Search

Abstract: Monazite growth and composition has been modeled in the system K2O–Na2O–CaO–MnO–MgO–FeO–Al2O3–SiO2–H2O–Y2O3–Ce2O3–P2O5–F and a pseudosection is presented that is contoured for monazite modal amount and YPO4 composition, garnet amount and YAG composition, and xenotime amount. Monazite amount and composition is strongly controlled by the presence of xenotime and when xenotime is absent, by the growth or consumption of garnet. Monazite grows continuously in the subsolidus region over most reasonable metamorphic P–T paths with YPO4 content generally increasing with increasing temperature, especially where xenotime is present. When melt is present, monazite is consumed with increasing melting and, in the absence of xenotime, decreases in YPO4 content. Two prograde forward models are presented to illustrate the effect of garnet fractional crystallization on the results. When xenotime is present and the modal amount of garnet is low (e.g., at low pressure), fractional crystallization has little effect on phase amount or composition. Where xenotime is absent and garnet is more abundant (e.g., at intermediate to high pressure), garnet fractionation dominates both monazite growth history and composition. In particular, YPO4 content of monazite drops to nearly zero at intermediate grades in a rock in which garnet has sequestered the Y as YAG component. This Y may be released by diffusion at higher temperature and promote monazite growth with a higher YPO4. Therefore, the growth history of monazite as inferred from YPO4 zoning may be quite complex and reflect different reactions and processes, even in a simple bulk composition. [Copyright &y& Elsevier]

Abstract: Instrumental and spectral characteristics germane to chemical dating of monazite have been tested using the Cameca SX-100 at Rensselaer Polytechnic Institute. Statistical analysis demonstrates that, for trace element analysis, equal counting time on peak and background is required for optimal statistical precision, thus rendering impractical the procedure of fitting the entire spectrum to obtain background values. Energy shifts require shifting the detector voltage window between peak and background positions, and it is concluded that the differential auto PHA mode works optimally for this. Analyses of Pb-free phosphates, silicates, and oxides are used to measure spectral interferences with the PbMα peak and background positions. Backgrounds were modeled using both linear and exponential fits. It was found that the difference in background counts using the two fits varies with each of the five spectrometers examined, and that the high-pressure (3bar) detectors show larger differences in exponential vs. linear peak-minus-background (P-B) values than the low-pressure (1bar) detectors. In addition, every spectrometer requires a unique correction for every major element in monazite. An analytical protocol is presented that incorporates these results. This protocol was applied to several monazite standards to determine inter-spectrometer variability, and spectrometer reproducibility from session to session. It was found that the difference in composition (and age) between spectrometers on identical spots exceeds the 2 sigma standard error of the mean of composition (or age) on either spectrometer. This means that (a) additional sources of error beyond the counting statistics exist between spectrometers; (b) the precision of microprobe ages cannot be continuously improved by additional counting; and (c) the minimum realistic precision is on the order of ±2–3% for monazites with around 1500–2000ppm total Pb, or an additional absolute uncertainty of 20–50ppm Pb. [Copyright &y& Elsevier]

Monazite crystallization ages have been measured in situ using SIMS and EMP analysis of samples from the Bronson Hill anticlinorium in central New England. In west-central New Hampshire, each major tectonic unit (nappe) displays a distinctive P–T path and metamorphic history that requires significant post-metamorphic faulting to place them in their current juxtaposition, and monazite ages were determined to constrain the timing of metamorphism and nappe assembly. Monazite ages from the low-pressure, high-temperature Fall Mountain nappe range from c. 455 to 355 Ma, and Y zoning indicates that these ages comprise three to four distinct age domains, similar to that found in the overlying Chesham Pond nappe. The underlying Skitchewaug nappe contains monazite ages that range from c. 417 to 307 Ma. 40Ar/39Ar ages indicate rapid cooling of the Chesham Pond and Fall Mountain nappes after 350 Ma, which is believed to represent the time of emplacement of the high-level Chesham Pond and Fall Mountain nappes onto rocks of the underlying Skitchewaug nappe. Garnet zone rocks from western New Hampshire contain monazite that display a range of ages ( c. 430–340 Ma). Both the metamorphic style and monazite ages suggest that the low-grade belt in western New Hampshire is continuous with the Vermont sequence to the west. Rocks of the Big Staurolite nappe in western New Hampshire contain monazite that crystallized between c. 370 and 290 Ma and the same unit along strike in northern New Hampshire and central Connecticut records ages of c. 257–300 Ma. Conspicuously absent from this nappe are the older age populations that are found in both the overlying nappes and underlying garnet zone rocks. These monazite ages confirm that the metamorphism observed in the Big Staurolite nappe occurred significantly later than that in the units structurally above and below. These data support the hypothesis that the Big Staurolite nappe represents a major tectonic boundary, along which rocks of the New Hampshire metamorphic series were juxtaposed against rocks of the Vermont series during the Alleghanian. [ABSTRACT FROM AUTHOR]

