This compositional range is intermediate, and is one of the characteristics which distinguish Proterozoic anorthosites from Archean anorthosites (which are typically >An
80).
Proterozoic anorthosites often have significant
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
components in addition to plagioclase.
These phases can include
olivine
The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
,
pyroxene
The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...
, Fe-Ti oxides, and/or
apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of Hydroxide, OH−, Fluoride, F− and Chloride, Cl− ion, respectively, in the crystal. The formula of the admixture of ...
.
Mafic minerals in Proterozoic anorthosites have a wide range of composition, but are not generally highly
magnesian.
The trace-element chemistry of Proterozoic anorthosites, and the associated rock types, has been examined in some detail by researchers with the aim of arriving at a plausible genetic theory. However, there is still little agreement on just what the results mean for anorthosite genesis; see the 'Origins' section below. A very short list of results, including results for rocks thought to be related to Proterozoic anorthosites,
Some research has focused on
neodymium
Neodymium is a chemical element; it has Symbol (chemistry), symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth element, rare-earth metals. It is a hard (physics), hard, sli ...
(Nd) and
strontium
Strontium is a chemical element; it has symbol Sr and atomic number 38. An alkaline earth metal, it is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is exposed to ...
(Sr)
isotopic determinations for anorthosites, particularly for anorthosites of the Nain Plutonic Suite (NPS). Such isotopic determinations are of use in gauging the viability of prospective sources for magmas that gave rise to anorthosites. Some results are detailed below in the 'Origins' section.
High-alumina orthopyroxene megacrysts (HAOMs)
Many Proterozoic-age anorthosites contain large crystals of orthopyroxene with distinctive compositions. These are the so-called high-alumina orthopyroxene megacrysts (HAOM).
HAOM are distinctive because 1) they contain higher amounts of Al than typically seen in orthopyroxenes; 2) they are cut by numerous thin lathes of plagioclase, which may represent exsolution lamellae;
and 3) they appear to be older than the anorthosites in which they are found.
The origins of HAOMs are debated. One possible model
suggests that, during anorthosite formation, a mantle-derived melt (or partially-crystalline mush) was injected into the lower crust and began crystallizing. HAOMs would have crystallized out during this time, perhaps as long as 80–120 million years. The HAOM-bearing melt could then have risen to the upper crust. This model is supported by the fact that aluminium is more soluble in orthopyroxene at high pressure.
In this model, the HAOM represent lower-crustal cumulates that are related to the anorthosite source-magma. One problem with this model is that it requires the anorthosite source-magma to sit in the low crust for a considerable time. To solve this, some authors
suggest that the HAOMs may have formed in the lower crust independent of the anorthosite source-magma. Later, the anorthosite source-magma may have entrained pieces of the HAOM-bearing lower crust on its way upward. Other researchers consider the chemical compositions of the HAOM to be the product of rapid crystallization at moderate or low pressures, eliminating the need for a lower-crustal origin altogether.
Origins of Proterozoic anorthosites
The origins of Proterozoic anorthosites have been a subject of theoretical debate for many decades. A brief synopsis of this problem is as follows:
The problem begins with the generation of magma, the necessary precursor of any igneous rock.
Magma generated by small amounts of partial melting of the
mantle is generally of
basaltic
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% ...
composition. Under normal conditions, the composition of basaltic magma requires it to crystallize between 50 and 70% plagioclase, with the bulk of the remainder of the magma crystallizing as mafic minerals. However, anorthosites are defined by a high plagioclase content (90–100% plagioclase), and are not found in association with contemporaneous ultramafic rocks.
This is now known as 'the anorthosite problem.' Proposed solutions to the anorthosite problem have been diverse, with many of the proposals drawing on different geological subdisciplines.
It was suggested early in the history of anorthosite debate that a special type of magma, anorthositic magma, had been generated at depth, and emplaced into the crust. However, the
solidus
Solidus (Latin for "solid") may refer to:
* Solidus (coin)
The ''solidus'' (Latin 'solid'; : ''solidi'') or ''nomisma'' () was a highly pure gold coin issued in the Later Roman Empire and Byzantine Empire. It was introduced in the early ...
of an anorthositic magma is too high for it to exist as a liquid for very long at normal ambient crustal temperatures, so this appears to be unlikely. The presence of water vapor has been shown to lower the solidus temperature of anorthositic magma to more reasonable values, but most anorthosites are relatively dry. It may be postulated, then, that water vapor be driven off by subsequent metamorphism of the anorthosite, but some anorthosites are undeformed, thereby invalidating the suggestion.
The discovery, in the late 1970s, of anorthositic
dykes in the Nain Plutonic Suite, suggested that the possibility of anorthositic magmas existing at crustal temperatures needed to be reexamined.
However, the dykes were later shown to be more complex than was originally thought.
In summary, though liquid-state processes clearly operate in some anorthosite plutons, the plutons are probably not derived from anorthositic magmas.
Many researchers have argued that anorthosites are the products of basaltic magma, and that mechanical removal of mafic minerals has occurred. Since the mafic minerals are not found with the anorthosites, these minerals must have been left at either a deeper level or the base of the crust. A typical theory is as follows: partial melting of the mantle generates a basaltic magma, which does not immediately ascend into the crust. Instead, the basaltic magma forms a large magma chamber at the base of the crust and
fractionates large amounts of mafic minerals, which sink to the bottom of the chamber. The co-crystallizing plagioclase crystals float, and eventually are emplaced into the crust as anorthosite plutons. Most of the sinking mafic minerals form
ultramafic cumulates which stay at the base of the crust.
