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16.8.1: Almandine - Geosciences

16.8.1: Almandine - Geosciences


Almandine Garnet
Chemical compositionFe3Al2(SiO4)3

Iron aluminum silicate

Crystal systemCubic
HabitDodecahedra
CleavageNone
Hardness7.5
Optic natureIsotropic
Refractive index1.78-1.81
BirefringenceNone
Specific gravity3.95 - 4.20
LustreVitreous

Diagnostics

Figure (PageIndex{1}): Oval, faceted Almandine garnet

Color

Brownish red, red to violet/purple

Spectroscope

Almandine has a typical iron spectrum.

Figure (PageIndex{2}): usually at 504, 520, and 573nm, may also have faint lines at 423, 460, 610 and 680-690nm

Inclusions
  • Short rutile needles in 3 directions
  • Crystals surrounded by stress cracks
  • Irregular and rounded crystals
Polariscope

Almandine is isotropic but may have a small DR.

Specific gravity

Almandine sinks in all common heavy liquids.

Optical effects

Almandine may show asterism and chatoyancy.

Varieties

Rhodolite

Rhodolite is the purple (pinkish/violet) variety of pyrope-almandine.

Localities

India, USA, Brazil

Occurrence

Metamorphic rocks


16.8.1: Almandine - Geosciences

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Conditions for the origin of oxidized carbonate-silicate melts: Implications for mantle metasomatism and diamond formation

An experimental study on the origin of ferric and ferrous carbonate-silicate melts, which can be considered as the potential metasomatic oxidizing agents and diamond forming media, was performed in the (Ca,Mg)CO3-SiO2-Al2O3-(Mg,Fe)(Cr,Fe,Ti)O3 system, at 6.3 GPa and 1350–1650 °C. At 1350–1450 °C and ƒO2 of FMQ + 2 log units, carbonate–silicate melt, coexisting with Fe 3 + -bearing ilmenite, pyrope-almandine and rutile, contained up to 13 wt.% of Fe2O3. An increase in the degree of partial melting was accompanied by decarbonation and melt enrichment with CO2, up to 21 wt.%. At 1550–1650 °C excess CO2 segregated as a separate fluid phase. The restricted solubility of CO2 in the melt indicated that investigated system did not achieve the second critical point at 6.3 GPa. At 1350–1450 °C and ƒO2 close to CCO buffer, Fe 2 + -bearing carbonate–silicate melt was formed in association with pyrope-almandine and Fe 3 + -bearing rutile. It was experimentally shown that CO2-rich ferrous carbonate-silicate melt can be an effective waterless medium for the diamond crystallization. It provides relatively high diamond growth rates (3–5 μm/h) at P,T-conditions, corresponding to the formation of most natural diamonds.

Highlights

► Generation of ferric and ferrous carbonate-silicate melts in HPHT experiments. ► Metasomatic interactions, involving mantle silicates, oxides, CO2 fluid and melt. ► Ferrous carbonate-silicate melt in a role of a potential media for diamond growth. ► Ferric carbonate-silicate melt as oxidizing metasomatic agent in the mantle.


