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We have derived Fe abundances of 16 solar-type Pleiades dwarfs by means of an equivalent-width analysis of Fe Iand Fe IIlines in high-resolution spectra obtained with the Hobby-Eberly Telescope and High Resolution Spectrograph. Abundances derived from Fe IIlines are larger than those derived from Fe Ilines (herein referred to as overionization) for stars with Teff< 5400 K, and the discrepancy (?Fe = [Fe II/H]-[Fe I/H]) increases dramatically with decreasing Teff, reaching over 0.8 dex for the coolest stars of our sample. The Pleiades joins the open clusters M 34, the Hyades, IC 2602, and IC 2391, and the Ursa Major moving group, demonstrating ostensible overionization trends. The Pleiades ?Fe abundances are correlated with Ca IIinfrared triplet and H? chromospheric emission indicators and relative differences therein. Oxygen abundances of our Pleiades sample derived from the high-excitation O I triplet have been previously shown to increase with decreasing Teff, and a comparison with the ?Fe abundances suggests that the overexcitation (larger abundances derived from high-excitation lines relative to low-excitation lines) and overionization effects that have been observed in cool open cluster and disk field main-sequence (MS) dwarfs share a common origin. Curiously, a correlation between the Pleiades O I abundances and chromospheric emission indicators does not exist. Star-to-star Fe Iabundances have low internal scatter (< 0.11 dex), but the abundances of stars with Teff< 5400 K are systematically higher compared to the warmer stars. The cool star [Fe I/H] abundances cannot be connected directly to overexcitation effects, but similarities with the ?Fe and O I triplet trends suggest the abundances are dubious. Using the [Fe I/H] abundances of five stars with Teff> 5400 K, we derive a mean Pleiades cluster metallicity of [Fe/H] = +0.01 ± 0.02.

Oxygen abundances have been derived from the near-IR, high-excitation O I l7774 triplet in high-resolution, high signal-to-noise ratio spectra of 45 Hyades dwarfs using standard one-dimensional, plane-parallel LTE models. Effective temperatures of the stellar sample range from 4319 to 6301 K, and the derived relative O abundances as a function of Teff evince a trichotomous morphology. At Teff > 6100 K, there is evidence of an increase in the O abundances with increasing Teff, consistent with non-LTE (NLTE) predictions. At intermediate Teff (5450 K [?] Teff [?] 6100 K), the O abundances are flat, and star-to-star values are in good agreement, having a mean value of [O/H] = +0.25 +- 0.02; however, systematic errors at the [?]0.10 dex level might exist. The O abundances for stars with Teff [?] 5450 K show a striking increase with decreasing Teff, in stark contrast to expectations and canonical NLTE calculations. The cool Hyades triplet results are compared to those recently reported for dwarfs in the Pleiades cluster and the UMa moving group; qualitative differences between the trends observed in these stellar aggregates point to a possible age-related diminution of triplet abundance trends in cool open cluster dwarfs. Correlations with age-related phenomena, i.e., chromospheric activity and photospheric spots, faculae, and/or plages, are investigated. No correlation with Ca II H+K chromospheric activity indicators is observed. Multicomponent LTE "toy" models have been constructed in order to simulate photospheric temperature inhomogeneities that could arise from the presence of starspots, and we demonstrate that photospheric spots are a plausible source of the triplet trends among the cool dwarfs.