Lecture 2: Causes of metamorphism

Transcription

1 Lecture 2: Causes of metamorphism Metamorphism refers to a suite of processes that change the mineralogy, composition and texture of pre-existing materials this is a broad definition and certain industrial processes could be classified as metamorphism, although we generally see metamorphism as resulting from changes in pressure and temperature in the Earth s crust and mantle metamorphism always acts on a pre-existing material, such as a rock or sediment, which had a different mineralogy and structure a rock that has developed its characteristics by metamorphic processes is called a metamorphic rock Causes of metamorphism The Earth is dynamic as a result of plate tectonics, and materials formed under one set of conditions are therefore invariably subjected to another set of P, T and /or σ conditions. The response of the rock to these changes is called metamorphism development of extreme pressures in subduction progressive burial regional scale deformation heating around intrusions

2 Causes of metamorphism - conditions in the Earth Plate tectonic processes subject rocks to a wide range of metamorphic conditions with gradational boundaries to diagenesis and the igneous realm: Pressure (kbar) conditions not found on Earth Temperature ( o C) Dora Maira melting granite fluid-saturated Kokchetav quartz coesite graphite diamond Su-Lu fluid-absent granite melting Napier, Antarctica Scourian, Scotland fluid-absent fluid-absent basalt basalt melting melting 7 modified after Bucher and Frey 22, Spear 1993, Stein depth (km) Depth (km) mantle solid outer core inner core Temperature ( o C) mantle melting curve iron melting curve melt average conditions in the Earth Causes of metamorphism - three main settings 3 main settings for metamorphism: burial in sedimentary basins; regional scale collision zones (orogenic and subduction); and contact metamorphism development of extreme pressures in subduction progressive burial regional scale deformation heating around intrusions

3 Causes of metamorphism - conditions in the Earth Temperature ( o C) Pressure (kbar) 1 2 Dora Maira fluid-saturated granite melting Kokchetav Napier, Antarctica Scourian, Scotland depth (km) Mars burial metamorphism regional orogenic metamorphism conditions not found on Earth Su-Lu fluid-absent granite melting modified after Bucher and Frey regional subduction metamorphism contact or pyrometamorphism Causes of metamorphism - three main variables temperature: pressure: stress: leads to different minerals, larger minerals, and eventually melting of the rock preference for lower volume minerals, compaction of the rock and loss of volatiles (mostly H2O and CO2) makes textures and fabrics in rocks as minerals align themselves Any system will counteract the change it is subjected to: le Chatelier s principle The overall effect is to change a precursor/protolith to a sequence of successively higher grade metamorphic rocks.

4 From protolith to metamorphic rock The overall effect of changing conditions is to change a protolith to a sequence of successively higher-grade metamorphic rocks: prograde metamorphism. clay (pelite) spotted slate hornfels phyllite schist gneiss migmatite mylonite Causes of metamorphism - three main variables temperature: pressure: stress: leads to different minerals, larger minerals, and eventually melting of the rock preference for lower volume minerals, compaction of the rock and loss of volatiles (mostly H2O and CO2) makes textures and fabrics in rocks as minerals align themselves let s have a closer look at each one

5 Pressure in the Earth Lithostatic pressure is the pressure exerted by the overlying column of material: P = ρ g z in Pa = kg m -1 s -2 = 1-5 bar where: ρ = density of the overlying material ( kg m -3 ) g = gravitational acceleration (~ 9.8 m s -2 ) z = column height, or depth ( m ) ρgranite ρbasalt ρperidotite ρlimestone ρpelite 27 kg m -3 3 kg m kg m kg m kg m -3 1 km exerts a P of: 264 bar 294 bar 323 bar 245 bar 255 bar Density of rocks is not constant with changing P and T, but depends on mineralogy and especially on their volatile content Pressure in the Earth ρgranite ρbasalt ρperidotite ρlimestone ρpelite 27 kg m -3 3 kg m kg m kg m kg m -3 Semprich et al. 21 IJES

6 Pressure in the Earth P (bar) density ρgranite ρbasalt ρperidotite ρlimestone ρpelite 27 kg m -3 3 kg m kg m kg m kg m -3 Calculated with PerpleX Temperature in the Earth The change in temperature with depth in the Earth is called a geotherm. This geotherm is different for different geo-tectonic settings: Temperature ( o C) Pressure (kbar) conditions not found on Earth fluid-absent granite melting fluid-absent basalt melting depth (km) 1. shield 2. present-day continents 3. Archaean geotherm 4. active rift

7 Temperature in the Earth Can calculate a steady-state geotherm (for a static crust) using: -A z T = 2 Q + A D + z + 2 o C 2 k k k where: A = heat production in the crust ( μw m -3 ) k = thermal conductivity (W m -1 K -1 ) z = depth ( m ) Q = mantle heat flow into the crust ( W m -2 ) D = crustal thickness ( m ) Typical values for these variables: A granite A gabbro A peridotite 1-3 μw m -3.1 μw m -3.1 μw m -3 k = W m -1 K -1 lower for higher T Q = 2-4 mw m -2 D = 3 km stable crust; 7 km thickened Temperature disturbances Steady-state geotherms are disturbed by a variety of processes including plate-tectonic transport of rocks and heat input by intrusion of magma active transport of cold rocks into the hot mantle doubling of the crust by collisional stacking extra source of heat to add to geotherm

8 P-T paths for rocks in disturbed geotherms For a steady-state geotherm, there is a known P + T at any depth: expect an equilibrium mineral assemblage + texture at any depth However, when the geotherm is disturbed, the T at a given depth changes with time and so the rocks are subjected to a metamorphic P-T path from Bucher and Frey 22 Stress in the Earth In addition to temperature and pressure, the third main variable that controls the appearance of metamorphic rocks is stress pressure is a uniform force that acts on a rock and compresses it or pulls it apart. When this force field is unequal in different directions, we call it stress a rock responds to this unequal pressure by deforming, or yielding (e.g. flattening), and we call this response to stress strain when this stress is accompanied by movement, we call the response of the rock shear In fact, shear is a general term that describes all types of rock deformation, including that resulting from simple flattening, which is called pure shear Plate tectonic processes introduce stress by forcibly moving masses of rock, but stress even develops in sedimentary basins during burial.