Fluorine and Chlorine in Alkaline Magmas
This is a collaborative project with Dr. Norman Charnley, University of Oxford. We are using the electron microprobe at Oxford to determine the fluorine and chlorine content of micas, amphiboles, apatites, and titanites from various alkaline provinces. To date we have studied the Monteregian Hills, Quebec (Canada) and White Mountain, New Hampshire (USA) alkaline provinces, the Beemerville alkaline complex, New Jersey (USA), the Arkansas Alkaline Province, Arkansas (USA), the Archean A-type granites of the Kola Peninsula (Russia), and the Chilwa province (Malawi). Preliminary analysis has indicated a complex relationship between the evolution of the magmatic systems, the sources, and the presence/absence of other halogen-containing minerals. We are in the early stages of data analysis. As results and models are developed they will appear on this page and in the literature.
Eby, G. N. and Charnley, N. (2010) Fluorine and chlorine in alkaline rocks and A-type granites. In Ramo, O. T., Lukkari, S. R., and Heinonen, A.P. (eds.) 2010. International Conference on A-type Granites and Related Rocks through Time (IGCP-510). Helsinki, Finland, August 18-20, 2010. Abstract Volume 26-28.
Eby, G.N. and Charnley, N.
The role of halogens, particularly fluorine, in the
petrogenesis of A-type granites and related rocks has been a subject of
continuing discussion in the petrologic literature. The determination of the
halogen content of a magma is a significant challenge. Whole-rock F and Cl
concentrations, even for fine-grained rocks, may not represent the halogen
content of the magma. Melt inclusions, when available, can yield the halogen
content for a magma at a particular point in its evolution. Halogen containing
minerals, most notably biotite and apatite, have been used to estimate the HF
and HCl fugacity (in the case of biotite) or directly the F and Cl melt
concentrations (using apatite). Here we report the preliminary results from an
ongoing study of the halogen content of magmas using the F and Cl content of
biotite, apatite, and amphibole to estimate the F and Cl content of the
Determination of F and Cl in
In minerals, F and Cl are generally determined using the electron microprobe (EMP).
Particularly in the case of F, such determinations are far from routine. We have
investigated the analytical variables and have come to the following
conclusions: (1) F and Cl should be determined using a defocused beam (5 to 10µm)
and a low accelerating potential, (2) standardization should be done on a
F-containing mineral that is not sensitive to counting times (we use fluorite),
(3) F and Cl should be determined at the start of an analytical sequence, (4)
the calculated values are sensitive to the correction routine used to reduce the
data – ZAF vs PAP vs etc. and (5) a F-standard should be interspersed with
unknowns to measure the variability of the F determination. For measurements
made on the same EMP, a reasonable analytical error for F is +/- 10% relative
and for Cl +/- 5% relative at concentrations several standard deviations above
the detection limit.
Estimating F and Cl melt
concentrations from mineral chemistry. Based on experimental results,
Munoz (1984, 1992) published a set of equations that could be used to estimate
the HF and HCl fugacity of vapor and melt co-existing with biotite. The HF and
HCl fugacities can be converted to concentrations using the algorithms of
Piccoli and Candela (1994). Icenhower and London (1997) experimentally
determined F and Cl partition coefficients for biotite/melt equilibria at 640 to
680oC. The Munoz (1992) and Icenhower and London (1997) models yield
similar results for biotites with mg# = 60 to 20. For mg#s <20, there is a
significant discrepancy between F values calculated by the two models. The F
algorithm of Icenhower and London (1997) was not calibrated for mg#s <20 and
this model should not be used for biotites with mg#s <20. In order to have
consistent results, in our investigations we have used the model of Munoz (1984,
1992). For Cl, the two models yield similar results. In the case of F the
calculations are sensitive to temperature and an independent temperature
estimate is required. The Cl calculation is sensitive to
both temperature and pressure. For apatite we use the most recent results
of Webster et al. (2009).
Currently there is no model relating the distribution of
F and Cl between amphibole and melt. We have developed an empirical model for F
partitioning between amphibole and melt. Melt F
concentrations are estimated using the measured concentrations of F in apatite
co-existing with amphibole. Based on these measurements we propose that the
following (preliminary) equations can be used to estimate the F concentrations
of a melt. The calculation was done for amphibole-apatite
pairs that are inferred to have co-crystallized between 950 and 850oC.
DFamph/Fmelt = 1.03 + 0.06*mg#
DFamph/Fmelt = 1.03 + 0.06*mg#
The possible effect of pressure is unknown.
The possible effect of pressure is unknown.
Chilwa alkaline province.
