Earth Materials Exam 2

indirect vs direct constraints on the composition of the mantle

Indirect constraints- Seismic observation - mantle is mostly solid 

Direct constraints-

-Upper mantle - Ophiolites (mostly complete slivers mantle shoved onto the continents), Alpine peridotites (incomplete slivers of mantle shoved onto continents), Dredge sample from mid-ocean ridges

- Deeper mantle - Mantle nodules in basalts and Xenoliths in kimberlites

fertile’ and ‘depleted’ mantle

Fertile mantle is original and unmelted and depleted mantle is likely derived from partial melting of lherzolite to produce basaltic melt (refractory residuum)

partial melting

raise temperature and lower pressure (subduction zones add water) - increasing melt generation, melt droplets may become interconnected and separate from the source rock

Magmatic differentiation

separation and concentration of elements from a large volume of magma into a smaller volume - More mafic minerals crystallize first, leftover liquid is more felsic

fractional crystallization, volatile transport, magma mixing, and country rock assimilation

Fractional Crystallization - Crystallization of grains that then separate from melt (disequilibrium crystallization process) - More mafic minerals crystallize first, leftover liquid is more felsic

Volatile transport-  incompatible elements go into vapor phase instead of melt (‘liquid-vapor fractionation’)

Magma Mixing - Mixing of two magmas result in third magma that has composition of both 

Country rock assimilation - magmas may assimilate or partially melt portions of country rock as they rise - Magma inherits chemical characteristics of country rock

Mid-ocean ridge (MOR) volcanism

Divergent plate boundary and most abundant form of volcanism. Consists of series of spreading centers linking by transform faults - produces MOR basalts

NMORB - normal morb from depleted mantle (harzburgites and dunites)

EMORB - enriched morb from fertile mantle (lherzolites)

mantle material partially melts as it rises in response to reduced pressure

ophiolite and its structure

Ophiolites- Cross Section of MOR Lithosphere

Layer 1 - pelagic sediments (clay)

Layer 2 - basaltic pillows and dikes (plagiogranite)

Layer 3 - gabbros- the moho (crust/mantle boundary)

Layer 4 - assorted cumulate ultramafic rocks 

Volcanism at subduction zones

Subducted plate shoved under overriding plate - Melting in overlying mantle wedge due to rising fluids from slab

Generated melts rise, fractionate, interact with overriding plate, and may eventually erupt

Oceanic SZ - Mostly produced volcanic rocks and rocks with mafic to intermediate compositions

Continent SZ - intermediate to felsic extrusive and intrusive igneous rocks

Island and continental arcs

Island arc volcanism - ocean/ocean collision - forms above a subduction zone 

Continental arc volcanism - ocean/continent collision - forms along the margin of a continent where oceanic crust subducts beneath continental crust

‘Granite’ vs ‘granitoid’

Broad umbrella term to describe felsic to intermediate, coarse-grained, plutonic igneous rocks

- contains QAP - usually produced in continent- continent collision (convergence)

- commonly contain parts of country rock

anatexis

Partial melting of crust - need either thickened crust or increased heat flow- Caused by H2O liberation from breakdown of micas in metasedimentary rocks

peraluminous vs metaluminous vs peralkaline.

Peraluminous - Al2O3 content exceeds all other minerals (biotite, garnet, andalusite)

Metaluminous - Na2O, K2O, and CaO exceeds Al2O3 content (pyroxene, hornblende)

Peralkaline - not much Al2O3 (aegirine)

Melt source of S-I-A-M

S - sedimentary 

I - igneous (mafic to intermediate)

A - anorogenic granite - plate interior or rifting 

M - Mantle 

Intraplate volcanism

takes place within a plate instead - partially molten mantle heated by rising mantle plume

OC

Ocean island basalts

Expressed in landscape as volcanic ocean island chains or seamounts (isolated oceanic volcanoes) - tholeiitic and alkaline

CC

Continental Flood Basalts (CFBs) or ‘Traps

Represent outpourings of massive amounts of basaltic lava

Tholeiitic

mantle plumes in intraplate volcanism

plume is often invoked as the cause of volcanic hotspots- Melting triggered by adiabatic rise of plume- driven materials

continental flood basalts, climate, and history of life on Earth.

