Lecture 4 MORB petrogenesis 53127 67789

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Lecture 4: MORB petrogenesis
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Outline
Overview of igneous petrogenesis
Mid-Ocean Ridges – how are they characterized?
MORB – where and how do they form? 
Geochemical variations in MORB (major elements, trace elements and isotopic characteristics)
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Igneous Petrogenesis
Mid-ocean ridges
Continental rifts
Island Arcs
Active continental margins
Back-arc basins
Ocean Islands
Intraplate hotspot activity, carbonatites, or kimberlites
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Mid-ocean ridges
Mid-ocean ridges produce ~ 21 km3 of lava per year 
~60% of the earth’s surface is covered with oceanic crust
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Mid-ocean Ridges
Spreading rate influences thermal structure, physical structure, crustal thickness and amount of melting
Chart1
		-2
		0
		0
		0.3
		1.5
		2
		7
		7
		10
Water
Moho
Mantle = altered peridotite
Layer 3 = gabbro
Layer 2 = extrusives
Layer 2a = dikes
Layer 1 = sediment
Vp (km/s)
Vp (km/s)
Depth (km)
Sheet1
		
		
		
		Depth (km)		Vp (km/s)
		-2		1.5
		0		1.5
		0		2
		0.3		5.1
		1.5		5.6
		2		6.5
		7		7
		7		8
		10		8.5
Sheet1
		
Water
Moho
Mantle = altered peridotite
Layer 3 = gabbro
Layer 2 = extrusives
Layer 2a = dikes
Layer 1 = sediment
Vp (km/s)
Vp (km/s)
Depth (km)
Sheet2
		
Sheet3
		
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Spreading rate and structure
Slow-spreading Mid-Atlantic Ridge
Fast-spreading East Pacific Rise
Thermal structure is warmer
Crust is thicker, lithosphere is thinner
Higher degrees of melting
Sustained magma chambers and volcanism
Less compositional diversity
Thermal structure is cooler
Crust is thinner, lithosphere is thicker
lower degrees of melting
Episodic volcanism
Higher compositional diversity
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The Axial Magma Chamber: original model 
Semi-permanent 
MORB magmas are produced by fractional crystallization within the chamber
Periodic reinjection of fresh, primitive MORB
Dikes upward through extending/faulting roof
Crystallization at top and sides  successive layers of gabbro (layer 3) “infinite onion”
Dense olivine and pyroxene crystals  ultramafic cumulates (layer 4)
Moho?? Seismic vs. Petrologic
Figure 13.16. From Byran and Moore (1977) Geol. Soc. Amer. Bull., 88, 556-570. 
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After Perfit et al. (1994) Geology, 22, 375-379. 
A modern concept of the axial magma chamber beneath a fast-spreading ridge
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Model for magma chamber beneath a slow-spreading ridge, such as the Mid-Atlantic Ridge
Most of body well below the liquidus temperature, so convection and mixing is far less likely than at fast ridges
numerous, small, ephemeral magma bodies occur at slow ridges
Slow ridges are generally less differentiated than fast ridges - no continuous liquid lenses, so magmas entering the axial area are more likely to erupt directly to the surface
After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216. 
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Oceanic Crust and Upper Mantle Structure 
Geophysical studies
Mantle xenoliths
Ophiolites: uplifted oceanic crust + upper mantle
Lithology and thickness of a typical ophiolite sequence, based on the Samial Ophiolite in Oman. After
Boudier and Nicolas (1985) Earth Planet. Sci. Lett., 76, 84-92.
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Rock types in the mantle
Peridotite is the dominant rock type of the Earth’s upper mantle
Lherzolite: fertile unaltered mantle; mostly composed of olivine, orthopyroxene (commonly enstatite), and clinopyroxene (diopside), and have relatively high proportions of basaltic ingredients (garnet and clinopyroxene). 
Dunite (mostly olivine) and Harzburgite (olivine + orthopyroxene) are refractory residuum after basalt has been extracted by partial melting
Wehrlite: mostly composed of olivine plus clinopyroxene. 
wehrlite
lherzolite
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Ocean Crust Geology
Modern and ancient pillow basalts
Glassy pillow rinds are used to infer original melt compositions
P. Asimow
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Magma: mixture of molten rock, gases and mineral phases, produced by mantle melting
Mantle melts between ~800-1250ºC due to:
Increase in temperature
Decrease in pressure
Addition of volatile phases
Adiabatic rise of mantle material with no heat loss – decompression melting
Mid-Ocean Ridges
Partial melting
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A model for mantle melting
Several models are possible of how and where the melt is extracted and what happens to it during transport
This average melt is primary mid-ocean ridge basalt (MORB).
Hot mantle starts melting at deeper depths, thus has a larger melt triangle or area over which melting occurs than a cooler mantle 
Mantle rising nearer axis of plume traverses greater portion of triangle and thus melts more extensively 
Hot mantle
cool mantle
Asimow et al., 2004
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Igneous rock classification by composition
 There are several classifications, of individual rocks or rock suites.
 By silica percentage:
%SiO2	Designation	% Dark Minerals	Designation	Example rocks	
>66	Acid		<40		Felsic		Granite, rhyolite	
52-66	Intermediate	40-70		Intermediate	Diorite, andesite
45-52	Basic		70-90		Mafic		Gabbro, basalt	
<45	Ultrabasic	>90		Ultramafic	Dunite, komatiite	
The common crystallization sequence at mid-ocean ridges is: olivine ( Mg-Cr spinel), olivine + plagioclase ( Mg-Cr spinel), olivine + plagioclase + clinopyroxene
After Bowen (1915), A. J. Sci., and Morse (1994)
(plagioclase)
(olivine)
(clinopyroxene)
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“Fenner-type” variation diagrams for basaltic glasses from the Afar region of the MAR. From Stakes et al. (1984)
The major element chemistry of MORBs
MORBs are the product of fractional crystallization, melt aggregation, seawater interaction and crustal contamination
MgO contents are a good index for fractional crystallization (typically, more primitive melts have higher MgO)
Data is often “corrected” back to 8 wt% MgO to estimate primary melt compositions and to compare data sets
Increased fractional crystallization
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Global systematics
The values of regionally-averaged Na8 (i.e., Na2O concentration corrected to 8% MgO), Fe8, water depth above the ridge axis, and crustal thickness show significant global correlations.
Where Na8 is high, Fe8 is low
Where Na8 is high, the ridges are deep
Where Na8 is high, the crust is thin
Deep ridges
Shallow ridges
Na8 is an incompatible element, thus an indicator of mean extent of melting. 
Fe8 is an indicator of mean pressure of melting. 
Axial depth is an indicator of mantle temperature, extent of melting, and crustal thickness combined – see slide #5
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Synthesis of global systematics
The global correlation implies that extent of melting and pressure of melting are positively correlated, on a global scale. This relates to the mantle potential temperature.
If melting continues under the axis to the base of the crust everywhere, then high potential temperature means: long melting column  high mean extent of melting  low Na8 and high crustal thickness  shallow axial depth; high mean pressure of melting  high Fe8. Cold mantle yields the opposite.
P. Asimow
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Spider diagram of crust vs mantle
Workman and Hart, 2005
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Figure 13-15. After Perfit et al. (1994) Geology, 22, 375-379. 
A modern concept of the axial magma chamber beneath a fast-spreading ridge
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Generating enriched signatures in MORB
Low degrees of melting
Mantle source enrichment
 
