Large, long-lived, and entirely molten magma chambers once existed in
Earth's crust

Date:
March 14, 2022
Source:
University of the Witwatersrand
Summary:
Geologists have come up with multiple lines of evidence indicating
that the Bushveld Complex in South Africa functioned as a 'big
magma tank' in the ancient Earth's crust.



FULL STORY ==========================================================================
An international group of researchers led by geologists from Wits
University in Johannesburg have come up with multiple lines of evidence indicating that the Bushveld Complex in South Africa functioned as a "big
magma tank" in the ancient Earth's crust. This research was published
as a paper in Scientific Reports.


========================================================================== Professor Rais Latypov from the School of Geosciences at Wits University
says "While re-examining thin-sections of Bushveld chromitites, we noticed
a very puzzling observation: chromite often occurs as individual grains
that seemingly 'suspended' within matrix minerals. This observation leads
us to a critical question: why have the chromite grains failed to sink
towards the chamber floor despite being much denser than the host melt?"
To answer this question, the researchers have studied chromitite in
three- dimensions (3D) using high-resolution X-ray computed tomography
and revealed that nearly all chromite grains are closely interconnected
to form a single continuous 3D framework. "This gave us an answer
to the above question: chromite grains are not able to settle freely
towards the chamber floor simply because they are all bound together in self-supporting 3D frameworks attached to the chamber floor," says Dr
Sofya Chistyakova from the School of Geosciences at Wits University.

There is only one process that may result in the formation of such 3D frameworks of chromite crystals. This is an in situ self-nucleation and
growth of chromite grains, for example, when all new chromite grains
nucleate and grow on pre-existing chromite grains directly at the chamber floor. This happens from the parental melt that is saturated in chromite
as the only crystallising phase.

"This logically brought us to a long-known Cr mass balance issue --
normal basaltic melts contain only a very small amount of Cr so that the formation of thick chromitite layer requires extraction of Cr from a very
large volume of liquid that must be present as a thick melt layer in the chamber. Simple mass balance calculations indicate that a 1 metre thick
layer of chromitite will require a magma column of 2km to 4km thick,"
says Latypov.

Latypov and his co-authors believe that the enormous lateral extent
of chromitite layers indicate that during the formation of massive
chromitites the Bushveld chamber was operating as a giant magma body of
more than 400km in diameter, with a column of the resident melt likely attaining a few km in thickness. "This conclusion is at odds with a
currently emerging school of thought is that such large, long-lived
and largely molten magma chambers are non-existent in Earth's history,"
says Latypov.


========================================================================== Story Source: Materials provided by University_of_the_Witwatersrand. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Illustration_of_a_big_tank_magma_chamber ==========================================================================


Link to news story: https://www.sciencedaily.com/releases/2022/03/220314105642.htm

--- up 2 weeks, 10 hours, 50 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)