40AR/39AR AGES ON INTERCALATED SILICIC TUFFS PROVIDE PRECISE AGES FOR STEENS BASALT LAVAS: IMPLICATIONS FOR FLOOD BASALT EFFUSION RATES, RELATION TO MIOCENE CLIMATIC OPTIMUM, AND THE AGE OF THE STEENS GEOMAGNETIC REVERSAL

To establish causality between flood basalt eruptions and extinction events and global environmental effects recorded by isotopic excursions in marine sediments, accurate and precise ages for the flood basalts are required. But low K content and absence of zircon make flood basalts difficult to date. We illustrate how ⁴⁰Ar/³⁹Ar feldspar ages for silicic tuffs intercalated with and overlying sections of Steens Basalt, the earliest lavas of the mid-Miocene Columbia River Basalt Group, provide high-precision ages for lavas, and, for the first time, make it possible to resolve age differences within a section of these flood lavas. The stratigraphically lowest tuff, a fall deposit, yielded an age of 16.694±0.012 Ma, and the uppermost, the Tuff of Oregon Canyon ignimbrite, is 16.573±0.007 Ma (FCs = 28.201 Ma, ±1). Based on calculated accumulation rates of Steens Basalt lavas between dated tuffs, we estimate that the Steens geomagnetic reversal occurred at 16.61 Ma, and that the total duration of Steens Basalt volcanism in the area was ~190 ka, from ~16.74 to 16.55 Ma. The resulting estimate of average effusion rate of ~0.17 km³ a^-1 is only about one-tenth that of the Siberian Traps, but is nearly double the rate at Pu’u O’o, the longest-lived, most voluminous historic eruption of Kilauea Volcano. Our new 16.74 Ma estimate for the initiation of Steens Basalt volcanism does not match with an estimated age of ~16.9 Ma for the beginning of the Miocene Climatic Optimum (MCO); either there are inconsistencies between the ages of basalts, geomagnetic reversals, and/or orbitally tuned sedimentary sections, or other forcing mechanisms initiated the MCO. Given our modest estimated eruptive rates, even with a generous allowance for degassing of related intrusions and host rocks, Columbia River Basalt volcanism is unlikely to have initiated the MCO, but the CO₂ emitted may have intensified the MCO and delayed reglaciation of Antarctica (Armstrong McKay et al., 2014). The combination of short-term cooling effects of S gases and the small but longer-term heating effects of CO₂ emitted during Columbia River flood basalt eruptions superimposed onto cycles of primary productivity, silicate weathering, and carbon sequestration may account for the large-amplitude d18O and d13C variations observed during the MCO (Holbourn et al, 2015).