Wednesday, June 26, 2013

Mantle degassing in the Rockies

As you bathe in Colorado's hot springs, your legs may be being tickled by bubbles containing mantle gas, seeping through the whole thickness of the continent to the surface. Their presence far from plate boundaries, where most of the interaction between mantle and surface occurs, has been a surprise to tectonicists. 

These mantle gases are found mixed with groundwater, and their rise through the crust is linked to basement penetrating faults, providing a direct connection for mantle and crustal fluids to mix. The fluids then emerge as hot springs or travertine marble terraces, generally linked with the same extensional tectonics that produced the faults. Mantle contributions were found to increase with seismic activity, and lower again afterwards. The team has named them xenowhiffs, foreign gases in groundwater, after the mantle xenoliths brought up in volcanic eruptions.

The results came from measuring the ratios of helium and carbon dioxide across 25 hot springs and a variety of travertine deposits throughout the Rockies. Helium has two main isotopes, called He3 (light) and He4 (heavy). The heavy isotope is created by radioactive decay of metals like uranium, that are mostly concentrated in the crust. The lighter one is a relic of the Earth's formation. Most of the crust's light helium has been lost to space during post Archaean crustal reworking, so the only He3 left on Earth is believed to lie in deep mantle reservoirs. A high proportion of He3 relative to He4 if held to indicate mantle contributions, whether found in lavas, volcanic gases or groundwater. These results were then compared with seismic tomographic maps of earthquake wave speeds under the western USA to compare mantle gases with magma reservoirs. Low wave speeds tend to indicate the presence of magma.

Over a quarter of the helium in the groundwater, and a whopping three quarters of the CO2 was found to come from a mantle source, transported by fluids that had moved though more than 50Km of continental crust. Due to the complex tectonic history, ascribing exact sources for these gases is hard. Some may come from the recent extension related magmatism in the basin and range province, some may be due to the subduction of the Farallon plate and the Laramide Orogeny, which hydrated the mantle, lowering its melting point, resulting in magma that is visible on seismic tomograms. This mountain building event also heated and thinned the continental keel. Another possible contribution may be the consequences of part of the subducting Farallon slab breaking off and sinking into the deeper mantle as its minerals pass through the eclogite pressure transition and metamorphose to denser forms.

The pervasive presence of mantle gases in groundwater throughout the western USA has several implications. Carbon sequestration plans may not mitigate atmospheric CO2 as efficiently as we hope, because if mantle gases can pass through the whole thickness of the continent via faults and seepage, and the whole west is seismically active, then no formation may be truly safe from leakage events if used to store man made CO2. The ratios suggest that the mantle gases are taking less than three million years to pass through the crust. The relative contributions of fault and seepage based transport will have to be estimated, though with careful assessment procedures sequestration should work, since gases have been injected into reservoirs with favourable geometry without signs of significant leakage. It must also be emphasised that the amount of CO2 degassing from the mantle is minute relative to anthropogenic emissions, so like volcanoes, this process cannot be blamed for climate change.

A second problem is related to contamination of water resources, since these mantle gases bring dissolved metals such as arsenic and uranium with them, that then end up in the groundwater. Some of the springs had higher levels of arsenic than permitted for human or agricultural use. As the easily mined aquifers of the west, the Oglala in particular, become depleted, people are going to be seeking water elsewhere, and the risk that this metal rich water will be used is high. This will either require costly separation of the metals, or accepting the type of serious health problems that plague Bangladesh, ever since NGO's dug wells into an arsenic bearing aquifer back in the 70's and 80's.

Tectonically, these results imply that the entire mantle beneath the western USA is degassing heterogeneously. Similar results have been found near the San Andreas fault and other places in California and Nevada. By contrast, springs above the thick craton of the Canadian Shield show no similar geochemical connection between mantle and surface. This type of research offers vital clues to the nature of the complex interactions between asthenosphere, mantle and crust, in a tectonically complex area of extension, accreted terranes and recent orogenies.

Image credit for Pinkerton Hot Spring: E.R. Pape, via state geothermal data project.

Original paper, paywall access:

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