Gabbro will also be present in the deep plutons that form when magma chambers that feed basaltic eruptions crystallize. Large volumes of gabbro are present beneath extensive flood basalts such as the Columbia River flood basalts of Washington and Oregon and the Deccan Traps of India.
Close-up view of gabbro: Magnified view of the gabbro shown in the photograph at the top of the page. Gabbro can be polished to a brilliant black luster. Brightly polished gabbro is used to make cemetery markers, kitchen counter tops, floor tiles, facing stone, and other dimension stone products. It is a highly desirable rock that stands up to weathering and wear.
In the dimension stone industry, gabbro is sold under the name " black granite. The most common use of gabbro is as a crushed stone or aggregate. Crushed gabbro is used as a base material in construction projects, as a crushed stone for road construction, as railroad ballast, and anywhere that a durable crushed stone is needed as fill. The best way to learn about rocks is to have specimens available for testing and examination. Gabbro sometimes contains economic amounts of some relatively rare metals.
Gabbros containing significant amounts of the mineral ilmenite are mined for their titanium content. Other gabbros are mined to yield nickel , chromium , or platinum. Article by: Hobart M. Find Other Topics on Geology. Maps Volcanoes World Maps. Hardness Picks. Rock, Mineral and Fossil Collections.
Flint, Chert, and Jasper. Tumbled Stones. Fluorescent Minerals. One relates to the heat involved. Mafic magma is much hotter than felsic magma. Because of this, it is easier for the basaltic lavas to reach the surface while still in the liquid phase. Felsic magma, starting much cooler, generally crystallizes before reaching the surface. Therefore, more basalt than gabbro, and more granite than rhyolite. Another reason is the internal crystalline structure of the silicate minerals.
Mafic magmas crystallize to form relatively simple atomic structures isolated tetrahedra and single chains , and therefore flow easily the higher mafic temperatures also contribute to this ease of movement.
These minerals are in the 4 component normative system Ol-Ne-Cpx-Qtz, shown here as a tetrahedron. The basalt tetrahedron can be divided into three compositional volumes, separated by planes. The plane Cpx-Plag-Opx is the critical plane of silica saturation. Compositions that contain Qtz in their norms plot in the volume Cpx-Plag- Opx-Qtz, and would be considered silica oversaturated. Basalts that plot in this volume are called Quartz Tholeiites.
The plane Ol - Plag - Cpx is the critical plane of silica undersaturation. Normative compositions in the volume between the critical planes of silica undersaturation and silica saturation are silica saturated compositions the volume Ol - Plag - Cpx - Opx.
Silica saturated basalts are called Olivine Tholeiites. Alkali Basalts, Basanites, Nephelinites, and other silica undersaturated compositions lie in the silica undersaturated volume. Note that tholeiitic basalts are basalts that show a reaction relationship of olivine to liquid which produces a low-Ca pyroxene like pigeonite or Opx.
Both olivine tholeiites and quartz tholeiites would show such a relationship and would eventually precipitate either Opx or pigeonite. The critical plane of silica undersaturation appears to be a thermal divide at low pressure. This means that compositions on either side of the plane cannot produce liquids on the other side of the plane by crystal fractionation. To see this, look at the front two faces of the basalt tetrahedron. These two faces are laid out side by side in the diagram below.
Experiments conducted on natural basalt compositions were run until liquids were found to be in equilibrium with at least Ol, Cpx, and Plagioclase.
Such liquids would plot on a cotectic surface in the four component system represented by the basalt tetrahedron. These compositions were then projected from plagioclase onto the front two faces, shown here, to find the projection of the Ol-Plag-Cpx cotectic the boundary curve along which Ol, Plag, Cpx, and Liquid are in equilibrium. Experimental liquid compositions fairly well define the projection of this cotectic onto the two front faces as seen here. Although some information is lost in the projection, we can still treat these two phase diagrams in the same way that we treat normal phase diagrams.
For compositions on this side of the diagram, the liquid composition would then change along the boundary curve until it eventually precipitated Nepheline. A composition projecting into the Ol-Cpx- Qz triangle, on the other hand, would first crystallize plagioclase then crystallize Olivine. But these compositions would then follow a path away from the Ol-Cpx join toward the Qz corner of the diagram and would eventually precipitate Opx.
As this experimental data shows, the join Ol-Cpx which is the projection of the critical plane of silica undersaturation is a thermal divide at low pressure. Thus, low pressure crystal fractionation of tholeiitic basalts cannot produce silica undersaturated basalts and low pressure crystal fractionation of silica undersaturated basalts cannot produce tholeiitic basalts.
Mantle peridotite, because it contains Opx, would plot in the Ol-Cpx-Opx part of the diagram. Note that at low pressure, only silica oversaturated liquids could be produced by melting such a peridotite. Furthermore, because of the low pressure thermal divide, silica undersaturated liquids could never be produced unless the peridotite had a silica undersaturated composition i.
High pressure experiments reveal a solution to this problem. In these experiments basaltic material was placed in a sandwich between layers of peridotite. The peridotite sandwiches were then placed in an experimental apparatus at various high pressures and temperatures and then quenched.
Compositions of the liquid in equilibrium with the peridotite at various pressures was then determined. These results are plotted in a somewhat different projection shown here.
This plots molecular norms in the triangle Qz-Ol - Ne. This is equivalent to the base of the basalt tetrahedron, where the critical plane of silica saturation is represented by the Opx - Plag join, and the critical plane of silica undersaturation is represented by the Ol - Plag join.
