Xenotime Grains Research Articles

Potential minerals for determining U–Th–Pb chemical age using electron microprobe

After a decade of studies and development, it is now accepted that reliable U–Th–total Pb isochron ages can be calculated for monazite using an electron microprobe at μm scale, either directly on thin sections or on separated grains mounted in polished section. The potential for determining U–Th–Pb chemical ages from other U- and Th-enriched phases has been investigated compared to chemical monazite-dating results for which individual spot-age precisions of 20 to 100 Ma can be achieved from individual spot analyses. Using isochron plots for monazite, the age homogeneity of a given population of data can be assessed and, depending upon the number of analyses ( n ∼ 50), a precision of 5 to 10 Ma can be obtained. The U content in xenotime widely varies from less than 0.1 wt.% up to 3 wt.%, but Th rarely exceeds 1 wt.%. As a consequence, the amount of radiogenic Pb produced during a given period remains significantly lower for xenotime than for monazite, leading to a lower precision (± 20 Ma) on the mean ages. Xenotime, however, appears to remain as a closed system, but common Pb must be carefully checked. Furthermore, the electron-microprobe technique (EPMA) allows controlling any age discrepancy on xenotime grains as small as 10–20 μm that cannot be dated by other isotopic methods. Such xenotime ages can be useful when studying the monazite–xenotime equilibrium. The electron microprobe is not the most reliable method for dating zircon since U and Th concentrations are generally low and common Pb is not negligible. Nevertheless, the spatial resolution of EPMA coupled with isotope methods allows conclusive in situ studies about radiogenic Pb mobility and metamictization. Thorite does not seem suitable for dating with either isotope methods or EPMA because of continuous radiogenic Pb loss. Conversely, the oxide phases, thorianite and baddeleyite are robust minerals with closed systems. They are rather rare and seem to incorporate negligible common Pb, making EPMA a method of choice for dating them. For thorianite, the precision on the mean age can be similar as that obtained for monazite, or even better, while the precision for baddeleyite cannot be significantly better than 20 to 50 Ma due to the limited amount of U (∼ 0.1%) and the lack of Th.

Monazite and xenotime petrogenesis in the contact aureole of the Makhavinekh Lake Pluton, northern Labrador

High-temperature (700–900°C) metamorphism in the contact aureole of the Makhavinekh Lake Pluton (MLP), northern Labrador, led to the growth of monazite and xenotime during progressive replacement of regional garnet-bearing assemblages (M1) by lower-pressure symplectitic coronas of orthopyroxene + cordierite (M2). In the inner aureole (<500 m from the contact), where M1 garnet is strongly resorbed, high-Y+HREE monazite (XY+HREE 0.14–0.18) occurs as small isolated grains and as discontinuous rims on partially resorbed pre-M2 monazites that were liberated from garnet. Xenotime also occurs as small isolated grains within M2 coronas. Ion-microprobe dating of thin, high-Y rims indicates that new monazite growth occurred during M2. Monazite–xenotime miscibility-gap temperatures are consistent with Al-solubility-in-orthopyroxene thermometry estimates, indicating that peak temperatures in the inner aureole are accurately recorded and preserved by monazite. M2 monazite records, therefore, the temperature and timing of M2 metamorphism. Two net-transfer reactions, modelled using singular value decomposition in the system P-Y-HREE-LREE, are proposed to account for the growth of M2 phosphates: (1) 38 Grt1 + 1 Mnz1 = 1.13 Mnz2 and (2) 737 Grt1 + 1 Ap = 1 Mnz2 + 3.4 Xno2. Reaction (1) conserves P and gave rise to locally coronitic high-Y overgrowths on partially resorbed pre-M2 monazite, whereas reaction (2) accounts for the growth of small new monazite and xenotime grains. Both reactions were highly localized within individual M2 coronas due to slow intergranular diffusion accompanying fluid-undersaturated metamorphism in the MLP aureole. Similar monazite-forming reactions are expected in other polymetamorphosed granulites.