2 Methods

2.1 Starting Material

The depleted peridotite composition (DP) used in this study is the average composition of the global cratonic xenoliths with Mg# 92 (Lee et al., 2011 Luguet et al., 2015 Maier et al., 2012 Pearson & Wittig, 2013 ). The volatile-bearing silicic melt (SM) that has been used as the metasomatic agent is similar to a partial melt composition derived from the average composition of global subducting sediments (GLOSS) obtained at 3.5 GPa and 900°C in a study by Hermann and Spandler ( 2008 ). Based on the amount of carbon that can be dissolved in a model slab-derived rhyolitic melt at subarc depths (Duncan & Dasgupta, 2014, 2015 Ghiorso & Gualda, 2015 ), 5 wt.% CO2 was added to the partial melt composition along with 9.4 wt.% H2O to understand the effect of mixed volatiles on the mineral phase stability as a consequence of subduction influence during craton formation. To understand how the proportion of the infiltrating silicic slab-derived melt would affect the modes or composition of the volatile-bearing phases, two different bulk compositions (Bulk 1: 90 wt.% DP + 10 wt.% SM and Bulk 2: 95 wt.% DP + 5 wt.% SM) were investigated. We chose our bulk compositions such that bulk H2O <∼1wt.% so that the experiments are fluid undersaturated or produce very small fraction of excess fluid/hydrous melt near the solidus (Mandler & Grove, 2016 Tsuno & Dasgupta, 2012 ). Bulk starting compositions, Bulk 1 and Bulk 2 along with the composition of the base peridotite (DP) and the volatile-bearing silicic melt (SM) are reported in Table 1. The experimental bulk compositions (Bulk 1 and 2) are also compared with natural peridotite xenoliths and previous experimental compositions in Figure 1. It is evident from Figure 1 that bulk compositions in our study plot toward the high Mg# and low CaO, low Al2O3 end of the composition space. Bulk 1 and Bulk 2 have different K2O contents, which might affect the stability or abundance of the potassic hydrous phases of our interest.

Starting composition SiO2 TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O K2O P2O5 CO2 H2O Total Mg#
a a Base-depleted peridotite (DP).
DP
45.04 0.09 1.17 0.4 6.99 0.12 45.02 0.92 0.07 0.17 0.02 0 0 100 91.98
b b Rhyolitic sediment melt mix (SM) is the average composition of partial melt from global subducting sediments derived at 3.5 GPa and 900°C from the experimental study of Hermann and Spandler ( 2008 ) with 5 wt % CO2 added to take into account the amount of dissolved CO2 slab-derived sediment may contain based on CO2 solubility experiments of Duncan and Dasgupta ( 2014, 2015 ).
SM
62.93 0.3 12.34 0 0.75 0.03 0.34 1.02 3.22 4.44 0.21 5 9.41 100 44.68
c c Bulk 1 mixture consists of 90 wt % of DP and 10 wt % of SM.
Bulk 1
46.83 0.11 2.29 0.36 6.37 0.11 40.55 0.93 0.38 0.6 0.04 0.5 0.94 100 91.9
d d Bulk 2 consisting of 95 wt % DP and 5 wt % SM.
Bulk 2
45.93 0.1 1.73 0.38 6.68 0.12 42.78 0.92 0.23 0.39 0.03 0.25 0.47 100 91.94
  • Note. Major element compositions of starting mixes used in this study. FeO* indicates all Fe reported as FeO.
  • a Base-depleted peridotite (DP).
  • b Rhyolitic sediment melt mix (SM) is the average composition of partial melt from global subducting sediments derived at 3.5 GPa and 900°C from the experimental study of Hermann and Spandler ( 2008 ) with 5 wt % CO2 added to take into account the amount of dissolved CO2 slab-derived sediment may contain based on CO2 solubility experiments of Duncan and Dasgupta ( 2014,2015 ).
  • cBulk 1 mixture consists of 90 wt % of DP and 10 wt % of SM.
  • dBulk 2 consisting of 95 wt % DP and 5 wt % SM.

To prepare the starting mixes, reagent grade oxides and carbonates were first dried by firing SiO2, TiO2, Al2O3 overnight at 1000°C, Fe2O3 at 800°C, MnO at 400°C, CaCO3 at 200°C, and K2CO3 and Na2CO3 at 110°C. P2O5 was kept in a desiccator. For synthesis of the depleted peridotite (DP) composition, various oxides and carbonates were added in predetermined proportion in an agate mortar and ground under ethanol for an hour to homogenize the mix. The mixture was then left overnight and for ethanol to evaporate. The powder was then collected in a clean Au-lined alumina crucible and the mix was decarbonated and reduced at 1000°C in a CO-CO2 gas mixing furnace for 24 h at logfO2∼ FMQ-2. The rhyolitic sediment melt mix (SM) was prepared in two steps. The first step was similar to the preparation of the DP mix, where the oxides SiO2, TiO2, Fe2O3, MnO, MgO, P2O5 were added in the predetermined proportion, and ground under ethanol for 45 min to homogenize it. After the ethanol evaporated, the mixture was fired at 1000°C in a CO-CO2 gas mixing furnace for 24 h (at logfO2∼ FMQ-2) in order to reduce Fe 3+ to Fe 2+ followed by addition of Na2CO3, CaCO3, K2CO3, and Al(OH)3 so that the desired proportion of these oxides are added to the mix along with 5 wt.% CO2 and 9.4 wt.% H2O. The mix was ground again in ethanol for an hour to homogenize it and then left overnight to dry. It was later collected and stored in a glass vial in a 110°C drying oven.

