scholarly journals An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions

2009 ◽  
Vol 114 (B12) ◽  
Author(s):  
J. M. Warren ◽  
N. Shimizu ◽  
C. Sakaguchi ◽  
H. J. B. Dick ◽  
E. Nakamura

The problem of compositional heterogeneity in the Archaean upper mantle is evaluated from the existing data of isotopic compositions (Sr, Pb and Nd), trace element abundances (rare earth elements (r.e.e.)) and some critical major element chemistry in Archaean igneous rocks. To assess a large-scale mantle heterogeneity, we have emphasized the chemical characteristics of undoubtedly mantle-derived komatiites. The initial isotopic compositions of Sr and Pb ( I Sr and I Pb ) in Archaean igneous rocks show a significant variation, suggesting that a long-term heterogeneity in Rb/Sr and U /Pb ratios might have existed in their mantle sources. In contrast, the available I Nd values show little variation from the expected evolution in a chondritic uniform reservoir. These contrasting results, i.e. the variable I Sr and I Pb values as against the uniform I Nd values, are in part attributed to the fact that mobile-refractory pairs in the Rb/Sr and the U /Pb systems but only the refractory-refractory pair in the Sm/Nd system have been involved in geochemical differentiation. The r.e.e. distributions in komatiitic rocks are not uniform. Many show strong depletion in light r.e.e. The degree of light r.e.e. depletion in these komatiitic rocks is comparable with that found in present mid-ocean ridge basalts. Regarding the major element chemistry, we have demonstrated that the CaO / Al 2 O 3 ratios are significantly different in the komatiitic suites of two adjacent cratons (Kaapvaal and Rhodesia) in southern Africa. Both trace and major elements data suggest that the Archaean upper mantle was heterogeneous, but whether the heterogeneity truly represents a long-term phenomenon remains to be resolved by more isotopic analyses.


2021 ◽  
Vol 12 (1) ◽  
Author(s):  
A. Y. Yang ◽  
C. H. Langmuir ◽  
Y. Cai ◽  
P. Michael ◽  
S. L. Goldstein ◽  
...  

AbstractThe plate tectonic cycle produces chemically distinct mid-ocean ridge basalts and arc volcanics, with the latter enriched in elements such as Ba, Rb, Th, Sr and Pb and depleted in Nb owing to the water-rich flux from the subducted slab. Basalts from back-arc basins, with intermediate compositions, show that such a slab flux can be transported behind the volcanic front of the arc and incorporated into mantle flow. Hence it is puzzling why melts of subduction-modified mantle have rarely been recognized in mid-ocean ridge basalts. Here we report the first mid-ocean ridge basalt samples with distinct arc signatures, akin to back-arc basin basalts, from the Arctic Gakkel Ridge. A new high precision dataset for 576 Gakkel samples suggests a pervasive subduction influence in this region. This influence can also be identified in Atlantic and Indian mid-ocean ridge basalts but is nearly absent in Pacific mid-ocean ridge basalts. Such a hemispheric-scale upper mantle heterogeneity reflects subduction modification of the asthenospheric mantle which is incorporated into mantle flow, and whose geographical distribution is controlled dominantly by a “subduction shield” that has surrounded the Pacific Ocean for 180 Myr. Simple modeling suggests that a slab flux equivalent to ~13% of the output at arcs is incorporated into the convecting upper mantle.


2010 ◽  
Vol 11 (5) ◽  
pp. n/a-n/a ◽  
Author(s):  
Pietro Armienti ◽  
Daniela Gasperini

2002 ◽  
Vol 3 (1) ◽  
pp. 1-24 ◽  
Author(s):  
J. J. Standish ◽  
S. R. Hart ◽  
J. Blusztajn ◽  
H. J. B. Dick ◽  
K. L. Lee

1992 ◽  
Vol 97 (B4) ◽  
pp. 4461 ◽  
Author(s):  
E. Bonatti ◽  
A. Peyve ◽  
P. Kepezhinskas ◽  
N. Kurentsova ◽  
M. Seyler ◽  
...  

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