Lifetimes of rhyolitic magma systems: high temporal resolution magma evolution of the Quaternary Acigöl Complex, Anatolia (Turkey)
Final Report Abstract
Protracted pre-eruptive zircon residence frequently detected in continental rhyolites conflicts with thermal models indicating briefer magma cooling durations if scaled to erupted volumes. In the frame of the present project we performed combined U-Th and (U-Th)/He zircon ages from the Acigöl rhyolite field, which is part of a Quaternary bimodal volcanic complex in central Anatolia. Unlike other geochronometers, this approach dates crystallization and eruption on the same crystals, allowing for internal consistency testing. Despite the overall longevity of rhyolite volcanism at Acigöl, and systematic trends of progressive depletion in compatible trace elements and decreasing zircon saturation temperatures, we find that zircon crystallized in two brief pulses corresponding to eruptions in the eastern and western part of the field during Middle and Late Pleistocene times, respectively. For Late Pleistocene zircon, resolvable differences exist between interior (average: 30.7 ± 0.9 ka; 1σ) and rim (21.9 ± 1.3 ka) crystallization ages. These translate into radial crystal growth rates of ~10^-13 – 10^-14 cm/s, broadly consistent with those constrained by diffusion experiments. Rim crystallization and (U-Th)/He eruption ages (24.2 ± 0.4 ka) overlap within uncertainty. Evidence for brief zircon residence at Acigöl contrasts with many other rhyolite fields, suggesting that protracted zircon crystallization in, or recycling from, long-lived crystal mushes is not ubiquitous in continental silicic magma systems. Instead, the span of pre-eruptive zircon ages is consistent with autochthonous crystallization in individual small-volume magma batches that originated from basaltic precursors. Finally, we can refine some of the conclusions in Druitt et al. (1995) regarding the possibility of a large magma body underlying the Acigöl rhyolite field. Druitt et al. postulated a “basaltic shadow zone” (based on the absence of any basaltic vents in the region enclosed by Middle to Late Pleistocene rhyolite domes) to represent a low-density reservoir of silicic magma, or hot intrusives. However, the well-resolved hiatus (~125 ka) between rhyolite eruptions, and rapid zircon crystallization time-scales (~10^3 - 10^4 a) weakens arguments in favour of a persistent subterranean magma chamber. By the same token, we argue that the distribution of rhyolite domes is more regular than previously thought and that a clear-cut distinction exists between Middle and Late Pleistocene eruptive centers that shifted focus from east to west.
Publications
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(2010) Sr-Nd-Pb isotopic and age constraints on the origin of the Acigöl complex, Cappadocia. Tectonic Crossroads, AGU Speciality Meeting 2010, Ankara, Turkey
Siebel, W., Schmitt, A.K., Danisík, M., Aydin, F., Kiemele, E., Jahn, S.
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(2011) Acigöl rhyolite field, Central Anatolia (part I): high-resolution dating of eruption episodes and zircon growth rates. Contributions to Mineralogy and Petrology 162/6 1215-1231
Schmitt, Axel K.; Danišík, Martin; Evans, Noreen J.; Siebel, Wolfgang; Kiemele, Elena; Aydin, Faruk & Harvey, Janet C.
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(2011) Acigöl rhyolite field, Central Anatolia (part II): geochemical and isotopic (Sr-Nd-Pb, δ18O) constraints on volcanism involving two high-silica rhyolite suites. Contributions to Mineralogy and Petrology 162/6 1233-1247
Siebel, W.; Schmitt, A. K.; Kiemele, E.; Danišík, M. & Aydin, F.
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(2011) Eruptive history and chemical evolution of the Acigöl volcanic field, central Anatolia, Turkey, based on geochemical and isotopic (Sr-Nd-Pb, δ18O) constraints and ion microprobe zircon analysis. Conference Supplement to Geochimica et Cosmochemica Acta, Goldschmidt Conference 14-19 August 2011, Prague, Czech Republic
Siebel, W., Schmitt, A.K., Danisík, M., Aydin, F., Kiemele, E.
