Temperature gradient metamorphism

THE TEMPERATURE GRADIENT METAMORPHISM OF SNOW MODEL AND FIRST VALIDATIONS USING X-RAY MICROTOMOGRAPHIC IMAGES... 

EXPERIMENT OF TEMPERATURE GRADIENT METAMORPHISM 3.1 Sample Preparation... 

Snow is a porous medium formed of air, ice crystals and small amounts of chemical impurities. Because ice has a high vapor pressure (165 Pa at -15°C, 610 Pa at 0°C), the vertical temperature gradient that is almost always present within the snowpack generates sublimation and condensation of water vapor that change the size and shape of snow crystals. This results in changes in physical variables such as density, albedo, heat conductivity, permeability and hardness. These physical changes have formed the basis for the definition of snow metamorphism. ... 

Next to the temperature gradient inside the snowpack, an important driving force for snow metamorphism is wind, that lifts, transports and redeposits snow crystals, changing snowpack mass and density " and deposits aerosols inside the snowpack.Wind and temperature are climatic variables that determine metamorphism and snowpack physical properties such as albedo and heat conductivity. These properties affect the energy balance of the snow-atmosphere and of the soil-snow interfaces, which in turn affect climate. 

The impact of the temperature gradient on metamorphism explains many of the features of Figure 1. The typical HGM-type metamorphism of the taiga snowpack eventually transforms most of the snowpack into depth hoar, " while the QIM-type metamorphism of the maritime and Alpine snowpacks forms, in the absence of melting, layers of small rounded grains 0.2 to 0.4 mm in diameter. However, considering the effects of other climate variables such as wind speed is necessary to explain features such as the presence of windpacks formed of small rounded grains in the tundra snowpack. 

The grain growth almost always observed during metamorphism results in a decrease in snow SSA. The rate of decrease greatly affects snow albedo and e-folding depth, considered over large spatial and temporal scales. The initial decrease is very fast, with a factor of 2 decrease in I to 2 days. Experimental and field studies have quantified the rate of decrease of snow SSA as a function of temperature and temperature gradient." In all cases, the best empirical fit of SSA decay plots was of the form ... 

During a snowfall, the snow crystals accumulate on the ground and gradually form a complex porous medium constituted of air, water vapour, ice and sometimes liquid water. This ground-lying snow transforms with time, depending on the physical parameters of the environment. This process, called metamorphism, can be divided into three main types the wet snow metamorphism, the isothermal metamorphism, and the temperature gradient (TG) metamorphism. 

The model should now be tested on a larger set of temperature gradients, by using the other images provided by X-ray microtomography. If confirmed, it offers really interesting outcomes for 3D time-lapse numerical simulations of the TG metamorphism. 

In this study, to clarify the effect of water vapor, we chose a wide temperature range (-65 °C to -12 °C), for which the water vapor concentrations differed by a factor 1000 between the lowest and highest temperatures, and carried out snow metamorphism experiment under high temperature gradient in a cold room. The density change with water vapor was measured and the effects of water vapor transport on crystal growth, and on density change were examined experimentally. 

Snow metamorphism under high temperature gradient... 

The relevance of the remarks on sulfur content is that, for reasons explained above, it is usually a valid index of the salinity of the environments of deposition. It was remarked earlier that the Eastern and Interior provinces have experienced different temperature/pressure/time histories. It should be added that coals of the Rocky Mountain, Pacific and Alaskan provinces most probably experienced yet further sets of conditions of metamorphism a locally increased geothermal gradient that produced relatively high temperatures at relatively low depths of burial and hence at relatively low pressures of overburden. 

The established regularities in the evolution of the composition of fluids also explains some particulars of the metamorphism of iron-formations of other t) es. In particular, the frequently observed disappearance of hematite in oxide rocks metamorphosed in amphibolite and granulite facies conditions can be caused by reduction by volatiles, especially by easily diffused hydrogen. Acceleration of diffusion with rise in temperature and increase in concentration gradients of volatiles leads to the appearance of more uniform mineral associations in place of formerly extremely variegated banded sequences. 

Hodges K. V., Hames W. E., and Bowring S. A. (1994) Ari Ar age gradients in micas from a high-temperature-low-pressure metamorphic terrain evidence for very slow cooling and implications for the interpretation of age spectra. Geology 22(1), 55-58. 

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