Crustal contamination

Honma et al. (1991) have shown that the Okinawa Trough basalts have significantly high K, Rb and Sr contents and D/H, 0/ 0 and Sr/ Sr ratios than N-Morb have and these are due to generation of magma from normal-type mantle peridotite modified by component from the subducted slab and crustal contamination. 

Briqueu, L. Lancelot, J. R. (1979). Rb-Sr systematics and crustal contamination models for calc-alkaline igneous rocks. Earth Planet. Sci. Letters, 43, 385-96. 

As the sannaites have moderate to high Mg (27-54), Nb/U (26-48), and no xenoiiths (all consistent with minimal crustal contamination) they can be used to assess the nature of the mantle source. Regardless of the variation between groups, the low Sr/ Sr368Ma values suggest that the sannaites (and syenites) were derived from partial melting in the mantle, and the nci values indicate that the mantle source was depleted relative to bulk earth. 

Sr/ Sr ratios that correlate with an increase in Si02 and decrease in Sr content. In contrast, a mantle melt, which evolves only through differentiation unaccompanied by interaction with crustal material, will have an 0-isotope composition that mainly reflects that of its source region, independent of variations in chemical composition. In this latter case, correlated stable and radiogenic isotope variations would be an indication of variable crustal contamination of the source region (i.e., crustal material that has been recycled into the mantle via subduction). 

In conclusion, although crustal contamination has been an important evolutionary process at most of the Aeolian volcanoes, it seems inadequate to explain the geochemical and isotopic variations observed from calc-alkaline to potassic mafic melts at both local and regional scales. Therefore, many studies concluded that the mafic magmas with different enrichment in potassium reflect primary compositions which were generated in a heterogeneous mantle source (e.g. Ellam et al. 1988 Ellam and Harmon 1990 De Astis et al. 1997, 2000 Francalanci et al. 2004). 

Crustal-like geochemical and isotopic signatures for most rocks along the Italian peninsula (Fig. 10.4), requiring crustal contamination of their mantle sources, a process that can be only accomplished by subduction processes ... 

Francalanci L, Manetti P, Peccerillo A (1989) Volcanological and magmatological evolution of Stromboli volcano (Aeolian Islands) the roles of fractional crystallization, magma mixing, crustal contamination, and source heterogeneity. Bull Volcanol 51 355-378... 

Hart SR, Hauri EH, Oschmann LA, Whitehead JA (1992) Mantle plumes and entrainment isotopic evidence. Science 256 517-520 Hawkesworth CJ, Vollmer R (1979) Crustal contamination vs. enriched mantle 143Nd/144Nd and 87Sr/86Sr evidence from the Italian volcanics. Contrib Mineral Petrol 69 151-165... 

Luais B (1988) Mantle mixing and crustal contamination as the origin of the high-Sr radiogenic magmatism of Stromboli (Aeolian Arc). Earth Planet Sci Lett 88 93-106... 

Gasparon M., Hilton D. R., and Varne R. (1994) Crustal contamination processes traced by helium isotopes— examples from the Sunda arc, Indonesia. Earth Planet. Sci Lett. 126, 15-22. 

The problem of crustal contamination is particularly acute for low mg continental flood basalts and smaller volume continental tholeiitic basalts, both of which have low trace-element concentrations (see Sections 3.03.3.2.3 and 3.03.3.3). The issue is less critical for many smaller volume continental rocks, such as kimberlites and alkali basalts, which have much higher abundances of many trace elements. As a result of their high strontium and neodymium content, for example, the isotopic compositions of these elements in kimberlites and alkali basalts are relatively insensitive to modification during crustal contamination. Conversely, the osmium and lead concentration of basaltic magmas are so low that these isotope systems are particularly vulnerable to modification by interaction with cmstal rocks (McBride et al, 2001 Chesley et al, 2002) hence these systems provide relatively sensitive indicators of crustal assimilation. 

Glazner A. F. and Farmer G. L. (1992) Production of isotopic variability in continental basalts by cryptic crustal contamination. Science 255,12-1A. 

---