  Alkaline vents (AV) are hypothesized to have been a setting for the emergence
of life, by creating strong gradients across inorganic membranes within chimney
structures. In the past, 3-dimensional chimney structures were formed under
laboratory conditions, however, no in situ visualisation or testing of the
gradients was possible. We develop a quasi-2-dimensional microfluidic model of
alkaline vents that allows spatio-temporal visualisation of mineral
precipitation in low volume experiments. Upon injection of an alkaline fluid
into an acidic, iron-rich solution, we observe a diverse set of precipitation
morphologies, mainly controlled by flow-rate and ion-concentration. Using
microscope imaging and pH dependent dyes, we show that finger-like precipitates
can facilitate formation and maintenance of microscale pH gradients and
accumulation of dispersed particles in confined geometries. Our findings
establish a model to investigate the potential of gradients across a
semi-permeable boundary for early compartmentalisation, accumulation and
chemical reactions at the origins of life.
