Komatiites

Komatiites are volcanic rocks with abundant olivine and pyroxene and little or no feldspar. They also contain large magnesium oxide concentrations (greater than 18 percent). They are most abundant in the lower portion of exceedingly old piles of volcanic rocks. Economic deposits of nickel, copper, platinum group minerals, antimony, and gold have been found in some komatiites.

Komatiite Composition

Komatiites are unusual volcanic rocks. They contain mostly large grains of olivine (magnesium and iron silicate mineral) and pyroxene (calcium, iron, and magnesium silicate mineral) scattered among fine mineral grains that at one time were mostly glass. The glass was unstable and in time slowly converted into individual minerals. The larger minerals often form elongated grains composed of olivine, pyroxene, or chromite (a magnesium-chromium oxide mineral at the top of some lava flows) called skeletal grains. No other rocks form these skeletal grains. The skeletal grains are believed to have formed by quick cooling at the top of the lava flow. The glass must have also formed by quick cooling of the lava, often in contact with water. Because most igneous rocks contain feldspar (calcium, sodium, and potassium aluminum silicate minerals), the lack of feldspar in komatiites also makes them unusual. They also contain magnesium concentrations (magnesium oxide greater than 18 percent) higher than most other igneous rocks. Most important, komatiites are extremely silica-poor rocks (ultramafic rocks) that melt at temperatures hotter than any present-day lava, and currently do not erupt.

Komatiite Lava Flows

Each individual lava flow of a komatiite may be a meter to tens of meters thick. A given lava flow can often be traced for a long distance without much variation in thickness. Some of the lava flows contain rounded or bulbous portions called pillows. Pillows can be formed only by extrusion of the lava into water. As lava breaks out of the solid front of a flow, it is quickly cooled into a rounded pillow. Since this process continually takes place, numerous pillows form as the lava advances. The more magnesium-rich komatiites occur as lava flows in the lower portions of these volcanic piles. They gradually become less magnesium-rich in the younger or upper portions of these volcanic piles.

The mineralogy and observed mineral shapes can vary vertically through a given lava flow. The most spectacular and beautiful komatiite lava flows are the ones in which the upper portions contain the skeletal olivine and pyroxene grains. Skeletal grains have an extremely distinctive texture and may resemble chains, plates, or feathers. Some grains grow as large as 3 centimeters. The thickness of these lava flows varies from about 1 to 20 meters. In one type of vertical variation, there are abundant fine minerals formed from the original glass at the top of these flows, along with irregular fractures caused by quick cooling at the top of the lava. Underlying this quickly cooled zone is a layer of large skeletal grains of olivine and pyroxene that increase in grain size downward. These skeletal crystals probably form very rapidly in the lava by growth from the top downward.

The lower portion of these komatiite flows often contains abundant olivine grains that are much more rounded and less elongated than those of the upper zone. The more rounded grains are believed to have formed by the slow crystallization of the olivine from the liquid lava. As the olivine formed from the liquid, the grains slowly sank and piled up on the bottom of the flow until the lava solidified. The base of the lava also may contain very fine-grained, skeletal grains of olivine with irregular cracks, much like the top of the flow. Presumably, the base of the flow formed by quick cooling, as did the top portion.

Observing several cross-sections shows how komatiite flows can vary. There is, for example, a considerable difference in the amount of the flow that contains the skeletal minerals and the flow that contains the more rounded minerals. Most komatiite flows do not contain any skeletal minerals. Instead, they consist mostly of the more rounded mineral grains and have irregular fractures extending throughout the lava flow. These differences could be caused by different cooling rates or by the lava’s viscosity (how easily it flows). Other flows are composed mostly of rounded pillows.

Komatiite lava flows are often interbedded with rocks composed of angular fragments that came from all parts of the lavas. Some of these rocks show features that might be found along the edge of a body of water, such as ripples formed by wave action. These rocks have formed by the action of waves breaking up some of the solidified lavas and reworking them after they were deposited. (Rocks that have been reworked by water are sedimentary rocks.)

Komatiite Occurrence

Komatiites were discovered in the late 1960s in exceedingly old Archean rocks (more than 2.5 billion years old) in Zimbabwe and South Africa. Only a few komatiites occur in younger rocks. The youngest known komatiite, only about 70 million years old, and the only one less than 500 million years old, is located on Gorgona Island off the coast of Colombia. Komatiites typically occur in the lower or older portions of vast piles of volcanic rocks containing many layers of different lava flows. Komatiites gradually become less abundant farther up the volcanic pile and in younger volcanic rocks. Other dark, feldspar-rich, volcanic rocks called basalt (calcium-rich feldspar and pyroxene rock) are interlayered with komatiites. Basalts gradually become more abundant in the younger volcanic flows, along with more light-colored and more silica-rich rocks such as andesite. Small amounts of sedimentary rocks may be interlayered with the volcanic rocks. Sedimentary rocks are derived by water reworking the volcanic rocks. Komatiites eventually disappear in the upper portions of these volcanic piles.

Many komatiites are located in rather inaccessible regions. One area that is accessible is the Vermilion district of northeastern Minnesota. The age of the district is 2.7 billion years old. Here, there are no true komatiites with magnesium oxide concentrations greater than 18 percent. There are, however, very magnesium-rich basalts that contain the skeletal crystals found in true komatiites. All the rocks have been buried deep enough that the original minerals were changed or metamorphosed to new minerals in response to the high temperature and pressure. The temperature of metamorphism was still low enough that the original igneous relations may still be observed.

