What We Are Studying Now: Investigating Zircon’s Preservation of Ultrahigh-Pressure Metamorphism

By Liam Houlgate ’21 and Holly Harris ’22

Holly Harris ’22 and Liam Houlgate ’21 spent the fall semester of 2020 examining the unique physical and chemical features of the mineral zircon found within rock samples collected from Greece's Rhodope Metamorphic Complex to determine if these grains preserve a history of metamorphism at extreme conditions.

To do this work, they collaborated virtually with Emily Peterman, an associate professor in the EOS department. Harris and Houlgate present a summary of their findings here.

Topographic map of Greece with red outlined box showing Rhodope Metamorphic Complex
Figure 1. Location of Rhodope Metamorphic Complex in Eastern Greece. Red box marks location of Figure 2.

"When a plate subducts, it reaches progressively higher pressures and temperatures, resulting in metamorphism, or the transformation of minerals. In some instances, the subducting slab reaches pressures and temperatures that are so extreme that rock undergoes what is known as ultrahigh-pressure (UHP) metamorphism, a phenomenon that is largely restricted to the last 20 percent of Earth’s history.

While conventional studies have focused on analyzing garnet and diamond within UHP terranes, these minerals are commonly partially replaced and/or modified (which is termed overprinting) upon return from extreme conditions. Therefore, the record of extreme metamorphism is erased as rocks return to the surface from depths  of greater than 100 kilometers.

Geologic map of Rhodope Metamorphic Complex

Figure 2. Geologic map of Rhodope Metamorphic Complex. Samples in this study come from Kimi and Xanthi (stars).

This semester, we have been testing the hypothesis that evidence of UHP is preserved in the mineral zircon. If so, our work provides a new tool for reconstructing the processes that occur deep within subduction zones. Our research builds upon the work of Kitrea Takata-Glushkoff ’19, Manlio Calentti ’20, Tessa Peterson ’20 and Katelyn Cox ’21, who traveled with Peterman to Eastern Greece (see map, Figure 1) to collect samples from the Rhodope Metamorphic Complex and process them for subsequent analysis.

Peterman collected geochemical and geochronological data from zircon from Kimi, Xanthi and Sidironero (see geologic map, Figure 2) during her sabbatical, and we have spent the semester interpreting the results.

Just as tree rings provide snapshots of Earth’s past climate, zircon similarly records a pressure and temperature history through the growth of metamorphic rims surrounding its igneous core (see Figure 3). We examined the physical attributes of zircon using images that were collected using a cathodoluminescence (CL) detector on the scanning electron microscope (SEM) at Bowdoin (see Figure 4). We classified our observations to define three different domains preserved within each zircon—core, transition zone, and rim (see Figure 5). We then analyzed geochemical data measured from spots 10 micrometers in diameter (One tenth the diameter of the average human hair!) in order to identify trends among these domains. 

The core and rim domains yield patterns consistent with trends identified for igneous and metamorphic growth (Rubatto, 2002). However, the transition zone domain yields distinctly different, yet characteristic, results that we suggest are evidence of UHP metamorphism. The transition zone is remarkably consistent across all three localities in Greece, and yet it varies in size from a few nanometers to several micrometers. Zircon grains with a narrow transition zone have significantly larger metamorphic rims and were subjected to higher temperatures. While these data suggest that the transition zone can be overprinted at high temperatures, the fact that evidence of this domain remains suggests that zircon can be a robust indicator of UHP metamorphism.

Many questions still persist regarding UHP metamorphism. However, our work has further illuminated the complex dynamics of the subduction and exhumation process. Next steps of the project will include the investigation of the processes that occurred to produce this transition zone and to see if it exists in other UHP terranes. We have felt humbled by the extent of what nature is capable of as nanoscale crystals have continually informed our understanding of large-scale tectonic events."

“What We Are Studying Now” is a news showcase for the work of current undergraduates in Earth and Oceanographic Science at Bowdoin College.

References Cited

Krenn, K., Bauer, C., Proyer, A., Klötzli, U., & Hoinkes, G. (2010). Tectonometamorphic evolution of the Rhodope orogen. Tectonics 29, TC4001.

Kunz, B.E., Regis, D., & Engi, M. (2018) Zircon ages in granulite facies rocks: decoupling from geochemistry above 850°C? Contributions to Mineralogy and Petrology, 173:26.

Rubatto, D. (2002) Zircon trace element geochemistry: partitioning with garnet and the link between U–Pb ages and metamorphism. Chemical Geology 184:123–138.