Jörn-Frederik Wotzlaw

High-precision zircon U/Pb geochronology by ID-TIMS using new 1013 ohm resistors

Accessory mineral U–Pb geochronology by isotope dilution thermal ionization mass spectrometry (ID-TIMS) requires precise and accurate determinations of parent–daughter isotope ratios. The small sample size, particularly with respect to radiogenic Pb (Pb*), requires highly sensitive ion detection systems. Most studies therefore employ either secondary electron multipliers (SEMs) or Daly photomultipliers that provide low background noise and high sensitivity but have a limited linear range and require dynamic peak-hopping. We here evaluate the application of new 1013 ohm resistors in a Faraday cup amplifier feedback loop for the static collection of all Pb isotopes (sample and tracer) with 202,205,206,207,208Pb measured on Faraday cups and 204Pb measured in the axial SEM of a Thermo Scientific™ TRITON™ Plus TIMS instrument. We demonstrate long-term stability of the amplifier gain calibration using a secondary Nd standard and test short- and long-term stability and reproducibility of amplifier baselines. Accurate calibration of static detector arrays is demonstrated by repeated analyses of synthetic and natural U–Pb standards (ET100, Temora-2 and AUS_Z7_5) with variable Pb* (0.551 to 699 pg) and comparison with conventional dynamic ion counting data. Excellent agreement between the two detector systems for all analysed standards suggests that our static measurement routine with 1013 ohm resistors produces accurate and precise U–Pb isotopic data with superior external reproducibility. We anticipate that this new technique will push the frontiers of high-precision U–Pb geochronology and may represent a crucial advancement in the quest towards inter- and intra-laboratory reproducibility at the 0.01% level.

Zircon petrochronological evidence for a plutonic-volcanic connection in porphyry copper deposits

Bridging the gap between the plutonic and volcanic realms is essential for understanding a variety of magmatic processes from caldera-forming eruptions to the formation of magmatic-hydrothermal ore deposits. Porphyry copper deposits are commonly associated with large and long-lived volcanic centers, but the temporal and dynamic link between mineralized intrusions and volcanic eruptions has remained controversial. Based on the combination of (1) high-precision zircon U-Pb geochronology and trace element geochemistry with (2) plagioclase textures, we discovered an intimate connection between an ignimbrite eruption and a nearby world-class porphyry deposit (Bajo de la Alumbrera in the late Miocene Farallon Negro Volcanic Complex of Argentina). Our results indicate that the magmatic-hydrothermal deposit and explosive volcanism were derived from a common magma reservoir that evolved over a minimum duration of 217 ± 25 k.y. before the final eruption. We show that the volcanic pile represents the inverted magma reservoir, recording systematic differences in plagioclase textures and juvenile clast content from bottom to top. This tight temporal and geochemical link suggests that deposit formation and volcanic eruption were both triggered by the same injection of a volatile-saturated primitive magma into the base of the magma chamber. A time gap of 19 ± 12 k.y. between porphyry mineralization and the onset of explosive volcanism indicates a minimum duration of magma reservoir rejuvenation that led to the explosive eruptive event. Catastrophic loss of volatiles by explosive volcanism terminated the ore-forming capacity of the upper-crustal magma chamber, as evidenced by the intrusion of a syn-eruptive barren quartz-feldspar porphyry. Our results demonstrate that porphyry copper deposits provide critical information to understand how volatiles control the fate of hydrous magmas between pluton formation and explosive volcanism.

ID-TIMS U–Pb geochronology at the 0.1‰ level using 1013 Ω resistors and simultaneous U and 18O/16O isotope ratio determination for accurate UO2 interference correction

We document recent advances in analytical protocols that employ 1013 Ω resistors in the Faraday cup amplifier feedback loop for high-precision U–Pb geochronology by isotope dilution thermal ionisation mass spectrometry (ID-TIMS). We present a direct comparison of high-precision U–Pb data obtained using different detector systems, including dynamic peak hopping on a secondary electron multiplier and static multicollection routines that use 1013 Ω resistors in the amplifier feedback loop. The high-ohmic resistors also allow the precise and accurate measurement of the minor 272(UO2) isotopologues (mostly 238U18O16O) in a static multicollection routine, which permits precise and accurate determination of the 18O/16O ratio of UO2 molecules during the U-isotope ratio measurement without compromising signal intensity on the more abundant UO2 isotopologues. This enables the precise determination of U abundances in zircons by isotope dilution with within-run correction of isobaric interferences from the minor UO2 isotopologues, thereby eliminating one of the major sources of uncertainty in ultra-high-precision U–Pb data sets. This approach permits the determination of single U–Pb dates with uncertainties <0.2‰ and corresponding weighted mean dates with uncertainties <0.1‰ (n ∼ 6 to 10). Furthermore, the ability to use different combinations of detector setups for the same analysis within the same mass spectrometer allows for direct comparison of independently calibrated detector systems.

