Rare Earth Elements in Apatite as a Monitor of Magmatic and Metasomatic Processes: The Ilímaussaq Complex, South Greenland - 2015

Uranium (Nuclear Minerals) / Exploration, Mining, Marketing, & Acquisitions

Zirner, and 4 co-authors, reported in 2015 that textural and compositional variations of apatite from the plutonic Ilímaussaq complex in South Greenland and its surrounding country rocks track magmatic and metasomatic processes. Detailed back-scattered electron (BSE) imaging reveals various types of apatite textures including (i) growth zonation (concentric, oscillatory as well as sector zonation) formed during magmatic differentiation, (ii) resorption and overgrowth textures due to fluid/melt induced metasomatic overprint of precursor apatite and (iii) replacement textures indicating the destabilization of apatite in favor of monazite.

In the least evolved rocks of the Ilímaussaq complex, apatite incorporates rare earth elements and Y (REY) mainly via the coupled substitution Ca²⁺ + P⁵⁺ = REY³⁺ + Si⁴⁺. In the more evolved peralkaline rocks and in metasomatically overprinted rocks, however, the coupled substitution 2 Ca²⁺ = REY3+ + Na+ becomes increasingly relevant, and apatite incorporates successively more LREE compared to HREE as shown by increasing (La/Gd)N and (Gd/Yb)N ratios. Similarly, at the contact between the Ilímaussaq rocks and the granitic country rocks, the metasomatic effect of the emplacement of the Ilímaussaq melts is preserved in partly resorbed precursor apatite, which is overgrown by REY-enriched apatite with higher (La/Gd)N and (Gd/Yb)N ratios compared to apatite from granites more distant from the contact.

The authors concluded that apatite textures and compositions are suitable to track both primary magmatic and later fluid-present metasomatic processes. The incorporation of REY in apatite is not only dependent on the amount of REY present but also depends largely on Na activity in the apatite-precipitating melts and fluids.  Apatite textures and compositions from the Ilímaussaq complex (South Greenland) are a powerful tool in deciphering their magmatic history as well as metasomatic processes taking place over a large temperature and time interval. During magmatic differentiation, apatite incorporates successively more REY. In the least evolved rocks of the complex (augite syenites), REY incorporation in apatite follows mainly the coupled substitution mechanism Ca²⁺ + P⁵⁺ ↔ Si⁴⁺ + REY³⁺. With increasing differentiation, however, the substitution Ca²⁺ ↔ Na⁺ + REY₃⁺ becomes more important such that it is the principal REY incorporating coupled substitution mechanism in the more evolved peralkaline rocks of the complex. Both coupled substitution mechanisms can incorporate similar amounts of REY into the apatite. Hence, not only the absolute REY concentration in the melts influence the REY concentration in apatite, but also the availability of other cations, such as Na and Si, involved in the two dominant coupled substitutions.

In many of the investigated samples, magmatic apatite shows variable overgrowth, replacement and zonation textures, invariably enriched in REY and several other trace elements. These are most likely related to late magmatic to hydrothermal processes that formed during the metasomatic overprint with highly evolved melts and/or fluids.
Very rarely, monazite was found to replace precursor apatite. Based on textural observations, we suggest that the formation of monazite at the expense of apatite is probably a result of fluid-induced alteration of apatite via coupled dissolution–reprecipitation processes.