The starting mixes (Bulk 1 and Bulk 2) were then prepared by weighing out predetermined proportions of the depleted peridotite and the rhyolitic sediment melt mix, homogenizing the mixes in an agate mortar under ethanol, and finally drying the mixes by storing in an oven. Homogeneous mixture of siliceous melt and depleted peridotite was prepared to simulate grain scale, porous infiltration of melt in a peridotite matrix similar to previous studies (Mallik & Dasgupta, 2014 Mallik et al., 2016 ).

2.2 Experimental Procedure

Experiments were performed using a piston cylinder (PC) and an 1100 ton Walker-style multi-anvil (MA) apparatus in the Experimental Petrology Laboratory at Rice University, Houston. The PC experiments were done using a half-inch BaCO3/MgO pressure media and graphite furnace assembly following the calibration and procedure described in previous studies (Duncan & Dasgupta, 2015 Eguchi & Dasgupta, 2017 Tsuno & Dasgupta, 2011 ). The MA experiments were conducted using a 18 mm MgO-Al2O3-SiO2 Walker-style castable assembly, following the calibration reported in Ding et al. ( 2014 ) and adopted in subsequent studies (Li et al., 2016 Tsuno & Dasgupta, 2015 ). A Type-C thermocouple oriented axially with respect to the heater and located next to the capsule was used to monitor and control the temperature for both PC and MA experiments. Pressure and temperature uncertainties are estimated to be ± 0.1 GPa, 12°C for PC and ± 0.3 GPa, 10°C for the MA experiments.

Gold capsules (2 mm outer diameter) were used to contain the homogeneous starting mixes in both PC and MA experiments. The packed capsules were welded shut using a graphite arc welder or PUK welding machine. Weight loss due to welding measured for each capsule with Bulk 1 or Bulk 2 was ≤0.6% and 0.04% relative to the unwelded capsule for the graphite arc welder and the PUK welder, respectively. Experiments using Bulk 1 were performed at 950°C at 2 GPa, 950–1,175°C at 3 GPa, and 950–1150°C at 4 GPa. As evident from Figure 2, these P-T conditions are relevant for the CLM (Boyd et al., 1993, 1997 Carswell et al., 1979 Ehrenberg, 1982 Kopylova et al., 1999 Lee & Rudnick, 1999 Nixon et al., 1981 Pearson et al., 1995 Rudnick et al., 1994 Winterburn et al., 1990 ). In order to study the change in modes and compositions of the volatile-bearing phases as a function of melt:rock ratio, three additional experiments were performed at 950°C at 2 GPa, 1050°C at 3 GPa, and 1100°C at 4 GPa with Bulk 2. To constrain the effects of volatile-bearing siliceous melt metasomatism on depleted peridotite, a melt-free experiment with DP was also performed at 3 GPa and 950°C (Table 2). In all experiments, the desired pressure was attained first and then the samples were heated to the temperature of interest at a rate of 100°C/min. Run durations for the experiments varied from 3 to 7 days depending on the temperatures of interest, with lower temperature experiments typically run longer.

The pressure-temperature conditions of our experiments. The solid square symbols represent the experiments with starting composition Bulk 1. The solid squares with a * beneath them represent experimental P-T conditions with starting composition Bulk 2. The dotted grey lines represent conductive geotherms that correspond to different surface heat flow. The green-shaded region marks the P-T conditions beneath continents based on xenolith thermobarometry data from Lee et al. ( 2011 ). The grey band indicates the depth range where MLD occurrences are reported.

