Daniel R Viete | Johns Hopkins University (original) (raw)

Papers by Daniel R Viete

Large earthquakes occur in rocks undergoing high-pressure/low-temperature metamorphism during sub... more Large earthquakes occur in rocks undergoing high-pressure/low-temperature metamorphism during subduction. Rhythmic major-element zoning in garnet is a common product of such metamorphism, and one that must record a fundamental subduction process. We argue that rhythmic major-element zoning in subduction zone garnets from the Franciscan Complex, California, developed in response to growth-dissolution cycles driven by pressure pulses. Using electron probe microanalysis and novel techniques in Raman and synchrotron Fourier transform infrared microspectroscopy, we demonstrate that at least four such pressure pulses, of magnitude 100–350 MPa, occurred over less than 300,000 years. These pressure magnitude and time scale constraints are most consistent with the garnet zoning having resulted from periodic overpressure development-dissipation cycles, related to pore-fluid pressure fluctuations linked to earthquake cycles. This study demonstrates that some metamorphic reactions can track individual earthquake cycles and thereby opens new avenues to the study of seismicity.

Short-duration regional metamorphism is a recently observed and poorly understood phenomenon in m... more Short-duration regional metamorphism is a recently observed and poorly understood phenomenon in metamorphic geology. In this review, it is defined as metamorphism on time scales that limit length scales (of the associated thermal anomaly) to significantly less than the thickness of the orogenic crust (<10 myr) or subducted oceanic lithosphere (<5 myr). Without appealing to exceptional heat sources, thermal models have been unable to account for peak metamorphic temperature during collisional orogenesis and subduction. This observation, combined with restricted time scales for regional metamorphism, suggests that metamorphic facies series can record atypical and transient thermal conditions (related to punctuated and localized heat advection and/or production), rather than normal, ambient conditions for the tectonic setting to which they are allied. High-precision geochronology can resolve short-duration metamorphic estimates of 1 – 10 myr. However, diffusion geospeedometry typically yields extremely short metamorphic durations (<1 myr); tools in metamorphic geology may have matured to the point that the discipline is beginning to recognize episodicity and criticality in deep processes. New, very high-precision petrochronology techniques offer great potential to probe the veracity of extremely short metamorphic durations being obtained from diffusion geospeedometry. Benchmarking of these new very high-precision petrochronology techniques must become a priority for metamorphic geology.

Single-shot laser ablation split stream (SS-LASS) petrochronology offers spatial resolution of < ... more Single-shot laser ablation split stream (SS-LASS) petrochronology offers spatial resolution of < 1 μm per (surface) analysis. This enables the technique to interrogate metamorphic zircon overgrowths that are very thin, or that preserve a composite age structure. To demonstrate the advantages of SS-LASS, the technique was applied to metamorphic zircon overgrowths in five rocks from the Cordillera de la Costa, Venezuela. In these rocks, SS-LASS was able to decipher discrete, short-duration (< 10^6 yr) zircon growth events at c. 33.0, c. 28.3, c. 23.0 and c. 18.2Ma. Comparison with existing geo/thermochronology suggests that the SS-LASS dates represent (hydro)-thermal events that mark distinct episodes of tectonism affecting the northern margin of South America.

With an external error of 5% incorporated into isotope ratios (as required to achieve satisfactory analytical precision), SS-LASS dates for reference materials were accurate to within 1.5% of published values. The SS-LASS technique is thus, in favorable cases, capable of providing < 10^6 yr resolution for Cenozoic rocks. Methods in ‘geospeedometry’—which employ forward models to reproduce observed diffusion length scales—have obtained anomalously short time scales of 10^4–10^6 yr for metamorphism. The high spatial and temporal resolution of SS-LASS offers tremendous promise for investigating the veracity of these claims.

Discussion of 'Metamorphic P-T and retrograde path of high-pressure Barrovian metamorphic zones n... more Discussion of 'Metamorphic P-T and retrograde path of high-pressure Barrovian metamorphic zones near Cairn Leuchan, Caledonian orogen, Scotland' D. R. Viete, G. J. H. Oliver and S. A. Wilde comment: First, we would like to commend Aoki et al. (2013) on a careful study and thought-provoking manuscript.

New SHRIMP U/Pb zircon ages of 472.2 ± 5.8 Ma and 471.2 ± 5.9 Ma are presented for the age of pea... more New SHRIMP U/Pb zircon ages of 472.2 ± 5.8 Ma and 471.2 ± 5.9 Ma are presented for the age of peak metamorphism of Barrovian migmatites. 40Ar/39Ar ages for white mica from the Barrovian metamorphic series are presented, and are recalculated using recently-proposed revisions to the 40K decay constants to allow more precise and accurate comparisonwith U/Pb ages. The 40Ar/39Ar ages are found to vary systematically with increasing metamorphic grade, between c. 465 Ma for the biotite zone and c. 461 Ma for the sillimanite zone. There is no evidence for any significant metamorphic heating during the first 15 Myr of the Grampian Orogeny (before c. 473 Ma) or the final 4 Myr (after c. 465 Ma). The Barrovian metamorphism occurred over a period of ~8 Myr within the ~27-Myr Grampian Orogeny. The Barrovianmetamorphismrecords punctuated heating, was temporally and spatially associated with large-scale bimodal magmatism, and developed within crust that was not overthickened. The temporally distinct nature of the Barrovian metamorphic episode within the Grampian Orogeny, and its heating pattern and tectonic context, are not consistent with significant heat contribution from thermal equilibration of overthickened crust. Rather, the Barrovian metamorphism records a transient phase of crustal thermal disequilibrium during the Grampian Orogeny. Temporal and spatial association with Grampian bimodal magmatism is consistent with production of the Barrovian metamorphic series within the middle crust as the result of advection of heat from the lower crust and/or mantle. The Barrovian metamorphic series—the classic example of ‘orogenic regional metamorphism’—did not form in response to crustal thickening and thermal relaxation, but appears to record large-scale contact metamorphism.

