ESTIMATING P-T CONDITIONS OF GARNET GROWTH WITH ISOCHEMICAL PHASE-DIAGRAM SECTIONS AND THE PROBLEM OF EFFECTIVE BULK-COMPOSITION

2005 ◽  
Vol 43 (1) ◽  
pp. 35-50 ◽  
Author(s):  
D. K. Tinkham ◽  
E. D. Ghent
Keyword(s):  
Phase Diagram ◽  
Bulk Composition ◽  
Garnet Growth ◽  
Effective Bulk Composition

scholarly journals Ca-rich garnets and associated symplectites in mafic peraluminous granulites from the Gföhl Nappe System, Austria

Solid Earth ◽  
2018 ◽  
Vol 9 (3) ◽  
pp. 797-819 ◽  
Author(s):  
Konstantin Petrakakis ◽  
Nathalie Schuster-Bourgin ◽  
Gerlinde Habler ◽  
Rainer Abart
Keyword(s):  
Early Stage ◽  
Bulk Composition ◽  
Element Distribution ◽  
Garnet Growth ◽  
Distribution Maps ◽  
Compositional Structure ◽  
Garnet Composition ◽  
Show Evidence ◽  
Isothermal Decompression ◽  
Diffusion Profiles

Abstract. Mafic peraluminous granulites associated with the mantle-derived peridotites of the Dunkelsteiner Wald provide evidence of the tectono-metamorphic evolution of the Gföhl Nappe System, Austria. They contain the primary assemblage garnet + Al-rich clinopyroxene + kyanite. Large Ca- and Mg-rich garnets are embedded in a granoblastic matrix of Al-rich clinopyroxene, Ca-rich plagioclase and minor hornblende. They were partially replaced by different generations of symplectites: (a) corundum + sapphirine + spinel + plagioclase formed around kyanite inclusions, (b) orthopyroxene + spinel + plagioclase ± hornblende formed at their rims and (c) clinopyroxene + orthopyroxene + spinel + plagioclase ± hornblende formed within cracks. Large garnets show complex compositional structure comprising several repeatedly occurring garnet types, which are characterized by specific compositions. The areal extent and the cross-cutting relations observed in element distribution maps allowed for the derivation of the relative timing of the formation of the different garnet types. The compositional features of the garnets indicate post-formational modification by intra-crystalline diffusion and metasomatic agents. The garnet composition isopleths in equilibrium assemblage diagrams are in line with compositions modification as indicated by the element distribution maps. They confirm the deviation of composition from equilibrium for all garnet types. Furthermore, at least the youngest garnet types show evidence of metasomatic (Fe + Mg) loss affecting their Ca content. Pressure–temperature (P–T) estimates are based on equilibrium assemblage diagrams that reproduce satisfactorily the observed mineral assemblages and measured mineral compositions. Criteria for checking the existence of preserved equilibrium compositions are suggested. The results call into question the invariability of the assumption that the Ca content and/or zoning in garnet preserves primary P–T information from garnet growth in every case. Recrystallization and compositional readjustment of the reactive garnet volume during symplectite formation led to the development of pronounced, secondary diffusion-induced zoning profiles overprinting the different garnet types and post-dating the complex garnet compositional structure. The primary assemblage is stable between 760 and 880 °C and pressures > 11 kbar. The bulk composition of the crack symplectites is almost isochemical to the oldest, broken-down garnet type. These symplectites were formed above 730 °C and pressures between 5 and 7.5 kbar. The rocks studied underwent more or less isothermal decompression from pressures above 11 to ∼ 6 kbar at temperatures of about 800 °C. Crack and rim symplectites were formed after decompression during the early stage of approximately isobaric cooling under conditions of low differential stress. Due to limited availability of fluids promoting symplectite formation, the timescale of symplectite formation calculated from secondary diffusion profiles associated with crack symplectites is shown to be geologically very short (< 0.5 ka).

Metamorphic evolution of the Petersen Bay assemblage, Ellesmere Island: What can we learn about Pearya - Laurentia accretion?