All experimental liquid compositions are surrounded by a shaded field indicating the pressure at which the experiments were run. To see how this projection works, let's look at the experiments run at 30 kb pressure. At 30 kb pressure the first liquids to form from melting of peridotite would be those farthest away from the peridotite.
Note that these liquids would be highly silica undersaturated. With increasing degrees of melting the liquid composition would change along a path toward the Ol - Plag join and eventually become silica saturated they would enter the Opx - Plag - Ol compositional triangle. Melting at other pressures would follow a similar path. With increasing degrees of melting they would change first toward silica saturation then towards the original peridotite.
Note that as pressure is reduced, the lowest degree of melting produces less silica undersaturated melts. In fact, at pressures of 8 and 10 kb the first liquids produced are silica saturated liquids, and at 5 kb the first liquids are silica oversaturated. These experimental results bring out two important points: The critical plane of silica undersaturation is a thermal divide at low pressures less than about 10 kb and is not a thermal divide at higher pressures.
Silica undersaturated liquids are favored by high pressure of melting and low degrees of melting. Conversely, silica saturated to oversaturated liquids are favored by higher degrees of melting and low pressure. We will refer to these two important points in the following discussion of the various oceanic settings. Occurrence The Oceanic Ridges are probably the largest producers of magma on Earth. Yet, much of this magmatism goes unnoticed because, with the exception of Iceland, it all takes place below the oceans.
This magmatism is responsible for producing oceanic crust at divergent plate boundaries. This is the trend that would be expected from fractional crystallization involving the removal of early crystallizing olivines and pyroxenes from a tholeiitic basaltic liquid.
Note that the trend is often referred to as an Fe-enrichment trend. During this sequence the olivines and pyroxenes are expected to become more Fe enriched which would tend to cause the trend to bend somewhat. But at the peak of Fe enrichment it appears that the liquids have become so rich in Fe that an Fe-rich phase, like magnetite, joins the early crystallizing mineral assemblage. Fractionation of this Fe-rich mineral assemblage would then cause Fe to become depleted in successive liquids, driving the liquid compositions toward rhyolite.
Thus, crystal fractionation appears to be responsible for the main variety of rocks found at Iceland. Here we turn our attention to what the major element chemistry of MORBs tells us about their origin.
The projected phase diagram shown here is the system Ol-Cpx-SiO 2 , the front face of the basalt tetrahedron. Arrows on the boundary curve show direction of falling temperature. These intersecting boundary curves are also shown at pressures of 10, 15, and 20 kb. Note that the composition of the first liquid produced shifts away from the SiO 2 corner of the diagram with increasing pressure.
Since Cpx would be the first solid phase to disappear during melting, further melting of peridotite at any of these pressure would produce liquids with compositions that lie along the Ol - Opx boundary curve at each pressure. Let's imagine that a liquid is produced at a pressure of 20 kb by partial melting of peridotite. Let's further specify that the melting produces a liquid with a composition at the tip of the arrow on the Ol-Opx phase boundary at 20 kb.
If this liquid is then brought to low pressure, near 1 atm. Olivine fractionation at low pressure will eventually cause the liquid composition to reach the Ol-Cpx boundary curve.
Further fractionation involving the removal of olivine and Cpx will cause the liquid composition to change along the 1 atm boundary curve. But, note how the compositions of the most primitive least fractionated MORB glasses restrict the pressure conditions under which the primary unfractionated MORB glasses could have formed by melting of peridotite.
Small degrees of melting at 15 kb could produce liquids that could fractionate by removal of olivine at low pressure to produce the primitive MORB glasses, and higher degrees of melting at pressures from 15 to 20 kb could produce such liquids as well.
But, compositions of partial melts of mantle peridotite at pressures less than 15 kb are inconsistent with the composition of MORB glasses. Thus, it appears that primary MORB liquids can only be produced by melting of peridotite at pressures in excess of 15 kb. On the diagram the concentration of each trace element in the rocks MORBs in this case is divided by the concentration estimated for the Bulk Earth.
The trace elements are plotted in order of decreasing incompatibility, from most incompatible to less incompatible. Note that this is similar to the REE diagrams we have discussed previously. Note that the most incompatible trace elements are depleted relative to the less incompatible elements. To see why such a pattern suggests that these rocks were derived from a depleted mantle source, we must explore further. Imagine that we start out with a mantle that has the composition of the Bulk Earth.
Such a mantle will have trace element concentrations equal to the bulk earth, and therefore the normalized trace element concentrations will all plot at a value of 1.
If we melt this mantle, for example to produce early crust, then the incompatible trace elements in the residual rock will show lower values than in the original bulk Earth. The most incompatible elements will show the most depletion. This would produce what we call a depleted mantle. If we melt this depleted mantle the incompatible trace elements will be preferentially partitioned into the melt. At higher degrees of melting, the trace element pattern of the liquid produced will be closely parallel to the depleted mantle source rock, but all concentrations will be higher.
Thus, MORBs, which show a depletion in the most incompatible elements likely formed by melting of a "depleted mantle", that is a mantle that had suffered a melting event sometime in the past. Another isotopic system to consider is the Sm - Nd system. Note that both of these elements are REEs. In this system the parent radioactive isotope is Sm.
Note that in the incompatible element diagram above, the parent Sm isotope in a depleted mantle has higher concentrations than the daughter Nd isotope. Thus, in a mantle depleted in the most incompatible elements at some time in the past will produce more of the daughter Nd isotope than the original undepleted Bulk Earth composition.
Note that this relationship is just the opposite of the Rb-Sr system.
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