N-MORB: normal MORB
T-MORB: transitional MORB
E-MORB: enriched MORB
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Isotope systematics of MORB
Radiogenic isotope systems (Sr, Nd, Pb) are used to see mantle enrichments due to relative compatibilities of radiogenic parents and daughters 
e.g., 87Rb 87Sr, Rb is more incompatible than Sr so high 87Sr/86Sr ratios indicate an enriched source
Compared to ocean islands and subduction zones, MORBs
are relatively homogeneous
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Stable isotopes
Like radiogenic isotopes, stable isotope can be used to trace source enrichments and are not influenced by degrees of melting
Oxygen, boron, helium and nitrogen isotopes show very little variability in MORB, and are distinct from enriched OIB and subduction related lavas
Macpherson et al., 2000
Manus
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Craig and Lupton (1981)
He isotopes:
3He : key tracer of a primordial component
4He : representing a radiogenic component (U+Th decay)
3He anomalies at ridges is evidence for degassing of primordial gases from the earth
Typical 3He/4He ratios:
Crust : 0.01-0.05 RA
MORB : 8 ± 1 RA
Arcs: 5 - 8 RA
Hotspots: up to 37RA
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Relatively large (~ 5 km wide and 9 km deep)
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Recent seismic work has failed to detect any chambers of this size at ridges, thus causing a fundamental shift away from this traditional view of axial magma chambers as large, steady-state, predominantly molten bodies of extended duration 
Combines the magma chamber geometry proposed by Sinton and Detrick (1992) with the broad zone of volcanic activity noted by Perfit et al. (1994)
Completely liquid body is a thin (tens to hundreds of meters thick) and narrow (< 2 km wide) sill-like lens 1-2 km beneath the seafloor
Provides reflector noticed in detailed seismic profiles shot along and across sections of the EPR
Melt surrounded by a wider mush and transition zone of low seismic velocity (transmits shear waves, but may still have a minor amount of melt)
“Magma chamber” = melt + mush zone (the liquid portion is continuous through them)
As liquid  mush the boundary moves progressively toward the liquid lens as crystallization proceeds
Lens maintained by reinjection, much like the “infinite onion” 
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Reduced heat and magma supply
Steady-state eruptable magma lens is relinquished in favor of a dike-like mush zone and a smaller transition zone beneath the well-developed rift valley
With the bulk of the body well below the liquidus temperature, convection and mixing is far less likely than at fast ridges
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Decrease in MgO and relative increase in FeO  early differentiation trend of tholeiites 
Patterns are compatible with crystal fractionation of the observed phenocryst phases
Removal of olivine can raise the FeO/MgO ratio, and the separation of a calcic plagioclase can cause Al2O3 and CaO to decrease
SiO2 is a ~ poor fractionation index (as we’d suspected)
Na2O K2O TiO2 and P2O5 are all conserved and the concentration of each triples over FX range
This implies that the parental magma undergoes 67% fractionation in a magma chamber somewhere beneath the ridge to reduce the original mass by 1/3
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Recent seismic work has failed to detect any chambers of this size at ridges, thus causing a fundamental shift away from this traditional view of axial magma chambers as large, steady-state, predominantly molten bodies of extended duration 
Combines the magma chamber geometry proposed by Sinton and Detrick (1992) with the broad zone of volcanic activity noted by Perfit et al. (1994)
Completely liquid body is a thin (tens to hundreds of meters thick) and narrow (< 2 km wide) sill-like lens 1-2 km beneath the seafloor
Provides reflector noticed in detailed seismic profiles shot along and across sections of the EPR
Melt surrounded by a wider mush and transition zone of low seismic velocity (transmits shear waves, but may still have a minor amount of melt)
“Magma chamber” = melt + mush zone (the liquid portion is continuous through them)
As liquid  mush the boundary moves progressively toward the liquid lens as crystallization proceeds
Lens maintained by reinjection, much like the “infinite onion”