The Chilwa alkaline province is located in Malawi near the southern end of the
modern-day East African rift system. The province is lithologically diverse and
includes carbonatite, basanites and nephelinites, nepheline-sodalite syenites,
nepheline syenites, syenites, and granites. All these rocks were emplaced
between ca 130 and 120 Ma and are apparently petrogenetically related. The
Chilwa granites are classified as A1 granites (Eby, 1992) and were
presumably emplaced in an anorogenic setting. We have analyzed apatites,
biotites, and amphiboles from all of the rock-types except the carbonatite,
phonolites, and nephelinite/basanites.
Whole-rock F and Cl concentrations are variable. The
nephelinites/basanites are uniformly very low in Cl (near or below detection)
and have up to 0.6 wt.% F. The more evolved silicate rocks show a range in both
F and Cl concentrations of up to 0.8 wt.% F and 0.7 wt.% Cl. Cl/F ratios >1
are found for a number of the silica undersaturated felsic rocks (and some of
these do contain sodalite). For the syenites and granites F/Cl ratios are
usually less than 1. In the granites and syenites whole-rock F ranges between
0.1 and 0.4 wt.% and whole-rock Cl is usually less than 0.1 wt.%.
The apatites are all fluorapatites and Cl is at or below
the detection limit. To date, all calculated F-melt values fall in a restricted
range between 0.248 and 0.303 wt.%. On petrographic grounds apatite is an early
crystallizing mineral and is found as inclusions in feldspar and the mafic
silicates. There is no apparent relationship between the paragenetic association
and the F content. We conclude that the apatites are recording F content through
much of the solidification history of the melt and that there is little
variation in F. In almost all cases, the calculated melt F concentration is
greater than the F content determined in the whole-rock analysis.
The biotites present a somewhat different picture.
Calculated F melt concentrations range from 0.1 to 1.0 wt.%. In almost all cases
these values exceed the measured whole-rock concentrations. They are also
generally greater than the F melt concentrations determined from the apatite
chemistry. The biotites appear relatively late in the crystallization history of
the magmas and are thus recording F values for the residual melts. Calculated Cl
melt concentrations range from 0.02 to 0.2 wt.%. In most cases these values are
less than the measured whole-rock Cl concentrations. This observation leads to
the conclusion that Cl is a late, probably of hydrothermal origin, addition to
the rock, and that the sodalite found in some of the silica undersaturated
syenites is of hydrothermal origin.
Taken in total, for this A-type granite province,
magmatic F content is in the range of 0.2 to 0.4 wt.%, only becoming more
concentrated near the end stage of crystallization, and the magmatic Cl content
is generally less than 0.1 wt.%.
with Argentina A2 granites.
Dalhquist et al. (2010) recently reported biotite chemistry and calculated F and
Cl concentrations for A2-type granites from western Argentina. We
have obtained additional data for biotites from associated granite bodies. In
all cases, biotite is the principal halogen containing phase and appears to have
crystallized through much of the solidification range. All these granites have a
largely crustal source with a minor input of mantle material. For the San Blas
pluton, which contains late stage mineralization, F melt = 0.27 to 0.54 wt.% and
Cl melt = 0.03 to 0.11 wt.%. For the Achala batholith, which has associated U
mineralization, F melt = 0.13 to 0.33 wt.% and Cl melt = 0.02 to 0.06 wt.%.
These values are in the same range recorded for the A1-type granites
of the Chilwa alkaline province. At least in terms of this limited data set,
there seems to be no difference in the F and Cl content of A1 and A2
granites. This observation does call into question one of the assumed
characteristics of A-type granites derived from the crust, i.e., high F content.
Dalhquist et al. (2010) do report that other (none A-type) granites in the same
province have lower F contents. Hence, at least at the province level there is a
distinction between A-type and none A-type granites.
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G. N., 1992. Chemical subdivision of the A-type
granitoids: petrogenetic and tectonic implications. Geology 20, 641-644.
J., London, D., 1997. Partitioning of fluorine and chlorine between biotite and
granitic melt: experimental calibration at 200 MPa H2O. Contributions to
Mineralogy and Petrology 127, 17-29.
J. L., 1984. F-OH and Cl-OH exchange in micas with applications to hydrothermal
ore deposits. In: Bailey, S. W. (Ed.) Reviews in Mineralogy, Micas, vol. 13.
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J.L., 1992. Calculation of HF and HCl fugacities from biotite compositions:
revised equations. Geological Society of America, Abstracts with Programs 24,
P., Candela, P., 1994. Apatite in felsic rocks: a model for the estimation of
initial halogen concentrations in the Bishop Tuff (Long Valley) and Tuolumne
intrusive suite (Sierra Nevada Batholith) magmas. American Journal of Science
J. D., Tappen, C. M.., Mandeville, C. W., 2009. Partitioning behavior of
chlorine and fluorine in the system apatite-melt-fluid. II: Felsic silicate
systems at 200 MPa. Geochimica et Cosmochimica Acta 73, 559-581.
Red text represents modifications made to the meeting abstract.