INTRODUCTION
This report presents K-Ar and 40 Ar/ 39 Ar ages and supplementary data measured in support of geologic mapping at Medicine Lake volcano, northern California. Interpretation of the significance of these ages to be presented elsewhere (DonnellyNolan, in preparation) . Sample locations are shown in Figures 1 and 2 . Previous publications describing argon dating at and near the volcano include Brown and Mertzman (1979) , Luedke and Lanphere (1980) , Mertzman (1977 Mertzman ( , 1981 Mertzman ( , 1982 Mertzman ( , 1983 , McKee and others (1983) , Becker and others (1994) , Herrero-Bervera and others, 1994 , Donnelly-Nolan and others (1994 , 1996 ), and Turrin (1996 .
METHODS
All of the ages listed in Table 1 are conventional K-Ar ages measured in the USGS Menlo Park laboratory on whole-rock samples selected after thin-section examination. Decay constants (Steiger and Jager, 1977) are listed in Table 1 . K 2 O measurements were made by flame photometry after lithium metaborate fusion and dissolution (Ingamells, 1970) . Ar analyses were by isotope-dilution mass spectrometry using a high-purity (>99.9%) 38 Ar tracer and techniques described previously (Dalrymple and Lanphere, 1969) . All samples for Ar extraction were baked overnight at 280°C. Mass analyses were done on a 22.68 cm radius, multiple-collector mass spectrometer with a nominal 90° sector magnet, using automated data collection (Stacey and others, 1981; Sherrill and Dalrymple, 1980) . 40 Ar/
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Ar incremental-heating experiments in the Menlo Park laboratory (Table  2) were made on splits of approximately 100 mg of sample material. The resistanceheated furnace used to extract Ar is attached to the cleanup system and mass spectrometer described by Dalrymple (1989) . The furnace is modified from the design of Staudacher and others (1978) . Heating temperatures were controlled with an optical fiber thermometer. The fluence monitor for USGS 40 Ar/ 39 Ar analyses was 856003 sanidine, a secondary mineral standard with a reference age of 27.92 Ma.
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In an incremental-heating experiment, the sample is heated to a given temperature and an apparent age is calculated for the gas extracted at that temperature. In calculating an apparent age, it is assumed that the non-radiogenic Ar in a sample is atmospheric in isotopic composition. Analytical data for the USGS determinations are given in Table 3 . 40 Ar/ 39 Ar ages measured at the Berkeley Geochronology Center (Table 2 ) used the techniques described in Herrero-Bervera and others (1994) and in Turrin (1996) . Available analytical data are given in Table 4 .
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One additional table is included (Table 5 ) which compares ages determined on early rhyolite units of the volcano, both for this project and with previous work. plateau age=22 ± 13 ka; isochron age=43 ± 48 ka; inverse age=41 ± 12 ka; isochron intercept=294.6 ± 1.6; total gas age=20 ± 13 ka Ar initial = 296.1 ± 0.8 MSWD = 0.8 

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