Ocean basins: OIBs and MORBsOcean basins: OIBs and MORBs

Figure 14-1. After Crough (1983) Ann. Rev. Earth Planet. Sci., 11, 165-193.

- Next Wednesday review, midterm on April 2ndNext Wednesday review, midterm on April 2nd- Lab next weekLab next week

Topics:Topics:

Systematics for:Systematics for:- Mid ocean ridge basalts (oceanic plate)Mid ocean ridge basalts (oceanic plate)- Ocean island basalts (hotspots)Ocean island basalts (hotspots)

Updates/questionsUpdates/questions

Figure 13-3.Figure 13-3. Lithology and thickness of Lithology and thickness of a typical ophiolite sequence, based on a typical ophiolite sequence, based on the Samial Ophiolite in Oman. After the Samial Ophiolite in Oman. After Boudier and Nicolas (1985) Earth Boudier and Nicolas (1985) Earth Planet. Sci. Lett., 76, 84-92. Planet. Sci. Lett., 76, 84-92.

Layer 1: sedimentLayer 1: sediment

Layer 2: a&b: pillowsLayer 2: a&b: pillows

c: sheeted dikesc: sheeted dikes

Layer 3: a: Layer 3: a: transitional transitional gabbros gabbros

b: layered gabbros b: layered gabbros -- magma chamber-- magma chamber

Layer 4: ultramafics Layer 4: ultramafics

-- cumulates, then -- cumulates, then mantlemantle

Oceanic Crust and Upper Mantle StructureOceanic Crust and Upper Mantle Structure

Figure 13-4.Figure 13-4. Modified after Brown and Mussett (1993) Modified after Brown and Mussett (1993) The Inaccessible Earth: An The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall. London.Integrated View of Its Structure and Composition. Chapman & Hall. London.

Petrography and Major Element ChemistryPetrography and Major Element Chemistry

12

10

8

6

4

2

35 40 45 50 55 60 65%SiO2

%N

a2O

+ K

2O

Alkaline

Tholeiitic

Figure 7-2.Figure 7-2. After Bowen (1915), A. J. Sci., and Morse After Bowen (1915), A. J. Sci., and Morse (1994)(1994), , Basalts and Phase Diagrams. Krieger Publishers.Basalts and Phase Diagrams. Krieger Publishers.

Crystallization SequenceCrystallization Sequence

ConstantConstantpressurepressure

ConstantConstantcompositioncomposition

The major element chemistry of MORBsThe major element chemistry of MORBs

MgO MgO and FeO and FeO by by olivineolivine

AlAl22OO33 and CaO and CaO by by

cpxcpx

SiO2 SiO2 less in crystals less in crystals

NaNa22O, KO, K22O, TiOO, TiO22, P, P22OO55

all all : not in crystals: not in crystals

Figure 13-5.Figure 13-5. “Fenner-type” variation diagrams for basaltic glasses “Fenner-type” variation diagrams for basaltic glasses from the Afar region of the MAR. Note different ordinate scales. from the Afar region of the MAR. Note different ordinate scales. From Stakes et al. (1984) From Stakes et al. (1984) J. Geophys. Res., 89, 6995-7028.J. Geophys. Res., 89, 6995-7028.

Major elementsMajor elements

Trace Element ChemistryTrace Element Chemistry

Figure 13-10.Figure 13-10. Data from Data from Schilling et al. Schilling et al. (1983) (1983) Amer. J. Amer. J. Sci., 283, 510-586.Sci., 283, 510-586.

