The radiometric dating evidence for the timing and duration of volcanism for the Steens through Wanapum Basalt of the Columbia River Basalt Group is critically reviewed here. K-Ar dates generally underestimate the age of crystallization, though one important exception is detected, where excess argon led to dates that were too old. The 40 Ar/ 39 Ar results on whole-rock basalts from 1980 through 2010 are examined for statistical validity of plateau sections, as well as alteration state of the material dated. In most instances, listed ages are shown to be invalid. The 40 Ar/ 39 Ar total gas (fusion) ages are, in general, not accurate estimates of the time of formation of these rocks. The 40 Ar/ 39 Ar ages on plagioclase separates from basalts yield good estimates of the extrusion age of the lavas. New 40 Ar/ 39 Ar ages on whole-rock basalts are presented that are in good agreement with the plagioclase ages. Various forms of the geomagnetic polarity time scale for mid-Miocene time are examined, along with the ages of lavas and their magnetic polarity. The main sections of the Columbia River Basalt Group (Imnaha through Wanapum Basalt) were formed in ~0.5 m.y. between 16.3 and 15.8 Ma. Steens Basalt extrusion occurred about ~0.1 m.y. before the Imnaha Basalt and appears to have been a precursor to the more voluminous volcanism noted in the Columbia River Basalt Group.

Alteration of undisturbed igneous material used for argon dating work, is the most common cause of incorrect (low) estimates of the time of crystallization. Identification of alteration has relied on qualitative and subjective (optical) methods. For 40 Ar/ 39 Ar dating, I introduce a new parameter—the alteration index (A.I.)—to quantitatively assess alteration. This looks to the quantity of 36 Ar (atmospheric argon) released in such studies. A non-dimensional parameter is used, relating the 36 Ar levels to that of 39 Ar for K-rich phases (K-feldspar, biotite, whole-rock basalt), and to 37 Ar for Ca-rich phases (plagioclase feldspar and hornblende). Water contains large amounts of dissolved argon derived from the atmosphere. During chemical weathering, 36 Ar carried by water is introduced into the silicate phases of rocks. All common alteration minerals contain water; their 36 Ar contents are ∼100–1000 times higher than in anhydrous silicate phases. Incipient alteration, undetected by current tests, is unequivocally recognized by the A.I. method. In 40 Ar/ 39 Ar stepheating studies, the plateau steps (if any) release argon from the least altered sites. The A.I. of plateau steps for fresh, subaerial, material yields the cut-off value for detecting alteration. Partial loss of 40 Ar* from altered samples may result in statistically acceptable plateaus that underestimate the true crystallization age by ∼2–10%. Many ages are invalid as accurate estimates of the age of crystallization (a) based on statistical analysis of the apparent ages on plateau/isochron plots and/or (b) ages derived from altered phases within the sample. At subduction zones, the hydrated slab cycles substantial quantities of (atmospheric) argon into the mantle. Monitoring 36 Ar levels in fresh (mafic and intermediate) rocks should serve as a sensitive tool in elucidating the role of water driven off the subducted slab in triggering magmatism in convergent zone settings.

Alteration of undisturbed igneous material used for argon dating work often results in inaccurate estimates of the crystallization age. A new quantitative technique to detect alteration has been developed (see Baksi, this volume , Chapter 15), utilizing the 36 Ar levels observed in rocks and minerals. The method is applied to data in the literature for rocks linked to hotspot activity. For subaerial rocks, argon dating results are critically examined for the Deccan Traps, India. The duration of volcanic activity and its coincidence in time with the K-T boundary are shown to be uncertain. The bulk of dated seafloor material (recovered from the Atlantic, Indian, and Pacific oceans) proves to be altered. Ages determined using large (hundreds of milligram) samples are generally unreliable, due to inclusion of altered phases. Such analyses include studies suggesting an age of ca. 43 Ma for the bend in the Hawaiian-Emperor chain. More recent attempts, using much smaller sub-samples (∼10 mg) that have been acid leached to remove alteration products, are generally of higher reliability. Plagioclase separates sometimes yield reliable results. However, many whole-rock basalts from the ocean floor yield ages that are, at best, minimum estimates of the time of crystallization. Most “rates of motions,” calculated from hotspot track ages, are shown to be invalid. Seafloor rocks are recovered at considerable expense but often are not suitable for dating by the argon methods. Most are severely altered by prolonged contact with sea-water. A method is recommended for testing silicate phases prior to attempts at argon dating. This involves a quantitative determination of the 36 Ar content of the material at hand; dating phases without pretreatment—leaching with HNO 3 for material containing ferromagnesian phases, and HF for feldspars—is strongly discouraged.

Hotspot activity has been invoked to explain a number of geological observations and phenomena. The genetic relationship between hotspots and continental flood basalts, as well as “tracks” in many oceans, appears to be commonly accepted by Earth scientists. One critical test that must be applied to such connections or hypotheses is that the pertinent radiometric data must be robust. Herein I critically reexamine some sets of data in this regard. The pertinent 40 Ar/ 39 Ar step-heating data must (1) satisfy rigorous statistical tests for validity and (2) be based on material that can be shown to be fresh or minimally altered. I show that most age data published recently for the Isle of Mull (British Tertiary Igneous Province) are invalid as proper estimates of the crystallization age. I apply a similar mode of examination to rocks thought to represent the tracks of (1) the Yellowstone hotspot, Pacific Northwest, USA, (2) the Tristan da Cunha and Great Meteor hotspots, Atlantic Ocean, and (3) the Kerguelen and Ré-union hotspots, Indian Ocean. Few, if any, valid crystallization ages were recovered. These hotspot tracks cannot be temporally defined. Conclusions based on the data rejected herein, in particular those pertaining to the extrapolated paths of such tracks and the calculation of plate velocities, should be subject to critical scrutiny.

The K-Ar data for various sections of the Columbia River basalt (Imnaha, Picture Gorge, and Grande Ronde Basalt formations) have been reevaluated, utilizing the atmospheric argon contents of the rocks to identify the least altered samples. These ages, together with the results of paleomagnetic studies on these same sections, were then fitted into the known magnetostratigraphy of the Columbia River basalt. Finally, these findings are integrated with the geomagnetic polarity time-scale for mid-Miocene times. The Imnaha Basalt was formed ~17.2 Ma, and the Picture Gorge Basalt ~ 16.0 Ma. The Grande Ronde Basalt was extruded between ~16.9 and 15.6 Ma, with >50 percent of the total volume (magnetostratigraphic units R 2 –N 2 ) formed within ~300,000 yr, around 15.8 Ma.