Abstract: Analysis of monazite-bearing lithologies from the Precambrian Honey Brook Upland (HBU) and overlying metasedimentary Paleozoic Chester Valley Sequence (CVS) (SE PA, USA) reveals overprinting of primary major and accessory phase parageneses by texturally and compositionally disparate secondary accessory phase parageneses. Two-pyroxene temperatures of 915–945 °C for reconstituted pyroxene reflect emplacement temperatures of felsic plutonic rocks (opdalite, charnockite) prior to Mesoproterozoic metamorphism. Monazite in metavolcanic felsic gneiss yields three age domains at 1009±4 Ma (2 s.e.), 965±6, and 876±10 Ma. The first two domains record metamorphism of the HBU after anorthosite intrusion; peak monazite–xenotime temperatures for the monazite core domain are ∼700 °C, and high Th/U values in the second (overgrowth) age domain likely reflect a second high-T monazite growth episode. Formation of cummingtonite coronas on orthopyroxene in opdalite constrains maximum ∼1010 Ma metamorphic temperatures in the “granulite-facies” terrane to 730–740 °C. Evidence of increased Cl fluid activity in the ∼965 Ma metamorphism includes higher Cl content of matrix apatite relative to garnet-included apatite (metavolcanics), and Cl-bearing K-hornblende succeeding cummingtonite in coronal overgrowths (opdalite). Extreme monazite Th/U values (75–250) in the rim domain suggest growth during low-T hydrothermal alteration. In the opdalite, secondary singe-grain monazite and monazite+xenotime metasomites in apatite yield ages of 714±24 and 586±88 Ma, temperatures of 325–425 °C, and are interpreted to reflect thermal disturbances associated with late Proterozoic plutonic and volcanic activity in the Upland. This thermal disturbance may be recorded by Rb–Sr age of 567 Ma for biotite from a HBU gneiss. Monazite age domains in metaquartzite (378±28, 272±44 Ma) suggest that low-grade metamorphism (260–320 °C, Mnz–Xno thermometry) of the CVS is not a result of Taconian orogenesis. [Copyright &y& Elsevier]

Al2SiO5 reaction textures in aluminous schist and quartzite of the northern Picuris range, north-central New Mexico, record a paragenetic sequence of kyanite to sillimanite to andalusite, consistent with a clockwise P–T loop, with minor decompression near the Al2SiO5 triple-point. Peak metamorphic temperatures are estimated at 510–525°C, at 4·0–4·2 kbar. Kyanite and fibrolite are strongly deformed; some prismatic sillimanite, and all andalusite are relatively undeformed. Monazite occurs as inclusions within kyanite, mats of sillimanite and centimetre-scale porphyroblasts of andalusite, and is typically aligned subparallel to the dominant regional foliation (S0/S1 or S2) and extension lineation (L1). Back-scatter electron images and X-ray maps of monazite reveal distinct core, intermediate and rim compositional domains. Monazite–xenotime thermometry from the intermediate and rim domains yields temperatures of 405–470°C (±50°C) and 500–520°C (±50°C), respectively, consistent with the prograde to peak metamorphic growth of monazite. In situ, ion microprobe analyses from five monazites yield an upper intercept age of 1417 ± 9 Ma. Near-concordant to concordant analyses yield 207Pb–206Pb ages from 1434 ± 12 Ma (core) to 1390 ± 20 Ma (rim). We find no evidence of older regional metamorphism related to the ∼1650 Ma Mazatzal Orogeny. [ABSTRACT FROM AUTHOR]