SHRIMP U–Pb dating of diagenetic xenotime in the Stirling Range Formation, Western Australia: 1.8 billion year minimum age for the Stirling biota

The Stirling Range Formation in southwestern Australia contains discoidal fossils previously linked to the late Neoproterozoic Ediacaran biota and possible trace fossils interpreted to have been made by vermiform, mucus-producing, motile organisms. The age of the sedimentary succession was recently reported to be between 2.0 and 1.2 billion years old, based on U–Pb geochronology of detrital zircon and metamorphic monazite, respectively. Ion microprobe U–Pb geochronology of xenotime in sandstones from the same succession produces three sets of radiometric data. Detrital xenotime grains yield ages between 3120 and 2100 Ma (consistent with the detrital zircon data; between 3460 and 1960 Ma), whereas compositionally distinct xenotime overgrowths yield a weighted mean 207 Pb/ 206 Pb age of 1800 ± 14 Ma for the oldest population and 1662 ± 15 Ma for a younger population. The new data show that the Stirling Range Formation is more than 1.8 billion years old, some 600 million years older than the minimum defined by metamorphic monazite. The results show that the various megascopic fossils reported in the succession are between 2.0 and 1.8 billion years old, emphasizing their already controversial nature. If our interpretation of the fossils is correct, then motile multicellular (or possibly syncytial) organisms had already emerged in the late Paleoproterozoic, simultaneously with the first large alga-like multicellular eukaryotes.

Relationship between detrital zircon age-spectra and the tectonic evolution of the Late Archaean Witwatersrand Basin, South Africa

Abstract SHRIMP U–Pb detrital zircon age-spectra for 14 samples from the West Rand and Central Rand Groups of the Witwatersrand Basin indicate that felsic source rocks mostly formed at ∼3090–3060 and ∼3000–2870 Ma, with a smaller proportion formed prior to 3100 Ma. Provenance evaluation indicates that source rocks have largely vanished through erosion, such that greenstone terrains extending from the Amalia–Kraaipan Belt in the southwest to the Murchison Belt in the northeast are models of, but not the actual, source areas. Analysis of detrital zircon age-modes reveals that the West Rand and Central Rand Groups record different basins. The West Rand Group is characterised by age-subpopulations with a much-smaller range and less complex distribution than those in the Central Rand Group. Furthermore, detrital zircon age-spectra for the West Rand Group generally decrease in complexity up-section, whereas they become increasingly more-complex in the Central Rand Group. It is concluded that the source area for the West Rand Group is consistent with a passive-margin setting, with the decrease in complexity up-section of zircon age-spectra being the consequence of increasing maturity of the hinterland. Conversely, the source area of the Central Rand Group was far-more dynamic, with an increasing age-range of source rocks being exposed through time. Pulsed tectonic rejuvenation of the source area is implied. Contemporaneous emplacement, uplift and erosion of granitoids supports a magmatic fold-thrust belt as the source, and a complementary retroarc basin setting. Detrital xenotime grains record the ages of high-U granitoids emplaced in the fold-thrust belt during deposition of the Central Rand Group, and are much closer to depositional ages than are detrital zircon grains. Change from a passive margin to a retroarc basin across the West Rand-Central Rand boundary is emphasised by change from simple to increasingly complex detrital zircon age-spectra. This study establishes that detrital zircon geochronology can overcome the problem of overly quartzose compositions in Archaean sandstones due to severe chemical weathering and authigenic alteration, thereby enabling distinction between passive-margin and retroarc-basin stages through systematic changes in the age complexity of the source area.

Geological and SHRIMP II U-Pb constraints on the age and origin of the Breves Cu-Au-(W-Bi-Sn) deposit, Caraj�s, Brazil

