Eruptive history and geochronology of Mount Mazama and the Crater Lake region, Oregon

Abstract

Geologic mapping, K-Ar, and 40 Ar/ 39 Ar age determinations, supplemented by paleomagnetic measurements and geochemical data, are used to quantify the Quaternary volcanic history of the Crater Lake region in order to defi ne processes and conditions that led to voluminous explosive eruptions. The Cascade arc volcano known as Mount Mazama collapsed during its climactic eruption of ~50 km 3 of mainly rhyodacitic magma ~7700 yr ago to form Crater Lake caldera. The Mazama edifi ce was constructed on a Pleistocene silicic lava fi eld, amidst monogenetic and shield volcanoes ranging from basalt to andesite similar to parental magmas for Mount Mazama. Between 420 ka and 35 ka, Mazama produced medium-K andesite and dacite in 2:1 proportion. The edifi ce was built in many episodes; some of the more voluminous occurred approximately coeval with volcanic pulses in the surrounding region, and some were possibly related to deglaciation following marine oxygen isotope stages (MIS) 12, 10, 8, 6, 5.2, and 2. Magmas as evolved as dacite erupted many times, commonly associated with or following voluminous andesite effusion. Establishment of the climactic magma chamber was under way when the fi rst preclimactic rhyodacites vented ca. 27 ka. The silicic melt volume then grew incrementally at an average rate of 2.5 km 3 k.y. -1 for nearly 20 k.y. The climactic eruption exhausted the rhyodacitic magma and brought up crystal-rich andesitic magma, mafi c cumulate mush, and wall-rock granodiorite. Postcaldera volcanism produced 4 km 3 of andesite during the fi rst 200-500 yr after collapse, followed at ca. 4800 yr B.P. by 0.07 km 3 of rhyodacite. The average eruption rate for all Mazama products was ~0.4 km 3 k.y. -1 , but major edifi ce construction episodes had rates of ~0.8 km 3 k.y. -1 . The longterm eruption rate for regional monogenetic and shield volcanoes was ~0.07 km 3 k.y. -1 , but only ~0.02 km 3 k.y. -1 when the two major shields are excluded. Plutonic xenoliths and evidence for crystallization differentiation imply that the amount of magma intruded beneath Mount Mazama is several times that which has been erupted. The eruptive and intrusive history refl ects competition between (1) crystallization driven by degassing and hydrothermal cooling and (2) thermal input from a regional magma fl ux focused at Mazama. Before ca. 30 ka, relatively small volumes of nonerupted derivative magma crystallized to form a composite pluton because the upper crust had not been heated suffi ciently to sustain voluminous convecting crystal-poor melt. Subsequently, and perhaps not coincidentally, during MIS 2, a large volume of eruptible silicic magma accumulated in the climactic chamber, probably because of heating associated with mantle input to the roots of the system as suggested by eruption of unusually primitive magnesian basaltic andesite and tholeiite west of Mazama. Sediment gravity-flow deposits (Hol.) Talus (Hol. and Pleist.) Landslide deposits (Hol.) Glacial deposits, undivided (Pleist.) Andesite of the E basin (Hol.) Lava Andesite of ...

Feedback between deglaciation, volcanism, and atmospheric CO2

“…This result is consistent with regional studies (e.g. Gardeweg et al, 1998;Jellinik et al, 2004;Bacon and Lanphere, 2006;Nowell et al, 2006;Licciardi et al, 2007), earlier inferences drawn from the Bryson and Bryson (1998) database of volcanic eruptions, and the volcanic index reconstructed from the Greenland ice core (Zielinksi et al, 1997). Depressurization associated with ablation of glaciers and ice caps could cause decompressional melting of the mantle sufficient to sustain a factor of five increase in volcanic output for 5 ky.…”

“…Large eruptions appear to have occurred with a quasi-100 ky period at Mount Mazama (Bacon and Lanphere, 2006), Western Europe (Nowell et al, 2006), and the South Eastern United Sates (Jellinik et al, 2004), indicating that at least some low-frequency volcanic systems are candidates for being paced by the glacial cycles. Increased eruptions during deglaciation, either directly forced by deglacial processes or paced to coincide with deglaciation, would lead to greater CO 2 emissions if they tap deep, pressurized magmatic reservoirs that still retain their CO 2 or if the eruption leads to depressurization of magmatic chambers and encourages the generation of new melt.…”

“…Bacon and Lanphere (2006) identify 60 km 3 of material erupted at Mount Mazama during the last deglaciation, almost half of the total mapped at that site over the last 400 ky, corresponding to a roughly 60 m thickness over a 1000 km 2 region. In addition, Bacon and Lanphere (2006) suggest that several times this amount was emplaced beneath Mount Mazama.…”

“…Bacon and Lanphere (2006) identify 60 km 3 of material erupted at Mount Mazama during the last deglaciation, almost half of the total mapped at that site over the last 400 ky, corresponding to a roughly 60 m thickness over a 1000 km 2 region. In addition, Bacon and Lanphere (2006) suggest that several times this amount was emplaced beneath Mount Mazama. Although an isolated study, this work indicates that the magma products from one volcano contributed about 1% (4× 60 km 3 /30,000 km 3 ) of the total melt (Zielinksi, 2000).…”

“…If subtle changes in weather and climate are sufficient to influence the timing of volcanism, even larger effects could be expected from the massive changes associated with deglaciation (Hall, 1982). Indeed, deglaciation is observed to coincide with increased volcanism in Iceland (Sigvaldason et al, 1992;Gudmundsson, 2000;Sinton et al, 2005;Licciardi et al, 2007), France and Germany (Nowell et al, 2006), the Western United States (Jellinik et al, 2004;Bacon and Lanphere, 2006), and Chile (Best, 1992;Gardeweg et al, 1998). Of these, the clearest demonstration comes from Iceland, where dates from lava flows (Sigvaldason et al, 1992), sulphate concentrations in Greenland ice cores (Zielinksi et al, 1997), and table mountains (Licciardi et al, 2007) are all consistent with increased volcanism during or following deglaciation.…”