Ocean Island Basalts

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/L19-OIB.html


A. Statistics

-Second largest igneous province

-Scattered islands and seamounts - some forming linear chains of successively older rocks

-Some are located on or near mid-ocean ridges - these generate aseismic ridges on one or both plates

-Mostly basalts: different from MORB; mantle source

-Interpreted as forming above hot spots

B. General composition

-Both alkaline (OIA) and tholeiitic (OIT) series present

-Many have voluminous OIT first, then later smaller amounts of OIA

-More variations among them than in MORB

-Many islands have a bimodal distribution: could be due to mixing

C. OIT

-Phenocrysts are mainly olivine (An80-88) + spinel

-Early ol reacts with liquid to form opx

-Enriched in incompatible elements, as compared to MORB

D. OIA

-Alkali content is mainly Na

-Phenocrysts are olivine (An34-87) + spinel - no opx reaction so olivine becomes Fe-rich

-Basalts differentiate to more evolved rocks: named according to An content of plagioclase; evolved rocks may be silica-undersaturated or over-saturated

-Very enriched in incompatible elements (> OIT)

II. Isotopic composition

A. Sr-Nd isotopes

-Upper left corner represents depleted mantle, lower right represents enriched mantle

-Mid-oceaqn ridges define the mantle array

-OIB plot mostly within array, but diverge at enriched end

-At the least, OIB represent a mixture between depleted & enriched mantle

B. Mantle heterogeneity

-DM = depleted mantle source

-BSE = bulk standard Earth: present day isotopic value of primordial chondritic mantle; may or may not exist as a separate reservoir.

-PREMA = prevalent mantle.

-EMI = enriched mantle I source

-EMII = enriched mantle II - similar Nd to EMI but much higher Sr :too high to be mantle-derived; involvement of continental crust/sediments

-HIMU = another reservoir, which seems unnecessary

III. Ph isotopes

A. Review

-238 > 206, 235 > 207, 232 > 206

-U, Th, Pb all incompatible => very low concentrations in mantle, high in crust

-Thus, mixing crust with mantle would strongly affect Pb values

B. A different Pb diagram

-Recall concordia, discordia > timing of origin and later events

-Geochron represents the 207/204 and 206/204 ratios of the presumed primordial chondritic Earth: BSE falls on this line; however, neither MORB nor OIB do!

C. See 6 reservoirs from Sr-Nd diagram

-DM has low values - expected since U, Th low in depleted mantle

-EMI = slightly enriched, EMII more so

-But, MORB and OIB don''t lie within DM-EMI-EMII triangle.

-Instead, they point toward a source with high Pb* ratios = HIMU

D. HIMU reservoir

-Represents a source with: high U and Th, low Rb; that originated far enough in the past that it had time to develop high values of Pb*

-What could this be? Subducted oceanic crust, metasomatized mantle, mantle that lost Pb to core

-DUPAL anomaly : data from Northern Hemisphere plot on mixing line between DM and HIMU = NHRL ; data from Southern Hemisphere plot above NHRL


-HIMU and EM reservoirs are too enriched to originate in the mantle => crustal sources: EMI: lower continental crust or oceanic crust; EMII, HIMU: upper continental crust/sediments

E. Mantle model for MORB and OIB

-Two layers separated by 660-km seismic discontinuity

-Upper layer ~ asthenosphere = DM = source of N-MORB

-Lower layer = mixture of EM, HIMU, +/- PREMA and BSE