Microstructural evidence for crystallization regimes in mafic intrusions: a case study from the Little Minch Sill Complex,Scotland

The magma forming the 20 m thick crinanitic/picrodoleritic Dun Raisburgh sill, part of the Little Minch Sill Complex of NW Scotland, comprised a mafic carrier liquid with a crystal cargo of plagioclase and olivine (1 vol%). The olivine component of the cargo settled on the floor of the intrusion while the more buoyant plagioclase component remained suspended during solidification, resulting in a relatively high plagioclase content in the centre of the sill. The settled olivine grains form a lower fining-upwards sequence overlain by a poorly sorted accumulation formed of grains that grew within the convecting magma. The accumulation of olivine on the sill floor occurred over 5–10 weeks, synchronous with the upwards-propagation of a solidification front comprising a porous (~?70 vol% interstitial liquid) plagioclase-rich crystal mush.     

Sorted circles in the maritime Subantarctic,Marion Island

Active sorted circles are quantitatively described for the first time for the maritime Subantarctic. Increasing altitude and hence increasing frost penetration are associated with systematic increases in pattern spacing, cell width, height of the fine centre relative to the border, and depth of sorting (but not the width of coarse margins). These results imply consistent dimensional scaling of circle parameters. It is argued, on the basis of the altitudinal trends in pattern morphology, the dimensional scaling of forms, and the morphology of the patterns, that differential frost heave in association with convection cells is responsible for circle formation. Copyright © 2003 John Wiley & Sons, Ltd.     

The effect of feldspar on quartz-H2O−CO2 dihedral angles at 4 kbar,with consequences for the behaviour of aqueous fluids in migmatites

An investigation was made of the effect of trace amounts of feldspar (Na and/or K) on dihedral angles in the quartz-H₂O-CO₂ system at 4 kbar and 450–1050°C. Quartz-quartz-H₂O dihedral angles in feldspar-bearing quartz aggregates are observed to be the same as those in pure quartz aggregates at temperatures below 500°C. Above this temperature, they decrease with increasing temperature until the solidus. The final angle at the inception of melting is about 65° for microcline-quartz-H₂O and microcline-albite-quartz-H₂O, and much less than 60° (the critical value for formation of grain-edge fluid channels in an isotropic system) for the albite-quartz-H₂O system. CO₂ was observed to produce a constant quartz-quartz-fluid dihedral angle of 97° in feldspar-bearing quartz aggregates at all temperatures studied. Also examined were the dihedral angles for the two co-existing supersolidus fluids in quartz aggregates. In all systems the quartz-volatile fluid angle is greater than 60°, whereas the quartz-melt angle is lower than 60°. Both super-solidus angles decrease with increasing temperature. The transition from nonconnected to connected poro- sity with increasing temperature observed in the quartz-albite-H₂O system some tens of degrees below the solidus (termed a permeability transition), if a common feature of rocks near their melting points, will play an important role in controlling the permeability of high-grade rocks to aqueous fluids. Received: 27 October 1993 / Accepted: 11 July 1994     

Textural Maturity of Cumulates: a Record of Chamber Filling, Liquidus Assemblage, Cooling Rate and Large-scale Convection in Mafic Layered Intrusions

Textural maturity describes the extent to which a rock has evolvedfrom the initial reaction-controlled texture towards texturalequilibrium controlled by the minimization of interfacial energy.Solidification in a magma chamber results in the formation ofan impingement texture by the random juxtaposition of planar-sidedgrains. Orthocumulates, in which the initial melt-filled poresare pseudomorphed by later-crystallizing phases, have an ophiticor intersertal texture immediately after complete solidification,which then evolves towards solid-state equilibrium by roundingof initially planar grain boundaries and an increase in themedian dihedral angle subtended at the junctions of two primocrysticgrains with the interstitial phase. The bulk of the increasein angle occurs just below the solidus temperature in kilometre-scalemafic plutons. Quantification of textural maturity via measurementof dihedral angle populations in troctolitic and gabbroic cumulatesfrom the Rum Eastern Layered Intrusion and the Skaergaard Intrusiondemonstrates that the rocks preserve a record of thermal eventsrelated to magma chamber replenishment and the onset of chamber-wideconvection. Textural maturity is also a function of the liquidusphase assemblage: for systems in which only olivine and plagioclaseare liquidus (i.e. cumulus) phases in the main magma body abovethe crystal mush, the texture is significantly less mature thanthat in systems in which clinopyroxene is an additional liquidusphase. The difference in textural maturity reflects differencesin the cooling and solidification rate, and demonstrates directlythat the liquidus phase assemblage plays a role in determiningthe thermal history of plutons. KEY WORDS: cumulates; dihedral angles; Rum; Skaergaard; textures     

Textures in Partially Solidified Crystalline Nodules: a Window into the Pore Structure of Slowly Cooled Mafic Intrusions

Abundant glass is present along grain boundaries in coarse-grained,glass-bearing, crystalline gabbroic and peridotitic nodulesentrained and erupted in lavas from Iceland, Santorini and MaunaLoa (Hawaii), even when the total porosity is less than a fewvolume per cent. The glass films vary from a few microns toa few tens of microns thick, and are associated with stringsof small lensoid grain boundary pockets formed by impingementduring crystal growth. Additional porosity occurs as extensiveliquid-filled pockets adjacent to included grains within oikocrystsand as large triangular pockets formed by impingement of planar-sidedgrains. Interstitial material within glass films, and the irregularityof film thickness along a single grain boundary, suggest thatthe present pore structure is representative of the pore structurebefore entrainment and eruption. Pore geometry is consistentwith a dominant control by crystal growth during solidification,with little or no evidence for control by minimization of internalenergies driven by textural equilibration. Similarities betweenliquid distribution in the crystalline nodules and that of late-stage,interstitial phases in fully solidified mafic cumulates fromthe Rum and Skaergaard intrusions demonstrate that the crystallinenodules provide information about the latest stages of solidificationin slowly cooled mafic plutons. The highly permeable networkof intersecting liquid films, lenses and pockets may promotein situ crystallization in the solidifying mush, explainingthe common presence of adcumulates in such intrusions. KEY WORDS: textures; liquid distribution; mafic cumulates; crystalline nodules