The Breves deposit in the Carajas Copper-Gold Belt, Brazil, a member of the Cu-Au-(W-Bi-Sn) group of deposits, contains about 50 Mt of 1.22% Cu, 0.75 g/t Au, 2.4 g/t Ag, 1,200 g/t W, 70 g/t Sn, 175 g/t Mo and 75 g/t Bi. It is hosted by sandstones and siltstones of the Aguas Claras Formation (minimum age of 2,681±5 Ma) in the roof zone of a complex, highly altered granite intrusion. The mineralisation is disseminated in a greisenized zone, resulting from alteration of probable monzogranites and syenogranites. The ore-bearing greisen contains abundant xenomorphic quartz in association with Fe-chlorite and muscovite. The gangue assemblage also includes fluorite, tourmaline, and minor amounts of monazite, xenotime, chlorapatite, thorite, zircon, calcite, siderite and bastnaesite. Copper mineralisation is dominated by chalcopyrite associated with pyrite, arsenopyrite, pyrrhotite and molybdenite. Gold particles, in equilibrium with native bismuth, are common as inclusions in chalcopyrite. The greisen contains sub-economic concentrations of tungsten and niobium that are related to the presence of ferberite, qitianlingite and Nb-rutile. SHRIMP II zircon dating of the host granites gives 207Pb/206Pb ages of 1,878±8 and 1,880±9 Ma for two phases, and a combined age of 1,879±6 Ma. SHRIMP II dating of monazite and xenotime grains in late- to post-mineralisation veins gives a combined 207Pb/206Pb age of 1,872±7 Ma, indistinguishable from the ages of the granites. This provides a genetic connection between the Breves deposit and the ca. 1.88 Ga A-type granite magmatism that typifies the Carajas Belt as part of a much larger, intracratonic magmatic province that extends over much of the Amazonian Craton. The recognition of this association has exploration implications, not only for the geophysical signature of the granite roof zones, but also for likely geochemical dispersion around the deposits of this type.

Fluid-induced nucleation of (Y + REE)-phosphate minerals within apatite: Nature and experiment. Part I. Chlorapatite

This study investigates chlorapatite [ideally Ca10(PO4)6Cl2)] from two spatially separated locations in the Ødegårdens Verk apatite mine, Bamble Sector, SE Norway, which was partially metasomatized under amphibolite-facies conditions. Relative to the original chlorapatite, metasomatized areas in sample DL136 are enriched in OH and F, depleted in Na, Si, and (Y + REE) and contain numerous inclusions (1-15 μm) of monazite and xenotime. In contrast, metasomatized areas in sample TN174 are enriched in F and Si, depleted in Na, show (Y + REE) abundances similar to the original chlorapatite, and host only a few monazite and xenotime grains.

Chemical Th–U–total Pb dating by electron microprobe analysis of monazite, xenotime and zircon from the Archean Napier Complex, East Antarctica: evidence for ultra-high-temperature metamorphism at 2400 Ma

The chemical Th–U–total Pb isochron method (CHIME) of dating was carried out on monazite, xenotime, zircon, and polycrase in six samples of ultra-high-temperature (UHT) granulites and one of pegmatite from five localities in the Napier Complex, Enderby Land, East Antarctica. Despite the chronological heterogeneity in many grains, the CHIME ages overall show strong evidence for a major event near 2420 Ma. Analyzed monazite and xenotime grains, except monazite from Mt. Riiser-Larsen, have a narrow range of CHIME ages: 2412±42 Ma from Beaver Island and 2429±36, 2421±18 and 2431±22 Ma from Reference Peak. These ages are indistinguishable from CHIME ages on zircon (2436±17 and 2420±20 Ma) and on central older domains of monazite (2404±54 Ma) from Mt. Riiser-Larsen, and on a zircon rim (2430±15 Ma) from Mt. Cronus. We conclude that the ca. 2420 Ma ages relate to the UHT metamorphism, but we do not exclude the possibility that lead loss has lowered the ages by some 50 Ma. This interpretation is consistent with a preponderance of ages obtained by other methods on the Napier Complex particularly the ca. 2475±25 Ma age for a generation of pegmatites coeval with the UHT event and containing the UHT mineral assemblage beryllian sapphirine–khmaralite+quartz. Events prior to 2500 Ma and after 2420 Ma have also left their imprint on the CHIME ages, for example, a 3646 Ma CHIME age suggests that a zircon core is inherited, whereas ca. 2200 and 2000 Ma CHIME ages in zoned monazite could be due to further lead loss during post-UHT activity. The much younger CHIME age of 1094±67 Ma for monazite from a pegmatite at ‘Zircon Point’, Casey Bay, confirms 1000–1100 Ma lead loss ages reported for this region, and could result from fluid activity in an outlier of Rayner activity near the boundary between the Rayner and Napier Complexes. Approximately 2500 Ma ages have been reported from granulite-facies (but not UHT) rocks in the Madras and Nilgiri blocks in South India; these blocks and the Napier Complex could have constituted a single structural unit by late Archean time.