Trace Element ChemistryTrace Element Chemistry

Range in MORB composition due to Range in MORB composition due to fractional crystallizationfractional crystallization

Modeling suggests ~ 60% fractional Modeling suggests ~ 60% fractional crystallizationcrystallization

MORBs have > 1 source type:MORBs have > 1 source type: N-MORBN-MORB E-MORBE-MORB Transition between them (T-MORB)Transition between them (T-MORB)

Conclusions about MORBsConclusions about MORBs

OIBs: ocean islands and seamountsOIBs: ocean islands and seamounts

Types of OIB MagmasTypes of OIB Magmas

Ol

Ne Ab

Opx

Q

Alkalin

e fie

ld

Tholeiitic field

Dividing line

12

10

8

6

4

2

35 40 45 50 55 60 65%SiO2

%N

a2O

+ K

2O

Alkaline

Tholeiitic

Hawaiian Stages-eruptive cycleHawaiian Stages-eruptive cycle

1. 1. Pre-shield stage: variable alkaline/tholeiiticPre-shield stage: variable alkaline/tholeiitic

2. 2. Shield-building stage:Shield-building stage: tholeiitic, 98% of the volcanotholeiitic, 98% of the volcano 3.3. Post-shield stage more alkaline, more differentiated Post-shield stage more alkaline, more differentiated

4.4. After ~1Ma break, post-erosional stage: highly After ~1Ma break, post-erosional stage: highly

alkaline and alkaline and

silica-undersaturated silica-undersaturated

magmasmagmas

Evolution in the SeriesEvolution in the Series

Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.

Trace Elements: REEsTrace Elements: REEs

Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.

Enriched

Enriched

DepletedDepleted

MORB-normalized Spider DiagramsMORB-normalized Spider Diagrams

Figure 14-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

Trace ElementsTrace Elements

Isotope GeochemistryIsotope Geochemistry Isotopes do not fractionate during partial Isotopes do not fractionate during partial

melting of fractional melting processes, so melting of fractional melting processes, so will reflect the characteristics of the sourcewill reflect the characteristics of the source

OIBs only cross oceanic plate, limiting OIBs only cross oceanic plate, limiting contamination (<-> continent); good contamination (<-> continent); good estimate of mantleestimate of mantle

Sr Isotope EvolutionSr Isotope Evolution

Figure 9-13. After Figure 9-13. After Wilson (1989). Igneous Wilson (1989). Igneous Petrogenesis. Unwin Petrogenesis. Unwin Hyman/Kluwer.Hyman/Kluwer.

Sm-Nd: Evolution opposite to Rb - SrSm-Nd: Evolution opposite to Rb - SrCtl Crust (enriched) = hi Ctl Crust (enriched) = hi 8787Sr/Sr/8686Sr, lo Sr, lo 143143Nd/Nd/144144NdNd

147147SmSm143143NdNd

by alpha decayby alpha decayhalf life 106 Gahalf life 106 Ga

Daughter Daughter moremore

incompatibleincompatible

143143Nd/Nd/144144Nd = (Nd = (143143Nd/Nd/144144Nd)Nd)oo

+ (+ (147147Sm/Sm/144144Nd)(eNd)(ett-1)-1)

MORB Sr - Nd IsotopesMORB Sr - Nd Isotopes

Figure 13-12.Figure 13-12. Data from Ito et al. (1987) Data from Ito et al. (1987) Chemical Geology, 62, 157-176; Chemical Geology, 62, 157-176; and LeRoex et al. (1983) and LeRoex et al. (1983) J. Petrol., 24, 267-318.J. Petrol., 24, 267-318.

MORB+OIB: Sr - Nd IsotopesMORB+OIB: Sr - Nd IsotopesRange in compositions = mantle arrayRange in compositions = mantle array

Every composition by mixing end-members:Every composition by mixing end-members:DM DM -depleted -depleted mantlemantleEM 1&2 EM 1&2 --enriched mantleenriched mantleHIMU HIMU - high - high = = 238238U/U/204204PbPb

238238U -> U -> 206206PbPbSo high So high 206206Pb/Pb/204204PbPb

MORB

OIB (colors)}

DMDM

HIMUHIMU

EM1EM1

EM2EM2

BSEBSE

AA Model for Oceanic Magmatism Model for Oceanic Magmatism

DMDM

OIBOIB

ContinentalContinental

ReservoirsReservoirs

EM and HIMU from EM and HIMU from crustalcrustal sources (subducted OC + CC seds) sources (subducted OC + CC seds)

Figure 14-10. Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993).