What is the Protolith of serpentinite?

Serpentinite: What Rock Did It Used to Be? A Geologist’s Tale

Serpentinite. It’s a rock that always catches my eye. That distinctive green, sometimes almost slimy feel… it’s hard to miss. The name itself, hinting at snakeskin, gives you a clue about its look. But have you ever stopped to wonder where this rock actually comes from? What was it before it became serpentinite?

Well, the story starts deep down, way below the surface, with a family of rocks called ultramafic rocks. Think of serpentinite as a makeover – a geological transformation of these mafic or ultramafic rocks. These rocks are packed with magnesium and iron, mainly in the form of minerals like olivine and pyroxene. The magic happens when these rocks meet water in a process called hydrothermal metamorphism. It’s like a spa day for rocks, but instead of relaxation, it’s a complete change of identity!

Meet the Usual Suspects: The Protoliths

While several ultramafic rocks could be the “before” picture for serpentinite, some are far more common culprits than others. Here are the prime suspects:

• Peridotite: This is your most likely answer. Peridotite is an ultramafic rock that’s mostly olivine, with a sprinkle of pyroxene. Think of it as the “plain Jane” of the ultramafic world, but with a secret potential for a stunning transformation. And it’s not just one type of peridotite either – harzburgite and lherzolite can also get the serpentinite treatment. Harzburgite? It’s mostly olivine and orthopyroxene. Lherzolite? It throws in some clinopyroxene for good measure.
• Dunite: Now, dunite is a special kind of peridotite. It’s practically all olivine – we’re talking 90% or more! Because it’s so rich in magnesium, it’s super prone to turning into serpentinite. Imagine a supermodel just waiting for the right stylist.
• Pyroxenite: Okay, this one’s a bit less common, but still in the running. Pyroxenite is mostly pyroxene, with just a little olivine hanging around. So, while it’s not the most frequent protolith, it definitely plays a role.

The Serpentinization Secret: How It All Happens

So, what is this “serpentinization” I keep mentioning? It’s basically a low-temperature geological process where those original minerals – olivine and pyroxene – react with water and transform into serpentine minerals. These serpentine minerals have cool names like antigorite, lizardite, and chrysotile. Plus, you can get other minerals like magnetite and brucite popping up too. This whole process usually happens where tectonic plates are bumping and grinding – especially in subduction zones and mid-ocean ridges, where there’s plenty of water to go around. Here’s a fun fact: during serpentinization, the rock can actually swell in size by up to 30%, while also becoming less dense.

Why Bother Knowing This?

Why should you care about the protolith of serpentinite? Well, for starters, it unlocks some amazing secrets about our planet!

• Tectonic Tales: Serpentinites are like geological detectives, giving us clues about past plate movements. Finding serpentinite in a region can tell us that there was once some serious tectonic action going on, like plates colliding.
• Mantle Mysteries: Since ultramafic rocks make up a big chunk of the Earth’s mantle, studying serpentinites helps us understand what’s happening way down deep inside the Earth.
• Hidden Treasures: Serpentinites can be associated with valuable mineral deposits, like chromite and magnesite. And, believe it or not, sometimes even asbestos.
• Life’s Origins? Get this – some scientists even think that serpentinization might have been important for the origin of life! The chemical reactions in hydrothermal vents could have created the perfect conditions for life to get started.

So, there you have it. The protolith of serpentinite is usually an ultramafic rock – most often peridotite, dunite, or pyroxenite. Through the fascinating process of serpentinization, these rocks from deep within the Earth are transformed into the eye-catching serpentinite we see on the surface. It’s a story of transformation, deep-Earth processes, and maybe even the very beginnings of life itself. Pretty cool, huh?