Pavement Geology: Ophicarbonate

You must have come across polished slabs of this dark green rock with white veins, used on stairways and as wall panels. It is an ophicarbonate.

The term encapsulates a range of geological processes, often separated by tens of millions of years. Oceanic lithosphere that is being generated by magmatism at mid ocean ridges, where plates diverge, is made up of igneous rocks arranged in a sequence. The top layer is basalt lava erupting and forming oceanic crust. Much lower down are ultramafic rocks like peridotites, composed mainly of the mineral olivine. In this lithospheric rock sequence, the boundary between the mantle and the crust is taken to be the transition from denser ultramafic rocks like peridotite to less dense feldspar containing rocks like a gabbro. Peridotites are rocks from the earth's mantle.   

As this newly formed lithosphere (tectonic plate) moves away from the active ridge, faulting can exhume these peridotites to shallower levels. The lithosphere is still warm and magmatic fluids and heated sea water hydrate and alter this peridotite to the mineral serpentine. Calcite can also form during these reactions, if these fluids are alkaline and bicarbonate rich. Such processes of serpentization have been observed, for example, at the Lost City hydrothermal vents in the Atlantic Ocean, where highly alkaline fluids are altering peridotite and precipitating large quantities of carbonate on the sea floor.

Being a part of a drifting tectonic plate, such serpentinized and carbonated peridotites eventually arrive at a subduction zone, where oceanic lithosphere is sliding underneath another plate. Slices of oceanic lithosphere including this altered peridotite gets scraped off along thrust faults and emplaced in a growing mountain chain. These fragments of oceanic lithosphere preserved on land along zones of plate convergence are called ophiolites. And the serpentine and calcite bearing altered peridotite is called an ophicarbonate.  

Alteration of the peridotite to serpentine and calcite can also occur during subduction by reaction with hydrothermal fluids expelled during metamorphism of buried sediments. During this alteration process the peridotite is crushed and acquires a broken fragmented appearance with veins of calcite surrounding blocks of serpentine. 

This example shows very clearly the brecciated nature of an ophicarbonate. You can observe fragments of green serpentine floating in and surrounded by large veins of calcite. 

Ophiolites and pockets of ophicarbonate are found all across the northern margin of the Indian tectonic plate from Ladakh and tracing the convergence zone southeastwards and south to the Naga Hills and the Andaman Island chain. 

Such deep sea processes are of interest to geologists who study the long term cycling of chemical elements on Earth. Alteration of  peridotites via carbonation reactions traps dissolved inorganic carbon in carbonate minerals like calcite and dolomite. The vast bulk of such altered rocks sink into the mantle along subduction zones, sequestering that carbon in the earth's interior for tens to hundreds of millions of years. Eventually that carbon may return to the surface as a gas via volcanic eruptions or geometrically bound as diamonds!

The next time you are climbing a stairway paved by this rock you can ponder on its fantastical journey from deep ocean to mountain front. And don't forget, you are stepping on a piece of the earth's mantle!

Articles: Earthquake Temperatures, Marshes, Ocean Drilling

Sharing some good stuff I came across recently.

1) Taking the temperature of faults. From AGU News. Rocks get hot as they slide past each other during faulting. Chemical changes in organic molecules and helium content in zircon crystals are sensitive to temperature changes. Scientists have used these to estimate short lived temperature rise during faulting. Understanding patterns and magnitude of earthquake generated heat informs us about earthquake intensity, heat dissipation, and fault movement history. 

2) Why a Marsh: "Neither land nor water, maybe both, a marsh is a balancing act, a collaboration between changing elements, uncertain by its very nature, in flux"

A beautiful long essay by Daniel Wolff and Dorothy Peteet on marshes and wetlands and the vital role they play in ecosystem functioning. Especially fascinating is the description of Piermont Marsh, along the banks of the Hudson river, north of New York City. Sediment cores going back a thousand years preserve a record of climate change and human modification of the landscape. The marsh was first exploited by Native Americans and then more extensively by European settlers. It is a detailed look at the wealth of information a wetland can yield about climate, ecology, and human disturbance of the environment.

3) Drilling Into the Sea Bed: This is a really interesting summary of scientific ocean drilling by Neena Notman . They are attempts going on to reach the earth's mantle by drilling into the sea bed. Although drilling in the deep ocean seems daunting, this makes more sense than trying to reach the mantle from land. The crust making up the continents is much thicker. Ocean bed drilling offers a short cut to the earth's mantle. We know about the mantle mostly through geophysical data. Actual samples of the mantle are rare. Recovering pristine pieces of the mantle will allow us to validate what we've gleaned from geophysics. Drilling is also going on along a subduction zone to understand the nature of fault interface and rock properties.