INVESTIGATING OH RETENTION IN NOMINALLY ANHYDROUS UPPER MANTLE MINERALS

Various mantle processes (e.g., melting and deformation) are controlled, in part, by the availability of H₂O. Values of mantle aH₂O may, in some cases, be inferred from the OH contents of nominally anhydrous minerals (NAMs) contained in mantle xenoliths. However, mantle NAMs may suffer OH loss during emplacement at earth’s surface. This study compares the OH contents of NAMs with values of aH₂O estimated from amphibole equilibria to determine if NAMs have retained mantle OH contents.

Nine amphibole-bearing xenoliths from two different regions, the southwestern U.S.A and Eastern Australia, were analyzed for this study. All samples contained olivine + orthopyroxene + clinopyroxene + amphibole + spinel. The chemistry of these phases was characterized using the electron microprobe to estimate values of temperature (T), pressure (P) and aH₂O. The P-T conditions of these nine samples ranged from 820 to 1000˚C and 1.0 to 1.6 GPa, respectively. In all cases, amphibole equilibria yield values of aH₂O ranging from 0.02 to 0.14. These values of aH₂O yield predicted olivine OH contents, ranging from < 7 to 43 wt. ppm, that are consistent with olivine OH contents measured using FTIR spectroscopy (< 7 to 44 wt. ppm). The OH-contents of the orthopyroxenes and clinopyroxenes ranged from 0 to 151 wt. ppm and 52 to 217 wt. ppm, respectively. The OH-contents of the three NAMs (olivine, orthopyroxene, and clinopyroxene) yield values for the partitioning of OH between co-existing phases that are generally consistent with partition coefficients obtained via experiments conducted at mantle P-T conditions.

In summary, we have observed: (1) values of aH₂O estimated from amphibole equilibria are consistent with olivine OH contents, and (2) agreement between values of OH partitioning between NAMs in our samples and those determined experimentally. These two observations indicate that even though our values of olivine OH contents (<45 wt. ppm) are low, given the range of values for mantle olivines (≈1 to 471 wt. ppm, Peslier, 2010, J. Volcan. & Geotherm. Res., v. 197, pp. 239-258), they likely reflect mantle conditions. This suggests that the uppermost mantle may be heterogeneous with respect to OH contents.