Biotite would be the first thing to spring to mind. Loads of good examples from experimental and natural studies. Search for fluorine in relation to biotite dehydration melting reactions

See the papers of JL Munoz from the 1970's. He did both lab and field work on fluorine in granitic melts and hydrothermal systems.  This work was instrumental to the understanding of and exploration for molybdenum deposits. He also did work on the role of F in hydrothermal gold transport but I doubt any of that was published.

Peter Modreski did an experimental study of partial melting of fluorine-rich phlogopite, which is more stable to greater depth than OH-bearing amphibole and mica. I think he did that research for his PhD from Pennsylvania State University. 

Flourine-rich magmas commonly are associated with back-arc and rift settings. Maybe the fluorine comes from partial melting of phlogopite in a deeply subducted slab, or from phlogopite in the mantle wedge above the slab, which might have been enriched in F by metasomatism above the slab, during current or previous episodes of subduction.

Donald Burt has also done a lot of work on topaz rhyolites and behavior of fluorine in felsic magmas. See Economic Geology, 1982, v77, p. 1818-1836.

Hi Rongqing,

This is an interesting one, and we've been very interested in this particularly from the perspective of A-type magmas which can have very high halogen (F in particular) contents. We would suggest that generally phlogopite is the main culprit, but depending on the prehistory of the lower crustal protolith amphiboles can be significant contributors as can phosphates including apatite and allanite in granulite terrains.

What this doesn't address however is the A-type liquids that are produced through fractionation from mafic magmas in plume related settings (granted these are often volumetrically minor but can be significant when considering mineralised systems) implying there is potential to source some component of halogens from the mantle- either metasomatised SCLM (again phlogopite could be a good source here) or even from a reservoir within the deeper convecting mantle.

cheers,

Bruce

Fluorine in felsic melts - lets look at the general geochemical background:

Lithophile (Goldschmidt 1958), semi-volatile fluorine is remarkably enriched in mantle volatiles, as evidenced by fluorine contents in phlogopite of kimberlites reaching 8500 ppm. Primitive mantle only contains 13-25 ppm F. Mantle-derived carbonatites and igneous alkali rocks also contain much fluorine, commonly in the form of fluorapatite segregations, which may be large enough to support important mines (e.g. Khibiny, cf. “Phosphate”). With an estimated crustal abundance of 500 ppm (range 270-800: Smith & Huyck 1999) fluorine is the most common trace element. Fluorine is a component of all magmas. Its concentration rises from mafic (400 ppm) to granitic rocks (700 ppm), although actual tenors depend on the individual degassing history of a magmatic body. Pelites display an average of 780 ppm and seawater only 1.3 ppm. In common rock-forming minerals, fluorine (F-) susbstitutes for OH-. Therefore, mica, amphibole, apatite, clay and other hydrous minerals are carriers of fluorine traces. Accordingly in most crustal rocks, fluorine is freely available. This explains why fluorite is a common gangue mineral in hydrothermal ore deposits and the frequency of fluorite deposits.

Although some mantle phlogopite certainly contain elevated concentrations of F, most granites are predominantly or entirely of crustal origin. In such crustal protoliths, irrespective of whether of igneous or sedimentary departure, tri- and dioctahedral micas constitute the main hosts of F. If the melting temperatures were sufficiently high to decompose those micas, significant amounts of F may be uptaken by the produced partial melt. Fractional crystallization may further increase the F concentration of a granitic melt. 

Dear Mr. Rongqing Zhang,

Dear colleagues,

I think all of the answers are correct as far as the source minerals are concerned. Fluorine is present in mica, amphibole, phosphates etc. And even exotic minerals like cryolite have to be added to the list in some cases. I interpret the question in a way a bit different: "Where did the F in a felsic melt under consideration come from". There are two principal pathways which have already been touched. It might be an intracrustal source or a subcrustal source. The minerals fluorite, topaz and F-bearing phyllosilicates on their own hardly disclose their derivation. You need to look at the rare elements and their host minerals associated with F-bearing minerals in the felsic melt. Based upon an ongoing review of such marker elements/ minerals one can conclude, e.g., from an increased Nb/Ta ratio or the presence of columbite and absence of tantalite that a subcrustal source of F in the felsic melt is more likely than vice versa. A similar conclusion can be drawn from the U/Th ratio and the presence or absence of refractory U- and Th minerals in the granite, when it comes to answer the question: "Where did the fluorine come from".

I am not sure if this was the real gist of the question (?).

With my best regards

Harald G. Dill

With my best regards

Harald G. Dill

Hi,

from which lithosphere domain the melt, and consequently the fluorine, was generated is a controversially debated issue. At least for initially F-rich magmas, most authors prefer a crustal origin. Regarding the use of the Nb/Ta ratio and the presence or absence of columbite/tantalite, I'm not convinced that these features may convincingly discriminate crustal from subcrustal magmas. I know a couple of high Nb/Ta granites for which all isotopic evidence supports derivation from a crustal rather than mantle source. 

Dear colleagues,

familiar with the geology of Central Europe, I agree with the comment that fluorine is concentrated in granites of crustal origin, contained in a wide range of micaceous phyllosilicates, even if in this particular Paleozoic setting we cannot cast aside the impact from the mantle. Attempts to constrain the crustal origin of granites by isotope studies of rock-forming minerals is one side of the coin, the concentration of rare elements such as Nb, Ta, F etc. in felsic melts is the other side and quite a different story. The geodynamic environments have to be determined by a wide range of methods from geology to geochemistry. I would like to refer to the "little brethren" of granite, the pegmatite. It is well entrenched in the literature, even if sometimes not properly handled, and used for the classification of pegmatitic melts: NYF (niobium-yttrium-fluorine-type) and LCT (lithium-cesium-tantalum-type) pegmatites are attributed to a mantle and crustal origin, respectively. This is beyond any doubt, because their position in the field well fits into geodynamic setting. Therefore the mineralogical outcome of this chemical approach as columbite and tantalite can also be applied to other felsic melts such as those under discussion.

