Revision of an open-split-based dual-inlet system for elemental and isotope ratio mass spectrometers with a focus on clumped-isotope measurements

In this work, we present a revision of an open-split-based dual-inlet system for elemental and isotope ratio mass spectrometers (IRMSs), which was developed by the division of Climate and Environmental Physics of the University of Bern 2 decades ago. Besides discussing the corresponding improvements we show that with this inlet system (NIS-II, New Inlet System II) external precisions can be achieved that are high enough to perform measurements of multiply substituted isotopologues (clumped isotopes) on pure gases. For clumped-isotope ratios 35/32 and 36/32 of oxygen, we achieved standard deviations of 3.4×10-9 and 4.9×10-9, respectively, that we calculated from 60 interval means (20 s integration) of pure-oxygen gas measurements. Moreover, we report various performance tests and show that delta values of various air components can be measured with precisions of the order of tens of per meg and higher with the NIS-II. In addition, we demonstrate that our new open-split-based dual-inlet system allows us to measure some of these delta values with significantly higher precisions than an NIS-I (precursor of the NIS-II) and conventional changeover-valve-based dual-inlet systems (tests performed with two dual-inlet systems built by Elementar and Thermo Finnigan). Especially, our measurements point out that our inlet system provides reliable results at short idle times (20 s) and that the corresponding data do not need to be corrected for non-linearity. However, the sample consumption of our open-split-based dual-inlet system is several orders of magnitude higher than that of changeover-valve-based ones (0.33 sccm versus 0.005 sccm; standard cubic centimetres per minute). Due to the successful preliminary tests regarding measurements of clumped-isotope ratios, we will continue our work in this area to perform clumped-isotope studies according to common practices.</p

Spontaneous formation of fluid escape pipes from subsurface reservoirs

Ubiquitous observations of channelised fluid flow in the form of pipes or chimney-like features in sedimentary sequences provide strong evidence for significant transient permeability-generation in the subsurface. Understanding the mechanisms and dynamics for spontaneous flow localisation into fluid conductive chimneys is vital for natural fluid migration and anthropogenic fluid and gas operations, and in waste sequestration. Yet no model exists that can predict how, when, or where these conduits form. Here we propose a physical mechanism and show that pipes and chimneys can form spontaneously through hydro-mechanical coupling between fluid flow and solid deformation. By resolving both fluid flow and shear deformation of the matrix in three dimensions, we predict fluid flux and matrix stress distribution over time. The pipes constitute efficient fluid pathways with permeability enhancement exceeding three orders of magnitude. We find that in essentially impermeable shale, vertical fluid migration rates in the high-permeability pipes or chimneys approach rates expected in permeable sandstones. This previously unidentified fluid focusing mechanism bridges the gap between observations and established conceptual models for overcoming and destroying assumed impermeable barriers. This mechanism therefore has a profound impact on assessing the evolution of leakage pathways in natural gas emissions, for reliable risk assessment for long-term subsurface waste storage, or CO2 sequestration

Resolving thermomechanical coupling in two and three dimensions: spontaneous strain localization owing to shear heating

International audienceNumerous geological processes are governed by thermal and mechanical interactions. In particular,tectonic processes such as ductile strain localization can be induced by the intrinsiccoupling that exists between deformation, energy and rheology. To investigate this thermomechanicalfeedback, we have designed 2-D codes that are based on an implicit finite-differencediscretization. The direct-iterative method relies on a classical Newton iteration cycle andrequires assembly of sparse matrices, while the pseudo-transient method uses pseudo-timeintegration and is matrix-free. We show that both methods are able to capture thermomechanicalinstabilities when applied to model thermally activated shear localization; they exhibitsimilar temporal evolution and deliver coherent results both in terms of nonlinear accuracyand conservativeness. The pseudo-transient method is an attractive alternative, since it candeliver similar accuracy to a standard direct-iterative method but is based on a much simpleralgorithm and enables high-resolution simulations in 3-D. We systematically investigate thedimensionless parameters controlling 2-D shear localization and model shear zone propagationin 3-D using the pseudo-transient method. Code examples based on the pseudo-transientand direct-iterative methods are part of the M2Di routines (R¨ass et al., 2017) and can bedownloaded from Bitbucket and the Swiss Geocomputing Centre website