Bunsen-Daly gap

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In the petrology of igneous rocks, the Bunsen-Daly compositional gap (more rarely just Daly gap ) denotes the absence or significant underrepresentation of volcanites with intermediate SiO 2 contents (especially 53–58%) in volcanics from ocean islands and rift valleys. The extraction of predominantly basic and acidic lavas with an SiO 2 content of 43–48% and 59–62% is also known as bimodal volcanism .

history

This phenomenon was first mentioned in 1851 by the famous German chemist Robert Wilhelm Bunsen in the course of researching the volcanic rocks of Iceland. Almost 75 years later, the eminent Canadian geologist Reginald A. Daly made similar observations in a treatise on the geology of the island of Ascension on the South Atlantic Ridge . In the further course of the 20th century, based on the work of these two scientists, the term "Bunsen-Daly gap" was established.

Occurrence

As a typical example, are basalt - trachyte or basanite - Phonolith Socialization on ocean islands, the geologically mostly Hotspot -Vulkanismus be attributed to the magmas of the asthenosphere come. The original interpretation of this bimodal distribution in the chemical composition of the volcanic rocks was that they are end members of a single magma series. For a long time, this was considered problematic because, according to the basic model of magma differentiation, fractional crystallization , as a continuously running process, approximately the same volumes of basic, intermediate and acidic rocks can be expected in a volcanic region. Examples of such ocean islands are the Canary Islands , Ascension , Iceland and nearly all islands in the Pacific Basin .

Bimodal volcanism occurs and has also occurred in rift fractures and other expansion zones, however due to the different origins of the magmas from the earth's mantle or from the lithosphere, which is why more than just one magma series must be assumed here. Examples are the East African Rift Valley or the Rhön .

Explanatory approaches

There is broad agreement that, from a geochemical point of view, miscibility gaps are responsible for this, but the exact sequence of magma formation in nature is controversial.

One explanation is based on the assumption that the basic and acidic volcanic rocks are not the end product of fractional crystallization, but rather different products of a multi-phase partial melting of the same area of ​​the earth's mantle, in which possibly even the Felsic magmas first and only later in one When the same area of ​​the mantle was heated up, basic magmas were formed, which would explain the geochemical relationship. Other authors simply suspect an incorrect sampling in which (due to their similarity to basic rocks) the intermediate rocks are underrepresented; however, this can be ruled out at least for some of the islands that have been investigated in more detail.

literature

  • Felix Chayes: The oceanic basalt-trachyte relation in general and in the Canary Islands. American mineralogist. Vol. 62, 1977 pp. 666-671 ( PDF 650 kB)
  • Sylvia E. Berg, Valentin R. Troll, Steffi Burchardt, Morten S. Riishuus, Michael Krumbholz, Ludvik E. Gústafsson: Iceland's best kept secret. Geology Today, Vol. 30, No. 2, 2014, pp. 54-60, doi : 10.1111 / gto.12042 (see alternatively S. Berg et al .: Silicic Magma Genesis in Neogene Central Volcanoes in Northeast Iceland. Geophysical Research Abstracts . Vol. 14, EGU2012-10678, 2012, PDF 38 kB )

Individual evidence

  1. ^ Robert Wilhelm Bunsen: About the processes of volcanic rock formation in Iceland. Annals of Physics. Vol. 159 (Poggendorf's Annalen der Physik und Chemie Vol. 83), No. 6, 1851, pp. 197-272 ( gallica.bnf.fr )
  2. ^ Reginald A. Daly: The Geology of Ascension Island. Proceedigs of the American Academy of Arts and Science. Vol. 60, No. 1, 1925, pp. 3-80 ( JSTOR 25130043 )
  3. ↑ http://www.nature.com/ngeo/journal/v6/n5/pdf/ngeo1781.pdf