The formation process of serpentinites releases geologic hydrogen, which has drawn much attention in recent years. We will highlight new studies in this edition of Core Elements that show serpentinites hold many critical clues to mantle-crust processes and resources.
Rasoul Sorkhabi
Editor, Core Elements
Serpentinites and Subduction
Francesca Leslie/ Shutterstock.com
Serpentinite—rich in serpentine minerals such as antigorite, lizardite, and chrysotile—forms by metamorphism of ultramafic (extremely rich in magnesium and iron) rocks reacting with hot water such as peridotite and dunite in mid-ocean ridges or subduction zones.
Driving the news: Two recent studies published in Earth and Planetary Science Letters show that serpentinization is a huge part of fluid circulation in subduction zones, with important implications for element cycling, earthquakes, arc volcanism, and magmatic rocks.
A report from Ecuador: Donoso-Tapia and colleagues published a petrological and geochemical study of serpentinites from the Raspas Metamorphic Complex generated by the subduction of the Pacific Ocean floor along the Ecuador Trench.
Study area:
The Raspas Metamorphic Complex is a fault-bounded ultramafic rock sequence of Early Cretaceous age exposed in the Amotape block in southwest Ecuador.
It is a complete, exhumed oceanic crust that preserves all slab components: ultramafic, mafic, and sedimentary.
The Moho boundary between the uppermost mantle and oceanic crust is exposed in the Raspas riverbed.
It is divided into the lower El Toro unit (metamorphosed ultramafic rocks with varying degrees of serpentinization often cut by mafic dykes) and the upper La Chilca unit (ecologies and blueschists).
What they found:
Warm subduction: Recorded pressure-temperature conditions in the rock samples indicated that the Raspas subducting slab experienced relatively warmer temperatures (13.5 C per kilometer) compared to many other subduction zones.
De-serpentinization: The authors refer to dehydration of serpentinites as de-serpentinization, which is recorded in the breakdown of titanium-clinohumite. This mineral forms by combining olivine, ilmenite, and water. Its breakdown is a water-releasing process. Petrological modeling reveals that de-serpentinization occurred at subsurface pressures of 1.3–1.6 gigapascals and temperatures of 620–650 C.
Re-serpentinization: The study also reveals a subsequent (“retrograde”) re-serpentinization in the rocks, as crust-derived metamorphic fluids (with distinct boron-11 isotopic signatures) infiltrated at varying fluid-rock ratios. Re-serpentinization occurred during the exhumation of the rocks at pressures of less than 1.3 gigapascal and temperatures of 320–400 C.
Non-volcanic earthquakes: The researchers extend these results to present-day active subduction zones and suggest that de-serpentinization reactions overlap with non-volcanic seismicity at depths of 25–25 km.
A new study from China: In another study in the same volume of Earth and Planetary Science Letters, Kan Li and colleagues reported the geochemistry of lizardite and antigorite serpentinites from the Mianlue Suture Zone in central China.
Study area: The Mianlue suture zone is an ophiolite mélange of Triassic age in the Quiling-Dabie Mountain that marks the boundary between the North and South China blocks.
What they found: The geochemical results show that the Mianlue serpentinites havehigher nitrogen concentrations (20–37 percent) compared to ocean-floor serpentinites (3–19 percent).
A message from AAPG Academy and AWS For Energy and Utilities
Join AAPG Academy and Amazon Web Services for Energy and Utilities for a freeupcoming webinar on 27 February at 9am CST to learn more about how cutting-edge AI applications are empowering geoscientists to drive critical business decisions.
Entitled From Data to Discovery: AI's Game-Changing Role in Geoscience, the webinar will showcase a presentation from expert speaker Chris Hanton, product lead-energy data insights AI and HPC application at AWS for Energy and Utilites.
Hanton will discuss how industry leaders are:
Using AI to transform complex data into actionable insights
Proterozoic Serpentinite from the Llano Uplift in Texas
Wikimedia Commons
The Llano Uplift is a hallmark of central Texas’ geology. Here, Proterozoic rocks of the Grenville orogeny (1300–1000 Ma) stand tall. They were juxtaposed against the Paleozoic rocks during the Pennsylvanian-age Ouachita orogeny (330–290 Ma) that produced the Llano Uplift.
One of the interesting features of the Llano Uplift is the Coal Creek Serpentinite, which is the focus of a research paper by Mosher and Gillis.
Coal Creek Serpentinite:
This serpentinite body exposed in the Llano Uplift measures 6 kilometers long and 2 kilometers wide on average.
It is thrust over the Coal Creek plutonic complex to the north and the Big Branch Gneiss to the south and east.
These rock packages are parts of a magmatic arc dated 1325–1275 Ma that later collided with Laurentia at 1150–1120 Ma.
The new study: The researchers describe the following three events in the evolution of Coal Creek Serpentinite:
Stage 1:
During the Grenville orogeny in Texas, harzburgite (a type of peridotite) underwent dynamothermal prograde metamorphism, which resulted in partial to complete serpentinization.
The researchers interpret the ultramafic protoliths of this metamorphism as the upper mantle portion of an obducted ophiolite, rather than an arc basement.
This metamorphism took place in 1118–115 Ma based on U-Pb ages. The peak metamorphic conditions ranged from 685–810 C and 0.8–1.0 gigapascal.
Stage 2: De-serpentinization.
This process occurred very gradually and via grain-by-grain replacement. It was so gradual that there was minimal volume change in the rock and the original foliation was preserved.
Stage 3: Post-tectonic serpentinization
The researchers highlight that these events took place at temperatures of 685–793 C and pressures of 0.64 gigapascals.
This serpentinization took place at very shallow depths after the Grenville orogeny.
The final tectonic emplacement of Coal Creek Serpentinite along thrust faults occurred during the Llano Uplift.
Why it matters: Coal Creek Serpentinite is one of the few and largest exposed Grenville-age serpentinites in the world. Moreover, Coal Creek Serpentinite has a relatively simple tectonic setup, while most of the Grenville-aged belt deformation is superimposed on older orogenic events or are overprinted younger ones.
Go deeper: Read the full article in the GSA Bulletin.
Sponsored
Celebrating Women in Geoscience
“Few geoscientists have worked the deep stratigraphy of Oman. It’s so new, and I wanted to be at the forefront of it... I took it as a challenge to come up with new ideas and look for greater prospectivity, and I did it”—Layaan Al Kharusi, Occidental Petroleum, Oman
Last week’s quiz question was: Of the 17 rare-earth elements, which is the rarest?
Answer: Lutetium (atomic number 71) and Thulium (69) with crustal concentrations of only about 0.5 parts per million are usually cited as the rarest REEs. However, Promethium (61) may be rarer, as it is found only as a uranium fission product with about 560 grams at a given time on Earth.
This week’s quiz question is: Serpentinite is the state rock of which U.S. state? What is the geologic reason behind that?
👍 If you enjoyed this edition of Core Elements, consider supporting AAPG's brand of newsletters by forwarding to a friend or colleague and signing up for our other newsletters here.
➡️ Was this newsletter forwarded to you? Subscribe to Core Elements here.
AAPG thanks our advertisers for their support. Sponsorship has no influence on editorial content. If you're interested in supporting AAPG digital products, reach out to Melissa Roberts.
.png?upscale=true&width=220&upscale=true&name=Rasoul-Sorkhabi-Headshot-Signature%20(1).png)
