markers in five widely spaced drill cores through the Skaergaard Layered Series suggests that the mushy layer was only a few metres thick at the UZa–b boundary in the centre and east of the floor, whereas it was ∼100 m thick on the floor near the western margin. There is no correlation between the efficiency of liquid expulsion (as recorded by bulk-rock P2O5 concentrations and the stratigraphic distribution of reactive symplectites) and the recorded mush thickness at the moment of apatite saturation, suggesting that existing models of adcumulate formation that depend on mush thickness need to be reconsidered.

Mt Bambouto Volcano, Cameroon Line: Mantle Source and Differentiation of Within-plate Alkaline Rocks


The Late Cretaceous–Quaternary Cameroon Volcanic Line (CVL) is a 1600 km long chain of volcanoes that straddles the continent–ocean boundary and extends from the Gulf of Guinea to the interior of the African continent. The magmatic activity started at 70 Ma and has continued until the present. The products of this magmatic activity are distinctive in terms of petrology and isotope geochemistry, the variety of volcanic rocks ranging from ultrabasic, alkaline to sub-alkaline lavas to highly evolved alkaline lavas with isotopic compositions indicating complex combinations of both sub-lithospheric (HIMU, EM, DMM) and lithospheric components (sub-continental lithospheric mantle and crust). We conducted a petrological and geochemical study of a set of volcanic rocks, sampled from the rim and interior of the Miocene Mt Bambouto caldera, one of the 12 main volcanic centres of the CVL. The rocks were analysed for their whole-rock major and trace element contents, 40Ar/39Ar ages and whole-rock Sr–Nd–Pb–Os isotopic compositions. Phonolites and quartz-trachytes of the Mt Bambouto caldera are derived by fractional crystallization of highly alkaline and moderately alkaline parental basic magmas, respectively. Assimilation of the shallow crust has affected both alkaline and subalkaline magmas, suggesting that the petrogenesis of the differentiated rocks cannot be explained by crustal contamination alone. Only minor amounts (usually less than 5%) of assimilation of upper crustal silicic rocks from the local Pan-African basement are required to produce the most differentiated compositions. The rocks with the highest crustal contribution are Q-normative trachytes from peripheral cones, as well as one Ne-trachyte. Mt Bambouto basic–ultrabasic rocks, including basanites and alkali-basalts with high 187Os/188Osi, might have experienced some crustal contamination, but it must have been a limited process. Some Mt Bambouto ultrabasic to basic rocks show large ion lithophile element enrichment, notably of Sr, Ba and P compared with Zr. These samples also have relatively radiogenic Sr and unradiogenic Pb isotopic compositions. Such compositions are similar to those of the high-Sr group identified by previous studies. Most of the basic rocks do not show such characteristics and are identified as a low-Sr group. We interpret the geochemical characteristics of the high-Sr group as resulting from the partial melting of a depleted mantle (DMM-like) peridotite source containing pyroxenite veins that had interacted with carbonatitic fluids. To test this hypothesis, we used a new modelling approach based on Monte Carlo simulation; this method has the advantage of deciphering how different mantle components interacted through time. Our modelling confirms the plausibility of a three-component source. In addition, it suggests that the carbonatitic fluid first mixed with the pyroxenititic component and the resulting melt interacted with a DMM-like mantle. Both high-Sr and low-Sr groups can be produced by such a mixing scenario but with a stronger contribution of the carbonatitic fluid for the high-Sr group. At the time of melting, these source components could have been located in a metasomatized region of the sublithospheric mantle (uppermost section of the asthenosphere) or in the sub-continental lithospheric mantle.