Kinetics of chlorination and microstructural changes of xenotime by carbon tetrachloride

Among the rare earth minerals, fluorides, phosphates, and oxides have received attention from the rare earth industry. Traditional methods of decomposition of these minerals, usually alkaline or acid processes, involve several operations. Another possibility to obtain lanthanide chlorides or oxychlorides is reacting the mineral with chlorinating agents, such as gaseous chlorine, hydrogen chloride, thionyl chloride, and carbon tetrachloride, reducing the operation costs and making the process less complicated. In this context, we investigated the decomposition of xenotime using carbon tetrachloride at temperatures from 873 to 1173 K and kinetic and mechanistic studies have been performed. Powder X-ray diffraction, scanning electronic microscopy, energy dispersive X-ray spectrometry, ultra-violet/visible spectroscopy, and thermal analysis techniques were used in this study. The results showed that the reaction follows the shrinking-unreacted-core model with formation of a product layer (lanthanide oxychloride), confirmed by powder X-ray diffraction. Moreover, microstructural changes of xenotime grains during the chlorination reaction were verified.

Monazite–xenotime thermochronometry: methodology and an example from the Nepalese Himalaya

Monazite–xenotime thermochronometry involves the integration of petrographic, geochronological, and geochemical techniques to explore the thermal evolution of igneous and metamorphic rocks containing these accessory minerals. The method is illustrated in this paper by application to an orthogneiss sample from the Everest region of the Nepalese Himalaya that contains leucogranitic segregations produced by in-situ anatexis. Observations of phase relationships and the internal structure of accessory minerals made using both transmitted light and electron microscopy revealed the existence of multiple generations of monazite and xenotime and guided microsampling efforts to isolate grain fragments of Himalayan (Tertiary) and pre-Himalayan age. Nearly concordant U–Pb isotopic ratios for 13 single monazite and xenotime grains ranged in age from 28.37 to 17.598 Ma, making determination of the timing of anatexis difficult without additional information. Presuming that monazite and xenotime were in equilibrium over that entire interval, temperatures estimated from the yttrium contents of dated monazites range from 677–535 °C. Only the highest temperatures are consistent with experimental constraints on the conditions necessary to produce anatectic melts of appropriate composition, implying that the ~25.4–24.8 Ma dates for the grains with high apparent equilibration temperatures provide the best estimates for the age of anatexis. Two monazite crystals yielded 207Pb/235U dates that are statistically indistinguishable from the 207Pb/235U dates of coexisting xenotime crystals, permitting the application of both quantitative Y-partitioning and semi-quantitative Nd-partitioning thermometers as a cross-check for internal consistency. One of these sub-populations of accessory minerals, with a mean 207Pb/235U date of 22.364±0.097 Ma, provides inconsistent Y-partitioning (641±39 °C) and Nd-partitioning (515–560 °C) temperatures. We suspect the discrepancy may be caused by the high Th concentration (6.12 wt% ThO2) in this subpopulation's monazite. The Y-partitioning thermometer was derived from experimental data for the (Ce, Y)PO4 binary and may be inappropriate for application to high-Th monazites. For the other subpopulation (mean 207Pb/235U date=22.11±0.22 Ma), the Y- and Nd-partitioning temperatures are indistinguishable: 535±49 and 525–550 °C, respectively. This consistency strongly suggests that the sample experienced a temperature of ~535 °C at 22.11 Ma. This finding is tectonically important because temperatures at higher structural levels were much higher (by ~100 °C) at the same time, lending support to earlier suggestions of a major structural discontinuity within the upper part of the Himalayan metamorphic core at this longitude. An additional finding of uncertain importance is that inherited monazite and xenotime yielded U–Pb discordia with indistinguishable upper intercept ages (465.5±8.7 and 470±11 Ma, respectively) and application of the Y-partitioning thermometer to the inherited monazites produced a restricted range of model temperatures averaging 470 °C. Whether or not these temperatures are geologically meaningful is unclear without independent corroboration of the assumption of equilibrium between the inherited monazites and xenotimes, but it appears that monazite–xenotime thermochronometry may be useful for "seeing through" high-grade metamorphism to extract temperature-time information about inherited mineral suites.

