Studies on modern oceanic lithosphere and ophiolites show that the petrogenesis of the oceanic mantle is complex. Its chemistry does not reflect simple trace element depletion by partial melting, and this complexity is highlighted by chemical and isotopic heterogeneities in the mantle, and by isotopic contrasts between mantle and crust. We present an overview of the present knowledge on isotopic heterogeneities of Sr, Nd and Os in present-day oceanic peridotites and Alpine-Apennine ophiolites. Compared to modern oceanic lithosphere, these ophiolites represent a fossil analogue of ocean-continent transitions and slow spreading settings. Previous studies show that mantle peridotites record significant isotopic heterogeneity, detectable on a wide range of length scales, much larger than observed in associated MORB. This reflects two major aspects. First, the MORB mantle source region is larger than peridotite sampling scales, and MORB represent aggregated melts that smooth and average mantle source heterogeneities, as supported by melt extraction models (Stracke and Bourdon, 2009). Second, isotopic heterogeneities in oceanic peridotites may result from a combination of processes, reflecting a long-lived history: i) old depletion events, ii) pyroxenite components in the mantle source, iii) recent pre- and/or post-melting metasomatism. In this context, we focus on two major issues: 1) What is the significance of extremely depleted isotopic signature found in oceanic peridotites - do they reflect a subcontinental origin, or preserved isotopic heterogeneities in the asthenospheric MORB source, inherited from old (1-2 Ga) depletion events, not sufficiently stirred by convection; 2) What is the length scale, amplitude and distribution of isotopic heterogeneities in the oceanic mantle, and their relation to lithologic heterogeneities like pyroxenites? Specifically, the effect of a pyroxenite component in creating small-scale chemical and isotopic heterogeneities in the mantle has often been advocated but not adequately investigated. New isotope studies of pyroxenite bands (clinopyroxenites and spinel websterites) and host peridotites from the External Liguride ophiolites (Northern Apennines, Italy) show that pyroxenites define a large Nd isotope range (143Nd/144Nd =0.512936-0.513401). Chemical and isotopic profiles through the pyroxenite-peridotite boundaries indicate that the interaction of the pyroxenites with the peridotite wall rock creates Nd isotopic variations resulting in an overall Nd isotopic range in the host mantle (143Nd/144Nd=0.512799-0.513544, initial EpsilonNd=3-16) that covers nearly the entire abyssal peridotite field. This suggests that pyroxenite components can induce extreme small scale isotopic changes, and strongly calls for detailed field-based studies in spatially-controlled settings in order to unravel the size and distribution of upper mantle isotopic heterogeneity. Stracke A., and Bourdon, B. (2009). GCA 73, 218-238.
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|Data di pubblicazione:||2011|
|Titolo:||Upper mantle isotopic heterogeneities - global overview with new results from Alpine-Apennine ophiolites|
|Autori:||Rampone, E; Borghini, G; Hofmann, A.W.; Class, C; Cipriani, Anna; Zanetti, A; Goldstein, S.L.|
|Nome del convegno:||2011 Fall Meeting, AGU|
|Luogo del convegno:||San Francisco, California|
|Data del convegno:||5-9 Dec. 2011|
|Volume:||EOS, Trans. AGU, Fall Meet. Suppl.,|
|Appare nelle tipologie:||Abstract in Rivista|
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