Space/Science — HabitableZone

Space/Science » in reply to re: olivine in earth is from recent eruptions

Oliving formation and destruction.
Nick Hoffman — (archived)

Olivine is a high-temperature mineral. It forms in lava flows as they cool (but only as fine grains, since they cool quickly). It also forms larger crystals when magma cools underground in a dyke. sill, or magma chamber. If magma erupts from a chamber while crystals are growing, the lava can include large crystals.

Olivine goes rusty fairly quickly - hundreds to tens of thousands of years. This depends strongly on temperature & water availability, weakly on local chemistry - salts etc. Products are iron oxides and clays...

Some work has been done on how long it would take to erode grains on a cold wet Mars, but more still to be done.

Underground, a thin basalt flow will lose all its Olivine quickly, but a thick sequence of flows will end up with semi-stable olivine - only the rims of the grains will be transformed and the cores remain intact. After many hundred million years, it's hard to find any left.

If what we see on Mars is the remnant of a larger fraction that got partially weathered, we should see:

a) lower amounts of total olivine
b) alteration products - distinctive ratios of clays etc. We see hardly any clay on Mars! (clays are included in "sheet silicates").

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