Prolonged CFB volcanism can influence global climate and drive mass extinction

flood basalts can release large amounts of carbon dioxide and other greenhouse gasses into the atmosphere, contributing to global warming and climate change

metamorphism

Changes to a pre-existing rock’s mineralogy, texture, and/or composition that occur predominantly in the solid state in response to changes in pressure (P), temperature (T), or geologic fluid composition (X)

Protolith

Unmetamorphosed parent rock - important to understand rock history and starting composition

isochemical metamorphism

chemistry of system doesn’t change a lot from protolith to metamorphosed rock

pressure, temperature, and geologic fluid composition

Temp - recrystallization - bigger crystals and dehydration (breakdown of previous minerals and growth of new stable minerals)

Pressure - feel pressure change before temperature, denser metamorphic minerals, deformation - hydrostatic pressure (equal) and deviatoric (variable stress)- creates foliation and lineation

Fluid composition - along grain boundaries, leave system due to lower density, transport chemical constituents

polymetamorphism

Metamorphic rocks with multiple metamorphic episodes episodes

Metamorphic P-T paths

Peak - P-T of highest metamorphic T conditions

Prograde - P-T prior to peak metamorphism

Retrograde - P-T after peak metamorphism

Influenced by tectonic setting

why peak metamorphic minerals are retained in metamorphic rocks

Fluids typically aren’t available on retrograde path to rehydrate the dehydrated peak mineral assemblage

Difficult to ‘unmetamorphosed’ a rock once it has been metamorphosed

Contact metamorphism

Of country rock by intruding plutons

High temp and low pressure

Regional Metamorphism

Convergent plate boundary

Orogenic - mountain building - Variable P-T conditions, commonly results in deformed met rocks

Burial - burying sediments causing diagenesis - Low P-T conditions

Ocean floor - mid ocean ridge - most common - High T, low P conditions

Hydrothermal Metamorphism

via Intruding Hot Fluids in Areas with Elevated Geothermal Gradients

High T, variable P conditions

Fault zone Metamorphism

Metamorphism Along Fault Zones

High P, variable T depending on depth, high degree of deformation

Can be brittle or ductile

Impact Metamorphism

Response to meteorite impact

Extremely high P, moderate to high T

NOT related to tectonics

barrovian metamorphic zones

Different zones based on first appearance of a given mineral

index mineral- appear in response to increasing metamorphic grade

Chlorite, biotite, garnet, staurolite, kyanite, to sillimanite

crystal form

Euhedral - idioblastic

Subhedral - hypidioblastic

Anhedral - xenoblastic

Porphyroblastic

large crystals in fine grained matrix

Relict

feature preserved from protolith

Slate

foliated and planar cleavage

Phyllite

foliated, shiny, planar cleavage

Schist

fine to coarse, foliated, shiny from micas (schistosity)

migmatite

partially molten with leucosome (light) and melanosome (dark)

Granofels

fine grained with isotropic texture, may have differential grain size

Skarn

Calc-silicate rock formed via contact and hydrothermal metamorphism in contact
aureoles

Contact metamorphism textures

Little to no deformation - usually granoblastic

Porphyroblastic → poikiloblastic → skeletal/web texture (increasing volume of inclusion minerals

regional metamorphism textures

• Multiple stages of deformation and metamorphism common

• Lineation (blocky), foliation (sheet), and crenulation (folding)

kinematic behaviors

Prekinematic - before movement Synkinematic - during movement Postkinematic - after movement

PTD

pressure, temperture, and deformation events

M- metamorphic events

D- deformation events

S- fabric (S0- compositional layers, S1- foliation, S2- crenulation)