The Milankovitch theory of climate change is widely accepted, but the registration of the climate changes in the stratigraphic record and their use in building high-resolution astronomically tuned timescales has been disputed due to the complex and fragmentary nature of the stratigraphic record. However, results of time series analysis and consistency with independent magnetobiostratigraphic and/or radio-isotopic age models show that Milankovitch cycles are recorded not only in deep marine and lacustrine successions, but also in ice cores and speleothems, and in eolian and fluvial successions. Integrated stratigraphic studies further provide evidence for continuous sedimentation at Milankovitch time scales (10 ⁴ years up to 10 ⁶ years). This combined approach also shows that strict application of statistical confidence limits in spectral analysis to verify astronomical forcing in climate proxy records is not fully justified and may lead to false negatives. This is in contrast to recent claims that failure to apply strict statistical standards can lead to false positives in the search for periodic signals. Finally, and contrary to the argument that changes in insolation are too small to effect significant climate change, seasonal insolation variations resulting from orbital extremes can be significant (20% and more) and, as shown by climate modelling, generate large climate changes that can be expected to leave a marked imprint in the stratigraphic record. The tuning of long and continuous cyclic successions now underlies the standard geological time scale for much of the Cenozoic and also for extended intervals of the Mesozoic. Such successions have to be taken into account to fully comprehend the (cyclic) nature of the stratigraphic record.

Understanding the time scales of magmatic differentiation, storage, and eruption of large-volume silicic magmas is a primary goal of igneous petrology. Within the Huckleberry Ridge Tuff (HRT; Idaho, USA), representing the earliest and largest caldera-forming eruption associated with Yellowstone volcanic activity, zircon morphological zoning patterns coupled to strongly correlated changes in Ti-in-zircon thermometry and trace element indicators of progressive differentiation provide a proxy record for the evolution of the HRT member B magma body. Tandem in situ and isotope dilution U-Pb dating of single zircon crystals demonstrates an absence of pre-Pleistocene xenocrysts, but reveals the presence of antecrysts recycled from pre-caldera rhyolites in the HRT magma. The petrochronologic interpretation of autocrystic zircon thermal, chemical, and temporal characteristics suggests that HRT member B differentiated over ∼10 k.y. prior to eruption at 2.0794 ± 0.0046 Ma as defined by new astronomically calibrated, single-crystal total fusion 40Ar/39Ar sanidine analyses. This refined eruption age demonstrates that the transitional polarity preserved by HRT member B does not record the Reunion subchron, but rather a separate, younger geomagnetic event. Our novel approach places the thermal and chemical regime of silicic magmas within a temporal context and demonstrates the rapid evolution of a large volume of silicic magma

Calc-alkaline plutons are the major crustal building blocks of continental margins, but are rarely exposed in oceanic island arcs. Two of the best examples are the ∼10 km wide Hidden Bay and Kagalaska plutons that intrude Eocene mafic volcanic–sedimentary rocks on Adak and Kagalaska islands in the central Aleutian arc. Twenty new Ar/Ar and U/Pb ages, coupled with published ages, show that the Hidden Bay pluton was intruded in multiple stages from ∼34·6 to 30·9 Ma, whereas the Kagalaska pluton was intruded at ∼14 Ma. The plutons largely consist of medium- to high-K2O hornblende-bearing cumulate diorite (53–55 wt % SiO2) and hornblende–biotite granodiorite (57–64 wt %), with lesser amounts of gabbro (50–52 wt % SiO2), leucogranodiorite (67–69 wt % SiO2) and aplite (76–77 wt % SiO2) that can generally be linked to each other by crystal fractionation. The compositions of these plutons are generally similar to those of continental plutons, except for more oceanic-like large ion lithophile element and isotopic signatures (87Sr/86Sr = 0·703–0·7033; ɛNd = 9·4–7·7) that reflect oceanic- rather than continental-type crustal contaminants. Chemical similarities between the Hidden Bay homogeneous gabbros and high-Al basalts in Adak Pleistocene-Holocene volcanoes indicate little temporal evolution in the general character of the mantle-derived basalts. Rather than a unique arc setting and distinctive magmas, formation of the Aleutian calc-alkaline plutons seems to require a sufficient crustal thickness (∼37 km) and a high enough water content to stabilize pargasitic hornblende amphibole in a relatively closed magma system that favors increasing K, Ti and H2O at the end of a magmatic cycle. This termination of magmatism coincides with a northward migration of the magmatic front that is inferred to be associated with fore-arc subduction erosion. In accord with Adak region crustal architecture based on seismic data, crystallization models for the plutons suggest that mantle-generated hydrous arc basalts fractionated olivine and clinopyroxene in the lower crust to form high-Al basaltic composition magmas that rose into the mid-crust, where gabbro and diorite crystallized to form the magmas that buoyantly rose into the upper crust and crystallized to form the volumetrically dominant granodiorite (58–63 wt % SiO2). The most important temporal changes in chemistry can be explained by fore-arc crust incorporated into the mantle wedge by fore-arc subduction erosion creating ‘adakitic’ signatures at times of northward arc migration and a change to a more continental subducted sediment component at the time of Plio-Pleistocene glaciation.

