HAFNIUM AND TEMPERATURE (FROM TITANIUM) VARIATIONS IN ZIRCONS: A POTENTIAL NEW TOOL TO EVALUATE PETROLOGIC PROCESSES IN MAGMAS

The ability to use Ti concentrations in zircons to measure the temperature (T) at which a specific cathodoluminescence (CL) zone in a zircon grew provides a new tool to evaluate petrologic processes in magmatic rocks. We have reported previously that a number of trace element concentrations and element ratios in zircons are correlated with T. Hf conc. in zircons increase with decreasing T because early-formed zircon prefers Zr over Hf leading to higher Hf/Zr ratios in the residual magma. We analyze 3-6 CL zones on several individual zircons from a single rock sample to develop a Hf vs. T plot for a sample. Ti concentrations are evaluated for reliability by measuring a suite of elements (F, Al, Ca, Fe) that help identify when the beam overlaps non-zircon components. The trends of data on these plots take several different forms. Individual samples can have roughly parallel trends starting at different Hf conc. We interpret this as reflecting different Hf/Zr in the sources of the parent magmas, and our current data sets suggest there can be regional patterns for the starting Hf/Zr. Evidence for magma mixing takes two forms: 1) Intermediate CL zones have higher Ti and lower Hf conc. than zones interpreted to have been formed earlier but all data lie along one Hf-Ti trend. This is interpreted to indicate recharge of a magma chamber by hotter pulses of magma from the same general parentage. 2) Outer, higher T CL zones surround inner, lower T zones but the data define two separate Hf-T trends. This suggests mixing of magmas of different parentage and is supported by other strongly contrasting geochemical characteristics of the CL zones such as the Eu anomaly. Some suites of zircons define a broader than typical Hf-T trend and data points from single zircons move to higher Hf but higher T and then back to a normal trend of lower T and higher Hf. This type of variation suggests changes in the activity of Ti or SiO₂ or possibly P_H2O. When zircon grows at the solidus, Hf increases but T becomes restricted producing a sharp break in the slope of the Hf-T trend. Late formed (subsolidus ?) zircon edges can exhibit extreme trace element enrichments with very high U and very low Th conc. (black CL). Very high and unrealistic calculated T's from these zones imply non-equilibrium uptake of trace elements, perhaps as incorporation of nm-scale inclusions of other phases.