PRELIMINARY INSIGHTS FROM THE FE ISOTOPE COMPOSITION OF GARNETS IN THE TIBES IRON SKARN

The Tibes iron (Fe) skarn in southern Puerto Rico formed during the later stages of Caribbean-North American collision. This event caused the igneous activity that led to the emplacement of the Tibes dioritic stock ca. 60 Ma. As the stock cooled, episodic injections of acidic magmatic-hydrothermal fluids entered the surrounding carbonate wall rock. Available elemental and isotopic compositions of garnet and magnetite ore from Tibes indicate Fe-rich to aluminum-rich fluid evolution with minimal meteoric contribution. The objectives of this study are 1) to determine the Fe isotope composition of well-characterized garnets in the Tibes Fe skarn, and 2) to assess the potential of the Fe isotope composition of garnets as a reliable geochemical tool for understanding the fluid evolution during skarn formation. Furthermore, the Tibes skarn presents a unique opportunity to characterize the genesis for a calcic-iron skarn that lacks the effects of continental accretion.

We present preliminary data for the bulk Fe isotope composition of garnets (n=17) from the Tibes skarn to characterize variation within (intra) and between (inter) samples. The assessment of intrasample variation was completed with the analysis of 4 garnets within one hand sample, which varied in grain size (diameters: 0.5 – 2cm). The bulk δ⁵⁶Fe values (relative to IRMM-14 ±2SD) for garnets within the same sample range from +0.19 ±0.06‰ to +0.12 ±0.03‰. Intersample variation was assessed by analyzing 13 garnets from a variety of populations within the Tibes skarn: garnet associated with magnetite ore (n= 4), garnet associated with marble (n=4), and garnet associated with hornfels (n=5). The variation in bulk garnet δ⁵⁶Fe (n=13) across these populations ranged between -0.12 ±0.03‰ and +0.12 ±0.06‰. There is less Fe isotope variation within a single garnet population.

These preliminary data provide the first look at the Fe isotope compositions of garnets of the Tibes skarn and indicates that bulk δ⁵⁶Fe values of garnets may be useful in tracing fluid-rock interaction during skarn formation. Trace element geochemistry and additional isotope analysis will be integrated in future work to test the hypothesis that bulk garnet Fe isotopes are useful records of fluid evolution during skarn formation.