MORB

See http://plate-tectonic.narod.ru/petrographyigneouslinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html
http://ijolite.geology.uiuc.edu/08SprgClass/geo436/436%20lectures/L17-18-MORB.html

I. Basics

A. Most extensive igneous province

-#2 in volume is convergent boundaries

-#3 in volume is continental rifts - melting at

-Cover 65% of Earth''s surface

-10 km3 erupted along 65,000 km of ridge per year

B. Eruption

-MOR has rift valley at top ~35 km wide

-MORB erupt along central fissure or erupt along bounding faults

-Eruptions: are symmetric wrt ridge axis; generate shallow-focus EQ; are associated with high heat flow; vary along ridge segments

C. Composition

-Range is small, though important distinctions exist

-Most are olivine tholeiites, with low Ti and K

-Restricted Si, increasing Fe, decreasing Mg; SiO2 restricted (47-51%) => not useful as differentiation index; use Mg # instead = 100 Mg / (Mg + Fe+2)

-Low concentrations of alkalis & incompatible elements

-High Cr and Ni

-REE diagram => depth <85 km

-Mode: a Aphyric, or Phenocrysts (Olivine/+/- Plagioclase/ccasionally augite

- trends suggest fractionation involving plag + cpx

- Pearce element ratios - a shows no cpx, b shows ol + plag

- no Eu anomaly => no plag fractionation

- wide variation =>

-Some magmas derive from a depleted source = N-MORB; some from a more enriched source = E-MORB (this distinction shows in REE

D. Other mid-ocean ridge rocks

-Hydrothermal alteration by seawater > spilite

-Associated minor rock: plagiogranite

II. Structure of the oceanic lithosphere

A. Problem: vast majority of MORB never sampled

-DSPS/ODP

-Even these don''t go deep into oceanic lithosphere

B. Ophiolite suites
-Def: distinctive combination of rocks believed to represent slices of oceanic lithosphere thrust onto continents during subduction (obduction)

-Sequence - bottom to top: ultramafic rocks (Harzburgite /Grades into dunite (ol, foliation becomes horizontal, includes chromite bodies up to 1 km long); layered gabbroic cumulates; sheeted dike complex ; pillow basalts; pelagic sediment

-Boundary between deformed ultramafics & layered gabbros represents Moho

-Thickness of ophiolite layers

-Ophiolites accepted as samples of oceanic crust

B. Model of MOR

Sheeted dikes => 100% extension, but underlying layered rocks undeformed; could be explained by expansion of magma chamber as new magma is emplaced

-Thus, a small magma chamber could produce vast amounts of dikes + undeformed layers

III. Origin of MORBs

A. Differentiation

-Observed concentrations of incompatible elements => higher degree of F.C. of olivine +/- plagioclase than is suggested by variation in major elements

-Also, concentrations of compatible elements too low to match % F.C. based on incompatible elements

-Suggests : 1) more than one parental magma, and/or 2) augite fractionation (pyroxene paradox)

-Hypothesis: augite F.C. produces evolved magma; periodic replenishment of magma chamber with more primitive melt could (1) resorb any augite phenocrysts, and 2) maintain MORB composition )

B. Composite interpretation of MORB genesis

-Divergent boundary: mantle rises adiabatically ; 15-40% decompression melting at 60-80 km depth = N-MORB; zone of melting ~100 km wide at base but narrows to as little as 3 km at plate boundary ; melt separates at ~30 km - fractionates in magma chamber at 1-2 km

-Depleted mantle extends to 660-km seismic discontinuity, enriched below

-Some enriched mantle is entrained in convective rise - generates E-MORB

C. Axial magma chamber

-Early interpretations pictured large, continuous magma chamber beneath ridge axis: but no seismic evidence for large volume of molten rock

-More recent idea: iIn situ crystallization near walls in mush zone ; thin discontinuous layer of melt along ridge

-This interpretation is for fast-spreading ridges.

-For slow -spreading ridges : small melt zone, discontinuous in space and time, only ~5% melt

D. Conclusions of a global study by Klein and Langmuir

- Trends caused by differences in "thermal regime" between ridge segments

- Temperature also controls thickness of crust and height of MOR

-3. Melting is fractional rather than equilibrium, i.e., melt separates efficiently,

-4. Local variations probably originate in mantle: pods of enriched mantle scattered through depleted mantle; more variation along slow-spreading ridges than hotter, faster ones.

- Hot spots affect chemistry of MORB