Изотопная композиция и мантийная неоднородность 

Isotopic composition
C bhttp://ijolite.geology.uiuc.edu/08SprgClass/geo436/436%20lectures/L19-OIB.html http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html

A. Sr-Nd isotopes

Upper left corner of diagram represents depleted mantle, lower right represents enriched mantle. Mid-ocean ridges define the mantle array. OIB plot mostly within array, but diverge at enriched end at the least, OIB represent a mixture between depleted and enriched mantle.

B. Mantle heterogeneity

DM = depleted mantle source , SE = 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


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, but neither MORB nor OIB do!

C. See 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 & 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


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