GLY 4310C WORD LIST FOR MIDTERM 2
Spring, 2012
The second exam will be on Monday, March 26, 2012 from 9-10:20 a.m. It will cover Chapters 4, 7, 10, 13, 14 of Winter, Homeworks 1, 2 and 3, and associated lecture material.
Igneous Structures and Field Relationships
Viscosity
Temperature
Composition
Volatile Content
Types of gases associated with magma
Late escape of gas leads to spattering
Textures associated with gas
Scoria
Pumice
Volcanic vent types
Shield
Strato- or composite
Domes (coulées)
Pressure ridges
Crypto
Fissure
Calderas
Resurgent
Maars
Phreatic explosion
Tuff rings
Tuff cones
Lava flows
Pahoehoe
Aa
Lava tunnels
Aphanitic texture
Columnar joints
Pillow basalts
Volcanoclastic
Pyroclastic
Tephra
Fall deposits
Plinian
Areal extent of large eruptions
Flow Deposits
Collapse of a vertical cloud
Lateral blast
Low pressure
Dome collapse
Pyroclastic Flows
Surges
Ignimbrites
Tuff
Welded tuff
Intrusive Igneous Deposits
Tabular
Concordent
Non-concordent
Sill
Dike
Multiple
Ring dikes
Cone sheets
Veins
Non-tabular
Plutons
Stocks
Plug
Batholiths
Lopolith
Laccolith
Border zones
Assimilation
Contact metamorphic aureole
Schlieren
Time of emplacement
Pretectonic
Syntectonic
Posttectonic
Depth of intrusion
Epizone
Mesozone
Catazone
Multiple intrusions
“Room problem”
Magmatic stoping
Zone melting or solution stoping
Ballooning
Hydrothermal Systems
Black and white smokers
Reaction Series and Melting Behavior
N.L. Bowen
Reaction principle
Bowen’s Reaction Series - based on study of basalts
Continuous reactions - felsic
Discontinuous reactions - mafic
Factors not considered in Bowen’s work
Oxygen fugacity
Thermodynamics review
Gibbs free energy
Isobaric system
Isothermal system
Clapeyron equation
Effect of increasing pressure on melting behavior
Effect of fluids on melting behavior
Fluid saturation
Le Châtlier’s Principle
“Dry” vs. “wet”
Bridging vs. Non-bridging oxygens
Effect of water on systems of different composition
Effect of carbon dioxide on systems of different composition
Generation of Basaltic Magma
Magma series
J.P. Iddings
Alkaline
Sub-alkaline
C.E. Tilley
Sub-alkaline divided
Calc-alkaline
Tholeiitic
Melting within the Mantle
Indirect Samples:
Ophiolites
Dredge samples from fracture zones
Nodules in basalts
Xenoliths in kimberlites
Stony Meteorites
Composition of the mantle - ultramafic
Rock types
Plagioclase lherzolite
Spinel lherzolite
Garnet lherzolite
Fertile vs. depleted (residuum)
Heating above normal geotherm
Radioactive heat
Hot spots
Melting by decompression
Adiabatic condition
Volatiles in the mantle
Mantle composed of anhydrous minerals
Phlogopite, amphibole, serpentine are alteration products
Role in seismic low-velocity zone
Formation of basalt from a chemically homogeneous mantle
A.E. Ringwood
Experiments on pyrolite
Lack of spinel at any pressure
Formation of alkaline and tholeiite basalts
Limited to the effects of partial melting
P. Wyllie - Effects of fractional crystallization
Production of nephlinite
Hirose and Kushiro
Effect of pressure on silica saturation
Primary, Parental, and Derivative Magmas
Saturation
Single
Multiple
Relation to position in phase diagrams
Forward vs. Reverse Methods
Criteria for showing that a magma is primary
Difficulty in proving a magma is primary
Formation of basalt from a chemically heterogeneous mantle
Fertile, enriched, and depleted xenolites
Trace element patterns
MORB - HREE enrichment
Suggests mantle is LREE depleted
OIB - no HREE enrichment
Derived from fertile mantle
Suggests mantle has at least two distinct compositions
Isotope patterns
143Nd/144Nd
87Sr/86Sr
Slope if isochrons for melt and residuum for each system
MORB vs. OIB
Mantle Circulation models
Traditional one-layer
Two layer, separation at 660 km
