Daniel R. Viete

The nature and origin of the Barrovian metamorphism, Scotland: diffusion length scales in garnet and inferred thermal time scales

Abstract: In this paper we examine the length scales of major element diffusion in garnet during the Barrovian metamorphism. The role of diffusion in the flattening of Mn zoning profiles in garnet with increasing metamorphic grade across the Barrovian metamorphic series is ambiguous. However, the loss of distinct Mn-defined secondary compositional zoning in Barrovian garnets with increasing metamorphic grade and preservation of geochemical textures around Mn-free inclusions within high-grade Barrovian garnets provide robust evidence for c. 1000 μm Mn diffusion in sillimanite-zone garnets during the Barrovian regional metamorphism. Sillimanite-grade garnets from the Barrovian metamorphic series also preserve c. 100 μm Mn diffusion textures between sillimanite-grade rim domains and lower-grade cores. Bimodality in diffusion length scales requires bimodality in thermal time scales. Length scales of chemical diffusion are considered within the context of recent duration estimates for the Barrovian metamorphism of a few million years. We conclude that heat associated with the Barrovian regional metamorphism accumulated following numerous, short time-scale heating events responsible for the smaller-scale diffusion textures. This self-similar thermal regime was likely accommodated by the operation of shear zones in the highest-grade regions of the Barrovian metamorphic series, which concentrated small-scale episodic heat sources such as sheeted magmas, fluids and/or mechanical work. Supplementary material: A table of quantitative electron microprobe analyses, an example of the calibration curve used to produce quantitative electron microprobe maps from count maps, and Y maps are available at http://www.geolsoc.org.uk/SUP18415.

The nature and origin of the Barrovian metamorphism, Scotland: 40Ar/39Ar apparent age patterns and the duration of metamorphism in the biotite zone

Abstract: A geochronological traverse across the Barrovian metamorphic series, Scotland, shows 40Ar/39Ar apparent age spectra that reflect the influence of progressive metamorphism during the Grampian orogenic episode. The lowest-grade units of the Barrovian metamorphic series retain pre-Grampian detrital ages as components of their white mica 40Ar/39Ar apparent age spectra. These relict ages are progressively obliterated in the direction of increasing metamorphic grade, with a Grampian-age 40Ar/39Ar step-heating plateau first occurring in the biotite zone. The microstructure at this point shows only limited recrystallization, suggesting loss of argon mainly by diffusion. Forward modelling of argon diffusion from white mica grains was therefore carried out, for various thermal histories and grain sizes, to match 40Ar/39Ar step-heating apparent age spectra patterns preserved within the biotite zone of the Barrovian metamorphic series. The results imply a thermal duration of between 1 and 10 Ma for Barrovian metamorphism in the biotite zone. Such short time scales for metamorphism place a limit on length scales for the heat sources responsible. Mid-crustal extensional ductile shear zones that crop out in the NE of the Grampian Terrane once focused narrow, Grampian-age heat sources (e.g. magmas, hot fluids, shear heating) that drove a brief thermal episode, resulting in the Barrovian metamorphism. Supplementary material: 40Ar/39Ar data tables and plots, including (1) data from analyses on flux monitors and plots used for J-factor determination, and (2) data from analyses on unknowns and 40Ar/39Ar step-heating spectra and inverse isochron plots for each unknown, are available at http://www.geolsoc.org.uk/SUP18442.

Lithospheric-scale extension during Grampian orogenesis in Scotland

Abstract This contribution presents a new model for the Grampian-age tectonothermal development of the Buchan Block and Barrovian-type regions to its west, in the Grampian Terrane, Scotland. The model has drawn on evidence gathered from field mapping, microstructural analysis, metamorphic petrology and mafic magma geochemistry to propose that emplacement of the Grampian gabbros and regional metamorphic heating associated with production of Barrovian- and Buchan-type units occurred during syn-orogenic (Grampian-age), lithospheric-scale extension. Extension followed lithospheric thickening associated with the initiation of Grampian orogenesis and was followed by renewed lithospheric thickening and termination of the extensional heating. Mantle melting to produce the Grampian gabbros of the Grampian Terrane was achieved by extensional thinning of the lithosphere and decompression melting of the asthenosphere at depths of less than 70 km. Advection of heat from the mantle with emplacement of the Grampian gabbros augmented elevated heat budgets associated with attenuation of isotherms during extension. Deposition of the uppermost Dalradian (the Whitehills and Boyndie Bay Groups and the Macduff Slates) occurred during Grampian-age lithospheric extension. A gently-dipping, mid-crustal detachment focused metamorphic heat sources and accommodated significant lithospheric-scale strain, allowing independent thermal evolution of units in its hanging wall (the Buchan Block) and footwall (Barrovian-type units).