2020 ◽  
Author(s):  
Karolina Kośmińska ◽  
Jane Gilotti ◽  
William McClelland ◽  
Matthew Coble
Keyword(s):  
Middle Devonian ◽  
Bulk Composition ◽  
Arctic Region ◽  
Ellesmere Island ◽  
The Arctic ◽  
Garnet Growth ◽  
Fold Belt ◽  
Weighted Mean ◽  
Northern Margin ◽  
Garnet Core

&lt;p&gt;The accretion of the Pearya terrane to the northern margin of Laurentia plays an important role in the paleogeographic reconstructions for the Arctic region. Earlier workers proposed a timing of its juxtaposition spanning from Late Silurian (Trettin, 1998) to Late Ordovician (Klaper 1992). In this study, we focus on the pressure-temperature-time (P-T-t) evolution of the Petersen Bay assemblage. This subduction related unit crops out between the crystalline basement of Pearya and volcano-sedimentary sequence of Clements Markham fold belt. The highest grade rocks, garnet-kyanite-bearing schist (sample 17-66) and garnet-kyanite-staurolite garbenschiefer (sample 17-64) were selected for P-T studies and in-situ monazite U-Pb dating by sensitive high resolution ion microprobe.&lt;/p&gt;&lt;p&gt;Thermodynamic modelling of sample 17-66 gives a P-T condition of 7.8-8.1 kbar and 590-610&amp;#176;C for garnet core formation, whereas a pseudosection calculated for the effective bulk composition indicates garnet rim growth at 8-9 kbar and 650-660&amp;#176;C. The QuiG Raman barometry coupled with Ti-in-biotite thermometry yield conditions of 6.5-7.5 kbar and 540-600&amp;#176;C for the garnet growth. The combination of QuiG barometry and Ti-in-biotite thermometry indicate garnet growth at 7.5-8 kbar and 500-550&amp;#176;C for the garbenschiefer sample.&lt;/p&gt;&lt;p&gt;Monazite shows distinctive zonation and 2, up to 3, domains were recognized based on textures and X-ray microprobe maps. For the sample 17-66, Monazite-I forms inclusions within garnet rims or cores of bigger matrix grains. It defines a weighted mean&amp;#160;&lt;sup&gt;206&lt;/sup&gt;Pb/&lt;sup&gt;238&lt;/sup&gt;U age of 397&amp;#177;2 Ma (n=18, MSWD=1.6). Monazite-II occurs in the matrix and gives an age of 385&amp;#177;2 Ma (n=19, MSWD=1.5).&amp;#160;Monazite-I from sample 17-64 yields a weighted mean &lt;sup&gt;206&lt;/sup&gt;Pb/&lt;sup&gt;238&lt;/sup&gt;U age of 394&amp;#177;2 Ma (n=11, MSWD=0.6). Monazite-II defines the age of 388&amp;#177;2 Ma (n=7, MSWD=0.8). Monazite-III was distinct only in garbenschiefer. It yields a younger age of 374&amp;#177;6 Ma (n=6, MSWD=3.1).&lt;/p&gt;&lt;p&gt;The P&amp;#8211;T data coupled with monazite dating suggest a Middle Devonian metamorphism of the Petersen Bay assemblage under amphibolite facies conditions. These new results suggest that the juxtaposition of the Pearya terrane, Petersen Bay assemblage and the Clemens Markham fold belt is Middle Devonian or younger, i.e. much younger than previously thought.&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;Klaper E.M. 1992. The Paleozoic tectonic evolution of the northern edge of North America: A structural study of Northern Ellesmere Island, Canadian Arctic Archipelago Tectonics, 11, 854&amp;#8211;870.&lt;/p&gt;&lt;p&gt;Trettin H.P. 1998. Pre-Carboniferous geology of the northern part of the Arctic Islands: Northern Heiberg Fold Belt, Clements Markham Fold Belt, and Pearya; northern Axel Heiberg and Ellesmere islands GSC Bulletin, 425, 401 p.&lt;/p&gt;

A revised phase diagram for the bornite-digenite join from in situ neutron diffraction and DSC experiments

2000 ◽  
Vol 64 (2) ◽  
pp. 213-231 ◽  
Author(s):  
B. A. Grguric ◽  
R. J. Harrison ◽  
A. Putnis
Keyword(s):  
Phase Diagram ◽  
Neutron Diffraction ◽  
Structural Phase ◽  
Bulk Composition ◽  
Phase Relations ◽  
Synthetic Sample ◽  
Scanning Calorimetry ◽  
Thermal Anomalies ◽  
Diffraction Patterns

