Lecture 4: MORB petrogenesis

1) 2) 3) 4)

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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2.

3.

4.

5.

6.

7.

Igneous Petrogenesis

Mid-ocean ridges Continental rifts Island Arcs Active continental margins Back-arc basins Ocean Islands Intraplate hotspot activity, carbonatites, or kimberlites

Mid-ocean ridges

Mid-ocean ridges produce ~ 21 km 3 of lava per year ~60% of the earth’s surface is covered with oceanic crust

Mid-ocean Ridges

Spreading rate influences thermal structure, physical structure, crustal thickness and amount of melting

Spreading rate and structure

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

Slow-spreading Mid-Atlantic Ridge

• Thermal structure is cooler • Crust is thinner, lithosphere is thicker • lower degrees of melting • Episodic volcanism • Higher compositional diversity

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

Hekinian et al. (1976) Contr. Min. Pet. 58, 107.

Figure 13.16.

From Byran and Moore (1977) Geol. Soc. Amer. Bull., 88, 556-570.

A modern concept of the axial magma chamber beneath a fast spreading ridge

After Perfit et al. (1994) Geology, 22, 375-379.

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 2 Rift Valley 4 Gabbro 6 Moho 8 10 Transition zone 5 Mush 0 Distance (km) 5 10

After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.

Oceanic Crust and Upper Mantle Structure

1) Geophysical studies 2) Mantle xenoliths 3) 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.

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. lherzolite wehrlite

Ocean Crust Geology

Modern and ancient pillow basalts Glassy pillow rinds are used to infer original melt compositions

P. Asimow

Magma: mixture of molten rock, gases and mineral phases, produced by mantle melting Mantle melts between ~800 1250ºC due to: 1) 2) 3) Increase in temperature Decrease in pressure Addition of volatile phases

Partial melting

Adiabatic rise of mantle material with no heat loss – decompression melting Mid-Ocean Ridges

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

Igneous rock classification by composition

• There are several classifications, of individual rocks or rock suites.

• By silica percentage: %SiO 2 rocks Designation % Dark Minerals Designation Example >66 52-66

Acid Intermediate

<40 40-70

Felsic Intermediate

Granite, rhyolite Diorite, andesite 45-52 <45

Basic Ultrabasic

70-90 >90

Mafic Ultramafic

Gabbro, basalt Dunite, komatiite

(plagioclase)

The common crystallization sequence at mid-ocean ridges is: olivine (  spinel), olivine + plagioclase (  Mg-Cr Mg-Cr spinel), olivine + plagioclase + clinopyroxene

(clinopyroxene) After Bowen (1915), A. J. Sci., and Morse (1994) (olivine)

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

“Fenner-type” variation diagrams for basaltic glasses from the Afar region of the MAR. From Stakes et al. (1984)

Increased fractional crystallization

Global systematics

• The values of regionally-averaged Na 8 (i.e., Na 2 O concentration corrected to 8% MgO), Fe 8 , water depth above the ridge axis, and crustal thickness show significant global correlations.

– Where Na 8 – Where Na 8 – Where Na 8 is high, Fe 8 is low is high, the ridges are deep is high, the crust is thin 3.5

Deep ridges 3.0

Na8.0

2.5

2.0

1.5

6 7 8 9 Fe8.0

10 Na 8 is an incompatible element, thus an indicator of

mean extent of melting

. Fe 8 is an indicator of

pressure of melting

.

mean

Shallow ridges 11 12 Axial depth is an indicator of mantle temperature, extent of melting, and crustal thickness combined – see slide #5

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 Na 8 and high crustal thickness axial depth; high mean pressure of melting  high Fe 8  shallow . Cold mantle yields the opposite.

Hot mantle Cold mantle sea level mean

P

axial depth 25% 20% 15% 10% 5% crust

F

mean

F

40% 35% solidus 1.5 GPa mean

P

30%

F

25% 20% 15% 10% 5% mean

F

solidus 4.5 GPa

P. Asimow

Spider diagram of crust vs mantle

Workman and Hart, 2005

A modern concept of the axial magma chamber beneath a fast spreading ridge Figure 13-15. After Perfit et al. (1994) Geology, 22, 375-379.

• • • 1) 2)

Generating enriched signatures in MORB

Low degrees of melting Mantle source enrichment N-MORB: normal MORB T-MORB: transitional MORB E-MORB: enriched MORB

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., 87 Rb 87 Sr, Rb is more incompatible than Sr so high 87 Sr/ 86 Sr ratios indicate an enriched source Compared to ocean islands and subduction zones, MORBs are relatively homogeneous

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

Manus

Macpherson et al., 2000

He isotopes:

3 He : key tracer of a primordial component 4 He : representing a radiogenic component (U+Th decay) 3 He anomalies at ridges is evidence for degassing of primordial gases from the earth

Typical 3 He/ 4 He ratios:

Crust : 0.01-0.05 R A MORB : 8 ± 1 R A Arcs: 5 - 8 R A Hotspots: up to 37R A

Craig and Lupton (1981)