Four distinct generations of monazite growth have been identified in samples from theth Chesham Pond Nappe, and three (monazite compositional domains 2, 3, and 4) have been correlated with both temperature and mineral assemblage. Domain 1 cores were interpreted previously to be detrital relics or vestiges of an earlier Acadian metamorphism. The four monazite domains have been dated by in situ isotope and chemical methods; the following are chemical ages of each domain (weighted average ±2 standard errors of the mean): 400 ± 10 Ma (domain 1); 381 ± 8 Ma (domain 2); 372 ± 6 Ma (domain 3); 352 ± 14 Ma (domain 4). Heating and cooling rates derived from combining monazite ages, monazite thermometry, and 40Ar/39Ar closure temperatures are approximately 10-15 °C/m.y. for heating from 470 to 740 °C, approximately 8 °C/m.y. for cooling from 740 to 375 °C, and approximately 1-2 °C/m.y for cooling from 375 to 150 °C. Temperature-time paths calculated with monazite ages and monazite thermometry indicate that (1) plutonism at ca. 400 Ma was the likely heat source for the formation of monazite domain 1 and (2) monazite domains 2-4 were produced during a regional low-pressure, high-temperature metamorphism active between 380-350 Ma. The regional metamorphism is ascribed to lithospheric mantle delamination, followed by asthenospheric mantle upwelling, which heated a wide area of the Merrimack basin (southwestern New Hampshire, central Massachusetts, central Connecticut) to temperatures in excess of 725 °C. Monazite ages in the Chesham Pond Nappe and adjacent structural units to the west constrain the commencement of nappe overthrusting to roughly 355 Ma. [ABSTRACT FROM AUTHOR]

We examine the factors controlling accuracy and precision of monazite microprobe ages, using a JEOL 733 Superprobe equipped with 4 PET crystals, and both 1-arm gas flow Ar X-ray detectors and sealed Xe X-ray detectors. Multiple PET crystals allow for simultaneous determination of Pb concentration on up to 3 detectors, and the effects of different detector gases on spectral form can be addressed. Numerous factors in the X-ray production, detection, and counting sequence affect spectral form, including: choice of accelerating voltage, changes in d-spacing of the diffraction crystal, use of X-ray collimation slits, and type of detector gas. The energy difference between ArKα X-rays and XeLα X-rays results in, for 1-arm Ar detectors, escape peaks of second-order LREE L line X-rays that cannot be filtered using differential mode PHA. The second-order LREE energies are passed to the counter and produce, for a 140 mm Rowland circle, several problematic interferences in the Pb region of a monazite wavelength-dispersive (WD) spectrum. WD monazite spectra produced with Xe detectors are free from second-order LREE interferences in the Pb region; escape peaks of the second-order LREE are filterable with differential mode PHA if Xe detectors are employed. Silicon, Ca, Y, Ce, P, Th, U, and Pb (2 spectrometers) are measured as part of the monazite microprobe dating protocol; ±2σ variations in elements fixed for ZAF corrections do not affect the age outside of analytical uncertainty. ThMα, UMβ, and PbMα are the analyzed lines of the age components. Corrections for interference of ThMζ1,2 and YLγ2,3 on PbMα are significant, but can be done precisely, and reduce the precision of the Mα analysis by a trivially small amount. ThMγ, M3-N4, and M5-P3 interferences on UMβ can be corrected, as well, but ThM5 and M4 absorption edges in high-Th samples make estimation of UMβ background... [ABSTRACT FROM AUTHOR]

Prograde suites of pelitic rocks were examined with electron microprobe and laser ablation inductively coupled plasma mass spectrometry to determine the systematics of element partitioning between coexisting monazite, xenotime, and garnet. Monazite grains that grew in equilibrium with xenotime are enriched in Y and Dy compared with monazite that grew in xenotime-absent assemblages. Y and heavy rare earth element contents of monazite coexisting with xenotime increase with rising temperature. Monazite–xenotime Y–Gd and Y–Dy partitioning is systematic within a metamorphic grade, and increases slightly with increasing metamorphic grade, suggesting that monazite–xenotime pairs approached partitioning equilibrium. Garnet and monazite in both xenotime-bearing and xenotime-absent assemblages show a strong ( R2 = 0·94) systematic relationship between inverse temperature and ln(KEq) for the net-transfer equilibrium YAG + OH-Ap + (25/4)Qtz = (5/4)Grs + (5/4)An + 3YPO4-Mnz + 1/2H2O, suggesting that garnet and monazite crystallized in compositional equilibrium. The following temperature–KEq relationship for the equilibrium above has been derived: [ABSTRACT FROM PUBLISHER]