Prof. Daniela Rubatto

Stable isotope analyses of metamorphic minerals to trace fluids in the crust:

Tracing the passage of fluids in rocks in not a trivial matter. Fluids commonly escape the rock leaving behind chemical traces or minute inclusions in the minerals they interacted with. Variations in the isotopic composition of water-soluble elements (such as oxygen and boron) can be used to trace the passage of aqueous fluids. Importantly, using an ion microprobe (e.g. SHRIMP), oxygen and boron isotopes in minerals can be analyzed at the same micro scale as chemical zoning and age, making it possible to reconstruct P-T-time-fluid path from complex metamorphic minerals.
As a group we are working on the measurements of oxygen isotopes in-situ in a range of minerals commonly found in metamorphic rocks.

• The measurement of oxygen isotopes in garnet by ion microprobe requires a reference frame to account for matrix effects due to the variable composition of garnet: we have defined the necessary correction algorithm for variations in grossular (Ca) spessartine (Mn) and andradite (Fe+3) components (Martin et al. 214). Applications to garnet from subducted crust have returned spectacular results and testify to the robustness of oxygen isotopes in garnet in this setting (e.g. Martin et al. 2014, PhD thesis Laure Gauthiez-Putallaz and Morgan Williams, Fig. 1).
• The diffusion of oxygen isotopes in garnet is under investigation experimentally (PhD thesis Mari Scicchitano).
• Monazite is a common chronometer for metamorphic terranes, and we have developed standards and protocols for matrix corrections to achieve accurate oxygen isotope analysis in situ (Rubatto et at. 2014). Fig. 2.
• Apatite is a common mineral in crustal rocks and has proven to be an excellent recorder of oxygen isotope signatures. The fractionation of oxygen isotopes between apatite-garnet and its record of metasomatism has been investigated in the blueschists of the Tansvali zone of Turkey (PhD thesis Laure Gauthiez-Putallaz) and the ore deposit of Cannington (Thesis Maxine Kerr).
• Rutile oxygen isotope analysis by ion microprobe is less straightforward. Whereas matrix effects are not expected large variations in measured values correlate to grain orientation (PhD thesis Laure Gauthiez-Putallaz).
• Serpentine minerals are ubiquitous when mafic and ultramafic rocks interact with aqueous fluids. We have investigated standards and matrix effects for the accurate analysis of oxygen isotopes in antigorite, lizardite and chrysotile by SHRIMP ion microprobe (PhD thesis Mari Scicchitano).

Boron is another element that is highly soluble and thus can be used to trace fluid-rock interactions. The analysis of boron isotopes by ion microprobe demands matrix matched standards for the minerals of interest (e.g. antigorite and mica) and improvement in the sensitivity and signal stability during measurement (PhD thesis Morgan Williams). 

Geochronology

Ion microprobe in-situ dating of U-Pb minerals (zircon, monazite, allanite, titanite, rutile, baddelejite) with a composite internal structure. Dating of minerals or mineral zones formed at different stages during the evolution of a rock unit in order to define detailed P-T-time paths and thus determine rates of geological processes (e.g. exhumation, subduction, cooling, melt residence and extraction). Combining different isotopic schemes in order to unravel complex geological evolutions (Fig. 3 and 4).

Metamorphic petrology of U-Pb accessory minerals

Stability of U-bearing minerals over a wide range of P-T-fluid conditions. Understanding the chemical variation of such minerals with varying P-T and fluid composition in order to link datable minerals and metamorphic conditions. Quantifying solubility and mobility of trace and major elements in metamorphic and hydrothermal fluids.

Research group Metamorphic Geochemistry