Engi reported in 2017 that monazite, xenotime, and allanite are REE1-minerals sensu stricto because lanthanides (La…Lu) and yttrium are critical constituents in them. Apatite does not require REE, but because it contains substantial REE in many rocks, it is included in this review. All four minerals also host unusually high radionuclide concentrations, notably Th and U, forming the basis of their utility as geochronometers.
This quartet of accessory minerals is playing an increasingly important role in petrochronology because they provide ways to link robust spot ages to petrogenetic (P–T) conditions so can lend petrogenetic context to chronology based on other minerals. Part I of this review assembles the basic requisites prior to integrative petrochronologic analysis. Individual characteristics of the four REE-minerals are addressed first, i.e., their crystal chemistry and stability relations. Thermobarometers and trace element geochemistry used for tracing petrogenesis are discussed next, and finally their chronology is summarized. Part II presents case studies to highlight the specific strengths of REE-minerals used to resolve the dynamics of a broad range of processes, from diagenetic to magmatic conditions. Finally, a brief section at the end outlines a few of the current challenges and promising perspectives for future work.
To introduce the four REE-minerals in style, let us recall the origins of their names. The three phosphates have well respected Greek grandparents, and allanite has solid Scottish roots, yet of all four of them show idiosyncracies in etymology or type material:
Apatite had long puzzled naturalists, as it shows great chemical and physical variability and can resemble other minerals. Once properly identified, Abraham Gottlieb Werner named it apatite. His reasoning referred to the Greek root á¼€πατá½°ω and giving the precise Latin translation: decipio. Taken literally, both mean “I deceive” or “I mislead”, which sounds like an apt confession from this mineral for having fooled humans so long. It is perhaps appropriate that the original material identified probably was fluorapatite.
Monazite derives from μουάζειυ = to be solitary. This name probably refers to monazite’s sparse occurrence in early discovered locations as well as its tendency to form isolated crystals rather than aggregates. I have been unable to trace this fully, but the now abandoned synonyms eremite (from ερημία = solitude) and kryptolite (= hidden rock) would affirm this interpretation.
Xenotime again has Hellenic roots, but with a twist, as its original name was kenotime. An early copy-paste error led to xenotime. Both names contain the stem τιμή = honor. But κευÏŒς means “vain”, and the vain honor refers to the false initial claim by Jöns Jacob Berzelius that xenotime was a new element, which he then correctly recognized to be a mineral (e.g., Berzelius 1824) but under a different name. The name xenotime, introduced later (Beudant 1832), must be considered a misnomer, for ξÎνος means foreign or odd, making little sense, except that it is odd to honor a mere spelling mistake. The first reported kenotime was from a pegmatite in Hidra, SW Norway.
Allanite is named for Thomas Allen (Thomson 1810), an Edinburgh mineralogist who first singled out the mineral from a load of materials he had acquired, but which had previously been pilfered by English warships (Secher and Johnsen 2008). The original collector was Karl Ludwig Giesecke, and the type locality is Aluk, SE Greenland (Ibler 2010; p.86–87).
CRYSTAL CHEMISTRY AND CONSEQUENCES
Mineral structures and compositions relevant to petrochronology are outlined here. Emphasis is placed on clarifying substitution mechanisms, since this is required to appreciate how these minerals incorporate and fractionate REE, U, Th, and other elements used in chronology and petrology. Which exchange vectors operate is essential also to formulate chemical equilibria that constrain conditions of growth or interaction with other minerals, melt or hydrous fluid. Among the REE three groups are often distinguished, i.e., light, middle, and heavy REE, where LREE: La…Nd, MREE: Sm, Eu, Gd, and HREE: Tb…Lu. Y has an ionic radius within the range of HREE and is usually included with these.
More complete and comprehensive mineralogical accounts include the following: Huminicki and Hawthorne (2002) should be consulted for phosphate structure in general; Ni et al. (1995) for monazite and xenotime, Clavier et al. (2011) for monazite, and Hetherington et al. (2008) for xenotime. For apatite refer to White et al. (2005), Hughes and Rakovan (2002), Pan and Fleet (2002) or, for an elegant overview, Hughes and Rakovan (2015). The
crystal chemistry of allanite is briefly summarized in Armbruster et al. (2006) and with more background in Gieré and Sorensen (2004).
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