Thursday, October 26, 2023

Photomicrograph: Mineral Filled Vesicle

I came across this stunning image of a mineral filled vesicle on the September 2023 cover of Geology. The rock sample was collected from the Louisville Seamount Chain in SW Pacific Ocean.

 Source: Elmar Albers 2023- Timing of carbon uptake by oceanic crust determined by rock reactivity.

Vesicles in igneous rocks are spherical holes formed by expanding gas bubbles. As lava erupts, dissolved gases bubble out. Lava solidifies fairly rapidly on exposure either to air or water. The bubble shape is retained as a small cavity. It gets filled with minerals when magmatic fluids and mineral saturated seawater or groundwater circulate and react with the rock. 

The basalt rock in this study is about 50-74 million years old. The calcite in the vesicle precipitated within 8 million years of eruption. Alteration of undersea basalt is a CO2 sink. Basalt reacts with seawater, trapping carbon in carbonate minerals. The calcium required for formation of carbonate minerals is provided by the alteration of minerals like plagioclase. The study is trying to estimate how long such carbonation reactions continue. Carbonated oceanic crust eventually sinks into the mantle at subduction zones sequestering carbon from the surface for hundreds of millions of years.

This particular vesicle is filled with carbonate (calcite) and clay. Notice the beautiful banding suggestive of pulses of mineral formation. Among the brown and white layers are white bands of faceted saw tooth calcite. And the upper part of the vesicle is filled with large irregular shaped crystals. Surrounding the vesicle is the 'groundmass', made up of tiny crystals of plagioclase feldspar, iron oxide, and volcanic glass. There is no scale in the picture, but my guess is that the vesicle is a few hundred microns across.

In a hand sample a vesicular basalt will look like the example below. This is from the Deccan Traps near Pune. 

The vesicles here are much larger than the first example. Many are empty. Some vesicles have a lining of tiny crystals. Carbonation of terrestrial basalts also constitutes a carbon sink.  Combating global warming and achieving net zero emissions will require, foremost, a steep reduction in emissions, but additionally also removing carbon dioxide from the atmosphere and safely storing it in long term reservoirs. Such carbon removal and sequestration projects are exploring the potential of basalts and related igneous rocks as a long term carbon sink. 

Thursday, October 19, 2023

Links: Volcanic Underworld, First Americans, Billion Year Old 3D Microfossils

Readings over the past few weeks.

1) Taking the First Steps Into a Newly Formed Volcanic Underworld: Maya Wei- Haas describes a fascinating landscape on the Canary Islands in the Atlantic Ocean. Volcanic eruptions and the transport of lava via underground tubes has formed a subterranean world of stacked lava tunnels and caves. Their mapping is ongoing and scientists hope to understand not just the details of volcanism and the hazards it poses, but also how life can colonize such nascent surfaces, powered by nutrients from minerals. As one of the scientists remarks- "lava tubes is a rare chance to watch an evolving ecosystem from time zero".

2) It looks like the 23ky old human footprints at White Sands are solid: What is the earliest securely dated evidence of people in the America's? In 2021, there was a report of human footprints from an ancient lake in New Mexico. Since the footprints themselves could not be dated, seeds of an aquatic plant that were found in the same layer were carbon dated to about 23 thousand  years ago. That result was greeted with caution. The main concern was that the seeds may have taken up much older lake water containing less of the radioactive isotope C14. This may have made the dated material look older than it actually was. 

Now, there has been more work on the geochronology of the site using two more independent lines of dating. The results agree with the previously estimated date of 23 thousand  years. ArcheoThoughts summarizes the dating methodologies. 

3) Discovery of oldest 3D-preserved microorganisms: Before organisms evolved the ability to build hard skeletons, their remains have been preserved as impressions on soft sediment or as chemical degradation products recognizable by a light carbon isotope signal. Stefanie Terp reports on a discovery of 3D preservation of microorganisms from a mine in Ukraine. They are 1.5 billion years old! 

Scanning Electron Microscopy reveals the filamentous structure of these creatures. They are most likely a variety of fungi. Groundwater in the granite environment in which they lived was saturated with aluminum and silica. The microorganisms were covered and entombed in micrometer thin layers of aluminum silicate, perfectly preserving their delicate structure.