Pyrope-almandine Garnet Research Articles

Archaeometric Investigation of a Hunnic-Period Sacrificial Assemblage from Nyergesújfalu, Hungary

Near Nyergesújfalu (Komárom-Esztergom County, Hungary), objects of a Hunnic-period sacrificial assemblage were found in 2021. The Hunnic period assemblage contains fifteen items altogether: four gold lunular mounts, six cellwork-decorated gold oval mounts, two cellwork-decorated gold suspension rings, two gold buckles and fragments of a scale-patterned gilded silver plate. The present study aims to determine the elemental composition of the metal alloy of the Hunnic-period objects and characterise the decoration techniques (gilding and garnet inlays) by using optical microscopy, handheld X-ray fluorescence spectrometry (hXRF) and scanning electron microscopy with energydispersive X-ray spectrometry (SEM-EDX). The gold objects, including their small parts such as the rivets of buckles and lunular mounts, sockets and filigree, were manufactured from a relatively goodquality gold alloy (>80 wt% Au). The fragments of the scale-patterned silver plate were manufactured from a high-quality silver alloy (>94 wt% Ag), similar to late Roman silver alloys characterised by high Ag content, and was decorated with fire (mercury) gilding. The garnets used for inlays are almandine and intermediate pyrope-almandine garnets. Based on their chemistry, the garnets belong to Group X and probably originate from the placer deposits of Sri Lanka.

Single-crystal UV/Vis absorption spectroscopy of aluminosilicate garnet: Part III. {Fe2+} + [Fe3+] → {Fe3+} + [Fe2+] intervalence charge transfer

AbstractThe various intervalence charge transfer (IVCT) mechanisms that can occur in silicate garnet, general crystal-chemical formula {X3}[Y2](Z3)O12, are not fully understood. The single-crystal UV/Vis/NIR absorption spectra of two different almandine-rich, spessartine-rich and grossular-rich garnets, as well as an intermediate almandine-pyrope garnet, were measured. Absorption was observed from roughly 15 000 to 30 000 cm–1. The spectra were deconvoluted and a very broad band with FWHM values ranging from 5000 to 7000 cm–1 (except in the case of one grossular where the FWHM is 8700 cm–1) and having an intensity maximum located between about 20 000 and 22 000 cm–1 in the visible region could be fit. Small weaker features located on this broad band were fit as well. The broad band is strongest in a nearly end-member composition almandine and weakest in a very grossular-rich iron-poor crystal. It is assigned to {Fe2+} + [Fe3+] → {Fe3+} + [Fe2+] IVCT. This is the first recognition of this type of electronic transition mechanism in different aluminosilicate garnet species. Photon-induced electron transfer probably occurs through an overlap of the d orbitals of Fe2+ and Fe3+ in their edge-shared triangular dodecahedral and octahedral coordination polyhedra, respectively. The two Fe cations with different formal charges should have markedly different energy potentials giving rise to asymmetric IVCT behavior. This, together with the relatively long Fe2+-Fe3+ distances (greater than 3.2 Å), could explain the higher energy of the IVCT in garnet compared to Fe2+ + Fe3+ → Fe3+ + Fe2+ IVCT mechanisms observed in other minerals. The latter typically have iron cations in octahedral or quasi-octahedral coordination. The IVCT in aluminosilicate garnet can occur in different species that grew under dissimilar P-T-X conditions. The resulting electronic absorption band affects color markedly, because it is centered at higher energies in the blue visible region. It remains to be determined why IVCT is observed in the spectra of some garnets but not others. The various proposed IVCT mechanisms in Ca-Ti-bearing and aluminosilicate garnets are reviewed and analyzed.

