Zircon Dating in Serpentinite from the Puerto Rico Trench: the Key to Mantle-Ocean Interactions in the Atlantic Realm



In this pilot study PIs Klein and Marschall will examine the hydrothermal alteration of rocks and minerals from the deepest part of the Atlantic Ocean, the Puerto Rico Trench, to elucidate the timing and mechanisms of mantle–seawater interaction (referred to as serpentinization) in the Atlantic realm.  Mantle rocks crop out in a broad variety of geologic settings from mid‐ocean ridges to subduction zones and have been recently discovered on other planetary bodies such as Mars and Ceres.  Serpentinization of mantle rocks is a key process in influencing the chemistry of the oceans, in providing nutrients for microbes, and possibly provided the essential chemicals for the emergence of life on early earth.

Location and timing of serpentinization bear important ramifications for our current understanding of global geochemical fluxes between the Earth’s mantle and the ocean.  Yet, it is unclear where and when serpentinization of mantle rocks now present in the Puerto Rico Trench occurred.  To this end Klein and Marschall will determine the age of hydrothermal zircon recently discovered in serpentinite from the Puerto Rico Trench using a combination of ion microprobe and cathodoluminescence techniques.  From the timing of zircon growth combined with well‐established palaeo spreading rates of the Atlantic, they will be able to reconstruct the location of hydrothermal activity.  There are three competing hypotheses of when, where and how serpentinization occurred, all of which have distinct implications for mantle‐seawater interaction.

The first hypothesis is that hydrothermal alteration of mantle rocks is a present‐day process that is currently ongoing in the Puerto Rico Trench.  If this holds true, Klein and Marschall will be able to provide, for the first time, key insights into the exposure and hydrothermal alteration of fresh mantle rocks in a geologic setting where slow spreading oceanic lithosphere is currently being subducted.  Serpentinization in the Puerto Rico Trench would provide the geofuels (such as hydrogen and methane) crucial to support complex microbial ecosystems in one of the least accessible and least studied areas of the oceans.

The second hypothesis is that hydrothermal alteration of the Earth’s mantle took place at a time when the seafloor now exposed at the PRT was formed at the Mid‐Atlantic ridge in the early Cretaceous, when the Atlantic ocean was less than 30 million years old.  If this hypothesis holds true, then Klein and Marschall’s rock collection from the Puerto Rico Trench contains the oldest hydrothermally‐altered mantle rocks ever collected from the seafloor.  These rocks would be ideally suited to conduct a systematic study addressing the long‐term weathering behavior of hydrothermally altered mantle rocks, which will then enable them to constrain the geochemical fluxes of key elements, such as carbon and sulfur between the mantle and the ocean over a timespan of 130 Ma.

The third hypothesis is that hydrothermal alteration of mantle rocks took place sometime between the early Cretaceous at the Mid‐Atlantic Ridge and the present day Puerto Rico Trench.  If this hypothesis holds true, this would imply that hydrothermal alteration took place in an off‐axis environment, similar to the renowned Lost City vent field.  It has been suggested that off‐axis hydrothermal alteration of mantle rocks such as at Lost City represents a significant, but to date un‐accounted sink for CO2 in the global carbon cycle.  Ergo, fluid‐rock interaction in the young Atlantic may have had a direct influence on our paleo climate.