A Strong Correlation Between Geochemical Indicators of Slab-Derived Fluids and Elevated Oxidation States of Mantle-Derived Arc Lavas

There has been debate in the literature as to whether the relatively high oxidation state of subduction-related lavas relative to mid-ocean ridge basalts is related to the influx of slab-derived fluids that trigger partial melting in the arc mantle wedge, or alternatively is the result of secondary effects such as differentiation and/or volatile degassing. In this study, we use a suite of 32 high-MgO (8-16 wt%) basalts, absarokites and minettes from across the Mexican volcanic arc (all erupted in the last 1 Myr) to evaluate whether indices of slab-derived fluids (e.g., Sr, Ba and K2O concentrations) correlate with oxidation state (ΔNNO = log10fO2 units relative to the Ni-NiO buffer). By restricting the data set to high-MgO lavas, the effect of differentiation is removed. Moreover, because all samples contain olivine, the presence or absence of iddingsite along olivine rims is used to distinguish samples that have experienced any post-eruptive alteration. Initially, a larger data set of 54 lavas was considered, for which ferrous iron was measured by titration (using the Wilson 1960 method, which recovers three USGS and one Canadian Survey standards), thereby allowing the bulk ferric-ferrous ratio of the sample to be determined and thus the ΔNNO value using the calibration of Kress and Carmichael (1991). These samples were then placed into three categories: (1) no iddingsite anywhere in the sample, (2) sparse iddingsite along some olivine rims, and (3) thick iddingsite rims on most olivine phenocryts. For medium-K2O basalts, the average ΔNNO value for group (1) and (3) is +0.2 and +1.3, respectively, which suggests the effect of extensive post-eruptive alteration is to increase the calculated ΔNNO value by ~1.1 log unit. For the rest of the study, only group (1) lavas were considered, namely those for which no sign of any iddingsite on any olivine crystal was observed. This narrowed the group of lavas to 32 pristine samples, which ranged over an order of magnitude in their concentrations of K2O (0.6 to 6.0 wt%), Sr (446-3075 ppm) and Ba (218-3230 ppm), all fluid-mobile elements that can be used as indices of the relative input of slab-derived fluids into the magmas. Similarly, the ΔNNO values of the 32 samples also spanned a large range, from -0.9 to +4.4, more than five orders of magnitude. A strong linear correlation between ΔNNO and the three indices of slab-derived fluid were found; R2=.70, .74, and .73 for K2O, Sr, and Ba, respectively. This supports the hypothesis that the source of the relatively high oxidation state of mantle-derived arc magmas originates from the influx of slab- fluids into the mantle wedge.