Joseph M. Pyle

Four generations of accessory-phase growth in low-pressure migmatites from SW New Hampshire

Abstract Mineral compositions and reaction textures found in migmatitic gneisses from near Gilsum, New Hampshire, constrain both peak metamorphic pressure (P) and temperature (T) conditions and the PT path. Large K-feldspar porphyroblasts indicate isobaric heating of the samples at P < 4 kb (Spear et al. 1999), and relict cordierite + garnet assemblages record the occurrence of biotite vapor-absent melting; garnet-biotite thermometry yields peak temperatures of 740 ℃ at 3.5 kbar. During melt crystallization, low-Ti biotite + sillimanite replaced cordierite, and production of muscovite indicates P > 4 kb on the cooling path. Four generations of monazite have been identified, three of which have been linked to specific whole-rock reactions. Monazite (4) (the last generation) was produced with xenotime ± apatite during melt crystallization and consumption of garnet and cordierite. Monazite (3) grew in a xenotime-absent mineral assemblage as garnet + muscovite reacted to form sillimanite + biotite. Monazite (2) grew in a xenotime-bearing ± garnet + biotite + chlorite assemblage, as xenotime and chlorite were consumed during garnet production. Monazite (1) has not been linked to a specific reaction; it may be detrital, record an earlier metamorphic event, or represent disequilibrium overgrowths of xenotime. YAG-monazite, YAG-xenotime, and monazite-xenotime thermometry for monazite generations (2)-(4) yield temperatures consistent with major-phase thermometers.

Contributions to precision and accuracy of monazite microprobe ages

Abstract 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-atm 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-atm 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 secondorder 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 theMα 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 problematic. Background fits for UMβ peaks show that linear vs. exponential fits for UMβ do not, in general, produce statistically significant differences in microprobe ages. However, linear vs. exponential background fits for PbMα peaks do produce significantly different ages, most likely because of (1) low Pb concentrations relative to U; (2) ThMζ1 interference on backgrounds between ThMζ1 and PbMβ; and (3) SKα and Kβ interference in S-bearing monazite. For 6-min analyses (3 min peak, 3 min background) at 25 keV and 200 nA, 1σ Pb precisions are approximately 3.4% at 1700 ppm and 9.5% at 750 ppm; at 15 keV, precision decreases by roughly 25% of the 25 keV value. These precisions are constant for fixed current, analysis time, and concentration, but the statistical precision of distinct populations of monazite grains (domains) is a function of the total number of analyses within the domain. Instrumental errors (current measurement, dead time, pulse shift, d-spacing change) add 1.10% to random errors, but errors caused by pulse shift and d-spacing changes can be accounted for and corrected. Decreasing accelerating voltage from 25 to 15 keV decreases ZAF correction factors by as much as 50% relative, but replicate age analyses of Trebilcock monazite at 15 and 25 keV are statistically indistinguishable. Grain orientation, miscalculated background intensity, uncorrected interferences, and surface effects also introduce systematic errors. Accurate background interpolation and interference correction reduces systematic error to approximately 5.20% in addition to random (counting) error. Microprobe ages (~420 Ma) and 208Pb/232Th SIMS ages (~430 Ma) of monazite from Vermont are in agreement to within ~10 m.y. The discrepancy between U-Th-total Pb microprobe ages and 208Pb/232Th ages is removed when the high background measurement for PbMα is shifted to the short-wavelength side of PbMβ, removing a possible ThMζ1 interference.

An empirical garnet (YAG) – xenotime thermometer

Abstract A pronounced negative correlation between the yttrium concentration in garnet ([Y]Grt) and temperature has been observed in xenotime (YPO4)-bearing metapelites from central New England, USA. The [Y]Grt decreases roughly two orders of magnitude (∼5500 to less than 100 ppm Y) over a 150 °C interval. A regression of ln([Y]Grt) against estimated reciprocal temperature yields the following relationship:with R2 = 0.97. The decrease in garnet Y content is most rapid over garnet- to staurolite-zone conditions (450–550 °C) and the thermometer has a precision of a few degrees in this range.

Monazite ages in the Chesham Pond Nappe, SW New Hampshire, U.S.A.: Implications for assembly of central New England thrust sheets

Abstract Four distinct generations of monazite growth have been identified in samples from the 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.

MetPetDB: A database for metamorphic geochemistry

We present a data model for the initial implementation of MetPetDB, a geochemical database specific to metamorphic rock samples. The database is designed around the concept of preservation of spatial relationships, at all scales, of chemical analyses and their textural setting. Objects in the database (samples) represent physical rock samples; each sample may contain one or more subsamples with associated geochemical and image data. Samples, subsamples, geochemical data, and images are described with attributes (some required, some optional); these attributes also serve as search delimiters. All data in the database are classified as published (i.e., archived or published data), public or private. Public and published data may be freely searched and downloaded. All private data is owned; permission to view, edit, download and otherwise manipulate private data may be granted only by the data owner; all such editing operations are recorded by the database to create a data version log. The sharing of data permissions among a group of collaborators researching a common sample is done by the sample owner through the project manager. User interaction with MetPetDB is hosted by a web-based platform based upon the Java servlet application programming interface, with the PostgreSQL relational database. The database web portal includes modules that allow the user to interact with the database: registered users may save and download public and published data, upload private data, create projects, and assign permission levels to project collaborators. An Image Viewer module provides for spatial integration of image and geochemical data. A toolkit consisting of plotting and geochemical calculation software for data analysis and a mobile application for viewing the public and published data is being developed. Future issues to address include population of the database, integration with other geochemical databases, development of the analysis toolkit, creation of data models for derivative data, and building a community-wide user base. It is believed that this and other geochemical databases will enable more productive collaborations, generate more efficient research efforts, and foster new developments in basic research in the field of solid earth geochemistry.