Implications for magma generation
Generation of tholeiite
Generation of alkaline basalt
Melting of dry lherzolite at depths > 200 km
Possible ΔV = -
Implications
Mid-Ocean Ridge Volcanism
Mid-Ocean Ridge
About 65,000 kms long
Found in Atlantic, Indian and Pacific Oceans
Averages 2000 kms wide
Often, but not always, bilaterally symmetric
Very high heat flow
Extensive hydrothermal system
Earthquakes common
Associated with normal faulting
Ridges in isostatic equilibrium
Spreading rates
Fast-spreading ridges
Morphology
Slow-spreading ridges
Morphology
Magma generation 5-20 km3 a-1
Oceanic Crust and Upper Mantle Structure
Layer 1
Pelagic Sediment
Layer 2A
Pillow Basalt
Layer 2B
Vertical Sheeted Dikes
Layer 3A
Isotropic Gabbro atop transitional gabbro
Layer 3B
Layered gabbros with cumulate textures
Layer 4
Ultramafic rock
Seismic Moho
Petrologic Moho
Differences between Ophiolites and Oceanic Crust
Obduction
MORB Petrography and Major Element Geochemistry
Differences from other basalts
Fenner variation diagram
Use of Mg rather than silica
Trends
Change in CaO/Al2O3 as crystallization proceeds
Clinopyroxene likely responsible for calcium removal
Pearce diagrams
Pyroxene paradox
Observations about MORB's
Magmas not completely uniform
Show chemical trends consistent with fractional crystallization of ol, plagioclase and sometimes cpx
Derivative magmas
Composition usually near low-pressure cotectic for ol-plag-cpx system
Mg#'s usually less than 65
Definition of Mg#
Incompatible element geochemistry
K, Ti, and P enriched by 200-300% in MORB
Slow spreading center - crystallization in the mantle
Fast spreading center - crystallization in the crust
Variations in incompatible elements suggest two source rocks
N-MORB = Normal
E-MORB = Enriched
Off-axis magmas more evolved than axis magmas
MORB Trace Element Chemistry
N-MORB has large LREE depletion
E-MORB has large LREE enrichment
HREE patterns for both are similar
T-MORB = transitional, and have values between N and E MORB's
Plots of La-Sm vs. Mg#
MORB Isotope Chemistry
N-MORB's have depleted mantle source
E-MORB's show more enriched values
T-MORB's show intermediate values
Petrogenesis of MORB's
Potential parent saturated with ol, opx, and cpx at 0.8-1.2GPa
Within spinel lherzolite field
Lack of HREE depletion excludes garnet lherzolite
No europium anomaly, expected if plagioclase lherzolite were the source
Trace element/Isotope geochemistry sets point of separation, not ultimate source region
Ultimate source depth
N-MORB, down to 80 kms
E-MORB, > 80 kms
Divergent plates create openings
N-MORB's melt by decompression 60-80 kms
15-40 Partial Melting
Partial melting terminates when heat losses to surface prevent further melting
Disappearance of cpx may terminate melting, because melting temperature jumps after cpx lost
Melt blobs separate from residuum at 25-35 km depth
Upward migration to axial magma chamber at 1-2 km depth
Axial Magma Chambers
Original Model
Narrow, up to 5 km wide
Depth to 9 km
Periodic injections of parental magma, followed by fractional crystallization
Dikes created by upward magma movement, creating Sheeted Dike Complex
Crystallization around periphery creates gabbros of Layer 3
Divergence would expand magma chamber, preventing total solidification
Cann called this the "Infinite Onion" model
Dense ol, opx settle, creating ophiolite layers of Layer 3, possibly in Level 4
Model is of "Open System" type
Problem: Magma chambers not observed seismically - must discard or modify model
Fast-spreading Ridge Model
Magma lens 10's to 100's of meters thick, < 2km wide, and 1-2 km deep
Sub-horizontal seismic reflector, compatible with seismic data from EPR
Mush region - solid enough to transmit S-waves