The 1·3 Ma Mesa Falls Tuff (MFT), the second and volumetrically smallest of the Yellowstone caldera-forming eruptions, was examined using a joint zircon petrochronological and sanidine ⁴⁰Ar/³⁹Ar approach to constrain the thermal and chemical evolution, autocrystic growth, antecrystic recycling, and eruptive age of the host magma. A total of 451 laser ablation inductively coupled plasma mass spectrometry in situ spot analyses collected from 323 zircon crystals from five pumice blocks and two welded ash-flow tuff samples provide trace element and Ti-in-zircon thermometry data, which are in turn complemented by high-precision ²⁰⁶Pb/²³⁸U dates from over 50 of those grains. Sanidine grains from two of the pumices were analyzed by incremental step-heating or total fusion ⁴⁰Ar/³⁹Ar dating techniques performed on single crystals using a multi-collector mass spectrometer, yielding an eruption age of 1·300 ± 0·001 Ma. Zircon dates range from 1·57 to 1·30 Ma. Rare grains older than 1·37 Ma may contain inherited cores recycled from the Huckleberry Ridge Tuff (HRT) or other associated smaller volume, effusive Yellowstone magmas; however, the bulk of the Mesa Falls Tuff crystal load cannot be attributed to a long-lived, residual Huckleberry Ridge Tuff magma body. Zircon compositions define trends of strengthening negative europium anomaly and increasing incompatible trace element concentrations over ∼150 °C of cooling. Crystals defining this full compositional spectrum range in age from 1·33 to 1·30 Ma; the dominant mode of 19 grains yields a mean crystallization age of 1·303 ± 0·002 Ma, within uncertainty of the sanidine ⁴⁰Ar/³⁹Ar age, attesting to the rapidity of magma accumulation, differentiation and crystallization prior to eruption. A subset of composite grains composed of extremely differentiated core compositions overgrown by Mesa Fall Tuff-like rims probably represents earlier solidified sidewall or roof accumulations later remobilized within the main Mesa Falls Tuff magma. Fractional crystallization modeling utilizing temperature-dependent zircon–melt partition coefficients is successful in reproducing the trends in incompatible trace element enrichment within zircon grains as a function of decreasing temperature and increasing europium anomaly. Zircon geochemistry thus provides a robust proxy for magma evolution, from the time of zircon saturation through differentiation and eruption. Integrated sanidine and zircon dates coupled with the thermochemical trends indicate that the Mesa Falls Tuff magmatic system differentiated over a period of < 30 kyr, with the bulk of zircon crystal nucleation and growth occurring within 10 kyr of eruption. These petrochronological studies of the MFT and HRT clearly illustrate that the long-term volumetric extrusive rate at Yellowstone (∼2 × 10^–3 km³ a^–1) is punctuated by episodes of much higher magmatic flux (∼2 × 10^–2 to ∼2 × 10^–1 km³ a^–1).