A microscopic cross-section of the newly discovered type of basalt

Geologists have discovered a new type of rock. While drilling deep into the seafloor, a team of scientists found a new form of basalt that’s vastly different from any other on Earth, and the planet hasn’t produced any more of it for millions of years.

It’s easy to assume that there aren’t that many types of rocks left to discover, but it turns out that the Earth is still hiding some up its sleeves – or, as the case may be, deep below the ocean.

The team, made up of scientists from all over the world, discovered the new rock in the Amami Sankaku Basin (ASB), in the ocean south of Japan. There, the researchers lowered their equipment 6 km (3.7 miles) below the surface to the seafloor, then drilled down another 1.5 km (0.9 miles).

At that depth, the team found a new type of basalt with a completely unique chemical and mineral makeup. It’s a tholeiite that’s low in titanium and potassium, and bears aluminum-rich gemstones called spinels.

“The rocks that we recovered are distinctly different to rocks of this type that we already know about,” says Dr. Ivan Savov, co-author of the study. “In fact, they may be as different to Earth’s known ocean floor basalts as Earth’s basalts are to the Moon’s basalts.”

The geologists say that these rocks were produced during large and very hot volcanic eruptions, back when the Ring of Fire was first forming some 50 million years ago. The magma that formed them was transported to the surface very quickly, from its source in the upper mantle where it was exposed to pressures between 0.7 and 2 GigaPascals.

The depth at which they were found, in terms of both the ocean and the crust, explains why these rocks haven’t been discovered before. The Earth stopped making these particular rocks millions of years ago – so you could consider them kind of a Limited Edition, long out of print.

“Now that we know where and how this rock type is formed, we anticipate that many other rocks that we know were originally formed by ocean floor eruptions will be re-examined and potentially alter our wider understanding of the basalt formation,” says Savov.

https://newatlas.com/science/new-rock-basalt-discovered-ocean/

https://tokyo3.org/forums/holiday/posts/1714538/

roughbarked said:

https://tokyo3.org/forums/holiday/posts/1714538/

But mine has the pretty picture.

roughbarked said:

https://tokyo3.org/forums/holiday/posts/1714538/

Michael V said:

Here’s the original paper. The second author was one of my Honour supervisors.

https://www.nature.com/articles/s41467-021-21980-0

In summary, the rocks indicate the extraction of a later melt from a peridotite that had already had a mid-ocean ridge basalt extracted from it. This occurred in a very early subduction setting.

The peridotite was in the upper plate. Water driven off the down-going slab (comprising ocean floor) would have fluxed the partial melting of the upper-plate peridotite.

Oh, here’s some information about Tholeiitic Basalts.

https://en.wikipedia.org/wiki/Tholeiitic_magma_series#Geologic_context

Thanks. :)

Nobody reads chat. Has to be in a thread. ;-) Thanks PermeateFree.

“Tholeiitic rocks are the most common igneous rocks on Earth, produced by submarine volcanism at mid-ocean ridges and make up much of the ocean crust. Tholeiitic basaltic magmas are initially generated as partial melts of peridotite (olivine and pyroxene) produced by decompression melting of the Earth’s mantle.”

“The magmatic character of early subduction zone and arc development is unlike mature systems. Low-Ti-K tholeiitic basalts and boninites dominate the early Izu-Bonin-Mariana (IBM) system. Basalts recovered from the Amami Sankaku Basin (ASB), underlying and located west of the IBM’s oldest remnant arc, erupted at ~49 Ma. This was 3 million years after subduction inception (51-52 Ma) represented by forearc basalt (FAB), at the tipping point between FAB-boninite and typical arc magmatism. We show ASB basalts are low-Ti-K, aluminous spinel-bearing tholeiites, distinct compared to mid-ocean ridge (MOR), backarc basin, island arc or ocean island basalts. Their upper mantle source was hot, reduced, refractory peridotite, indicating prior melt extraction. ASB basalts transferred rapidly from pressures (~0.7-2 GPa) at the plagioclase-spinel peridotite facies boundary to the surface. Vestiges of a polybaric-polythermal mineralogy are preserved in this basalt, and were not obliterated during persistent recharge-mix-tap-fractionate regimes typical of MOR or mature arcs.”

Nope, still not following it.

“Chains of explosive stratovolcanoes are characteristic of island and continental arcs such as the IBM and Andes, respectively. These chains form a volcanic front typically no closer than 100 km from the nearest trench, and lie ~105 km above the uppermost surface of a subducting lithospheric plate. These magmas are distinctive compared with the basalts of mid-ocean ridges (MORB) and those of isolated ocean islands (OIB) such as Hawaii.”

So far so good. But I prefer the article that PermeateFree linked to.

“The Earth stopped making these particular rocks millions of years ago”

That can’t be right, can it? The Earth produced rocks like these for billions of years and then stopped just 50 million years ago. Really unlikely on the plausibility scale.