If the trees were to burn, all of that carbon you worked so hard to sequester would be released, so it is not foolproof and decades of sequestration can be rolled back in hours. Whereas once the olivine grains are on the beach, it is going into the seafloor in a pretty much irreversible chemical reaction.

As for the rate of olivine weathering, it is the fastest of any major rock-forming silicate mineral and is further accelerated by the abrasions of rock, and by microbes. The rate of sequestration depends on the size of the particles, and how much is spread per each beach. Many of the calculations affirming the data have been measured from static piles of olivine tailings (pilings of rock that sit as "waste" in mines sites.) Those rates alone are impressive, but we are looking to utilize the beaches to further increase the rate of weathering to at least 20 microns per year. At that rate, a grain of olivine with a diameter of 100 microns will dissolve in around 5 years.

"Our experiments show that olivine grains when kept in motion weather fast because continuous mutual impacts remove reaction-inhibiting silica from the surface and tiny µm-size slivers are produced allowing a fast chemical reaction. The application of olivine and other (ultra)mafic minerals like serpentine in high-energy shallow marine environments can make a significant contribution in the fight against climate change. The counteracting effect on ocean acidification is immediate."

Here is the basic reaction:

Mg2SiO4 + 4 CO2 + 4 H2O −→ 2 Mg^2+ + 4HCO3^- + H4SiO4

CO2 is consumed, and Mg2+, Fe2+, H4SiO4 and HCO3^- are produced.

This paper has the best calculations and a lab experiment demonstrating the rate. https://climitigation.org/wp-content/uploads/2017/10/Rolling...

This one is good as well and discusses the tailing pilings and importance of microbes alone in possibly accelerating the rate up to 4,000x https://climitigation.org/wp-content/uploads/2017/10/Olivine...