Effect of joint orientation on the hydromechanical behavior of singly jointed sandstone experiencing undrained loading

The hydromechanical behavior of singly jointed sandstone under undrained triaxial conditions was studied. Maximum induced pore water pressure was observed to increase with increasing joint orientation from 0° to 45°, for all confining pressures considered. For a 60° joint orientation, a considerable drop in maximum induced pore water pressure was observed (relative to the neighboring 45° and 75° joint orientations), under 4 and 10 MPa of confinement. At 25 MPa of confinement, the drop in maximum induced pore water pressure for the 60° joint orientation was not so pronounced. These observations were explained in terms of failure mechanism (sliding versus shearing) for the different joint orientations. For sliding failure, pore water pressure at failure did not decrease significantly from the maximum value, whereas a marked pore water pressure decrease (after maximum) was observed in the process of sample loading when failure involved intact material rupture. Peak differential stress values at different joint orientations showed that the critical joint orientation was 60° and increasing confining pressure can reduce the influence of joints on the compressive strength of rock. Volumetric strain data showed that dilation-related volume increase at or close to failure is significant for joint orientations of 0°, 30°, 45°, and 90°, but not for orientations of 60° and 75° at confining pressures of 4 and 10 MPa. At 25 MPa of confinement, none of the samples showed a considerable dilation-related volume increase. The results illustrate the major influence of failure mechanisms (as governed by joint orientation, confining pressure, etc.) on the hydromechanical response of jointed rock.

Constitutive models describing the influence of the geometry of partially-spanning joints on jointed rock mass strength: Regression and fuzzy logic analysis of experimental data

Using existing experimental data from Uniaxial Compressive Strength (UCS) testing, constitutive models were produced to describe the influence of joint geometry (joint location, trace length and orientation) on the UCS of rock containing partially-spanning joints. Separate approaches were used to develop two models: a multivariable regression model, and a fuzzy inference system model. Comparison of model predictions to the experimental data demonstrates that both models are capable of accurately describing the UCS of jointed rock with partially-spanning joints using information relating to joint geometry. However, according to the statistical evaluation methods used for performance evaluation, the multivariable regression model was significantly more accurate. Analysis of predictions made by the fuzzy inference system model showed that it was capable of resolving certain peculiarities in the influence of partially-spanning joint orientation on the compressive strength of rock that, from rock mechanics and fracture mechanics theory, should be expected. The multivariable regression model, whilst more accurate, did not recognise these peculiarities. Due to the additional insight that can be gleaned from the fuzzy inference system modelling, we recommend the use of the fuzzy inference system constitutive model in combination with the multivariable regression model.

Comparative study of the hydromechanical behavior of intact, horizontally jointed, and vertically jointed rocks under undrained conditions

AbstractThis paper investigates the influence of the presence of a joint (vertically or horizontally oriented) on the hydromechanical behavior of sandstone under undrained triaxial conditions. A series of undrained triaxial experiments was performed on intact and horizontally and vertically jointed sandstone samples under different confining pressures and initial pore-water pressures. Joints in jointed samples were artificially created with rough and irregular surface profiles. An increase in pore-water pressure with increasing deviatoric stress was observed for all tests. Maximum induced pore-water pressure values were higher and occurred later in the jointed rock samples when compared to the intact samples at all confining pressures. This may be related to the efficiency of the response of the joint pore volume to loading when compared to that of the matrix pore volume. Moreover, the significance of new pore volume creation (by microcracking during failure) on sample pore-water pressure is dependent on ...