Xenolith

A Xenolith ( ξένος from Greek, xenos " foreign" and λίθος, líthos, "stone"; German foreign rock) is an inclusion ( inclusion) older country rock ( " a stone " ) in a volcanic or plutonic, whose emergence with the formation of volcanic rock Plutonits or in any way related.

Formation

With the accumulation of magma in the earth's crust or in the mantle and the ascent to the surface parts of the surrounding rock can be switched on or break off and are absorbed by the melt. Due to the high temperature of a magma these fragments ( xenoliths ) suffer kontaktmetamorphe overprinting or melting. Therefore xenoliths have long residence time in the magma often rounded edges or spherical shape. The size of xenoliths can be a few millimeters to tens of meters. Occasionally, collections of (mostly mafic ) minerals that crystallize early from the melt and accumulate due to their high density at the bottom of the magma chamber, as referred xenoliths, although it is actually not about foreign rock, but a cumulate be.

Includes a magma a significant amount of foreign rock, can alter the composition of the melt significantly by absorption of chemical elements from the xenoliths.

Importance for research

The study of xenoliths has a special meaning in the scientific study of igneous rock body.

Often these are in xenoliths to mitbeförderte from the underground fragments of host rock. Therefore, they provide a (in many cases the only ) means is to investigate deeper rock layers immediately. From the fossil content of the xenoliths can be inferred to the local layer sequence. Geothermobarometrische methods allow the estimation of the pressure and temperature conditions under which the xenoliths were formed. From this, the depth at which the melt was formed, determine. Metamorphic changes and the degree of rounding of the xenoliths give an indication of the magma temperature and the residence time. Ideally, can be calculated from these data, the rate of ascent of magma from the place of origin to the surface.

In today curled by erosion deep areas of a volcanic chimney itself xenoliths can be located, which are dropped in the formerly molten vent filling from higher areas. This Sinkschollen can provide information about which rock layers were pending at the time of igneous activity above the current level and thus serve as a tool for age determination ( the magma must be younger than the Sinkschollen ).