Part 5. Ultramafic Rocks

In the field, ultramafic rocks are mostly seen in the basal sections of ophiolites and as exhumed pieces of the upper mantle. Here's a famous outcrop at Helgehornvatnet in Norway:

The layering is formed by different proportions of olivine, opx, cpx, and garnet:

Which you can see nicely in this closeup:

To understand the petrogenesis of ultramafic rocks using the CaO-MgO-SiO system, we plot the phases using molar proportions on a triangle with one component at each apex

forsterite Mg₂SiO₄
enstatite MgSiO₃
tremolite Ca₂Mg₅Si₈O₂₂(OH)₂
forsterite olivine (Mg,Fe)₂SiO₄)
serpentine (the three polymorphs are chrysotile, lizardite, antigorite) contains (Mg,Fe)SiO(OH)
anthophyllite Mg₇Si₈O₂₂(OH)₂
talc Mg₆Si₈O₂₀(OH)₂

PT Region--Divariant Field
if there are 2 degrees of freedom (P and T), 2 + P=3 + 2, P=3; there are three phases in a divariant field
these three phases define a triangle on the CMS diagram
bulk composition lies within triangle
a different bulk composition defines a different 3-phase triangle at the same P&T

Mineral Reaction--Univariant Line
if there is 1 degree of freedom (P or T), 1 + P=3 + 2, P=4; there are four phases along a univariant line--a reaction
the four phases create crossing tie lines
bulk composition lies within the intersection of the two 3-phase triangles

Invariant Point
if there are 0 degrees of freedom, 0 + P=3 + 2, P=5; there are five phases at an invariant point

Reactions in the CMS(H) system at low pressures--the model system for peridotite

• antigorite is stable to ~400°C, and is equivalent to greenschist-facies conditions
• tremolite appears at conditions close to where antigorite disappears, and indicates amphibolite facies
• talc is stable to much higher temperatures of ~750°C, and breaks down at granulite-facies conditions
• diopside is stable at both low temperatures and high temperatures, separated by an interval in which tremolite is stable instead
• anthophyllite is stable only at amphibolite-facies conditions, and only to pressures of ~1.2 GPa, rendering it the only useful indicator of pressure in this group of minerals

Most ultramafic rocks span a very restricted range of bulk compositions and thus their constituent phases are also very restricted in composition. Because of this, ignoring Fe to calculate the phase relations is not a big simplification. This also means that the compositions of minerals cannot change much within their stability fields, the positions of the reactions do not move around very much with mineral composition, and the reactions are essentially discontinuous reactions.

Below are phase diagrams showing the stability fields of minerals in various bulk compositions of ultramafic rocks: