282-8 Garnet and Plagioclase Control on Apatite Trace Element Concentrations in Lower Crustal Xenoliths

Session: Crustal Petrology, Part II

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Abstract:

Apatite (Ca⁵(PO⁴)³(F, Cl, OH)) is a versatile and ubiquitous accessory mineral occurring in a wide range of crustal rocks. Apatite is useful because its broad and unique range of major and trace element configurations and concentrations can be used to determine parent melt chemistry, metamorphic conditions, and fluid and hydrothermal processes. Here, we investigate apatite trace element geochemistry as a potential qualitative proxy for the relative depth of crystallization and/or equilibration. To test this, we analyzed in-situ apatite major and trace element geochemistry and U-Pb isotopes of four ~1.7 – 1.4 Ga lower crustal Xenoliths with varying modal abundances of garnet, plagioclase, and pyroxene from the State Line Xenolith kimberlite field (Colorado, USA). We find that apatite major element geochemistry is invariant to the modal abundance of plagioclase and garnet. In contrast, apatite trace element geochemistry covaries with the modal abundance of plagioclase and garnet. Samples with high modal garnet yield elevated Sr/Y and a stronger positive Eu anomaly relative to the sample suite. Samples enriched in plagioclase and lacking garnet show low Sr/Y and a more negative Eu anomaly relative to the sample suite. Apatite U-Pb dates young with increasing garnet modal abundance and yield Stacey-Kramer lower-intercept ²⁰⁶Pb/²³⁸U corrected-dates which range from Mesoproterozoic (~1330 Ma) to Devonian (~360 Ma). Cathodoluminescence images show homogenous textures without clear features of recrystallization. The absence of apatite recrystallization textures or a younger metamorphic overprint suggests that the Mesoproterozoic dates reflect cooling, consistent with published apatite U-Pb data from lower crustal xenoliths in the Great Falls Tectonic zone to the north. The Devonian dates suggest prolonged residence at temperatures sufficient to diffuse Pb until cooling caused by emplacement of kimberlite diatremes which host the xenolith samples. One sample did not yield U-Pb dates due to extremely low concentrations of U (<0.1 ppm) which suggests either crystallization from a U poor melt or subsequent diffusion of U into a cogenetic phase during equilibration. Based on these observations, we suggest that the apatite trace element geochemistry may reflect the growth and stability of cogenetic mineral phases during crystallization and subsequent equilibration

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5819

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