Fe–Ni-rich Silicate Aggregates Formed after Sulfides in High-pressure Serpentinites


High-pressure serpentinites from Alpine Corsica, the Piedmont Zone (Western Alps), the Tso Morari dome (Himalaya), the Dominican Republic and Cuba (Greater Antilles) contain reddish (Ni,Fe)-rich silicate aggregates, a few hundred micrometers in size, which have not been reported so far from lower-pressure serpentinites, nor found there through systematic screening. The high atomic (Ni + Fe)/Si ratio (5 ± 4) of these aggregates, as well as their occurrence as coronas around (Ni,Fe)-sulfides, indicates their formation at the expense of sulfides and serpentine. The high (Ni + Fe)/Si ratio may suggest the presence of an Fe or Ni analogue of the rare serpentine-related minerals balangeroite or cronstedtite. However, compositional variations within and between aggregates, synchrotron X-ray diffraction data acquired on single aggregates, and transmission electron microscopy reveal that these aggregates are not a single phase but rather a mixture at the hundreds of nanometer scale of oxyhydroxides, mainly goethite (FeOOH) + (Ni,Fe)-rich serpentine ± chlorite ± reevesite [Ni6Fe2(OH)16(CO3)·4H2O] ± magnetite. In addition to microtexture and mineralogy, strong chemical variations at the border of the aggregates and along micro-veins suggest formation through a low-temperature alteration process. However, the occurrence of such pseudomorphic aggregates only in high-pressure serpentinites suggests that they were formed at the expense of a high-pressure silicate precursor of high M2+/Si ratio, still to be identified but which could be a main nickel carrier at depth in subduction zones.

Geochemical Distinction between Carbonate and Silicate Metasomatism in Generating the Mantle Sources of Alkali Basalts


Crustal metasomatism by subduction is considered as an important mechanism for generating mantle heterogeneity through infiltration of different metasomatic agents into the mantle. As a consequence of the subduction of oceanic crust (including oceanic basaltic rocks and seafloor sediments), both carbonate and silicate metasomatism are expected to occur at the slab–mantle interface in an oceanic subduction channel. This is demonstrated by an integrated study of major and trace elements, stable Mg-isotopes and radiogenic Sr–Nd–Hf isotopes in Cenozoic and Mesozoic alkali basalts from the West Qinling orogen, China. Although the two series of continental basalts show ocean island basalt (OIB)-like trace element distribution patterns and relatively depleted Sr–Nd–Hf isotope compositions, they exhibit differences in other geochemical variables. The Cenozoic basalts have low SiO2, but high CaO and MgO concentrations, and high CaO/Al2O3 ratios but low δ26 Mg values of –0·54 to –0·32‰. They have relatively high abundances of melt-mobile incompatible trace elements such as the light rare earth elements (LREE) and most large ion lithophile elements (LILE), but low abundances of K, Pb, Zr, Hf and Ti, with high (La/Yb)N values but low Ti/Eu and Hf/Sm ratios. In contrast, the Mesozoic basalts have relatively high SiO2, but low CaO and MgO concentrations, and low CaO/Al2O3 ratios, but high δ26 Mg values of –0·35 to –0·21‰. They have relatively low abundances of melt-mobile incompatible trace elements such as the LILE and LREE, but are relatively enriched in Zr, Hf and Ti, with low (La/Yb)N values but high Ti/Eu and Hf/Sm ratios. These observations indicate a significant difference in the composition of the mantle sources between the two periods of alkali basalt magmatism. Whereas the low δ26 Mg values of the Cenozoic basalts indicate the involvement of sedimentary carbonate in their mantle source, the high δ26 Mg values of the Mesozoic basalts point to a primary contribution from a silicate component. Metasomatic reaction of depleted mid-ocean ridge basalt (MORB)-source mantle peridotite with carbonate and silicate melts, respectively, at the slab–mantle interface in the Paleotethyan oceanic subduction channel are responsible for the generation of their mantle sources. Both carbonate and silicate melts have been incorporated into the depleted MORB-source mantle wedge producing ‘crustal metasomatism’ in an oceanic subduction channel. We suggest that the type of metasomatic agent in the mantle sources is the key to the composition of the resultant alkali basalts.

On the Association between Veining and Index Mineral Distributions in Barrow’s Metamorphic Zones, Glen Esk, Scotland


The concept of index mineral based metamorphic zones was first introduced by George Barrow in 1912 and the Barrovian metamorphic zones continue to be used as a framework for describing regional metamorphism. Pressure, temperature, and protolith composition are widely recognized as primary controls on index mineral distribution. Today, metamorphic fluid flow is also recognized as an important driver of metamorphic reactions. The aim of this study is to establish if and how metamorphic fluids control index mineral distribution during Barrovian metamorphism. We use samples from Barrow’s type locality in Glen Esk, SE Scottish Highlands, to study possible relationships between veining and index mineral distribution. In addition to petrographic and textural observations, we use whole-rock compositions, mineral compositions and oxygen isotope analyses. At low grade, in the chlorite zone and most of the biotite zone, no correlation between veining and index mineral distribution is seen. At higher grade, in the garnet and staurolite zones, index mineral abundance is shown to be higher adjacent to veins. These trends coincide with other mineralogical, chemical, and isotopic changes in the vein-proximal rock, indicative of fluid–rock interaction. Kyanite distribution is homogeneous in the kyanite zone. However, we show that this too relates to extensive fluid–rock interaction. Garnet-, staurolite-, and kyanite-bearing selvedges are common in the sillimanite zone. However, sillimanite distribution is unrelated to these selvedges, which supports models arguing that sillimanite formed during a separate metamorphic event. We infer fluid flow from high grade to low grade because the fluid was out of isotopic equilibrium with the lower grade rocks, but in equilibrium with the higher grade rocks. We conclude that fluid flow played a major role in the stabilization and distribution of Barrovian index minerals in Glen Esk, and that the importance of fluid flow was greater at higher metamorphic grades.

Understanding Degassing and Transport of CO 2 -rich Alkalic Magmas at Ross Island, Antarctica using Olivine-Hosted Melt Inclusions


Volatiles play an important role in magmatic and volcanic processes. Melt inclusions are a powerful tool to study pre-eruptive volatiles, but interpretation of their H2O and CO2 variations can be difficult. The H2O and CO2 contents of melt inclusions from nine basanites from Hut Point Peninsula, Mt Terror and Mt Bird on Ross Island, Antarctica, were studied to understand better the behavior of volatiles in the magmas and to provide insight into magma transport and storage processes. Ninety olivine-hosted (Fo78–-88) melt inclusions were examined along with the composition of the associated bulk-rock samples. The H2O (0·4–2·0 wt %) and CO2 (0·2–0·9 wt %, or 0·2–1·8 wt % after correction for vapor bubbles) variations in the melt inclusions cannot be explained by equilibrium degassing. A strong correlation between melt inclusion radius and H2O content for Hut Point samples indicates that diffusive loss of H+ has occurred. Based on vapor saturation pressure trends, it is inferred that a magma reservoir existed below Hut Point at a depth of ∼18 km, and by modeling diffusive loss of H+ for melt inclusions, it is shown that the magmas ascended from this depth in less than a year. Melt inclusions from Terror and Bird lack evidence of diffusive loss of H+ and there are no strong chemical indicators of CO2 fluxing. Compositional heterogeneities in melt inclusions indicate that magma mixing occurred, making it difficult to interpret H2O and CO2 trends. Melt inclusions from these volcanoes were entrapped polybarically, inconsistent with entrapment in a single storage region. Published analyses of 54 olivine-hosted (Fo53–83) melt inclusions in seven samples from Erebus volcano on Ross Island were re-examined for comparative purposes. Low H2O (∼0·1 wt %) and CO2 (0–0·2 wt %) contents and the evolved compositions of these indicate that Erebus magmas undergo shallow (<6 km) crystallization before eruption, probably in a shallow storage region. Magmas from the surrounding volcanoes show no sign of shallow storage.