Phosphorus coupling obfuscates lithium geospeedometry in olivine
Abstract
Lithium zoning provides a powerful geospeedometer for relatively short-lived magmatic processes in volcanic systems. However, the competing effects of crystal growth and subsequent diffusion complicate interpretations of trace element zoning patterns. Here, we use diverse analytical techniques (EPMA, LA-ICPMS, nanoSIMS) to untangle the records of growth and diffusion that are preserved by Li zoning in olivine. We utilize samples from the Keanakāko'i Tephra (Kīlauea, Hawai'i) because their magma storage and transport histories have already been well characterized. The trace elements Li, P, and Al are targeted because they show correlations despite contrasting behaviors during growth and diffusive re-equilibration. Lithium zoning exhibits two styles: 1) non-coupled, wherein Li (at 1-3 ppm levels) shows diffuse zoning over 10's to 100's of μm between the crystal core and rim with low (10's of ppm) and homogeneous P concentration, and 2) coupled, where sub-ppm level Li enrichments <40 μm wide are correlated with P spikes (100's of ppm) that serve as a blueprint of olivine crystal growth. Lithium enrichments consistently correlate positively with P-rich zones as a result of Li+ ions partially satisfying charge balancing requirements of P5+ ions in the olivine lattice. Correlation of Li peaks with Al are much less common. Diffusion models of non-coupled Li profiles yield timescales of hours to days that reflect late-stage magma mixing events that primed the volcanic system for eruption. This interpretation is also supported by <40 μm wide changes in Fo zoning at olivine rims and Fe-Mg disequilibrium between the rims and surrounding quenched glass. In contrast, modeling the coupled Li peaks, following the common approach used for Li in zircons, yields timescales of <3 minutes. These durations are far too short to represent the time elapsed following crystallization (inferred from P zoning) and are inconsistent with results from other geospeedometers (Fe-Mg, Ni, non-coupled Li). Thus, we suggest that broad zoning of non-coupled Li is suitable for geospeedometry. Enrichment peaks of coupled Li+P should be avoided for timescale modeling. The mixed behavior of trace elements in olivine highlight the need to resolve the disparate interpretations drawn from Li zoning in other silicate minerals such as zircon.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43A..06L
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 3618 Magma chamber processes;
- MINERALOGY AND PETROLOGY;
- 3651 Thermobarometry;
- MINERALOGY AND PETROLOGY;
- 3652 Pressure-temperature-time paths;
- MINERALOGY AND PETROLOGY