Spectroscopic Analysis for Harnessing the Quality and Potential of Gemstones for Small and Medium-Sized Enterprises (SMEs)

Introduction of modern technologies and methods and quality analysis for the gemstone industry are the main strategic initiatives of the Small and Medium Development Authority (SMEDA) of Pakistan. In this regard, four natural gemstones Quartz, Pyrope-Almandine Garnet, Black tourmaline, and Amethyst brought from Hunza valley Pakistan were analyzed by state-of-the-art spectroscopic techniques including EDX, UV-VIS, and FTIR spectroscopy. EDX revealed the traces of Fe, Mg, and Ca in Pyrope-Almandine garnet, Mg and Fe in Black tourmaline, Au and Ca in Amethyst. UV-VIS data revealed the values of Urbach energies 520, 210, 460, and 430 meV, and the values of direct bandgap energies 5.14, 6.12, 5.54, 5.74 eV, respectively. The higher structural disorder due to the presence of Fe and other impurities in stones except Quartz was attributed to the higher values of Urbach energies and decrease in band gaps: FTIR data Fe-O and Si-O stretching vibration in Pyrope-Almandine garnet, Si-O bending vibrations and O-H stretching vibration in Quartz, Si-O-Si bending and stretching vibrations and C=O stretching vibrations in Black tourmaline, Ca-O stretching vibrations and Si-OH weak-vibrations in Amethyst. Photoluminescence results also showed useful information in investigating the properties of gemstones.

Two contrasting P-T paths for metamorphic sole amphibolites of the Dinaride Ophiolite Zone (Krivaja-Konjuh ultramafic massif, Central Bosnia and Herzegovina) and their geodynamic implications

The Inner Dinarides, a part of the Alpine-Dinaride mountain chain in south-eastern Europe, are known for extended ophiolite-derived ultramafic massifs. The sampled rocks from the metamorphic sole of such a massif, the Jurassic Krivaja-Konjuh ultramafic massif in central Bosnia and Herzegovina, are mainly garnet amphibolites, with large grains of pyrope-almandine garnet commonly surrounded by kelyphite coronae, and pargasite amphibolites. The bulk-rock geochemistry points to mantle peridotite as the protolith for pargasite amphibolite and N-MORB for garnet amphibolite. Pressure-temperature (P-T) pseudosections were constructed in the MnNCKFMASHTO system and contoured by isopleths for the modal and chemical composition of minerals. On this basis, a counterclockwise P-T path with maximum P-T conditions of 1.2 GPa and 960 °C was deduced for pargasite amphibolite as a bottom part of an overriding plate. This part was cooled by subducting oceanic crust. In addition, the Jurassic intra-oceanic subduction enabled infiltration of hydrous fluid necessary to form the pargasite amphibolites. A clockwise P-T path with maximum pressure conditions at ca. 2.1 GPa (ca. 65–70 km depth) and temperatures of 810 °C was reconstructed for garnet amphibolite from an upper part of the subducting plate which was exhumed in a subduction channel and came in contact with the pargasite amphibolite. Thus, these amphibolites formed before they were involved in a metamorphic sole at the interface between continental crust and obducting ophiolite.

Geochemistry and mineral chemistry of quartz mica schists within Iseyin-Oyan Schist Belt, Southwestern Nigeria

Background/Objectives: The Iseyin-Oyan schist belt is made up metasedimentary rocks, gneisses, granites and pegmatite intrusions. The study was aimed at identifying the schist within this belt and assess their metamorphism, geochemical characteristics and tectonic origin. Methods: Detailed geologic field mapping was undertaken where rocks were located, studied in-situ and identified. Samples of the schist were prepared for petrographic studies. Mineralogical contents were determined using X-Ray Diffraction technique. Polished sections were studied for mineral chemistry using Scanning Electron Microscope-Energy Dispersive Spectroscopy. Rock samples were analysed using X-Ray Fluorescence Spectroscopy and Inductively Coupled Plasma Emission Spectrometry. Geochemical data were elucidated using diverse geochemical discrimination diagrams. Findings: The schists are quartz mica schists and occur in close association with amphibolites, intrusive granitoids and pegmatites. The Mineral assemblage indicates upper (at the western part) to lower (at the central part) amphibolite facies grade metamorphism in the area. Pyrope-almandine garnets occur in quartz mica schist at the western parts reinforcing higher pressure-temperature metamorphic conditions. The concentration (in %) of SiO2 ranged from 56.4-71.6; Al2O3, 13.7-21.1; Fe2O3, 2-8; MgO, 0.7-2.4; and K2O, 2.1-5.5 supporting the evidence for differential degrees of metamorphism. Large iron lithophile and high field strength elements are similar to the average upper continental crust. Pronounced negative Europium anomaly pointed to the major roles played by feldspars during the geological processes. Plagioclase ranged from albite-oligoclase and oligoclase- andesine. The precursors of the quartz mica schist are possibly arkosic and greywacke sands deposited within the active continental margins. Evidence of uplift and overturning suggested for the differential metamorphism may be due to these events usually associated with active continental margins. Applications: This study has identified the once named undifferentiated schist in the study area to be quartz mica schist with details in their grades of metamorphism elucidated. Keywords: Quartz mica schist; geochemistry; mineral chemistry; Iseyin-Oyan schist belt; precambrian basement complex

