Hydrogen in orthopyroxene records oxidation during hydration of the cold mantle wedge

The mechanisms influencing the oxidation state of arc magmas and their correlation with high volatile content remain highly debated. It has been suggested that the incorporation of hydrogen into Nominally Anhydrous Minerals (NAMs), particularly pyroxenes, in the mantle wedge may play a significant role in the oxidation of arc magmas in subduction zones [1]. To assess this potential link, samples from two contrasting settings were analyzed using infrared spectroscopy (FTIR): [i] ultramafic xenoliths from the Colorado Plateau (CP, USA), representing a unique direct sampling of the cold part of the mantle wedge (< 800ºC), and [ii] reference samples from the Kilbourne Hole (New Mexico), and San Quintín volcanic field (Mexico) which were not directly affected by the fluids responsible for the hydration of xenoliths from CP. 

CP samples exhibit significant lithological variability, ranging from Opx-poor harzburgites to lherzolites and pyroxenites, with hydration levels varying from nominally dry and devoid of hydrated minerals to chlorite-rich samples  (occasionally antigorite), and to a lesser extent, bearing amphibole and Ti-clinohumite. The extent of hydration nicely correlates with the transition from spinel to magnetite. Hydrogen content hosted in NAMs ishigher and more variable in samples from the CP  cold hydrated mantle wedge (Ol: 4-32 and Opx: 146-685 ppm wt% H₂O) compared to other localities  (Ol: 0-4 and Opx: 24-209 ppm wt% H₂O). Notably, absorption bands exhibit distinct signatures for the two sample groups. Olivines from other localities  display bands related to Ti-defect complexes, while CP olivines also present bands related to Si- and Mg-vacancies. Interestingly, CP orthopyroxene shows distinctive bands (3544, 3520, 3325, and 3060 cm^-1) previously observed at relatively high oxygen fugacities [1] consistent with the presence of magnetite in these samples. These bands are absent in samples outside CP. 

These observations suggest that NAMs can serve as sensors of redox processes occurring during the hydration of the cold mantle wedge and that fluids derived from the slabs have sufficiently oxidizing capacity to significantly alter its redox budget [2].

[1] Tollan & Hermann, Nat. Geosci. 12, 667–671 (2019). [2] Evans. Geology 34, 489–492 (2006).

Research funded by RUSTED project PID2022-136471N-B-C21 & C22 funded by MICIN/AEI/10.13039/501100011033