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Want to generate clean hydrogen from rocks?

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Extreme uncertainty in the thermodynamic drive to generate hydrogen (y axis, ~10 orders of magnitude!) exists at low T (< ~20˚C, blue) if the host ultramafic rock composition is unknown. Blindly drilling into an ultramafic formation calmly cooking away at low or moderate temperature is unwise.

Low T (blue), high pH, high hydrogen seafloor systems are the holy grail of seawater + ultramafic reactions (a la Lost City). pH in these systems (x axis) is vary sensitive to temperature (color). These ultramafic rocks are olivine rich lherzolites, and are capible of producing larger quantities of hydrogen below 100 ˚C than their high temperature counterparts ( > ~300˚C, orange) .

Cpx enhances hydrogen generation in lherzolites and harzburgites by supplying Ca, however relative pure cpx (black box) cannot generate nearly as much hydrogen as ol-rich ultramfics. This is because cpx contains an unfavorable ratio of Si to Mg (it is too silica rich).

Changes in ultramafic rock composition can drive a ~7 order of magnitude shift in hydrogen generation potential, even at ~200˚C (purple). You still don't want to drill blindly into an ultramafic body, even at this temperature.

Magnetite formation is maximized above ~200˚C, and is associated with maximum hydrogen generation because it stores much of the oxidized Fe that facilitates hydrogen formation, However, some dunites and harzburgites can stabilize minor amounts of magnetite down below ~40˚C (See Ely et al., 2023). Be careful reading too much into the presence of magnetite as a geo thermometer.

These hydrogen maximizing systems are the result of dunite and harzburgite alteration. If you are taking measurements of alteration fluids in an actively serpentinizing system above ~300˚C, they can be distinguished from other ultramafic hosts by relatively high dissolved Fe ( > ~100 µmol) and pH < 6. Be careful interpreting pH in recovered fluid samples that have been allowed to cool, pH can move a lot during cooling.

These hydrogen maximizing systems (dunites and harzburgites) sit near ~350˚C (dark orange). At higher temperatures (bright orange), hydrogen potential actually decreases. This is because the olivine present in the host rock is stable above ~350˚C, no longer providing its Fe to the alteration system to be oxidized, a process that must take place during hydrogen generation. The systems on the left contain no cpx (and thus less Ca), and the systems on the right contain cpx between 5 and ~15%.