OXYGEN ISOTOPE GEOCHEMISTRY OF PRIMITIVE CALC-ALKALINE MAFIC LAVAS IN THE SOUTHERNMOST CASCADES

Pleistocene to Holocene mafic lavas in the southernmost Cascades (SMC) include primitive (Mg# > 66) calc-alkaline basalts and basaltic andesites. Existing whole rock (WR) δ¹⁸O values for the lavas are higher and have a larger range (5.6 – 7.8‰) than typically associated with mantle-derived mafic lavas (5.5 ± 0.2‰) from oceanic settings. The high WR values of the SMC lavas have been ascribed to mantle source fluxing by H₂O-rich subduction-derived fluids (Borg et al., 1997) and mafic crustal contamination (Bacon et al., 1997). Alternatively, subsolidus alteration may have shifted the WR values, although the rocks are visably and petrographically pristine.

To address the issue of the high δ¹⁸O WR values, we made high precision (± 0.1‰) O isotope measurements on ol and plag separates for 26 SMC primitive calc-alkaline lavas by laser fluorination techniques. Preliminary observations include the following: 1) Most δ¹⁸O_ol values (5.3 - 6.3‰) are lower than predicted from WR values by equilibrium ol-melt fractionations at magmatic temperatures (~0.4‰). Thus, many of the WR data can not represent initial mantle values either because of crustal contamination or cryptic subsolidus alteration. 2) δ¹⁸O_ol values are elevated relative to NMORB (4.9 - 5.3‰), although they are not correlated with Mg# or any other index of differentiation. Furthermore, values for lavas erupted in the forearc are typically 0.2 - 0.3 ‰ higher than δ¹⁸O_ol values for lavas erupted along the arc front and backarc. This is consistent with previous models where fluid fluxing diminishes toward the east. 3) δ¹⁸O_plag values (6.0 - 10.5‰) for many rocks are considerably higher than predicted from equilibrium ol-plag fractionations (~0.8‰). These data along with plag textural evidence and quartz xenocrysts indicate that some magmas incorportated silicic continental crust. Simple mass balance calculations assuming an initial melt δ¹⁸O value of 6.0‰ and a crustal assimilant with δ¹⁸O=10.0‰ require a minimum of 20% crustal endmember in rocks with WR values greater than 6.8‰, which is inconsistent with their magnesian-rich nature. Hence, although some magmas are contaminated and the plag crystals are probably xenocrysts, large-scale assimilation of silicic crust is not the cause of the high WR values.