Subduction zones are places on the Earth where dense ocean crust descends (or subducts) beneath more buoyant continental or oceanic crust. Active subduction zones are responsible for most of Earth's large earthquakes and explosive volcanic eruptions. Ancient subduction zones now exposed at the Earth's surface provide important information on the processes, both physical and chemical, occurring within active subduction zones. Determining the timing and rates of past events in these ancient subduction zones, however, has proven extremely difficult. This proposed research will develop a new technique to determine ages on lawsonite, an important mineral that forms during metamorphism associated with subduction zones. Dating lawsonite, therefore, provides geologists the ability to determine the timing and rates of ancient subduction processes.
This research will develop and refine the technique of dating lawsonite by the Lu/Hf method. Lawsonite forms at distinct pressure and temperature conditions during the metamorphic history of a rock. Depending on the metamorphic conditions, lawsonite may grow before or after other high-Lu minerals such as garnet and epidote, which may effect the ability to date lawsonite by the Lu/Hf method. This proposal will test the technique in the Franciscan Complex of California and the Sivrihisar Massif of Turkey. The Franciscan Complex contains Lawsonite-bearing rocks that are of similar metamorphic age but that preserve different metamorphic histories. These rocks also contain both garnet and epidote, making the Franciscan Complex an ideal place to examine lawsonite growth with respect to these minerals. The Sivrihisar Massif contains garnet and lawsonite-bearing rocks that formed under a large range of pressures and temperatures and is therefore ideally suited to compare the relative closure temperatures of Lu and Hf in garnet and lawsonite. The dating will be combined with quantitative thermodynamic models to directly relate the ages to the metamorphic history of a given sample. The results of this study can be applied to other lawsonite-bearing localities and will further develop this powerful tool for dating subduction zone metamorphism.
Subduction zones represent sites where cold dense oceanic crust descends beneath more buoyant oceanic or continental lithosphere. Active subduction zones are responsible for most of Earth’s large earthquakes and volcanic activity. Ancient subduction zones now exposed at the Earth’s surface provide critical constraints on the physical and chemical evolution of the crust, the history and correlation of past tectonic events, and the timescales and rates of ancient subduction zone processes. Dating ancient subduction zones in order to constrain such processes, however, has proven extremely difficult because the minerals formed during subduction zone metamorphism are not suitable for dating by existing techniques such as 40Ar/39Ar and U-Pb geochronology. We recently proposed a method to date lawsonite, a critical index mineral of high-pressure, low-temperature metamorphism associated with subduction zones, by the Lu-Hf method. Lawsonite is a carrier of water and rare-earth elements into the mantle, a contributor to subduction zone seismicity, and is stable at conditions within subduction zones where other minerals suitable for geochronology, such as garnet, are not. The method provides a potentially powerful new tool for constraining subduction zone processes in a pressure-temperature window where few successful geochronometers exist. This project tested and further developed the method of Lu-Hf lawsonite and garnet geochronology to date subduction zone metamorphism from two ancient subduction zones: the Franciscan complex of California, USA and the Sivrihisar Massif, of Turkey. The two regions contain garnet and lawosnite formed under different metamorphic conditions and metamorphic paths, allowing us to test the role these factors play in dating lawsonite and garnet. In the Franciscan Complex, the ages of garnet-epidote amphibolite, garnet-epidote blueschist, garnet-lawsonite blueschist, and lawsonite blueschist range from ˜166-130 Ma and generally decrease with decreasing metamorphic grade, consistent with previous studies. Garnet-lawsonite blueschist/eclogite formed along an apparent prograde path at Ward Creek records an apparent age ˜152 Ma. Lower temperature lawsonite blueschist at Ward Creek, however, failed to provide a geologically significant date and likely reflects isotopic disequilibrium at low temperatures. The apparent temperature-time history from Franciscan Complex Lu-Hf ages most likely reflects samples derived from various portions of the subduction zone or that were subducted and metamorphosed at different times in the thermal evolution of the subduction zone. In the Sivrihisar Massif, lawsonite eclogite and garnet-lawsonite blueschist record distinctly different ages of 91.1 ± 1.3 Ma and 83.3 ± 1.8 Ma. The different ages date the timing of high-pressure metamorphism within each protolith and suggest that garnet-lawsonite eclogite metamorphism predated garnet-lawsonite blueschist metamorphism in these samples by ˜8 Ma. The age of lawsonite eclogite metamorphism extends the timing of high-pressure metamorphism and requires subduction initiation prior to 91 Ma. The results indicate that the Lu-Hf system provides a reliable tool for dating the wide range in pressure-temperature conditions of subduction zone metamorphism. Lawsonite dating, in particular, provides a reliable method by which to date low-temperature retrograde and prograde metamorphism in the absence of garnet. Lawsonite may not be ideal for geochronology if sufficient garnet coexists in the mineral assemblage, the lawsonite has very low-temperature stability, or if extremely fine-grained Hf phases such as zircon are present in lawsonite. In poly-metamorphic assemblages where the pressure and temperature can be estimated for separate garnet and lawsonite sub-assemblages, the age discrepancy between garnet and lawsonite may provide the ability to quantify the rates of heating or cooling and subduction or exhumation. This proposal would represented the first NSF funding for S.R. Mulcahy, an early career geoscientist. The proposal worked to advance undergraduate education in petrology by funding lab and fieldwork for undergraduate researchers. Funding for this research provided support for the isotope geochronology facility at Washington State University, which is widely used in the geoscience community for Lu-Hf, Sm-Nd, and U-Pb geochronology