A second example of a komatiite sequence is located at Brett’s Cove in Newfoundland . The komatiites there are much younger than most komatiites (formed during the Ordovician, about 450 million years ago). Those komatiites formed within layers of rocks called ophiolites, which are believed to be sections of ruptured and tilted oceanic crust and part of the upper mantle. The lower part of the ophiolites contains an olivine and/or pyroxene mineralogy thought to compose much of the upper mantle of the earth. Overlying the upper-mantle rocks are rocks of basaltic composition containing coarse crystals of olivine, pyroxene, and feldspar. Above these rocks are numerous basaltic lavas that were extruded at the surface and that built up large piles of lavas on the ocean floor. Some komatiite lavas are interbedded in the lower portion of these mainly basaltic lavas. Those komatiite flows have a lower zone rich in pyroxene and an upper pillow lava that contains skeletal crystals of pyroxene.

Study of Field and Chemical Characteristics

The characteristics of komatiites that are exposed at the surface are described carefully during a field study. These characteristics can suggest how komatiites form at the surface. For example, features such as abundant pillows indicate that the komatiite lava was extruded into water. A small amount of reworking of the lavas by moving water suggests that there was little time between eruptions. A sign of a rapid eruption is the spread of lava over a great distance and at a gentle flow rate.

In addition to the field characteristics of komatiites, geologists study their chemical characteristics in order to understand how they form and evolve. Komatiites are igneous rocks, so they form by the melting of another rock. Experiments using furnaces suggest that much of the rock called peridotite (olivine-pyroxene rock) must melt to form high-magnesium magmas. The magma then may evolve or change in composition by processes such as the crystallization of minerals from the magma, by dissolving some of the solid rock through which it moves, or by mixing with magma of a different composition. The komatiite may even change composition after the magma solidifies because of water vapor or carbon dioxide-rich solutions moving through the solid. It is difficult to assess the relative importance of these processes to modify the composition of a given komatiite.

Analyzing Elemental Concentrations

One way to study the mineral crystallization of a magma is to plot the elemental concentrations of several analyzed rocks from the same general area against another element, such as magnesium. If the concentration of all elements systematically increases or decreases with increasing magnesium concentrations, then the lavas were probably related by fractional crystallization, because olivine becomes poorer in magnesium and richer in iron as the magma crystallizes. The formation and settling of olivine from lava appears to explain the concentration of most elements. For example, magnesium, chromium, nickel, and cobalt all concentrate in lava-related olivine; thus, a plot of chromium, nickel, and cobalt shows smooth and systematic decreases when compared to magnesium. Elements such as calcium, titanium, aluminum, silicon, iron, and scandium are rejected from lava-related olivine; they gradually increase with decreasing magnesium. Some elements, such as sodium, potassium, barium, rubidium, and strontium, should likewise systematically increase with decreasing magnesium, as they are also rejected from lava-related olivine. Instead, these elements are greatly scattered when they are plotted relative to magnesium. Those elements are notorious for being moved by carbon dioxide or water-vapor-rich fluids. Thus, it is assumed that the scatter of these elements is a result of the movement of these fluids. The fractionation of these elements because of olivine crystallization, therefore, is obscured.

Some of the variations in elemental concentrations in komatiite lavas cannot be explained by crystallization or alteration processes. For example, two rare-earth elements like lutetium and lanthanum should not differ in ratio during olivine crystallization, as those elements are chemically very similar and olivine should not fractionate these elements. Nevertheless, some processes do produce unexplained variations in rare-earth contents.

Economic Value

Komatiites contain several types of economic deposits. They may contain important deposits of nickel sulfides, along with large concentrations of platinum group elements (such as platinum and palladium) and copper. The nickel sulfide was probably formed from a sulfide liquid that separated as an immiscible liquid from the komatiite liquid. This is similar to the way oil and water separate when they are mixed together. Nickel sulfide ores have been found in western Australia, Canada, and Zimbabwe. The most important deposits are found in the komatiite lava flows in the lower portion of a lava pile. The nickel sulfide ore is concentrated in a portion of the base of a lava flow where it is thicker than other portions of the flow. The immiscible and dense nickel-sulfide liquid may have settled in a thick portion of the flow that was not stirred as much as other portions of the flow. The platinum group metals have a stronger affinity for the sulfide liquid than for the komatiite liquid; consequently, they concentrate in the sulfide liquid.

Gold, antimony, and a few other elements are concentrated in some komatiite flows. Running water may alter and rework some komatiite lavas and form sedimentary rocks. Examples of these deposits occur northeast of Johannesburg in South Africa. There, the lower portion of the rock pile is mostly layers of successive komatiite or basaltic lava flows. This portion is overlaid by sedimentary rocks composed of mudrocks changed, at high temperature and pressure, into metamorphic rocks. Some quartz-carbonate rocks within the sedimentary rocks likely formed by the alteration of komatiites, probably because carbon dioxide reacted with other elements to form carbonates, leaving residual silica to form quartz. The quartz-carbonate rocks contain high concentrations of antimony in the mineral stibnite and small particles of gold. Solutions moving through the komatiites probably altered the komatiites, leaving the high concentrations of these elements.

Principal Terms

basalt: a dark rock containing olivine, pyroxene, and feldspar, in which the minerals often are very small

crust: the veneer of rocks on the surface of the earth

feldspar: calcium, potassium, and sodium aluminum silicate minerals

igneous rock: a rock solidified from molten rock material

metamorphism: the process by which a rock is buried in high temperature and high pressure, transforming the original minerals into new minerals in the solid state

olivine: a magnesium and iron silicate mineral

pyroxene: a calcium, magnesium, and iron silicate mineral

silicate mineral: a naturally occurring element or compound composed of silicon and oxygen with other positive ions to maintain charge balance

skeletal crystals: elongated mineral grains that may resemble chains, plates, or feathers

upper mantle: the region of the earth immediately below the crust, believed to be composed largely of periodotite (olivine and pyroxene rock), which is thought to melt to form basaltic liquids

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