metamorphic petrology terms

closed system

metamorphism of a protolith w/o addition or loss of mass

open system

metamorphism of a protolith w/ addition or loss of mass

contact metamorphism

changes over limited volumes of rock (high temp)

regional metamorphism

changes over large volumes of rock (high pressure & temp)

barrovian sequence

moderate temp/pressure, typical orogenic metamorphism

buchan sequence

low pressure/temp, typical contact metamorphism

index mineral

characteristic minerals that appear in a predictable sequence w/ increasing P/T

zone

area over which an index mineral occurs

isograd

boundary between zones

metamorphic facies

characteristic mineral assemblage produced in a particular protolith that was metamorphosed at a particular P/T

corollary

for a given P-T, different mineral assemblages will develop in different protoliths

idioblastic

euhedral grains

subidioblastic

subhedral grains

xenoblastic

anhedral grains

matrix

set of minerals with a similar grain size

porphyroblast

mineral growth focused on a few nuclei, associated with static recrystallization

porphyroclast

surviving large mineral grains (often k spars)

fabric

the repetitive appearance of any planar or linear feature in a rock

schistocity

foliation due to alignment of platy minerals (micas)

cleavage

very fine-grained version of a schistocity

gneissocity

foliation due to compositional layering/banding

dissolution

breakdown of bonds in the presence of a fluid

precipitation

crystallization of material from a supersaturated fluid

intracrystalline plastic deformation

distortion of crystal lattices

recovery

reduction in crystal defect density

recrystallization

formation of new grains with distinct grain boundaries and crystallographic orientations

grain boundary migration (dynamic recrystallization)

a defective grain gets consumed by a nearby defect-free grain

sub-grain rotation (dynamic recrystallization)

a grain pushes out its defects to the grain boundary and gets re-oriented

static recrystallization

rocks deform slowly, causing grain coarsening and grain boundary straightening

poikiloblast

porphyroblasts that have inclusions, showing rapid growth or low interfacial energy between host and inclusions

solid-solid net transfer reactions

type of discontinuous reactions that only involve solid minerals

polymorphic transformation

type of discontinuous reaction involving polymorphs (no formula change)

devolatilization reaction

either continuous or discontinuous, produces/consumes volatile species

ion exchange reaction

type of continuous reaction with a 1:1 exchange of ions between two phases

migmatite

crustal rocks that have been partially melted (anatexis)

diffusion

motion of individual atoms or molecules relative to a medium

advection

passive motion of atoms and molecules as part of a moving medium

pervasive fluid flow

flow along grain boundaries and through microfractures

channelized fluid flow

fluid flow concentrated along a few pathways

steady-state permeability

the long-term permeability at depth

dynamic/transient permeability

the permeability of rocks during certain events that cause higher strain rates (fault/shear zone movement, earthquakes)

brine

fluid with increased halogens (like cl- and f-) that cause enhanced mineral solubility

metasomatism

change in rock bulk composition through open system mass transfer

local equilibrium

when minerals that are not in equilibrium create a buffer contact between them

thermobarometry

calculation of an exact pressure/temperature using measured mineral compositions

geothermometer

a reaction that is insensitive to changes in pressure and sensitive to changes in temperature (steep slope)

geobarometer

a mineral that is insensitive to changes in temperature and sensitive to changes in pressure (shallow slope)

mineral assemblage diagram (pseudosection)

type of pressure-temperature grid that can be calculated from a precisely known bulk composition

metamorphic event

time when a particular mineral assemblage formed and equilibriated

deformational event

time when rocks accumulated strain in response to (deviatoric) stress

burial

rising pressure without significant change in temperature when lots of deformation occurs in rocks

exhumation

when a rock cools down and fluid inclusions get trapped within it

thermal relaxation

when the geothermal gradient smooths out over time from a sharp change

post-tectonic poikiloblast fabric

when the inclusions in a poikiloblast are continuous with and match the orientation/pattern of the surrounding minerals

pre-tectonic poikiloblast fabric

when the inclusions in a poikiloblast are discontinuous with and do not match the orientation/pattern of the surrounding minerals

syn-tectonic poikiloblast fabric

when the inclusions in a poikiloblast are continuous with the surrounding minerals, but have a different orientation/pattern