GLY 4310C                          WORD LIST FOR MIDTERM 2

Spring, 2019

The second exam will be on Thursday, March 21, 2019 from 12:30- 1:50 p.m. It will cover Chapters 4, 7, 10, 13, 14 of Winter, Homeworks 1, 2 and 3, the Silicate Mineral Association Study Sheet, and associated lecture material.

Igneous Structures and Field Relationships




Volatile Content

Types of gases associated with magma

            Late escape of gas leads to spattering

Textures associated with gas



Volcanic vent types


Strato- or composite

Domes (coulées)

Pressure ridges






Phreatic explosion

Tuff rings

Tuff cones

Lava flows



                        Lava tunnels 

Aphanitic texture

Columnar joints

Pillow basalts




Fall deposits


Areal extent of large eruptions

Flow Deposits

Collapse of a vertical cloud

Lateral blast

Low pressure

Dome collapse

Pyroclastic Flows




Welded tuff

Intrusive Igneous Deposits







Ring dikes

Cone sheets









Border zones


Contact metamorphic aureole


Time of emplacement




Depth of intrusion




Multiple intrusions

“Room problem”

Magmatic stoping

Zone melting or solution stoping


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



            C.E. Tilley

                        Sub-alkaline divided



Melting within the Mantle

            Indirect Samples:


                        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




                        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



                        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 = -


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


                     Slow-spreading ridges


          Magma generation 5-20 km3 a-1

Oceanic Crust and Upper Mantle Structure

          Layer 1

                     Pelagic Sediment

          Layer 2A & 2B

                     Pillow Basalt

          Layer 2C

                     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


MORB Petrography and Major Element Geochemistry

          Differences from other basalts

          Fenner variation diagram

                     Use of Mg rather than silica


                     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


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





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


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


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


Quill Pen Questions or comments?

Last updated: March 12, 2019