AbstractPhase relations along the join bornite (Cu5FeS4)-digenite (Cu8.52Fe0.12S4.88) have been redefined using a combination of in situ high-resolution neutron diffraction and differential scanning calorimetry (DSC). Time-of-flight neutron diffraction patterns were collected on a synthetic sample of bn90 at 16 temperatures between 35 and 350°C. This data is compared with data from a natural end-member bornite sample obtained in an earlier study under identical conditions. Phase relations along the bornite-digenite join are inferred from the temperature evolution of the lattice parameters and the intensity of subcell and supercell reflections of coexisting phases.The DSC scans over the temperature range 50–300°C were performed on a natural digenite sample and samples synthesized at 5 mol.% intervals along the join Cu5FeS4-Cu9S5. The thermal anomalies are correlated with structural phase transitions in componen phases and the solvus temperature for each bulk composition. A phase diagram topology is defined, which was consistent with both diffraction and calorimetric data, but in marked contrast to previous diagrams, shows a consolute point at X = Cu5FeS4 and T = 265°C. This temperature corresponds to that of the tricritical intermediate-high transition in bornite. Isothermal annealing experiments carried out on synthetic starting materials for up to 7 months showed coarsening behaviour consistent with the revised phase diagram topology.

scholarly journals Paleoproterozoic Metamorphism of the Archean Tuntsa Suite, Northern Fennoscandian Shield

Minerals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 1034
Author(s):  
Pentti Hölttä ◽  
Tiia Kivisaari ◽  
Hannu Huhma ◽  
Gavyn Rollinson ◽  
Matti Kurhila ◽  
...  
Keyword(s):  
Bulk Composition ◽  
Metamorphic Zircon ◽  
The Core ◽  
Garnet Core ◽  
Mineral Assemblages ◽  
Mineral Compositions ◽  
Temperature And Pressure ◽  
Pt Path ◽  
Effective Bulk Composition ◽  
Age Determinations

The Tuntsa Suite is a polymetamorphic Archean complex mainly consisting of metasedimentary gneisses. At least two strong metamorphic events can be distinguished in the area. The first took place at high temperatures in the Neoarchean at around 2.70–2.64 Ga, indicated by migmatisation and U-Pb ages of metamorphic zircon. During the Paleoproterozoic, metasedimentary gneisses were penetratively deformed and recrystallized under medium pressures producing staurolite, kyanite and garnet-bearing mineral assemblages. The suggested Paleoproterozoic PT path was clockwise where the temperature and pressure first increased to 540–550 °C and 6 kbar, crystallizing high Ca/low Mg garnet cores. The mineral compositions show that commonly garnet core was not in chemical equilibrium with staurolite but crystallized earlier, although garnet-staurolite-kyanite assemblages are common. The temperature and pressure increased to c. 650 °C and 8 kbars where staurolite and kyanite coexist. This was followed by decompression down to c. 550–600 °C and 3–4 kbars, shown by andalusite crystallization and cordierite formed in the breakdown of staurolite and biotite + kyanite. The observed garnet zoning where Mg increases and Ca decreases from the core to the rim was developed with both increasing and decreasing pressure, depending on the effective bulk composition. The U-Pb and Sm-Nd age determinations for monazite and garnet show that the Paleoproterozoic metamorphic cycle took place at 1.84–1.79 Ga, related with thrusting of the Lapland granulites onto the adjacent terranes and subsequent exhumation.

Performing process-oriented investigations involving mass transfer using Rcrust: a new phase equilibrium modelling tool

2019 ◽  
Vol 491 (1) ◽  
pp. 209-221 ◽  
Author(s):  
Matthew Jason Mayne ◽  
Gary Stevens ◽  
Jean-François Moyen ◽  
Tim Johnson
Keyword(s):  
Mass Transfer ◽  
Pore Space ◽  
Bulk Composition ◽  
Software Tool ◽  
Stable Phase ◽  
Phase Assemblage ◽  
Process Oriented ◽  
Point To Point ◽  
New Phase ◽  
Effective Bulk Composition

AbstractModern quantitative phase equilibria modelling allows the calculation of the stable phase assemblage of a rock system given its pressure, temperature and bulk composition. A new software tool (Rcrust) has been developed that allows the modelling of points in pressure–temperature–bulk composition space in which bulk compositional changes can be passed from point to point as the system evolves. This new methodology enables quantitative process-oriented investigation of the evolution of rocks. Procedures are outlined here for using this tool to model: (1) the control of the water content of a subsolidus system based on available pore space; (2) the triggering of melt loss events when a critical melt volume threshold is exceeded, while allowing a portion of melt retention; (3) the entrainment of crystals during segregation and ascent of granitic magmas from its source; (4) the modification of the composition of granite magmas owing to fractional crystallization; and (5) the progressive availability (through dissolution) of slow diffusing species and their control of the effective bulk composition of a system. These cases collectively illustrate thermodynamically constrained methods for modelling systems that involve mass transfer.

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