Post-caldera Volcanism at the Heise Volcanic Field: Implications for Petrogenetic Models

The Heise volcanic field is the second youngest caldera complex of the Yellowstone–Snake River Plain province (USA) and represents a polycyclic caldera system with rhyolitic volcanism extending over more than 2 Myr. The products of the Heise volcanic field include four regionally extensive ignimbrites, including the Blacktail Creek and Kilgore tuffs, which both have volumes estimated at >1000 km, separated by sequences of smaller volume tuffs, lavas and sedimentary deposits. Rhyolites from the Heise volcanic field are both normal-dO and low-dO, making it a key locality for investigating rhyolite petrogenesis. However, the occurrence of abundant young basaltic lava has limited our ability to fully characterise this volcanic centre, particularly in terms of post-caldera volcanism. Here we describe rhyolitic samples from both a >700 m thick section of drillcore within the Snake River Plain and the exposed outflow stratigraphy on the margins of the plain. Based on a combination of bulk-rock and mineral geochemical, isotopic, and geochronological evidence, we conclude that the rhyolites from the drillcore are not exposed at the surface, nor are the surficial rhyolites found in the drillcore. High-precision isotope dilution thermal ionisation mass spectrometry U-Pb geochronology dates the rhyolite at the base of the drillcore to 4 0248 6 0 0011 Ma, 0 4 Myr younger than the youngest caldera-forming ignimbrite at Heise, the 4 48 Ma Kilgore Tuff, whereas U-Pb secondary ionisation mass spectrometry dates the uppermost portion of rhyolite in the drillcore to 3 86 6 0 19 Ma. The combined geochemistry and stratigraphic relations suggest that the drillcore penetrates the intracaldera stratigraphy. The intracaldera rhyolites are compositionally and mineralogically similar to the outflow stratigraphy with high-temperature magmas (>800 C) persisting for the full >3 Myr history of the Heise centre. The dO values of pyroxene, sanidine, and quartz from the unaltered drillcore samples are consistent with high-temperature equilibrium and return magma dO values that are low (4 1–6 0‰ based on DO melt–sanidine of 0 6‰) but somewhat higher than the value for the preceding Kilgore Tuff magma of 3 3‰. Buried deep within the drillcore are also hydrothermally altered rhyolites with bulk dO ranging from –3 5‰ to þ1 0‰ (SMOW) with complex X-ray diffraction spectra revealing the presence of epidote, quartz and chlorite. These altered samples are, however, not markedly different in bulk major or trace elemental geochemistry from the unaltered Heise rhyolites. Rhyolite-MELTS models using these hydrothermally altered samples as potential assimilants can reproduce the compositions, mineralogy, and crystallinity of the low-dO Kilgore Tuff with 40–50% assimilation while also satisfying the mass balance constrained on the basis of dO. These results support a cannibalisation model for Heise volcanism while highlighting that the lowest dO rhyolites may require large amounts of extremely O-depleted hydrothermally altered material available for assimilation.

High-resolution stratigraphy and zircon U–Pb geochronology of the Middle Triassic Buchenstein Formation (Dolomites, northern Italy): precession-forcing of hemipelagic carbonate sedimentation and calibration of the Anisian–Ladinian boundary interval

Orbitally forced cyclic variations in sedimentary sequences provide evidence for short-term fluctuations of Earth climate and a tool for high-resolution timescale calibration. We here present stratigraphic and geochronological evidence for precession-forcing in Middle Triassic hemipelagic limestones of the Buchenstein Formation (Dolomites, northern Italy). High-resolution stratigraphy of several correlative sections of the Buchenstein Formation documents a coherent cycle pattern. Isotope dilution thermal ionization mass spectrometry zircon U–Pb geochronology of tuffs bracketing the cyclic interval reveals an average cycle duration of 18.5 ± 2.1 kyr, consistent with a shorter climatic precession cycle in the Middle Triassic compared with today. This suggests a predominantly precession-controlled climate in this low-latitude setting of the western Tethys and allows high-precision calibration of the Anisian–Ladinian boundary interval. From integrating cyclostratigraphic and U–Pb geochronological constraints, our best estimate for the age of the Anisian–Ladinian boundary is 241.464 ± 0.064/0.097/0.28 Ma. We also provide precise estimates for lithostratigraphic boundaries, biostratigraphic markers and magnetic reversals within the boundary interval. Stratigraphic intervals with elevated sedimentation rate record a sub-Milankovitch signal that may be equivalent to patterns in adjacent carbonate platforms such as the Latemar platform. The origin of this sub-Milankovitch signal remains unknown but highlights the potential to investigate shorter-term climatic variations in Mesozoic sedimentary sequences. Supplementary material: Isotope dilution thermal ionization mass spectrometry U–Pb and laser ablation inductively coupled plasma mass spectrometry trace element data tables as well as the results of the Bayesian age modelling are available at https://doi.org/10.6084/m9.figshare.c.3861817.