Validation of very high cloud droplet number concentrations in air masses transported thousands of kilometres over the ocean

The microstructure of orographic clouds related to the aerosol present was studied during the second Aerosol Characterisation Experiment (ACE‐2). Very high cloud droplet number concentrations (almost 3000 cm⁻³) were observed. These high concentrations occurred when clouds formed on a hill slope at Tenerife in polluted air masses originating in Europe that had transported the order of 1000 km over the Atlantic Ocean. The validity of the measured droplet number concentrations was investigated by comparing with measurements of the aerosol upstream of the cloud and cloud interstitial aerosol. Guided by distributions of the ratios between the measurements, three criteria of typically 30% in maximum deviation were applied to the measurements to test their validity. Agreement was found for 88% of the cases. The validated data set spans droplet number concentrations of 150–3000 cm⁻³. The updraught velocity during the cloud formation was estimated to 2.2 m s⁻¹ by model calculations, which is typical of cumuliform clouds. The results of the present study are discussed in relation to cloud droplet number concentrations previously reported in the literature. The importance of promoting the mechanistic understanding of the aerosol/cloud interaction and the use of validation procedures of cloud microphysical parameters is stressed in relation to the assessment of the indirect climatic effect of aerosols.     

Rapid high‐T decompression recorded by Archean granulites in the northern Wyoming Province: Insights from petrological modelling

This study places new constraints on the pressure–temperature (P–T) path and duration of high‐temperature (HT) metamorphism recorded by Archean granulite facies metasedimentary rocks from the northern Wyoming Province in the eastern Beartooth Mountains, MT and WY, USA. These rocks exist as m‐ to km‐scale xenoliths within a c. 2.8 Ga calc‐alkaline granitoid batholith. Different interpretations of the timing of HT metamorphism relative to batholith intrusion in previous works have led to ambiguity over the mechanism by which these rocks were heated (i.e. batholith intrusion v. a later, cryptic event). The P–T path recorded by these rocks and the duration of this path may be indicative of the heating mechanism but are not currently well constrained. Here, we combine phase equilibria thermobarometry and diffusion modelling of major element zonation in garnet in order to constrain the P–T path of HT metamorphism and the durations of different parts of this path. It is shown that these rocks record a tight, clockwise P–T path characterized by near‐isobaric heating at ~6.5–7 kbar to ?770–800°C, HT decompression to ~6 kbar, 780–800°C, followed by limited decompression while cooling. Diffusion modelling of major element zonation in garnet suggests that HT decompression was brief (likely <1 Ma), and that cooling rates following this decompression were on the order of 10–100°C/Ma. Substantial changes in apparent thermal gradient along this P–T path indicate that the rocks record a significant but short‐lived thermal anomaly that occurred in the Wyoming mid‐crust in the Late Archean.     

Fluid production rate during the regional metamorphism of a pelitic schist

Phase equilibria modeling of the pressure–temperature (P–T) path of regional metamorphism and associated fluid expulsion, combined with constraints on the timescale of garnet growth by Sm–Nd geochronology, elucidates the fluid production rate and fluid flux during Barrovian metamorphism of pelitic rocks from Townshend Dam, VT, USA. This modeling builds on a published companion study that utilized Sm–Nd geochronology of concentric growth zones in multiple garnet grains, to constrain the duration of garnet growth in a large sample of schist at Townshend Dam to 3.8?±?2.2 million years (Gatewood et al., Chem Geol 401:151–168, 2015). P–T pseudosections combined with observed mineral compositions constrain garnet growth conditions, and were utilized to construct P–T path-dependent thermodynamic forward models. These models determine that garnet growth was initiated at ~?0.6 GPa and ~?525 °C, with a roughly linear loading and heating P–T trajectory to >?0.8 GPa and ~?610 °C. Loading and heating rates of 2.4 km·Myear^?1 (with a range of 1.6 to 5.8 km·million year^?1) and 23 °C·million year^?1 (with a range of 14 to 54 °C·million year^?1), respectively, are consistent with model estimates and chronologic constraints for tectono-metamorphic rates during orogenesis. Phase equilibria modeling also constrains the amount of water release during garnet growth to be ~?0.7 wt% (or >?2 vol%), largely resulting from the complete consumption of chlorite. Coupling this estimate with calculated garnet growth durations provides a fluid production rate of 5.2 kg·m^?3·million year^?1 (with a range of 3.2 to 12.2 kg·m^?3·million year^?1) and when integrated over the overlying crustal column, a regional-scale fluid flux of 0.07–0.37 kg·m^?2·million year^?1. This range of values is consistent with those derived by numerical models and theory for regional-scale, pervasive fluid flow. This study signifies the first derivation of a fluid production rate and fluid flux in regional metamorphism using a direct chronology of water-producing (garnet-forming) reactions and can provide a framework for future studies on elucidating the nature and timescales of fluid release.