In short, the accumulation of zircon in biotite is inherent and based on the geochemical behaviour of Zr in a silicate melt. Zirconium is a fairly typical incompatible (HFS) element and therefore accumulates in the final components of magma crystallization along with biotite. Even potassium is incompatible to some extent (potassium is LILE).

Thank you, Dr. Matýsek. Of course, potassium and zirconium are highly incompatible, if mafic magmas are considered. But I meant silicic systems where Zr behavior is not much incompatible because of early crystallization of zircon. In many sparsely phyric rhyolites, we may observe zircon crystals which crystallize fairly close to liquidus temperature. And biotite may also be an early phase, too.

I suspect the reason of this zircon+biotite association might be a bit more sophisticated. For instance, there could be some preferential nucleation of zircon at the surface of a biotite crystal. But I never heard any speculations on these matters, except the statement that zircon indeed forms abundant inclusions in biotite.

Best regards,

Petr

A possible factor is the solubility of Zr in alkaline/saline fluids which similarly would promote/facilitate crystallisation of a hydrous phase like biotite.

Hi Petr,

The few workers who have commented on included zircon in igneous biotite have tended to invoke the mechanism initially quantified by Green & Watson (1982) for apatite inclusions in plagioclase and pyroxene. Low melt diffusivity of the included phase's essential structural constituent(s) (ESC's), combined with a very low partition coefficient of the included phase's ESCs in the growing host mineral, results in a locally saturated boundary layer surrounding the host phase and promotes heterogenous nucleation of the included phase.

Bacon (1989) generalized this process to a discussion of a wide range of accessory inclusions (including zircon). He argued for a reduction in included-phase diffusivity in a chemical boundary layer (CBL) around the growing phenocryst. CBLs form by depletion of the host ESC's and other components with KD>1. He notes that ferromagnesians such as biotite in silicic magmas would be particularly prone to host inclusions via this mechanism by enriching the CBL in silica, further reducing diffusivity and promoting local saturation of the included phase.

Elburg (1996) commented: "The preferential inclusion of zircon in biotite appears to be a common phenomenon in granites, and is thought to be the result of local saturation adjacent to growing crystals"

By contrast, Clemens (2003) wrote: "Restitic and magmatic zircons commonly act as heterogeneous nuclei for the precipitation of early magmatic crystals of mafic silicates such as pyroxenes, amphiboles and biotite. This is why so many zircons in granitic rocks are found included in biotite crystals, for example". Clemens et al (2017) also state that "the abundance of zircon inclusions in biotite" is characteristic of peraluminous S-type granites.

Hope this helps.

Bacon, Charles R. "Crystallization of accessory phases in magmas by local saturation adjacent to phenocrysts." Geochimica et Cosmochimica Acta 53.5 (1989): 1055-1066

Green, T. H., and E. B. Watson. "Crystallization of apatite in natural magmas under high pressure, hydrous conditions, with particular reference to ‘orogenic’rock series." Contributions to Mineralogy and Petrology 79.1 (1982): 96-105

Elburg, Marlina Augusta. "U-Pb ages and morphologies of zircon in microgranitoid enclaves and peraluminous host granite: evidence for magma mingling." Contributions to Mineralogy and Petrology 123.2 (1996): 177-189

Clemens, J. D. "S-type granitic magmas—petrogenetic issues, models and evidence." Earth-Science Reviews 61.1-2 (2003): 1-18.

Clemens, J. D., I. S. Buick, and A. F. M. Kisters. "The Donkerhuk batholith, Namibia: A giant S-type granite emplaced in the mid crust, in a fore-arc setting." Journal of the Geological Society 174.1 (2017): 157-169

Hi Graham,

Thank you so much! This model is quite convincing, and it explains many other similar cases.

Best regards,

Petr

Why is the presence of inclusions interpreted as evidence of co-crystallization?

Cannot the zircon be (much) older?

We found in Schwindinger et al. (2020, J.Pet.) that Zr was being lost by melt in migmatites during crystallization, by growth of zircon within the biotite-rich melanosomes. We did not speculate why this was the case other than it seem that biotite seems to be an appropriate substrate for heterogeneous nucleation of biotite.

Hi Bernardo,

If zircon is older than biotite, we must explain why growing biotite accumulates previously formed zircon crystals. Could you suggest any publications which describe this mechanism?

Hi Roberto,

Thank you, I also like this idea. However, it would be pertinent to check it experimentally. Also, melanosomes are frequently considered as a kind of a solid residue, not a direct product of crystallization from melt. The factors which affect the distribution of mineral grains in solid state reactions would likely be somewhat different from those of solid-melt systems.

Petr Tikhomirov You can definitely find examples of minerals displaying different behaviour with respect to trapping of pre-existing accessory phases. I wouldn't be surprised that one phase has more inclusions of a pre-existing mineral than another in the same rock. I will look for examples.

Thanks Bernardo, I would be grateful if you provide the appropriate references. Though, the hypothesis of local saturation in CBL looks very attractive, and it may be applied to many other cases. For example, zircon hosted in silicic volcanics frequently contains apatite inclusions. It would be difficult to imagine that acicular apatite (crystallized prior to zircon and evenly distributed in magma) was trapped by growng zircon, as both phases are not much abundant.

A chicken and the egg type of question. Although, the question applications are far more interesting than they seem since zircon is becoming more important in exploration and clearly the most suitable heavy mineral for the task as well. You should split the question depending on the zircon type (checks Corfu's paper and all references to hydrothermal origins). If you have SEM-CL, you will be able to narrow down and create your own literature to be more specific. The ideas wont come to you by "osmosis" from reading 100..00 papers.

Hi Leonid,

Of course, both zircon and biotite commonly crystallize at relatively late stages of evolution of mafic / intermediate magmas, and this may result in their spatial assemblage. I was somewhat puzzled by cases when biotite and zircon (and some other accessory phases as well) are spatially related in silicic systems with early biotite precipitation.

Zircon inclusion in biotite could indicate later crystallization of biotite nucleating earlier zircon grains. This is true for minerals in diverse igneous systems. However, if they are co-precipitating, there must also be other textural associations rather than only inclusion and host relationships.

Please correct me if I am wrong.