The chemical composition of REE-Y-Th-U-rich accessory minerals in peraluminous granites of the Erzgebirge-Fichtelgebirge region, Germany; Part II, Xenotime

Other| December 01, 1998 The chemical composition of REE-Y-Th-U-rich accessory minerals in peraluminous granites of the Erzgebirge-Fichtelgebirge region, Germany; Part II, Xenotime Hans-Juergen Foerster Hans-Juergen Foerster GeoForschungsZentrum Potsdam, Potsdam, Federal Republic of Germany Search for other works by this author on: GSW Google Scholar American Mineralogist (1998) 83 (11-12_Part_1): 1302–1315. https://doi.org/10.2138/am-1998-11-1219 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Hans-Juergen Foerster; The chemical composition of REE-Y-Th-U-rich accessory minerals in peraluminous granites of the Erzgebirge-Fichtelgebirge region, Germany; Part II, Xenotime. American Mineralogist 1998;; 83 (11-12_Part_1): 1302–1315. doi: https://doi.org/10.2138/am-1998-11-1219 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Xenotime, from a geochemically heterogeneous series of mildly to strongly peraluminous granites of the Erzgebirge, Germany, displays extended compositional variability with respect to abundances of HREE, Y, U, and Th. With few exceptions, the maximum and minimum concentrations for the lanthanides and actinides exceed those noted for this mineral from other geologic environments. Xenotime chemistry is dominated by the theoretical end-members HREE-PO 4 and YPO 4 (>90 mol% in total). Typical xenotime grains from the Erzgebirge granites and from other granites worldwide contain 70-80 mol% YPO 4 , and 16-25 mol% HREE-PO 4 . This study documents the occurrence of xenotime, abnormally rich in HREE in place of Y, with up to 45 mol% HREE-PO 4 . Substitutions of thorite-coffinite, (Th,U,Pb)SiO 4 , brabantite, (Ca,Th,U)(PO 4 ) 2 , and monazite, (La-Sm)PO 4 , are of minor importance, reaching maximum levels of 5-6 mol% each with a total contribution typically <<10 mol%. Typically, the incorporation of actinides in xenotime from these and other granites, as well as from other rocks, is dominated by thorite-coffinite substitutions. However, in some xenotime grains, the U and Th concentrations are largely accounted for by the substitution mechanism 2 (REE,Y) (super 3+) <-->(Th,U) (super 4+) +Ca (super 2+) . The total lanthanide and actinide contents in xenotime and host granite are not strongly correlated. However, formation of xenotime unusually rich in HREE occurs only in the A-type Limica granites that are strongly enriched in these elements. The shapes of HREE patterns of xenotime and host rock are similar which, combined with mass-balance calculations, indicate the importance of this mineral in affecting the HREE as well as Y evolution of late-stage granitic melts. Late xenotime may show strong Y/Ho fractionation, thus recording the decoupling of Y and the HREE, which can occur in the latest stages of crystallization of evolved granites and which may be associated with deuteric alteration. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Okuhinotsuchi granitic mass in the South Kitakami terrane: pre-Silurian basement or Permian intrusives.

