Phosphorus and aluminum zoning in olivine: contrasting behavior of two nominally incompatible trace elements

Abstract

Phosphorus zoning in olivine is receiving considerable attention for its capacity to preserve key information about rates and mechanisms of crystal growth. Its concentration can vary significantly over sub-micron spatial scales and form intricate, snowflake-like patterns that are generally attributed to fast crystal growth. Ostensibly similar aluminum enrichment patterns have also been observed, suggesting comparable incorporation and partitioning behavior for both elements. We perform 1-atm crystallization experiments on a primitive Kīlauea basalt to examine the formation of P and Al zoning as a function of undercooling − ΔT (− ΔT = Tliquidus − Tcrystallization) during olivine growth. After 24 h spent at Tinitial = 1290 °C (10 °C above olivine stability), charges are rapidly cooled to final temperatures Tfinal = 1220-1270 °C, corresponding to undercoolings −ΔT= 10-60 °C (with Tliquidus = 1280 °C). Compositional X-ray maps of experimental olivine reveal that only a small undercool-ing (≤ 25 °C) is required to produce the fine-scale enrichments in P and Al associated with skeletal growth. Concentration profiles indicate that despite qualitatively similar enrichment patterns in olivine, P and Al have contrasting apparent crystal/ melt mass distribution coefficients of = 0.01-1 and = 0.002-0.006. Phosphorus can be enriched by a factor > 40-fold in the same crystal, whereas Al enrichment never exceed factors of 2. Glass in the vicinity of synthetic and natural olivine is usually enriched in Al, but, within analytical uncertainty, not in P. Thus, we find no direct evidence for a compositional boundary layer enriched in P that would suffice to produce P enrichments in natural and synthetic olivine. Numerical models combining growth and diffusion resolve the conditions at which Al-rich boundary layers produce the observed enrichment patterns in olivine. In contrast, the same models fail to reproduce the observed P enrichments, consistent with our observation that P-rich boundary layers are insignificant. If instead, P olivine/melt partitioning is made to depend on growth rate, models adequately reproduce our observations of 40-fold enrichment without boundary layer formation. We surmise that near-partitionless behavior ( close to 1) of P is related to the olivine lattice being perhaps less stiff in accommodating P during rapid crystallization, and/or to enhanced formation of vacancy defects during fast growth. Our results confirm that P is a robust marker of initial rapid growth, but reveal that the undercooling necessary to induce these enrichments is not particularly large. The near-ubiquitous process of magma mixing under volcanoes, for instance, is likely sufficient to induce low-tomoderate degrees of undercooling required for skeletal growth.

Microstructural constraints on magmatic mushes under Kīlauea Volcano, Hawaiʻi

“…First, we test the hypothesis that lattice distortions are produced during an early phase of branching dendritic growth, followed by textural reequilibration during periods of reduced undercooling 5 . Due to a change in olivine-melt partitioning during rapid dendritic growth, high concentrations of phosphorous (P) are incorporated into the crystal structure 39 . While the external morphology of dendritic crystals evolves during subsequent textural reequilibration to obscure this early growth phase, slow diffusion rates preserve internal P enrichments 40 .…”

Forming Olivine Phenocrysts in Basalt: A 3D Characterization of Growth Rates in Laboratory Experiments

“…The present results show that growth rates estimated from 3D can attain these values without necessitating extreme undercooling conditions. A companion paper based on the same experiments (Shea et al, 2019) fully explores the consequences of rapid growth for P and Al, and shows that the boundary layer enrichment model works for certain elements like Al but not for P. Regardless of the peculiar behavior of P, the concentration of other elements is clearly affected by disequilibrium caused by rapid growth, which may even affect Fe-Mg contents in olivine (Figure 7). Our results relax the requirement for unrealistically large undercooling and may pave to way to better understanding the extent to which olivine major and trace element compositions can be controlled by disequilibrium processes.…”

“…It should be noted that aside from differences in experimental volumes and techniques used to estimate growth rates, the experimental strategy employed herein differs from that of Jambon et al (1992) in one major respect: they used closed melt inclusions as experimental vessels for olivine crystallization in order to isolate the process of growth from that of nucleation, while our olivine is allowed to grow "freely" in a much larger volume of melt. This strategy allowed them to explore growth even at high degrees of undercooling (− T = 150 • C), a region FIGURE 1 | (A) Temperature-fO 2 phase equilibria diagram calculated from MELTS using the Kīlauea 1820CE bulk composition at 1 atm (modified from Shea et al, 2019). Gray region shows the range in fO 2 imposed in the experiments.…”

Plagioclase population dynamics and zoning in response to changes in temperature and pressure

“…http://www.minsocam.org/ the coupled CaAl-NaSi substitution, and the slow intracrystalline diffusion of those species prevent compositional homogenization, thus compositional zoning is a proxy for the pressuretemperature-compositional history of a given plagioclase. This is in contrast to minerals with no major element sensitivity to those conditions, or those, like olivine, with fast internal diffusion that removes most major element compositional zoning (Shea et al, 2019).…”

“…http://www.minsocam.org/ Humphreys et al, 2006). Kawamoto (1992) shows that some patchy zoning in plagioclase may record initially skeletal habits, and Shea et al (2019) have shown that some euhedral olivine crystals form through infilling of initially skeletal growth.…”