peridotite

[포도원]

감람암 [橄欖岩, peridotite]

감람석이 주성분인 완정질(完晶質) 및 조립(粗粒)의 초고철질암(超苦鐵質岩)을 총칭하는 초염기성 심성암.

감람석 이외에는 휘석을 함유하며, 드물게는 각섬석 ·운모 ·석류석 ·크롬철강 ·스피넬 등을 함유하고 있는 경우도 있다. 감람암에는 보통 장석이 들어 있지 않으나, 다소의 사장석이 들어 있는 경우가 있다.

주성분은 광물의 조성에 따라 ① 두나이트(dunite):감람석이 99% 이상인 것, ② 웨를라이트(wehrlite): 단사휘석·감람석으로 이루어진 것, ③ 하즈버자이트(harzburgite): 사방휘석과 감람석으로 이루어진 것, ④ 러졸라이트(lherzolite): 거의 같은 양의 사방휘석·단사휘석과 감람석으로 이루어진 것, ⑤ 킴벌라이트(kimberlite): 사방휘석·단사휘석과 감람석 및 흑운모로 이루어진 것, ⑥ 코틀랜다이트(cortlandite): 각섬석과 감람석으로 이루어진 것 등으로 구분된다. 석류석을 비교적 많이 함유하는 것은 석류석감람암이라 한다.

감람암은 때로 다량의 크롬철석·석면·능고토석(菱苦土石)·니켈 또는 백금의 광석 등을 수반하기도 하여, 중요한 광상(鑛床)이 된다. 킴벌라이트에는 다이아몬드를 함유하는 것도 있다. 감람암은 독립된 관입암체(貫入岩體)를 이루기도 하지만, 염기성 관입암체의 한 상(相)으로 볼 수 있는 것도 있다. 또한, 어떤 종류의 화성암 중에 포유물(包有物)로서 산출되기도 한다. 감람암의 관입암체는 광역변성대(廣域變成帶)나 조산대(造山帶)에 많이 분포하는 듯하다. 감람석은 사문석으로 변질되기 쉬우므로, 감람암이 현저히 변질하면 사문암으로 된다.

감람암의 변질로 인해서 생기는 2차광물로는 사문석·활석·각섬석·탄산염 광물 등이 있다. 염기성 관입암체의 일부를 이루는 감람석은 마그마로부터 조기에 정출(晶出)한 고철질 광물이 집적하여 생긴 것, 즉 마그마 분화작용의 산물이라 해석된다. 그리고 어떤 종류의 화성암 중의 포유물로서 발견되는 감람암은 맨틀 물질로 생각된다.

Peridotite

A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic and ultrabasic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from the Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.

Peridotite is the dominant rock of the upper part of the Earth's mantle. The compositions of peridotite nodules found in certain basalts and diamond pipes (kimberlites) are of special interest, because they provide samples of the Earth's mantle roots of continents brought up from depths from about 30 km or so to depths at least as great as about 200 km. Some of the nodules preserve isotope ratios of osmium and other elements that record processes over three billion years ago, and so they are of special interest to paleogeologists because they provide clues to the composition of the Earth's early mantle and the complexities of the processes that were involved.

Distribution and location

Peridotite is the dominant rock of the Earth's mantle above a depth of about 400 km; below that depth, olivine is converted to a higher-pressure mineral. Oceanic plates consist of up to about 100 km of peridotite covered by a thin crust; the crust, commonly about 6 km thick, consists of basalt, gabbro, and minor sediments. The peridotite below the ocean crust, "abyssal peridotite," is found on the walls of rifts in the deep sea floor. Oceanic plates are usually subducted back into the mantle in subduction zones. However, pieces can be emplaced into or overthrust on continental crust by a process called obduction, rather than carried down into the mantle; the emplacement may occur during orogenies, as during collisions of one continent with another or with an island arc. The pieces of oceanic plates emplaced within continental crust are referred to as ophiolites; typical ophiolites consist mostly of peridotite plus associated rocks such as gabbro, pillow basalt, diabase sill-and-dike complexes, and red chert. Other masses of peridotite have been emplaced into mountain belts as solid masses but do not appear to be related to ophiolites, and they have been called "orogenic peridotite massifs" and "alpine peridotites." Peridotites also occur as fragments (xenoliths) carried up by magmas from the mantle. Among the rocks that commonly include peridotite xenoliths are basalt and kimberlite. Certain volcanic rocks, sometimes called komatiites, are so rich in olivine and pyroxene that they also can be termed peridotite. Small pieces of peridotite have even been found in lunar breccias.

The rocks of the peridotite family are uncommon at the surface and are highly unstable, because olivine reacts quickly with water at typical temperatures of the upper crust and at the Earth's surface. Many, if not most, surface outcrops have been at least partly altered to serpentinite, a process in which the pyroxenes and olivines are converted to green serpentine. This hydration reaction involves considerable increase in volume with concurrent deformation of the original textures. Serpentinites are mechanically weak and so flow readily within the earth. Distinctive plant communities grow in soils developed on serpentinite, because of the unusual composition of the underlying rock. One mineral in the serpentine group, chrysotile, is a type of asbestos.