This theory has many appealing features, of which one is the capacity to explain the chemical composition of high-alumina orthopyroxene megacrysts (HAOM). This is detailed below in the section devoted to the HAOM. However, on its own, this hypothesis cannot coherently explain the origins of anorthosites, because it does not fit with, among other things, some important isotopic measurements made on anorthositic rocks in the Nain Plutonic Suite. The Nd and Sr isotopic data show the magma which produced the anorthosites cannot have been derived only from the mantle. Instead, the magma that gave rise to the Nain Plutonic Suite anorthosites must have had a significant crustal component. This discovery led to a slightly more complicated version of the previous hypothesis: Large amounts of basaltic magma form a magma chamber at the base of the crust, and, while crystallizing, assimilating large amounts of crust.
This small addendum explains both the isotopic characteristics and certain other chemical niceties of Proterozoic anorthosite. However, at least one researcher has cogently argued, on the basis of geochemical data, that the mantle's role in production of anorthosites must actually be very limited: the mantle provides only the impetus (heat) for crustal melting, and a small amount of partial melt in the form of basaltic magma. Thus anorthosites are, in this view, derived almost entirely from lower crustal melts.
Lunar anorthosite
On the
Moon
The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
, anorthosite is the dominant rock type of the
lunar highlands
The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to "lunar science") is the structure and composition of the Moon, which is quite different from that of Earth. The Moon lacks a true atmosphe ...
which covers ~80% of the lunar surface. Lunar anorthosite is characterized as
ferroan anorthosite (FAN), or
magnesium anorthosite (MAN). Pristine lunar FAN is some of the oldest lunar rock and the original
cumulate of the
lunar magma ocean
The Lunar Magma Ocean (LMO) is the layer of molten rock that is theorized to have been present on the surface of the Moon. The LMO was likely present on the Moon from the time of the Moon's formation (about 4.5 or 4.4 billion years ago) to tens ...
, with the Mg-suite forming from later impacts and plutonism. However, debate exists on the magma ocean fractionation complicated by surface impact mixing with evidence potentially indicating MAN being older and more primitive.
Lunar anorthosite is associated with two other rock types:
norite
Norite is a mafic Intrusive rock, intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite, orthopyroxene, and olivine. The name ''norite'' is derived from Norway, by its Norwegian name ''Norge''.
Norite, also known ...
and
troctolite. Together, they comprise the "ANT" suite of moon rocks.
Archean anorthosites
Archean anorthosites represent the second largest anorthosite deposits on Earth. Most have been dated between 3,200 and 2,800 Ma, and commonly associated with basalts and/or greenstone belts.
Archean anorthosites are distinct texturally and mineralogically from Proterozoic anorthosite bodies. Their most characteristic feature is the presence of equant, euhedral megacrysts (up to 30 cm) of plagioclase surrounded by a fine-grained mafic groundmass. The plagioclase in these anorthosites is commonly An80-90.
Economic value
The material's high temperature resistance makes it valuable in applications in the aerospace and defense industries. It is mined as a source of aluminium. Its low alkali content also makes it a valuable raw material for
fiberglass
Fiberglass (American English) or fibreglass (English in the Commonwealth of Nations, Commonwealth English) is a common type of fibre-reinforced plastic, fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened i ...
, requiring minimal refinement or pre-processing.
Contents
The primary economic value of anorthosite bodies is the
titanium
Titanium is a chemical element; it has symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
-bearing oxide
ilmenite
Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printi ...
. However, some
Proterozoic
The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...
anorthosite bodies have large amounts of
labradorite
Labradorite (( Ca, Na)( Al, Si)4 O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect ( schiller).
Labradorite is an intermediate to calcic member of the plagioclase series. It ...
, which is quarried for its value as both a
gemstone
A gemstone (also called a fine gem, jewel, precious stone, semiprecious stone, or simply gem) is a piece of mineral crystal which, when cut or polished, is used to make jewellery, jewelry or other adornments. Certain Rock (geology), rocks (such ...
and a building material. Archean anorthosites, because they are
aluminium
Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
-rich, have large amounts of aluminium substituting for
silicon
Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
; a few of these bodies are mined as
ore
Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the concentration ...
s of aluminium.
Anorthosite was prominently represented in rock samples brought back from the Moon, and is important in investigations of
Mars
Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
,
Venus
Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...
, and
meteorite
A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...
s.
Soil development on anorthosite
In the Adirondack Mountains, soils on anorthositic rock tend to be stony loamy sand with classic
podzol
Podzols, also known as podosols, spodosols, or espodossolos, are the typical soils of coniferous or boreal forests and also the typical soils of eucalypt forests and heathlands in southern Australia. In Western Europe, podzols develop on heathlan ...
profile development usually evident. In the
San Gabriel Mountains
The San Gabriel Mountains () are a mountain range located in Los Angeles and San Bernardino counties, California, United States. The mountain range is part of the Transverse Ranges and lies between the Los Angeles Basin and the Mojave Desert ...
, soils on anorthosite have a dominance of 1:1 clay minerals (kaolinite and halloysite) in contrast to more mafic rock over which 2:1 clays develop.
Genesis Rock
The Genesis Rock (sample 15415) is a sample of Moon rock retrieved by Apollo 15 astronauts James Irwin and David Scott in 1971 during the second lunar extravehicular activity, EVA, at Spur (lunar crater), Spur crater on Earth's Moon. With a mass ...
"
See also
*
References
Bibliography
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External links
Anorthosite Complexes (web archive)
* ttps://web.archive.org/web/20031012172703/http://www.nasm.si.edu/galleries/attm/wl.an.2.html Anorthosite – Lunar Highland Rockbr>Lunar Anorthosite Specimen 60025 Photomicrographs
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Plutonic rocks
Archean
Proterozoic