USING MINERAL LIFETIME DIAGRAMS: PREDICTING OLIVINE GRAIN LIFETIMES ON EARTH AND MARS

The timing of geological processes provides critical information that can constrain theories about those processes. We have used a well-known relationship between grain lifetimes and the reciprocal dissolution rate along with reported olivine dissolution rates to create a diagram that allows rapid visualization of olivine grain lifetimes under a wide variety of conditions. Reported rates of olivine dissolution show that pH, fayalite/forsterite ratio, temperature, grain size, hydrodynamics, organic ligands, ionic strength, and the unexplained discrepancy between field and laboratory dissolution rates significantly affect olivine grain lifetimes, whereas dissolved carbonate, H₄SiO₄ (aq), Mg²⁺(aq), and grain coatings do not. Based on our analysis, a 1 mm forsterite grain could persist in a pH 5.5 soil solution at 298 K for no longer than 6,500,000 years. Interestingly, this approach correlates remarkably well with an analysis by Kowalewski and Rimstidt (2003) that determined average mineral grain lifetimes based on radiometric dates, which determined that an average olivine grain would persist for approximately 3.9 million years in natural environments. On Mars, where olivine grain lifetimes are of interest because remnant olivine is thought to indicate limited interaction with water implying that liquid water was not present on the surface for prolonged periods of time, this 1 mm grain could persist in pH 4 solutions at 273 K for over 1.2 million years. Similar diagrams could be developed for any mineral for which sufficient rate data exists.