Magma contamination

Fig. 3.5. S180 vs. 87Sr/86Sr (A) and Nb/Zr (B) diagrams for IAP rocks. Curved full lines are calculated mixing trends between Roman ultrapotassic magma and limestone (i.e. magma contamination trend). Dashed line is a mixing trend between mantle and marly sediments (source contamination). Numbers along lines indicate amounts of sediments involved in the mixing.

De Astis et al. (2000) and Calanchi et al. (2002b) noticed that calc-alkaline and HKCA basalts at Vulcano and Panarea have distinct trace element ratios (e.g. La/U, Rb/Zr, Zr/Nb) compared to the associated sho-shonitic and KS mafic volcanics. However, the rocks of the Calabro-Peloritano basement underlying the Aeolian volcanoes show compositions that resemble the calc-alkaline rather than shoshonitic and KS rocks this was interpreted to exclude a derivation of potassic rocks from calc-alkaline parents via crustal assimilation. The same conclusion was drawn by Frez-zotti et al. (2004), who modelled magma contamination processes using melt inclusions entrapped in metamorphic xenoliths as contaminants. 

Oxygen isotopic data on whole rocks show 5180 in the range + 5.0 to +5.9 (Marty et al. 1994). Boron isotopic compositions have small but significant variations (8nB -8%o to -3%o), probably derived from both deep (i.e. source contamination) and shallow (i.e. magma contamination) processes (Tonarini et al. 2001b). 

Fig. 10.5. Models showing Sr-0 isotope variation during mixing between upper crustal rocks (full triangle), mantle peridotite (star) and mantle-derived basaltic magma (full square). Solid line represents the trend of magma contamination by upper crust. Dotted line is mantle contamination trend. Dashed line represents contamination trend of Roman Province magmas by sedimentary carbonates. Numbers along the lines indicate amounts of crustal end-member. For discussion, see text.

While the effects of magma contamination and radiogenic production cannot be discounted completely from some of these samples, the general pattern seems consistent with a mantle region of lower He/" He ratios. As discussed above, it appears that these mantle domains directly provide the He seen in the continental lithosphere. In western Europe, the Cameroon Line, Meidob Hills and Kivu, data from other isotopic tracers in the host basalts are consistent with such a source, which also then provides a reasonable explanation for the xenolith He. However, such domains are sampled at only a small proportion of oceanic vents (see Graham 2002, this volume), and so it is unlikely that such a source has contributed to many of the other continental locations. 

The basalt dikes in Vestfjella analyzed by Fumes et al. (1982) also scatter in Fig. 14.15b, although four samples approach a mixing line between a hypothetical magma and a contaminant having a higher Sr/ Sr ratio than the magma (>0.707) and a lower strontium concentration (<315 ppm). If the basalt dikes were not altered by hydrothermal solutions, then the scatter of data points is presumably the result of magma contamination alone. 

Ma of the flows and dikes of Plogen and Basen range widely from 0.70401 to 0.70794 and from 0.511596 to 0.51259, respectively. These isotope ratios are not correlated with the reciprocals of the concentrations for strontium and neodymium, respectively, which effectively rules out magma contamination. In addition, secondary alteration of the flows is unlikely because neodymium is not a mobile element in weathering environments. Although the data points do not form mixing lines. Fig. 14.20 demonstrates that the initial ratios (at 180 Ma) and the 1/Nd... 

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. 

Defining, in the diagram (Ci2/C 1),iq vs l/Cliqtl, the slope sm of the mixing line between contaminant and initial magma as... 

Kent AJR, Stolper EM (1997) Contamination of ocean-island basalt magmas by a seawater-derived component at Loihi Seamount, Hawaii. EOS Trans AGU 78 F806... 

Magma types 2006). A significant part in formation of magmatic complexes accompaning riftogenesis belongs to the sources of different nature and to characteristics of the continental crust contaminated by those complexes. These very data accounted for the formation of a contrasting volcanism which is widely developed in the zone of the Central-Asian fold belt. The paper considers a bimodal volcano-plutonic complex of the end of the Late Cretaceous. It is spatially located within the continuation of the formations with similar composition which compose the Central-Asian fold belt. 

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