Oxygen isotope effects associated with substitution of Al for Fe in synthetic goethite: Some experimental evidence and the criterion of oxygen yield

Measurements of the fractional abundance (m) of high temperature nonstoichiometric (HTN) hydrogen in goethite are shown to predict the reduction in O2 yield associated with removal of HTN oxygen by routine pre-treatment of a sample with BrF5 at 22 °C. The predicted relationship between smaller O2 yields and m is linear and defines an “oxygen yield limit” (OYL), which is introduced for use in oxygen isotope analyses of goethite. The OYL sets lower limits for O2 yields that would be consistent with: (1) removal of all HTN oxygen during pre-treatment and (2) complete recovery of the goethite structural oxygen during subsequent reaction with BrF5 at 450 °C. Measured O2 yields that are less than those predicted by the OYL are anomalously low and indicate probable partial loss of structural oxygen during pre-treatment at 22 °C. After such partial loss, measured δ18O values of the residue are more negative than the actual δ18O of the total structural oxygen. Therefore, the OYL can be used to distinguish between robust and suspect δ18O analyses of goethite. This OYL criterion was applied to some newly analyzed synthetic goethites in which Al substitutes for Fe up to mole fractions (XAl) of ∼0.13. As expected, the robust δ18O values indicate that 1000 ln 18α (where, 18α = [18O/16Omineral]/[18O/16Owater]) increases with increasing XAl, but comparison of these results with the predictions of a published solid solution model suggests that the relationship between 1000 ln 18α and XAl is sensitive to very high pH and possibly to the concentration of aqueous Cl− during synthesis of goethite. The cause of the apparent sensitivity to Cl− remains unknown. The various relationships are: equation(A) T=42°C,pH=1.5–2.5,[Cl-]≈0.16M:1000ln18α=18±5XAl+1.8±0.3 equation(B) low-pH model,[Cl-]=0.00M:1000ln18α=13(±2)XAl+4.1(±0.3) equation(C) T=22°C,pH=14,[Cl-]=0.00M:1000ln18α=5(±3)XAl+1.6(±0.3) equation(D) low-pH model,[Cl-]=0.00M:1000ln18α=13(±2)XAl+6.5(±0.3) However, for natural freshwater goethites, the low-pH, no-Cl, solid solution model (e.g., isothermal equations B and D) best predicts the oxygen isotope effects associated with substitution of Al for Fe. Thus, for goethites from freshwater systems with acidic to circum-neutral pH, the following expression can be used to calculate end member goethite (FeOOH) δ18O values from the measured values of δ18O and XAl: δ18OFeOOH=δ18Omeasured-13(±2)XAlδ18OFeOOH=δ18Omeasured-13(±2)XAl Because the slope of the preceding expression appears to be relatively insensitive to temperature, the equation should be applicable over the range of temperatures at which goethite commonly crystallizes in natural systems (0 °C ⩽ T ⩽ ∼70 °C). However, the value of δ18OFeOOH is sensitive to temperature via the temperature dependence of 1000 ln 18α for pure goethite at acidic to circum-neutral pH in freshwaters as follows (T in degrees Kelvin): 1000ln18α=1.66×106T2-12.6 The results of this study affirm the need to correct measured δ18O values of goethite for the effects of substituent Al to obtain useful paleoenvironmental information, and they support the existing model for doing so.