Ti in Zircon Megacrysts From Kimberlite: Evidence for low Temperatures of Formation

The Ti concentration and oxygen isotopic composition of megacrystic zircon hosted by kimberlite is generally homogenous within a given pipe but varies between pipes. These zircons are a distinct suite widely accepted to be of mantle origin and characterized by large (1 - 15 mm, dia), rounded, gemmy, but fractured crystals, low U (<50ppm), and ZrO2 coatings. Titanium in zircon hosted by kimberlite from the Kaapvaal Craton of southern Africa, the Siberian Platform, and Brazil was analyzed on the CAMECA ims-1280 ion microprobe using a 4 nA (O-) primary beam (25 μm spot, 5 μm analysis area). NIST 610 glass (434 ppm Ti) and synthetic Ti-rich (800-1500 ppm Ti, EPMA) zircon (B. Watson) were used as standards. New, precise, laser-fluorination oxygen isotope data were obtained for zircons from Brazil, yielding mantle-like δ18O values (4.8 to 5.8 ‰) similar to published data for Africa (4.9 to 5.9 ‰) and Siberia (4.7 to 5.6 ‰, Valley et al. 1998 CMP 133:1). The zircons analyzed range from 2 to 53 ppm Ti (n=169 analyses, 44 zircons), and most zircons contain less than 20 ppm Ti. The recently calibrated Ti in zircon thermometer (Watson and Harrison, 2005, Science, 308:841) yields an average temperature of 740 ± 64° C (1SD). Independent thermometry of zircon megacrysts based on the associated MARID and other parageneses is poorly constrained, but includes estimates as low as 700° C. Because megacrysts are not generally found within a mineralogical context, the presence of rutile and a(TiO2) are not known. However, megacrysts have been found intergrown with rutile, and the temperature correction for reduced a(TiO2) is likely (<) 50° C. Most megacrysts in this study preserve fine-scale, oscillatory zoning in CL and are generally homogenous in their Ti content and oxygen isotopic composition, consistent with preservation of primary compositions. In samples from Siberia and Africa, Ti concentration appears to vary among kimberlite pipes, but in most cases not within pipes or within megacrysts. The variation in Ti from these zircons is on a similar scale among pipes in the same region as among regions across the craton. African zircons have a slightly higher δ18O than Paleozoic Siberian samples with similar Ti contents; one Mesozoic Siberian pipe has elevated and variable Ti. One zircon from Kimberley has small zones of elevated Ti not associated with CL zoning but most regions of the crystal contain the same Ti as unzoned megacrysts from the same pipe. Megacrysts from Brazil have higher and more variable Ti content and oxygen isotopic composition overlapping Siberian and African samples. A few zircons are zoned with decreasing Ti from core to rim and others have low-Ti homogenous CL regions that cross-cut earlier zonation, while others appear to be homogeneous. The average temperature in this study corresponds to ~28 kbar on a 40 mW/m2 cratonic geotherm suggesting that zircon megacrysts from all three cratons formed in the shallow lithospheric mantle at approximately the time of kimberlite eruption. Greater Ti variability in zircons from Brazil suggests either more heterogeneous conditions of formation or greater interaction with high-temperature fluids/magma.