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Recently we could show that dynamic nuclear polarization (DNP) enhancements > 50 could be obtained in aqueous solutions at a magnetic field strength of 9.2 T (corresponding to a 1H NMR frequency of 400 MHz and a electron spin Larmor frequency of 260 GHz)1-3. This could be achieved with a home-build liquid DNP spectrometer, which consists of a gyrotron microwave source, holo-metallodielectric microwave transmission system and microwave bridge and most importantly a double-resonance structure (260 GHz / 400 MHz) to avoid sample heating4-6. To rationalize this surprisingly large enhancements, the individual factors describing the Overhauser DNP enhancement, as the leakage, saturation and coupling factors, have been individual evaluated and experimentally determined. Here we will show experimental results on TEMPOL nitroxide radicals dissolved in a series of liquids and compare the individual factors and the observed DNP enhancements with theoretical predictions from translational diffusion and MD calculations7.

The isotopes of C and O are not randomly distributed within single phases such as CO2 gas and carbonates—in particular, the heavy isotopes of C and O tend to bond preferentially (‘clump’) at lower temperatures. Consequently, the measurement of the deviation from a random distribution of C and O isotope distributions in a single phase can be used as a thermometer, which is independent of other constraints (such as the isotopic composition of water). The 'clumped' isotope geothermometer has been used to measure paleotemperatures as far back as the Ordovician, the body temperature of dinosaurs, and the temperature of African grasslands during human evolution. As with other geothermometers based on homogeneous or heterogeneous equilibria, the clumped-isotope thermometer is susceptible to resetting (e.g., if the phase is reheated or experiences slow cooling). Thus, clumped-isotope “temperatures” of phases that have experienced complex thermal histories may, in fact, be closure temperatures, the interpretation of which requires quantification of the kinetics of redistribution of C and O isotopes as a function of temperature. To better constrain these kinetics, we performed experiments on natural optical calcite and carbonate-bearing apatite in rapid quench furnaces at a variety of temperatures and times and created to models to quantify the rates. Additionally, we measured clumping in cogenetic apatite and calcite from three carbonatites; these intrusive rocks cooled slowly from igneous temperatures and thus comparison of the apatite and calcite closure temperatures provides constraints on the differences in kinetics of resetting of clumping in these two phases. Using the results of our models and measurements we are able to calculate activation energies and estimate expected changes in apparent temperatures of samples for geologically plausible scenarios. As such, this study has relevance for our interpretation of ancient samples that have thermal histories involving any significant heating. Additionally, it provides the groundwork for using apatites and carbonates together in paleotemperature studies to determine if thermal overprinting could be present.