Tweaking of Earth’s geological timescales and concurrent mantle processes

• Main Author: Groen, Mirek
• Format: Book
• Language: English
• Published: Natural History Museum of Denmark, Faculty of Science, University of Copenhagen 2019
• Subjects: Greenland; Greenland ice core; Greenland Ice core Project; Greenland ice cores; ice core; Ice Sheet; Iceland; NGRIP; North Atlantic; North Greenland; North Greenland Ice Core Project
• Online Access: https://curis.ku.dk/portal/da/publications/tweaking-of-earths-geological-timescales-and-concurrent-mantle-processes(bd2a4adc-8c92-4bfa-b054-39186c7ea311).html; https://soeg.kb.dk/permalink/45KBDK_KGL/1ed7rpq/alma99123213867505763

High-precision analyses are fundamental for the detection of very small variations in isotopic compositions and the achievement of accurate age calculations. This thesis is a combination of work on W and Ar isotope analyses trying to address the origin of heterogeneous mantle reservoirs and obtain accurate ages of Quaternary volcanic eruptions. Variations in W isotopic compositions could be the result of changes in the Hf/W ratio by metal-silicate and crystal-melt fractionation, additions of extraterrestrial material and coremantle interaction. The variations observed can be divided into two groups. The first group is formed by the Archean complexes, which are believed to yield 182W excesses due to either early differentiation within the mantle during the first 60 Myr or a lack of chondritic material that was added to the bulk silicate Earth (BSE) by the so called late veneer. Others show that by recycling proto-crust in the primitive mantle (PM) isotopic variations are mixed and over time fading away. We studied two age groups of rocks from the Isua Supracrustal Belt (ISB) and did not find evidence for the process of homogenisation of the isotopic signatures that was earlier confirmed by for example Nd isotopes. Moreover, the results obtained from 3.7-3.8 Ga supracrustal rocks and younger 3.4 Ga intrusions reveal no significant difference in W isotopic ratios and suggest that late-stage metamorphic events triggered mobilisation of W. However, by the use of another refractory element Zr it is possible to reconstruct the W isotopic composition before alteration. The 182W value of 12 ppm confirms earlier studies corresponding to a mantle source that received approximately 50% of the late veneer budget. The second group consists of the modern rocks with a deep mantle source sampled by hot spots. In previous work, the origin of 182W deficits are subject of debate. It is suggested that the core is interacting with the mantle at the base of mantle plumes, but an alternative theory comprises the existence of Hadean reservoirs, which are preserved in the deep mantle. We studied submarine and subaerial volcanic rocks from the Hawaiian Archipelago and our observations not only confirm the presence of 182W deficits, but also prove that another reservoir yielding a 182W excess and ambient mantle are components of the mantle plume. The Icelandic hotspot was used in a more applied study on re-calibration of geological timescales. Volcanic eruptions potentially cover large areas with volcanic ash, which are providing time-markers in geological records. Ice core chronology models are based on annual layer counting, but ages seem to be underestimated in the deeper part of ice sheet due to an increasing accumulated error with depth by compaction of ice layers. However, the volcanic ash, found as tephra layers in ice cores, is too fine grained to be suitable for 40Ar/39Ar dating. The widespread North Atlantic Z2 ash, found in the North Greenland Ice Core Project (NGRIP) ice core at a depth of 2,359.45 m, could be linked to the Thórsmörk Ignimbrite on Iceland. Based on glass fiammé in the ignimbrite, we report an astronomically calibrated 40Ar/39Ar age of 56.2 0.5 ka (2). This is not only approximately 800 years older than the annual counted layer age, but also nearly 5 times more precise. By using our 40Ar/39Ar age as a “golden spike” we propose a series of revised ages for climate events recorded in Greenland ice cores.