Run # P (GPa) T (°C) Run duration (days) ol opx cpx gt spinel amph Phl mgs melt ∑r 2
DP
B338 3 950 6 48.5(6) 41.5(9) 0.9(7) 9.1(4) + - 0.9(1)
Bulk 1
B405 2 950 6 49.3(5) 35.5(3) 2.4(6) - + 6.7(4) 5.1(2) 0.98(4) 0.07(1)
B342 3 950 6 50.5(8) 33.1(9) 0.05(2) + 9.4(1) 6.0(3) 1.0(0) 0.4(3)
B373 3 975 7 49.6(4) 36.3(7) 0.1(8) + 7.5(1) 5.6(2) 1.0(0) 0.14(6)
B352 3 1000 5 45.6(1) 44.7(7) 0.01(4) + 3.6(6) 5.8(2) + 0.05(3)
B353 3 1025 6 49.5(5) 39.4(3) 0.1(9) + 5.3(8) 5.7(8) + 0.2(2)
B363 3 1050 5 50.6(8) 36.4(9) 1.9(8) + 5.9(6) 5.2(9) + 0.04(2)
B402 3 1100 5 47.0(8) 40.1(7) 3.2(6) 2.1(6) 7.6(3) + 0.07(1)
B379 3 1175 6 41.4(8) 44.1(3) 3.3(6) 0.3(8) 3.3(7) + 0.23(8)
MA131 4 950 3 44.5(5) 38.9(7) 3.9(4) 5.8(8) 5.9(1) 1.00(0) 0.7(5)
MA136 4 1000 4 48.3(8) 36.8(7) 2.7(4) 5.7(4) 5.4(4) 1.00(0) + 0.20(5)
MA124 4 1050 5 47.5(3) 40.0(3) 2.7(1) 4.7(2) 5.1(1) 0.00(0) + 0.11(3)
MA119 4 1100 3 51.1(8) 35.3(3) 2.6(3) 5.8(4) 5.2(4) + 0.5(1)
MA125 4 1150 3 47.2(5) 39.8(7) 3.9(4) 3.7(4) 5.4(3) + 0.47(6)
Bulk 2
B413 2 950 6 60.8(6) 27.2(6) 2.89(5) + 6.5(7) 2.6(3) 0.60(5) 0.28(7)
B410 3 1050 5 61.9(4) 29.3(4) 4.9(3) + 2.2(5) 3.5(2) + 0.16(4)
MA171 4 1100 5 62.2(8) 26.6(4) 3.7(3) 2.4(3) 5.0(4) + 0.12(2)
  • Note. Numbers in the parentheses represent the ±1σ error determined by propagating the errors in each oxide by Monte Carlo simulations (n = 20). For example, 59.8(4) is 59.8 ± 0.4 wt. % “+” sign indicates the presence of a phase, but due to small size the phase could not be analyzed and therefore not included in the mass balance calculations. Absence of a phase is denoted by “–”. opx: orthopyroxene cpx: clinopyroxene phl: phlogopite ol: olivine amph: amphibole mgs: magnesite: gt: garnet melt: visible presence of partial melt. ∑r 2 —sum of residual squares obtained from mass balance of oxides. In order to check the robustness of our mass balance calculations, SEM images (for run # MA 131) were processed using the software ImageJ to estimate phase abundances. The modal abundance for each phase based on textural analysis lies within one standard deviation of values obtained by mass balance of oxides.

The experiments were terminated by cutting off power to the heater. After slow decompression of the assemblies, the locations of the capsules with respect to the thermocouples were verified for every experiment. The recovered capsules were mounted in epoxy, then exposed by grinding using 240–1200 grit silicon carbide paper, and polished on a dry nylon/velvet microcloth using 1–3 μm diamond powder. Repeated vacuum impregnation with low viscosity epoxy was performed to minimize sample loss while polishing. Any liquid lubricant such as water or organic solvents was avoided during polishing to minimize loss of fragile carbonate and hydrous mineral and/or melt phases.