Chemical Th-U-total Pb isochron (CHIME) ages were determined on monazite, xenotime and zircon from coarse-grained leucogranite, quartzose pegmatite and residual arkose (weathered granite) of the Okuhinotsuchi granitic mass (Okuhinotsuchi Mass) in the South Kitakami terrane, Northeast Japan. Leucogranite, characterized by higher K2O/Na2O and lower CaO/(Na2O+K2O) ratios, yields latest Permian ages of 250-260 Ma for monazite and/or zircon. Some of the granite contains xenocrysts of Th-rich monazite or zircon that yields late Silurian to early Devonian ages of 400-410 Ma. Quartzose pegmatite or quartz vein, cutting in places leucogranite and rarely sediments of the Lower-Middle Silurian Okuhinotsuchi Formation, yields unequivocal CHIME ages of 240-250 Ma for Th-poor monazite, zircon and/or xenotime. Residual arkose that occurs immediately below the unconformity, is more disintegrated, altered and depleted in K2O than leucogranite. The arkose contains Th- and U-bearing minerals with different CHIME ages: (1) both Th-rich (420 Ma) and Th-poor (260 Ma) monazites, (2) zoned xenotime grains with 410-420 Ma core mantled by 350 or 250 Ma rim, (3) euhedral ca. 250 Ma zircon, or (4) rounded inherited zircon grains of ca. 500-1000 Ma. All the CHIME age data demonstrate that the Okuhinotsuchi Mass is unlikely to be the pre-Silurian basement. The CHIME geochronology also reveals that the leucogranite of the Okuhinotsuchi Mass is a remnant of the source rock for some Permian granitoid clasts in the Usuginu Conglomerate in the South Kitakami terrane, whose provenance has long been an open problem.

Provenance and U-Pb geochronology of the Mesoproterozoic Belt Supergroup (northwestern United States): implications for age of deposition and pre-Panthalassa plate reconstructions

The Mesoproterozoic Belt-Purcell Supergroup is a thick succession of siliciclastic and carbonate sedimentary rocks in the northwestern United States and adjacent Canada. Recent stratigraphic studies provide strong evidence for an enclosed basin setting with a tectonically active western margin. The age of the western (present coordinates) landmass is unknown, but has been inferred to be different from North American basement that borders the Belt basin on the basis of whole rock Sm-Nd studies. We report U-Pb isotopic analyses of individual detrital zircon, monazite, and xenotime grains separated from westerly derived clastic units in the Belt Supergroup as well as Sm-Nd isotopic data for the same grains where appropriate. These data provide new constraints on the age and isotopic character of the western landmass. Most U-Pb data are concordant or slightly discordant; exceptions are noted below. The Revett Formation (Ravalli Group, lower Belt Supergroup) contains grains with concordant ages of 1590–1600 Ma and a grain with a discordant 207Pb/ 206Pb age of 1780 Ma: the ca. 1600 Ma age is very unusual in North America and effectively dates “Belt Island”, long postulated to be the Revett source in what are now terranes of southeast Washington and northeast Oregon that were accreted in the Mesozoic. Detrital zircons from the Missoula Group (Bonner and Mount Shields formations; upper Belt Supergroup) form a distinct group at 1670–1859 Ma with two ca. 2.6 Ga grains. U-Pb ages from monazite range from 1642 to 1786 Ma, and together with Nd isotopic data ( T DM model ages of 2.05–3.36 Ga) suggest variable interaction with previously differentiated crust. The Buffalo Hump Formation in northeast Washington may correlate with the Bonner Formation or may lie unconformably on the Belt Supergroup; it contains detrital zircons of ca. 1840 Ma and, surprisingly young, 1070–1244 Ma (several grains). Monazites have U-Pb ages of 1753–1774 Ma and Nd signatures similar to the Bonner Formation data. Several major conclusions can be drawn from these data. The ages of 1590–1600 Ma have no known possible source within western North America (although 1576 Ma augen gneiss is present in the Priest River Complex northwest of the Belt basin), and the 1642–1786 Ma ages are uncommon to rare in Precambrian rocks to the immediate south and east; together these data support previous Sm-Nd whole rock and stratigraphic conclusions of a dominantly western source for the Belt basin. The ages in the Buffalo Hump Formation can be interpreted in two ways. Either the Buffalo Hump correlates with the Bonner Formation and therefore the top of the Belt Supergroup is substantially younger (< 1070 Ma) than previously inferred (1250 Ma) and may overlap with Grenville-age tectonic activity. Alternatively, the Buffalo Hump Formation may represent a new unit that is younger than the Belt. Irrespective of this issue, these data help to constrain proposed pre-Paleozoic plate reconstructions; from the 1070–1244, 1590–1600 and 1642–1786 Ma source ages and related Nd data, we suggest that the basement blocks of south-central Australia (Gawler, Musgrave, Willyama, Arunta and possibly Mount Isa) were joined to the western side of Laurentia, adjacent to the Belt basin, prior to Neoproterozoic formation of the Panthalassa Ocean.