26.3: The hard rocks - mantle and crust

MANTLE: Peridotite to serpentinite

The upper mantle is made of a rock called peridotite. Peridotite is dominated by the mineral olivine, though some peridotites also have plenty of pyroxene and plagioclase in them. Peridotite formed long ago when the early Earth cooled off and solidified in its uppermost reaches. Thought it is solid, the mantle still flows slowly. Warm mantle peridotite rises due to its low density, while cold peridotite sinks due to its higher density. This relative motion sets up convection related “cells” in the mantle that are thought to be one of the important driving forces that power the surficial movement of lithospheric plates, including the manifestation of convergent motion that we call “subduction.”

Not all of the lithospheric mantle necessarily gets recycled during subduction. The process of “underplating” can cause slivers of the subducting plate (including lithospheric mantle) to be ripped off and added to the underside of the overriding plate. This ‘subduction accretion’ builds up the accretionary wedge from below, and is the major source of mantle rock introduced into the subduction complex.

When the peridotite is broken off and mixed into the accretionary wedge, it can come into contact with water, and metamorphose. This creates serpentinite, a metamorphic rock dominated by the mineral serpentine. Serpentine is a hydrated magnesium silicate. Serpentinite is the California ‘state rock,’ though because some of the minerals in serpentinite (such as chrysotile) have an asbestiform habit, it makes lawyers itchy, and there are periodic proposals to replace its official ‘state rock’ legal status.

Figure \(\PageIndex{1}\): Pillow lavas are a signature of underwater eruption. Here, relatively lightly-metamorphosed (greenstone) meta-basalts are exposed on Black Sands Beach in Marin Headlands.

OCEANIC CRUST: Basalt & its three metamorphic descendants

When the hot mantle is decompressed, it is capable of partial melting, which derives a mafic magma form the ultramafic source rock. Estimates are that mafic magmas derive from somewhere between 1% and 25% partial melting of source peridotite.

The resulting magma could cool at depth to generate a gabbro, or could be erupted on the seafloor to make basalt. When lava erupts at the bottom of the ocean, the high heat capacity of the surrounding seawater wicks heat away efficiently. This chills the edge of the lava instantly, forming a solid crust resembling a loaf of bread.

Figure \(\PageIndex{2}\): The formation of pillow structures occurs when basaltic lava erupts underwater.

The interior of the lava “loaf” is still molten, and if it is under sufficiently high fluid pressure, it will crack open the crust and squirt another lobe of fluid lava out into the water, repeating the process. Played out time and time again over geological time, this results in a pile of “pillow” shaped features. These “pillow lavas” are a therefore a primary volcanic structure; They are the signatures of seafloor eruptions of new oceanic crust.

When basalts are subducted, they serve as particularly good recorders of the pressure and temperature they encounter. If they are shallowly subducted, they will make greenstone. If they are subducted a bit deeper still (with high pressure), they will transform into blueschist, and if they are subducted really deep (with the highest pressure), eclogite will form.

Figure \(\PageIndex{3}\): A plot showing the various metamorphic facies that result from different combinations of temperature and pressure. Note that blueschist and eclogite result from relatively low-temperature but relatively high-pressure metamorphism: conditions we would expect in a subduction zone.

Figure \(\PageIndex{4}\): Greenstone-facies meta-basalt, showing the preserved forms of lava pillows, Jenner, California.

Greenstone is the lowest grade of subduction related metamorphism. Very few of the basalts seen today in the Franciscan complex are still basalt. Many/most have been metamorphosed to make greenstone. When the minerals that make up basalt are subjected to depths around 20 to ~35 km in a subduction zone, the pressure and temperature are sufficient to make the plagioclase and pyroxene react and generate the minerals chlorite and actinolite and sometimes (if water is present), epidote. The jet black basalt is now turned a pale green in color, and this is quite a common sight in the California Coast Ranges. If tectonic shear stresses are low, large-scale primary features such as lava pillows may be preserved, despite the metamorphic recrystallization that has reorganized the rock on smaller scales.

Blueschist is the rock that results when basaltic protoliths are driven deeper into the subduction zone, to perhaps ~35 to 50 km depth. The rate of subduction is greater than the rate of thermal equilibration; as a result the subducted basalt soon finds itself at conditions where the pressure is quite high, but the temperature is still relatively cold. These are the conditions at which the minerals lawsonite and glaucophane form. Metabasalts in the Angel Island Terrane have seen these conditions, and where basalt was present, it has been recrystallized to make blueschist.

It’s important to note that blueschist cannot form from “just any rock.” It must have basalt (or greenstone, which is of course made of the same blend of elements) as its protolith. If a sandstone is subducted to the same depths and temperatures, it cannot make a blueschist, as it doesn’t have the right starting ingredients. Such a sandstone is said to have been metamorphosed “at blueschist facies,” but it is neither blue nor a schist. We can still find crystals of lawsonite in it if we examine it in thin section (under a petrographic microscope), but to the naked eye, it frequently looks just like a normal sedimentary sandstone!

Eclogite represents the highest set of temperatures and pressures; the protolith basalt’s atomic ingredients reorganize to form distinctive “Christmas tree” colored minerals: the rich green of omphacite (a kind of pyroxene), and the cranberry-red of garnet (the variety called pyrope). Eclogites are astonishingly dense, which is perhaps not surprising considering they form to be stable at pressures corresponding to depths of at least ~50 km!

Figure \(\PageIndex{5}\): Eclogite consists of green omphacite (a variety of pyroxene) and prominent red pyrope (a variety of garnet). This outcrop appears on Ring Mountain, near Tiburon, California.

Eclogite is very dense. Depending on the location sampled, the density of eclogite ranges from 3.45 to 3.75 g/cm\(^3\). Recall that the mantle’s peridotite clocks in at around 3.3 g/cm\(^3\), so this means that once eclogite forms, it can act as a dead weight, pulling on the rest of the subducted plate. This downward pull is thought to be a primary driver of “slab pull,” a force partially responsible for the motions of lithospheric plates.