A mineralogical and microstructural study of 7 eucrites (A-881394, Y-791195, Y-981617, Y-790266, Y-791186, Y-792510, Y-793591).

第3回極域科学シンポジウム/第35回南極隕石シンポジウム 11月29日（木） 国立国語研究所 2階講

Toward an understanding of disequilibrium dihedral angles in mafic rocks

[1] The median dihedral angle at clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, Θcpp, is generally lower than equilibrium (109° ± 2°). Observation of a wide range of mafic bodies demonstrates that previous work on systematic variations of Θcpp is incorrect in several important respects. First, the spatial distribution of plagioclase compositional zoning demonstrates that the final geometry of three-grain junctions, and hence Θcpp, is formed during solidification (the igneous process): sub-solidus textural modification in most dolerites and gabbros, previously thought to be the dominant control on Θcpp, is insignificant. Θcpp is governed by mass transport constraints, the inhibiting effects of small pore size on crystallization, and variation in relative growth rates of pyroxene and plagioclase. During rapid cooling, pyroxene preferentially fills wider pores while the narrower pores remain melt-filled, resulting in an initial value of Θcpp of 78°, rather than 60° which would be expected if all melt-filled pores were filled with pyroxene. Lower cooling rates create a higher initial Θcpp due to changes in relative growth rates of the two minerals at the nascent three-grain junction. Low Θcpp (associated with cuspate clinopyroxene grains at triple junctions) can also be diagnostic of infiltration of previously melt-free rocks by late-stage evolved liquids (the metasomatic process). Modification of Θcpp by sub-solidus textural equilibration (the metamorphic process) is only important for fine-grained mafic rocks such as chilled margins and intraplutonic chill zones. In coarse-grained gabbros from shallow crustal intrusions the metamorphic process occurs only in the centers of oikocrysts, associated with rounding of chadacrysts

Textural immaturity of cumulates as an indicator of magma chamber processes: infiltration and crystal accumulation in the Rum Eastern Layered Intrusion

<p>Textural maturity describes the extent to which rocks have approached textural equilibrium. For igneous rocks, this is a function of the time-integrated thermal history in the sub-solidus which, for cumulate rocks, generally translates to the cooling history via the rate of solidification of the accumulating crystal mush. The most straightforward way to quantify the textural maturity of orthocumulates (<em>sensu lato</em>) is to measure the angle subtended at the corners of interstitial grains. Texturally mature cumulates have angle populations with median values of <em>c</em>. 120°, whereas texturally immature cumulates have lower medians, inherited by the interstitial phase, which infills either an impingement texture in the accumulating mush or a texturally equilibrated pore topology. Textural observations of allivalites from the Rum Layered Suite demonstrate that stratigraphically localized excursions towards textural immaturity are the result of either transient, relatively rapid, solidification rates (perhaps a result of the arrival of cold crystal-laden bodies of fluid on the chamber floor) or late-stage infiltration of fully solidified cumulates by evolved liquids expelled upwards by compaction of underlying horizons. These two processes have distinct textural and compositional signatures. </p

The Traigh Bhan na Sgurra Sill, Isle of Mull: Flow localization in a major magma conduit

Field evidence points to localization of magma flowing in a Tertiary doleritic sill on the Isle of Mull, Inner Hebrides, Scotland. Regions of the sill in which flow was short-lived have chilled margins, a narrow or absent metamorphic aureole, and alignment of plagioclase crystals resulting in pronounced, flow-parallel, lineations on fracture surfaces (Type I regions). Prolonged flow resulted in blocky, coarse-grained dolerite with no chilled margins, and an extensive metamorphic aureole (Type II regions). The distributions of Type I and Type II regions shows no spatial pattern, with stagnant solidifying sections immediately adjacent to sections with contemporaneous active and sustained flow. The transition between Type I and Type II regions occurs as the sill thickness exceeds 3.5 m. Our observation that sustained flow was only possible in regions of the sill thicker than 3.5 m is consistent with previously published theoretical models which predict a critical sill thickness in the range 2·2-5 m. Regions of the sill narrower than this experienced a single injection of magma. Simple models to determine flow duration from the width of the metamorphic aureole demonstrate that progressive focusing of flow into wider parts of the conduit created discrete channels active for up to 5 months

Melt-rich segregations in the Skaergaard Marginal Border Series: Tearing of a vertical silicate mush

The Marginal Border Series (MBS) of the Skaergaard Intrusion crystallised on the steeply dipping sidewalls of the magma chamber. Melt-rich segregations, previously described as the 'wavy pyroxene rock' are a key feature of the otherwise homogeneous outermost parts of the MBS. The lens-shaped segregations consistently strike parallel to the chamber wall and have a moderate dip towards it. The shape, size, grain size and mineralogy of the segregations evolve systematically away from the chamber wall. The segregations become bigger, more widely spaced, chemically more evolved and more irregular in shape with increasing distance from the margin. The segregations were previously interpreted as a nucleation effect parallel to the thermal gradient. However, they dip toward the margin, not parallel to it. We offer a new interpretation: that the segregations represent tearing of poorly consolidated crystal mush during localised sagging of the vertical solidification front. The tears form during a process analogous to 'hot tearing' of metal alloys, although the driving force for tearing is probably gravitational collapse of the vertical solidification front, or perhaps contemporaneous faulting during chamber filling. Small, regular tears formed in the thinner, more rapidly cooled outer parts of the MBS, while the larger, irregular tears formed in the coarser grained, inner parts. Movement of interstitial liquid fed the cracks after tearing, and the liquid crystallised as relatively evolved, coarse-grained segregations. The mineral chemistry and texture of the segregations suggest that the tearing occurs relatively early in the crystallisation history. © 2010 Elsevier B.V

Imprinted textures on apatite: A guide to paleoporosity and metamorphic recrystallization

Fluids play a key role in the transfer of both heat and mass within the crust and in the attainment of chemical equilibrium in metamorphic rocks, controlling both reaction rates and deformation mechanisms. However, our understanding of fluid movement through the deep crust is limited because rocks readily recrystallize, and fluids are invariably lost from grain boundaries. Little is known about how textures evolve during metamorphism since minerals rarely preserve much textural history that can be directly related to grain-boundary processes. Here we report on a study of the surfaces of apatite within gneisses. A variety of textural features is preserved on these surfaces, including imprinted texture of fossil grain boundaries. This texture is linked to deformation that caused a temporary excursion to fine grain size in the surrounding silicates. The geometry of paleoporosity is also recorded by microhollows on the apatite surface and gives a unique insight into both fluid behavior within the crust and the textural evolution of rocks