Zirconium Partitioning in Mafic Rocks as a Tool for Interpreting Zircon Geochronology
Matthew J. Kohn and Stacey L. Corrie
The mineral zircon (ZrSiO4) is commonly dated to determine when rocks form. During continent-continent collisions, rocks metamorphose, dramatically changing mineral assemblages and abundances. In past studies, zircons that formed during metamorphism have been used to infer the timing and rates of collisional processes. However, sparse information exists about how zirconium (Zr), the primary element that stabilizes zircon, partitions among major minerals. Because these minerals and their Zr content continually change, little is as yet known about the fate of zircon during metamorphism – under what circumstances does zircon grow (because Zr contents of other minerals decrease), and when is it consumed (because Zr contents of other minerals increase)? More specifically, which reactions liberate Zr and produce zircon, and which ones consume Zr and zircon? By identifying how Zr is partitioned among minerals during metamorphism, we can determine how and why zircon forms. To answer these questions, we embarked on two endeavors. First, we quantified Zr partitioning among major metamorphic minerals for the first time. We selected minerals that can be found in a typical rock with the composition of a basalt (oceanic crust) at varying degrees of metamorphism and measured Zr concentrations using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Analyses were collected using instrumentation housed in the Department of Geosciences, Boise State University. Second, we modeled the mineralogical development of a basaltic rock composition through a range of pressures and temperatures to determine mineral abundances at different metamorphic conditions. One important reaction that we investigated involves the breakdown of plagioclase feldspar to form sodium-rich clinopyroxene and calcium-rich garnet. This marks a major mineralogical transition from amphibolite (moderate-pressure) mineral assemblages to eclogite (high-pressure) mineral assemblages, and many metamorphic zircon ages have been attributed to this reaction. We have determined, however, that clinopyroxene and garnet contains at least 10 times more Zr than plagioclase, so in order for clinopyroxene and garnet to form, more Zr is required than the plagioclase can supply. Thus, the transition to eclogite facies should cause zircon to dissolve, rather than grow, and attributing a metamorphic zircon age to this reaction appears erroneous. We are currently exploring other key metamorphic reactions to infer more likely mechanisms for metamorphic zircon formation.