Peridotite: The Fabric of the Mantle

It's the generic name for the coarse-grained, ultraMafic rock that makes up the bulk of the earth's upper mantle.

Ultramafic is chemically defined as:

• $\mathrm{SiO}{\phantom{A}}_2$ < 45% (Very low silica compared to granite or basalt)
• $\mathrm{MgO}$ > 18% (extremely high magnesium)
• High in Iron, Chromium and Nickel

Main three minerals

1. Olivine (> 40% of rock): The matrix that hold the bulk of magnesium and iron. It is the residue that remains stable at highest temperatures.
2. Orthopyroxene (Opx): A low-clacium pyroxene. It is the second most refractory (heat-resistant) mineral.
3. Clinopyroxene (Cpx): A high-calcium pyroxene. This is the most important phase. The crystal structure is flexibale enough to hold incompatible elements like Sodium, Aluminum and Titanium. It is the first that breaks down in a melt and enter the magma.

Peridotite family

1. Lherzolite: Olivine + Opx + significant Cpx (>5%). Has not yet undergone extensive partial melting.
2. Harzburgite: Olivine + Opx (Cpx is absent). What remains after melting of Lherzolite and extraction of basalt. The Cpx is melted away and risen to form the crust.
3. Dunite: > 90% Olivine. The end-member, it forms either from extreme melting or as a cumulate.

Alteration: Serpentinization

Peridotite is unstable at the surface. When it comes into contact with water (seafloor), it undergoes serpentinization.

Olivine + Water -> Serpentine + Magnetite + Hydrogen + Heat

Massive amounts of Hydrogen gas and Methan is released, this can support deep-sea microbial life, without sunlight.

Question and Answers

What are the three distinct mineral phases that define the classification of Peridotite?::Olivine, Orthopyroxene (Opx), and Clinopyroxene (Cpx).

Geochemically, why is Clinopyroxene (Cpx) the most critical mineral in Peridotite?
?
It hosts the "incompatible" elements (Ca, Al, Na, Ti). When the mantle melts, Cpx breaks down first, releasing these elements into the melt (magma).

What is the process called when Peridotite reacts with water at low temperatures, and what gas is released?::Serpentinization; it releases Hydrogen gas (${\text{H}}_2$).

What defines an "Ultramafic" rock chemically?::Very low Silica (${\text{SiO}}_2<45\mathrm{\%}$) and high Magnesium ($\text{MgO}>18\mathrm{\%}$).