Shallow crust magma with a dash of salt and peperite

by Allison Mills
Tuesday, September 2, 2014

A mafic dike (right) cuts across a salt formation, exposed in the Herfa- Neurode Mine in Germany. The bulbous, pillowy peperite (center, left) formed when a sill intruded carnallite salt, causing water loss and instability in the salt. Credit: Nick Schofield.

Magma and salt are not often paired on the menu of geology. But, occasionally, the two do mix — with surprising results. Researchers explored these interactions in a new study, the first to examine how magma emplacement occurs in salt formations.

The standard model of magma emplacement assumes that it is always done in a brittle fashion, says Nick Schofield, an igneous and petroleum geologist at the University of Aberdeen in Scotland who led the study, which was published in Geology. Brittle emplacement, as in a typical dike intrusion, happens when a fracture splits open and is filled with rock and magma. But that’s not the case in the salt layers that Schofield and his colleagues examined. Instead, the salt and magma both appear to interact as viscous fluids.

The secret ingredient to understanding this nonbrittle emplacement: A bit of peperite, a distinct rock that looks like a frozen lava lamp.

Peperitic texture is often observed where lava flows hit wet, sandy beaches, for example. Because the two materials are viscous, they form blobs of volcanic rock where the sand squishes around them, Schofield says. This rounded, pillowy texture is quite different from the ripped-up rock common in intrusive breccias. Schofield’s team noted peperite in a drift cut in the Herfa-Neurode Mine in central Germany where a small sill branches off from a main dike and pushes through a layer of carnallite, a salt with potassium, magnesium and some water in its structure.

“Magma intruding salt — it’s a bit of a weird thing,” Schofield says, especially because it doesn’t conform to classic intrusion models. The team attributes this deviation to the salt composition. “For these hydrous [salt] layers where these intrusions are, it’s about the chemistry and its interaction with the heat of the magma.”

Carnallite cooks a little differently than other salts heated by magma. It’s considered mechanically weak, and past studies have examined it as a highly ductile material. But that’s not the only reason magma can push it around. Halite, which has no water, doesn’t melt until it is about 800 degrees Celsius or warmer. But carnallite starts losing the water integrated in its crystal lattice at temperatures above about 140 degrees Celsius. Losing water destabilizes the carnallite, turning it into a mush that flows with the magma, creating a lumpy sill intrusion. This kind of lateral displacement does not translate into equal upward displacement, however, and Schofield and his colleagues noted that the observed sill caused minimal deformation and doming in the above halite layers.

That, coupled with the fact that nonbrittle emplacement lacks the seismic signature of its fracturing, brittle counterpart, led Schofield and his fellow researchers to think this process could be happening on a regional scale today and is yet undetected. They note that the volcanically active Afar region in Africa has thick subsurface salts in close association with volcanism and could be a site of modern-day nonbrittle emplacement. The site has mostly been studied using seismic and remote sensing technologies. This process not only sheds light on the hard-to-see “plumbing system” of volcanic activity, but may have influenced the evolution of salt basins containing oil and gas reserves.

There are certainly a number of salt basins in the world where this may be happening, says Olivier Galland, a geophysicist studying shallow magma emplacement at the University of Oslo in Norway who was not involved in the study. Galland says plenty of those basins have volcanic intrusions, which is why he finds it “a bit surprising that there hasn’t been more study of this.” The Schofield study calls attention to a need to re-examine existing models, Galland says, and “demonstrates again that magma emplacement in the shallow crust is not always like a simple fracture.”

Still, he says, “the implications for the large scale depend on the different settings.” Galland has worked in the Neuquén Basin in Argentina where he observed similar structures and peperitic texture in the rocks there — not in the salt layers, but rather in the weak organic-rich shale layers. Although he has not studied these in detail, Galland says the observations noted in the new study could hold “in many other sedimentary formations.” In fact, much of Schofield’s previous work has documented this nonbrittle emplacement in coal, shale, sandstone and limestone.

Before the process is assumed to occur across the board, though, both Galland and Schofield suggest that places like the Neuquén Basin and other magma-impacted salt basins such as ones in Afar and Angola require specific studies of their own. “Indeed,” Galland says, nonbrittle emplacement “has very good potential and is a nice idea, so now it’s time to go beyond the idea and collect data and really test this hypothesis.”

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