Isotopes in a stand-alone study will hardly give proper results and I consider such ratios as a supplement to a wide range of geoscientific methods including among others the columbite/tantalite couple. Even the isotope ratios Sm/Nd which often are used for the F system, mainly for fluorite yield results that show the interaction of the mineralizing fluids with the lithology passed through in the course of their ascent. During our isotopes studies we could not discriminate the primary source rocks of the fluids.  The approach taken here is a more statistical one, based upon a proven geodynamic setting, a common host rock, and a special mineral association of accessory minerals which translates into some rare element ratios.

Taking a closer look at the "giant fluorine" concentration or most exotic types, e.g., Amba-Dongar in Gujarat State, India , Okurusu, Namibia , Mountain Pass, USA , Speewah, Australia, Rock Canyon Creek , Canada or the exotic cryolite deposit of Ivigtut, Greenland, will always show us the ultimate mantle source. We cannot ignore deep-seated rift environments in Brazil (mainly topaz), Africa (mainly fluorite and F in REE carbonates), Greenland (cryolite), while we are sitting in Central Europe next to a collisional orogen. Therefore I reiterate the usefulness of such marker elements/minerals which in combination with a sound geological work can be of assistance in getting a bit closer to the answer where the fluorine in felsic melts come from.

Best regards

H. G. Dill

Mantle-derived fluorine in felsic melts, in fluids and in the resulting ore, for example, in the giant W-Sn-F-U-Nb-Ta-SEE and base metal-Mo province in Eastern Asia. Hu & Zhou (2012) write that “helium and argon isotope characteristics of ore fluids suggest that the seemingly S-type parental granites result from mixing of crustal and mantle melts”.

Panasqueira in Portugal is characterized by near-horizontal tungsten ore veins in the apical part of a muscovite-albite leucogranite and in its roof. The vein paragenesis comprises quartz and wolframite, some cassiterite and arsenopyrite, sulphides of Fe, Zn, Cu, and Sn, apatite, siderite, Ca-Mg carbonates and fluorite. Mineralizing fluids were moderately saline and strongly influenced by the organic-rich country rocks. And they exhibit a high mantle component in 3He/4He/36Ar space (Moura et al. 2014).

In recent years, the often surprising role of the mantle in metallogeny is increasingly revealed. Exciting, is it not?

Hu, R.-Zh. & Zhou, M.-F. (2012) Multiple Mesozoic mineralization events in South China – an introduction to the thematic issue. Miner. Deposita 47, 579-588.

Moura, A., Dória, A., Neiva, A.M.R. et al. (2014) Metallogenesis at the Carris W–Mo–Sn deposit (Gerês, Portugal): Constraints from fluid inclusions, mineral geochemistry, Re–Os and He–Ar isotopes. Ore Geology Reviews 56, 73–93

Dear colleagues,

a short note on the source of fluorine in felsic melts. Apart from the solid chemical compounds, including silicates and phosphates, elemental fluorine has been proven only recently (Schmedt auf der Günne, J. , Mangstl, M., Kraus, F. (2012) Angew. Chem., 124, 7968–7971). Fluorine is released from the lattice of fluorite as being affected by radiation damage. Dark blue to black fetid fluorite is the source of elemental or in mineralogical terms "native fluorine", a mineral which to my knowledge has not yet been accepted by IMA. The study was done at Munich University, Germany, by chemists and as such it might have by-passed and not come to mineralogists ´and petrographers´ attention. For a better identification and discrimination between dark blue normal fluorite and dark blue to black fetid fluorite which has partially been deprived of some of its fluorine, I have attached the RAMAN spectra of dfferent multicolored fluorite (DILL, H.G. and WEBER, B. (2010) Accessory minerals of fluorite and their implication concerning the environment of formation (Nabburg-Wölsendorf fluorite district, SE Germany) -With special reference to fetid fluorite (“Stinkspat”).- Ore Geology Reviews 37: 65-86). In granitic systems (hot granites) I suspect this distorted F system of being another source which not has been drawn much attention to so far. It may have a wider appeal than expected (?).

Best regards

H.G.Dill

Remarkable!

Abundances of fluorine in the continental crust and meteorites are concluded to be about 553 ppm and 59 ppm respectively. So that, the average fluorine content for granite is close to 800 ppm, and reachs values between 1 and 7 wt.% in evolved felsic rocks such as granitic pegmatites, Li-F granites and ongonites. Mineralogically, the principal fluorine-containing minerals are biotite, muscovite, amphibole, Li-bearing micas (zinnwaldite, lepidolite), and accessory minerals including cryolite, topaz, fluorapatite, fluorite, sphene, microlite, pyrochlore, tourmaline, bastnäsite, amblygonite, spodumene, etc. Magmas rich in fluorine can be derived from metamorphic rocks where decomposition of biotite and other F-bearing minerals provides fluorine, whereas, the fractional crystallization processes generate magmas enriched with continuously increasing concentrations of fluorine and related volatile mineralizing elements (water, boron, chlorine).

mica and amphibole dehydration melting in both continental crust and mantle