Melt up to 30%
In-situ crystallization (Langmuir)
Seismically very slow
Transition region
Seismically slow
Mush-Transition boundary a rigidus
Solidification exceeds 50%
Assemblage behaves as crystalline aggregate
Off-axial intrusions and extrusions
Help to explain rapid thickening of layer 2A
Melt region has gaps at fracture zones
Model is of "Open System" type
Slow-spreading Ridge Model
Heat flow much less than fast ridges
Persistence of magma chambers doubtful
Dike-like mush zone with a small transition zone replace magma lens of fast-ridge model
May have small, ephemeral magma bodies along ridge - Infinite Leak model
Less differentiation than fast-spreading ridges
Polybaric fractionation or plagioclase accumulation may occur in magma blobs
Model is of "Closed System" type
"Global averages" - work from C.H. Langmuir laboratory
Averages for approximately 100 km segments - removes local variations
Conclusions:
Global correlations are controlled by differences in thermal regime, not magma composition differences
Thermal regime along ridge segment controls quantity and composition of MORB's
Melts are extracted from depth without low-pressure re-equilibration
Local trends vary from global average, and are different in slow and fast-spreading environments
Hot spots play an important role in some MORB chemical composition - Azores, Iceland
Oceanic Intraplate Volcanism
OIB - Ocean Island Basalt
OIT - Ocean Island Tholeiite
OIA - Ocean Island Alkaline Basalt
Hot spot
Aseismic ridge
Kinks in hot spot trails
Hawaiian Island volcanism
Pre-shield
Shield-building
Post-shield
Post-erosional
OIT Chemistry
K2O, TiO2, and P2O5 higher thn MORB
Al2O3 lower than MORB
OIA Chemistry
Much greater variability than OIT or MORB
Comparison of island basalt chemistry
Tholeiitic - Iceland
Silica Undersaturated alkaline - Tristan de Cunha
Silica saturated alkaline - Ascension
Origin of OIT
Less extensive partila melting than MORB
Melting of less depleted mantle
Origin of OIA
Complex melting processes
Very heterogeneous mantle
Both
OIB Trace Element Chemistry
LIL enrichment (Large-Ion Lithophile) - Used to evaluate:
Source composition
Degree of partial melting and composition of residual phases
Subsequent fractional crystallization
HFS - High-Field Strength Elements
Compatible elements - Ni, Cr
REE elements
LREE - Light rare earth elements
La/Sm slope
E-MORB, OIA, & OIT have negative slopes
N-MORB has a positive slope
HREE - Heavy rare earth elements
Garnet effects HREE
Incompatible elements
Spider diagrams
LIL enriched
HFS enriched
OIB Isotope Chemistry
Mixing patterns in multi-reservoir systems
Possible reservoirs
DM - Depleted Mantle
BSE - Bulk Silicate Earth
PREMA - PREvalent Mantle
EMI - Enriched Mantle I
EMII - Enriched Mantle II
HIMU - High μ, where μ = 238U/204Pb
Nd/Sr Isotope data
U/Pb and Th/Pb isotope data
NHRL - Northern Hemisphere Reference Line
Origin of HIMU
Subducted, recycled oceanic crust
Localized loss of lead to earth’s core
Deep metasomatism
DUPAL - Dupré and Allègre
Indian Ocean Volcanoes
Plot above NHRL due to enrichment from either EMI or EMII
Geographic distribution of Pb Isotope Anomalies
Petrogenesis of OIB
Different source than N-MORB
Possibility:
Mantle below 660 km
How can crustal rock be subducted
Conversion of oceanic crust to eclogite
Upper mantle material cold, and therefore denser
Accumulation of plates around 700 km?
Some subducted material may reach core-mantle boundary
Source of hot-spot plumes?
OIA generated by plume volcanism
Possible melting of basalt/peridotite mixture
5-15% partial melting yields OIA
15-30% partial melting yields OIT
Less if eclogite from basalt is melting
Questions or comments? mailto:warburto@fau.edu
Last updated: February 20, 2012