Cin-Ty Lee | Geology Reports

USGS2026

The fluorine dichotomy: Amphibole control on fluorine availability during magmatic differentiation

Experimental and observational evidence suggests that water and fluorine (F) are incompatible in silicate minerals and originate from similar sources in the mantle. This, in turn, suggests that F and water should coevolve in magmatic systems. However, hydrous subduction zone (“arc”) magmas are depleted in F relative to anhydrous magmas. We explore this apparent dichotomy in fluorine’s behavior by examining amphibole, which preferentially incorporates F into its crystal structure. By developing a parametrized fluorine amphibole/silicate melt partition coefficient for F, we show that F concentrations in calculated equilibrium melts in arc systems do not increase with differentiation. This indicates that F behaves compatibly in hydrous arc magmas due to sequestration in amphibole-bearing arc cumulates. In dry magmatic systems, where amphibole does not crystallize, we show that F behaves incompatibly during differentiation, resulting in F enrichment in evolved magmas. These results have implications for the formation of ore deposits associated with elements that complex with F, such as rare-earth elements.

Volcanica↗

USGS2024

The chlorine evolution of arc magmas and the crustal water filter

Degassing of water from magmatic systems is key to transporting metals from magmas to form ore deposits, but elements like chlorine, through the formation of anion complexes, can be important in solubilizing and mobilizing these metals into water-rich fluids. Reconstructing the Cl systematics of evolving magmas is thus an important step towards understanding the origins of ore deposits, but the magmatic record is not well preserved because Cl can be lost during degassing. Here, we reconstruct the pre-degassing history of Cl in subduction zone (arc) magmas through amphiboles, which incorporate Cl directly into their crystal structures, preserving pre-eruptive magmatic signatures. Amphibole-reconstructed Cl contents indicate that magmatic differentiation can lead to a 4-fold increase in concentration due to Cl's incompatible behavior. The amphibole-reconstructed Cl contents of arc magmas are also significantly higher than values reported from melt inclusions, suggesting that many melt inclusions may have been trapped after magmas had already lost some Cl. We show that such Cl loss is likely associated with preferential partitioning of Cl into hydrous fluids degassed from the magma during crustal storage or ascent. The extent of Cl depletion can thus be used to estimate how much water was lost during early degassing. If Cl is important to certain ore deposits, magmatic water content may play an indirect role. Magmas too rich in water will lose water and hence Cl at greater depths, rendering such magmas less able to transport metals to the upper crust. By contrast, drier magmas may not produce enough Cl-rich fluids to mobilize metals. Thus, magmas with intermediate water contents may produce enough Cl-rich fluids at the right depths for certain types of ore deposits.

Earth and Planetary Science Letters↗