Solid Earth

3.4 Formation of oceanic crust

Video: Formation of oceanic crust

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Photo: Pillowbasalts of the Oman ophiolite, Middle Cretaceous, Wadi al Jizzi, Oman, 2015

Chapter 3.4

Formation of oceanic crust

Fig. 3.4.1: Melt formation in the upper asthenosphere beneath a spreading zone. (Meschede, unpubl., 2021)

The magmatic melt from which the oceanic crust forms is an extract from the upper asthenosphere (Fig. 3.4.1). With the pressure and temperature conditions at the top of the asthenosphere, some minerals are already molten while others remain solid. This creates a melt that has a different composition than the mantle rock from which it comes.

Mantle rocks belong to the group of peridotites, which consist primarily of the minerals olivine and pyroxene and, to a small amount, plagioclase (feldspar mineral group). Plagioclase is only stable in the upper part of the asthenosphere, deeper down it becomes unstable due to the high pressure and is replaced by spinel. Pyroxene comes with two variants, diopside (clinopyroxene, monoclinic crystal class) and enstatite (orthopyroxene, orthorhombic crystal class). Diopside and enstatite have different melting temperatures, with the diopside melting in the upper asthenosphere while the enstatite still remains solid.

The peridotites are essentially composed of the minerals diopside, enstatite, olivine and spinel or plagioclase. The most important rocks of this group are lherzolite (named after the town of Lherz in the Pyrenees, France), harzburgite (after Bad Harzburg, Germany) and dunite (after Mount Dun in New Zealand), each of which contains different proportions of the minerals. The differences arise from melt formation, in which only certain minerals melt while others remain solid. The mantle rock at greater depths corresponds to a lherzolite. In the partial melt beneath the spreading zone, mainly diopside and plagioclase are melted out of the lherzolite, while enstatite and olivine remain largely solid. As a result, the solid portion of the asthenosphere below the spreading zone becomes depleted of diospide and becomes increasingly richer of enstatite. When the asthenosphere solidifies toward the sides near the spreading center, it forms harzburgite, which is typical of the upper layers of the lithospheric mantle. At very high degrees of melting, nests can be formed that consist almost exclusively of olivine. The resulting rock is called dunite. It consists of over 95% olivine.

Fig. 3.4.2: Section through a spreading zone in which new oceanic crust is being formed (Meschede, unpubl., 2021, modified after Frisch & Meschede, 2021)

The basaltic melt extracted from the mantle accumulates in a magma chamber in the spreading zone (Fig. 3.4.2). The melt contains mainly diopside and plagioclase and, in small proportions, olivine. However, the basaltic melt has a completely different composition than the mantle rock, which is essentially due to the different melting behavior of the individual minerals.

Essentially, the oceanic crust is made up of three layers: at the top are the basalts extruded at the seafloor, which are usually present as pillow basalts. Below these are the so-called dike-in-dike structures, which caused by their layered behaviour are also described as sheeted dike complex. These dikes represent the feeder channels of the lava that extreude at the seafloor. When the lava flow stops, the lava in the dikes also cools. The rock in it is called dolerite. The third and thickest layer consists of gabbro. This rock has the same chemical and mineralogical composition as dolerite and basalt. Except that the gabbro is completely crystallized, while basalt contains mostly glass. The dolerite often also contains glass, but is already partially crystallized, albeit with very small crystals.

When molten rock comes into contact with seawater, it is chilled. There is no time for crystals to grow. Only those that were already floating in the melt can be recognized as crystals in the basalt.

Fig. 3.4.3: Pillow basalts at 2500 m water depth at the spreading center of the East Scotia Ridge (ROV dive PS119-ROV445, 27.04.2019, MARUM Bremen)

Fig. 3.4.4: Pillow basalts in Wadi al Jizzi, Oman, Middle Cretaceous (85 mill. yrs.). Scale = 2 m. Photo: Meschede.

The examples of pillow basalts shown here were recorded during an expedition of RV Polarstern by the MARUM Bremen with an ROV (diving robot) at an active spreading center of the East Scotia Ridge in the South Atlantic at a water depth of 2500 m (Fig. 3.4.3); the other example comes from the Wadi al Jizzi in Oman and shows Middle Cretaceous pillow basalts (85 mill. yrs.; Fig. 3.4.4).

Fig. 3.4.5: Formation of pillow basalts at the seafloor (from Frisch & Meschede, 2021)

When pillow basalts are formed, the lava is pressed upwards through a feeder dike and the rounded shapes form in the water directly at the outlet point. As more lava pushes in from below, the pillows that have just formed burst open and the lava flows out again, creating more pillows (Fig. 3.4.5). This continues until the supply of liquid lava runs dry.

Fig. 3.4.6: Dike-in-dike structure of a sheeted dike complex, Wadi al Hibi, Oman, Middle Cretaceous (85 mill. yrs.). Photo: Meschede.

Dike-in-dike structures (Fig. 3.4.6) are known as sheeted dike complexes. Sheeted dike, because they look like they are layered structures like we know from sediments. But it is not stratification in this sense. They are formed as feeder channels in which the liquid lava rises from the magma chamber to the surface and flows out there. At some point the lava flow stops and the lava in the dike cools down. The cooling occurs very quickly at the edges, creating a thin, chilled margin made of volcanic glass. It often happens that a dike penetrates into an already existing dike that has just been formed and splits it into two halves. This is then referred to as half dike, which can be recognized by the fact that, in contrast to complete dikes, they have a chilled margin only on one side.

Fig. 3.4.7: Gabbro, Belledonne, Chamrousse massif, France. Cambrian (496 mill. yrs.). Photo: Meschede.

Fig. 3.4 8: Layered gabbro, Wadi al Abyad, Oman, Middle Cretaceous (85 mill. yrs.). Photo: Meschede.

The third and thickest layer consists of gabbro. This rock has the same chemical and mineralogical composition as dolerite and basalt. Except that the gabbro is completely crystallized, while basalt contains mostly glass. Gabbros can be uniformly grained (Fig. 3.4.7) or layered (Fig. 3.4.8), but this should not be confused with dike-in-dike structures of the sheeted dike complex.

Fig. 3.4.9: Alternating layer of gabbro/peridotite, at the crust/mantle boundary (fossil Moho), Wadi al Abyad, Oman, Middle Cretaceous (85 mill. yrs.). Photo: Meschede.

At the boundary to the lithospheric mantle, the Moho (Mohorovičić discontinuity, named after Andrija Mohorovičić), there can be alternating layers of mantle rocks (peridotites) and gabbros (Fig. 3.4.9), which can have irregular shapes in a zone of hundreds of meters . The brownish rock in the picture is peridotite, the lighter layers consist of gabbro.