Uniaxial Compressive Strength (UCS) testing of Hawkesbury sandstone was carried out at various te... more Uniaxial Compressive Strength (UCS) testing of Hawkesbury sandstone was carried out at various temperatures between 25 and 950 °C to explore the mechanical response of the sandstone to significant changes in temperature, as expected for enhanced geothermal energy systems, nuclear waste disposal and underground coal gasification. The UCS testing results demonstrate a mechanical dependence on temperature whereby the compressive strength and elastic modulus for the sandstone increases with increasing temperature for temperatures less than c. 500 °C and decreases with increasing temperature for temperatures greater than c. 500 °C. X-ray diffraction analyses performed on material from the failed 25 and 950 °C specimens highlights a distinct difference in mineralogy between the two specimens that has been related to mineralogical changes in the sandstone cement with heating. Progressive dehydroxylisation of kaolinite in the sandstone cement at temperatures beyond 500 °C appears to have enabled transformation plasticity, explaining the weakening and softening of the sandstone that was observed with increasing temperature beyond 500 °C. Transformation plasticity is mineralogy dependent and thus its influence on mechanical behaviour of rock will vary with bulk mineralogy and the relative distribution of mineral species. Comparison of the results from the UCS testing to those obtained from similar experimental work carried out on different sandstone units highlights variability in the response of rock to heating. The study provides a word of caution regarding the need for accurate understanding of the influence of temperature on the mechanical behaviour of the specific rock unit considered for a given elevated temperature engineering application. Such understanding requires consideration of the geological history of the rock in addition to its physical properties and mineralogy.

In this paper we examine the length scales of major element diffusion in garnet during the Barrov... more 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 _m Mn diffusion in sillimanite-zone garnets during the Barrovian regional metamorphism. Sillimanite-grade garnets from the Barrovian metamorphic series also preserve c. 100 _m 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.

A geochronological traverse across the Barrovian metamorphic series, Scotland, shows 40Ar/39Ar ap... more 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.

This contribution presents a new model for the Grampian-age tectonothermal development of the Buc... more 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).

Carbon dioxide displays a strong affinity for coal due to its propensity to adsorb to the coal su... more Carbon dioxide displays a strong affinity for coal due to its propensity to adsorb to the coal surface. The process of CO2 adsorption on coal causes lowering of surface energy and, it is hypothesised that an associated decrease in surface film confinement results in a decrease in material tensile resistance. Following the results of work carried out on the mechanical influence of CO2 on brown coal under in situ conditions [Viete DR, Ranjith PG. The effect of CO2 on the geomechanical and permeability behaviour of brown coal: implications for coal seam CO2 sequestration. Int J Coal Geol 2006;66(3):204–16], a theoretical explanation is proposed for the perceived lack of a weakening effect with the adsorption of CO2 to coal at significant confining pressures. We propose that at significant hydrostatic stresses, resistance to failure is otherwise provided (by external confinement) and the effects of adsorptive weakening are concealed. Our model predicts that adsorptive weakening, fracturing under in situ stresses, and associated permeability increases are not an issue for coal seam CO2 sequestration for sufficiently deep target seams. Lowering of the elastic modulus of coal upon introduction of CO2 may proceed by means other than surface energy lowering and could well occur irrespective of the depth of sequestration. The effect of elastic modulus lowering under in situ conditions would be beneficial for the long-term retention of sequestered gases.

Theory from fracture mechanics and thermodynamics coupled with the results of experimental studie... more Theory from fracture mechanics and thermodynamics coupled with the results of experimental studies provides evidence to suggest that the adsorption of carbon dioxide on coal causes a decrease in the coal strength. Coal weakening by the introduction of CO2 to a coal seam may induce fracturing, causing a permeability increase under in situ conditions. Such effects present significant implications for proposals regarding the storage of CO2 in coal seams.

A uniaxial and triaxial laboratory study was carried out to explore the effects of the adsorption of CO2 on the compressive strength and permeability of southeast Australian brown coal. Comparison of the stress–strain response of air-saturated and CO2-saturated specimens revealed a compressive strength decrease in the order of 13% and an elastic modulus decrease of about 26% for the uniaxial testing, but no significant strength or elastic modulus decrease for the triaxial testing. The absence of an adsorptive effect on the mechanical behaviour of the triaxial specimens may have been due to an insufficient saturation period under simulated ground conditions, or due to mechanical variability in the brown coal test specimens, however, further testing is required to reveal the reason for the apparent negligible strength reduction with CO2 adsorption at the higher confinement. Carbon dioxide outflow measurements during the stress–strain process demonstrated an initial permeability decrease with pore closure, followed by a significant increase in specimen permeability with fracturing.

Issues that require consideration in the application of these results to coal seam CO2 sequestration include: whether the expected regional and localised in situ stresses are sufficient to initiate fracturing with adsorptive weakening; how coal properties (e.g. rank, moisture content) are likely to affect the geomechanical influence of CO2 adsorption, and the expected magnitude of the proposed fracture related permeability increase.