I'm currently researching a I-type calc-alcaline granite with microgranular mafic enclaves. The 87Sr/86Sr ratio of the granite is 0.703-0.706 and the enclaves 0.705-0.706. From the geochemical analysis, it seems that there were two different magma chambers and at some point they mixed, but kept their chemical and isotopic identity. Also when I plot the εNd- 87Sr/86Sr, it is increasing.
As you know, the mafics (e.g. Gabbro, Basalt, or Dolerite) have calcic plagioclase in contrast to the sodic one in granites. Moreover, the pyroxene type in mafics is mostly augite, which contains Ca+2. Sr+2 tends to substitute Ca+2 due to their similar electron configuration. So, the substituted Ca+2 by Sr+2 in mafics is more than that in granites. Also, you should consider that 87Sr can form from 84Sr, 86Sr, and 88Sr during nucleosynthesis, and from 87Rb by radioactive decay. So, you should take 87Rb analyses in the two rock types into consideration.
Sincerely,
Mostafa Masoud
I think that the introduction of this paper will help you for more comprehension
Clow, D. W., Mast, M. A., Bullen, T. D., & Turk, J. T. (1997). Strontium 87/strontium 86 as a tracer of mineral weathering reactions and calcium sources in an alpine/subalpine watershed, Loch Vale, Colorado. Water Resources Research, 33(6), 1335-1351.
Conforming with the fundamental assumption made by Masoud, I think for your sample, there might be effects of residual mantle wedge (mafic component) and crustal components in the sample. A primarily depleted mantle wedge, upon melting can give rise to radiogenic Sr Enriched but Rb depleted (which might be incorporated more into the felsic counterpart, more fluid induced melting) residual mafic component. However, the range mentioned is within similar analytical precision, so they might be depicting cogenetic or same sources for the whole assemblage.
(Considering little effect of mixing on the Isotopic signature, as mentioned)
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