2.3 Analysis of the Run Products

The polished experimental samples were imaged and analyzed using JEOL JXA-8530F HyperProbe Electron Probe Microanalyzers at NASA Johnson Space Center (JSC) and Rice University, Houston, TX. The 3 GPa experiments for Bulk 1 were analyzed at JSC, while the 2 and 4 GPa experiments have been analyzed at Rice University, along with all the experiments for Bulk 2. Mineral phases were identified using energy dispersive spectroscopy (EDS) and concentrations of major and minor elements were determined using WDS spectroscopy. Melt was identified texturally by its occurrence along mineral grain boundaries and triple junctions. Accelerating voltage of 15 kV and ZAF correction was used for the WDS analyses. Silicate phases such as olivine, orthopyroxene, clinopyroxene, and garnet were analyzed using a 1 μm diameter beam and 20 nA beam current, whereas phlogopite was analyzed using a 5 μm wide beam and a beam current of 5 nA. Amphibole was analyzed using a 2–3 μm wide beam depending on the size of the grain and a beam current of 20 nA. For magnesite, a defocused beam of 2–5 μm was used. Almandine (Al), biotite (K), chromite (Cr), diopside (Ca), jadeite (Na), olivine (Si, Fe Mg), rhodonite (Mn), and rutile (Ti) were used as analytical standards. Olivine, diopside, orthopyroxene, almandine, biotite, and dolomite of known compositions were analyzed in each probe session as secondary standards to ensure accuracy of the analytical data.


16.8.1: Almandine - Geosciences

All articles published by MDPI are made immediately available worldwide under an open access license. No special permission is required to reuse all or part of the article published by MDPI, including figures and tables. For articles published under an open access Creative Common CC BY license, any part of the article may be reused without permission provided that the original article is clearly cited.

Feature Papers represent the most advanced research with significant potential for high impact in the field. Feature Papers are submitted upon individual invitation or recommendation by the scientific editors and undergo peer review prior to publication.

The Feature Paper can be either an original research article, a substantial novel research study that often involves several techniques or approaches, or a comprehensive review paper with concise and precise updates on the latest progress in the field that systematically reviews the most exciting advances in scientific literature. This type of paper provides an outlook on future directions of research or possible applications.

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to authors, or important in this field. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.


16.8.1: Almandine - Geosciences

All articles published by MDPI are made immediately available worldwide under an open access license. No special permission is required to reuse all or part of the article published by MDPI, including figures and tables. For articles published under an open access Creative Common CC BY license, any part of the article may be reused without permission provided that the original article is clearly cited.

Feature Papers represent the most advanced research with significant potential for high impact in the field. Feature Papers are submitted upon individual invitation or recommendation by the scientific editors and undergo peer review prior to publication.

The Feature Paper can be either an original research article, a substantial novel research study that often involves several techniques or approaches, or a comprehensive review paper with concise and precise updates on the latest progress in the field that systematically reviews the most exciting advances in scientific literature. This type of paper provides an outlook on future directions of research or possible applications.

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to authors, or important in this field. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.


Digital [email protected]

The equation of state (EoS) of a natural almandine74spessartine13pyrope10grossular3 garnet of a typical composition found in metamorphic rocks in Earth’s crust was obtained using single crystal synchrotron X-ray diffraction under isothermal room temperature compression. A third-order Birch-Murnaghan EoS was fitted to P-V data and the results are compared with published EoS for iron, manganese, magnesium, and calcium garnet compositional end-members. This comparison reveals that ideal solid solution mixing can reproduce the EoS for this intermediate composition of garnet. Additionally, this new EoS was used to calculate geobarometry on a garnet sample from the same rock, which was collected from the Albion Mountains of southern Idaho. Quartz-ingarnet elastic geobarometry was used to calculate pressures of quartz inclusion entrapment using alternative methods of garnet mixing and both the hydrostatic and Grunëisen tensor approaches. QuiG barometry pressures overlap within uncertainty when calculated using EoS for pure endmember almandine, the weighted averages of end-member EoS, and the EoS presented in this study. Grunëisen tensors produce apparent higher pressures relative to the hydrostatic method, but with large uncertainties.

Keywords

Garnet Equation of state Solid solution Host inclusion elastic geobarometry QuiG


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