Recycling oceanic crust: Quantitative constraints

Abstract

[1] Recycled ancient oceanic crust with variable amounts of aging, or inclusion of sediments of differing types and origins has often been invoked as a source for present-day ocean island basalts (OIB), but the current evidence remains largely qualitative. Previous quantitative modeling has shown that much has to be learned in order to better understand the implications of crustal recycling on mantle heterogeneity. Here, we present new model calculations incorporating recent constraints on subduction-zone processes and the composition of subducted sediments. Modeled compositions of the recycled oceanic crust vary widely as a function of the recycling age and composition of the oceanic crust. HIMU-type sources can only be created by recycling igneous oceanic crust if it has undergone substantial modification during subduction. Although the required modifications are qualitatively consistent with dehydration processes in subduction zones, the many uncertainties prevent a precise estimate of the isotopic composition of ancient recycled igneous crust. Inclusion of sediments increases the isotopic variability and although the resulting Sr and Nd isotopic signatures can be similar to enriched mantle (EM) signatures, the Pb isotopic composition of EMtype OIB is difficult to reconcile with the presence of sediment in their sources. The large variability of modeled compositions of the subducted crust suggests that if mantle heterogeneity is largely formed by crustal recycling, each OIB is likely to have a unique isotopic composition resulting from specific combinations of composition, age and subduction modification of the subducted crust. Given the variability of the recycled components, a small number of relatively well-defined enriched compositions can only be explained if either the subduction processing of oceanic crust is a far better defined process than observation would seem to indicate, or, the intramantle disaggregation and mixing of compositionally diverse recycled materials is surprisingly efficient.

Constraints from melt inclusions and their host olivines on the petrogenesis of Oligocene-Early Miocene Xindian basalts, Chifeng area, North China Craton

“…Since the incompatible trace element concentrations of the surrounding depleted mantle are much lower than those of recycled oceanic crust (e.g., Stracke et al 2003), their ratios of the stage II pyroxenite with less than 50 % peridotite are nearly similar to those of recycled oceanic crust. We used the average compositions of Zambezi Belt group I and II eclogites reported in John et al (2004), a good estimate for average recycled oceanic lithosphere (Pfänder et al 2007), as the compositions of the recycled oceanic crust.…”

“…However, this conventional view is challenged, because melting of pyroxenite, another lithology primarily composed of pyroxene, garnet and possibly olivine (olivine mode \40 %; Kogiso et al 2004), can also produce basalts (pyroxenite melts; e.g., Hofmann and White 1982;Weaver 1991;Hauri and Hart 1993;Hirschmann and Stolper 1996;Hauri 1996;Hofmann 1997;Lassiter and Hauri 1998;Stracke et al 2003Stracke et al , 2005Ren et al 2004Ren et al , 2005Ren et al , 2006Ren et al , 2009Kogiso et al 2004). Compared with peridotite, pyroxenite has lower solidus and shorter melting interval, corresponding to greater melting depth and higher melt productivity (e.g., Yasuda et al 1994;Takahashi et al 1998).…”

Origin of hotspots in the South Pacific: Recent advances in seismological and geochemical models

“…Typical trace element abundances for HIMU lavas (Mangaia), EM1 lavas (Pitcairn), and EM2 lavas (Tahaa) are after White and Duncan (1996), Eisele et al (2002), and Hanyu et al (2011a). The compositions of N-MORB (Hofmann, 1988), bulk oceanic crust (Stracke et al, 2003), and typical sediment (GLOSS; Plank and Langmuir, 1998) are shown for comparison. Trace element abundances of dehydrated bulk oceanic crust are calculated using mobility coefficients for dehydration (Kogiso et al, 1997b). tions; this suggests that the HIMU reservoir must be geochemically very uniform (Stracke et al, 2005).…”

“…However, the isotopic compositions of some mantle reservoirs resemble those of oceanic crust, continental crust, and sediment; therefore, some geochemists favored a model that considers the subduction of these surface materials (e.g., Zindler and Hart, 1986;Weaver, 1991;Hauri and Hart, 1993;Woodhead and Devey, 1993;Eisele et al, 2002;Stracke et al, 2003;Jackson et al, 2007). For instance, isotopic compositions of EM2 overlap those of terrigenous sediments, derived primarily from the continental crust.…”

“…We have presented recently obtained seismic images showing the low- Stracke et al (2003) velocity superplume extending from the CMB to 1000 km depth and the narrow low-velocity plumes above this depth. It has to be analyzed whether the seismic anomalies can have a thermal origin.…”