Cymrite as mineral clathrate: An overlooked redox insensitive transporter of nitrogen in the mantle

We study the phase relations and mineral chemistry in the systems muscovite–NH3–N2-H2O and eclogite + muscovite–NH3–N2-H2O at 6.3–7.8 GPa, 1000–1200 °C, and oxygen fugacity (fO2) from ∼IW (Fe–FeO) to ∼ NNO (Ni–NiO) equilibria. The quenched H2O-bearing fluids differ in nitrogen speciation from NH3-rich to N2–rich, and the respective N2/(NH3+N2) ratio varies from <0.1 to ∼ 1. N-bearing K-cymrite is obtained in association with a kyanite-garnet-jadeite ± muscovite ± corundum assemblage in the muscovite–NH3–N2-H2O system and coexists with pyrope-almandine garnet and omphacite in the eclogite + muscovite–NH3–N2-H2O system. The presence of an N-bearing fluid in the studied systems stabilizes the K-cymrite structure. Muscovite does not convert to K-cymrite in the absence of NH3–N2-bearing fluid up to 7.8 GPa and 1070–1120 °C. According to FTIR and Raman spectroscopy, K-cymrite in equilibrium with an N-rich fluid can capture a huge amount of nitrogen in cages of its framework, mainly as N2 molecules at fO2 ∼NNO and predominantly as NH3 molecules at fO2 ∼IW. The storage capacity of K-cymrite with respect to nitrogen increases from 2.9 to 6.3 wt% with increase of fO2. FTIR spectroscopy of muscovite equilibrated with K-cymrite shows that the clathrate mechanism of nitrogen entrapment by aluminosilicates (as neutral N2 and NH3 molecules) is much more efficient than the K+ → (NH4)+ substitution. The structure of N-bearing K-cymrite (K,(NH4+))[AlSi3O8]·(N2,NH3,H2O) determined using X-ray single-crystal diffraction is very similar to that of H2O-bearing K- and Ba-cymrites. It includes aluminosilicate layers consisting of double six-membered tetrahedral rings and cation sites statistically occupied with K+, Ba2+ and (NH4)+ on the six-fold symmetry axis in interlayer space. The N2 and NH3 molecules are located near the cage centers and, unlike H2O molecules, are included in the coordination environment of the cations. Our study confirms that NH3- and N2-rich K-cymrite may be stable in metapelites and can act as a redox insensitive carrier of nitrogen to >250 km mantle depths in downgoing slabs. The stability field of N-rich K-cymrite in the presence of an N2–H2O–NH3-bearing fluid is inferred to be P ≥ 4 GPa in metasediments rich in K-feldspar and P ≥ 6 GPa in those containing phengite. As the slab material sinks deeper than 250–300 km where N-bearing K-cymrite may lose stability, the releasing nitrogen may migrate to metal-saturated mantle and become stored there in γ−Fe, Fe3C, metal melt, or even iron nitride phases.

A combined cathodoluminescence and electron backscatter diffraction examination of the growth relationships between Jwaneng diamonds and their eclogitic inclusions

To fully understand the implications of the compositional information recorded by inclusions in diamond it is vital to know if their growth was syn- or protogenetic and the extent to which they have equilibrated with diamond forming agents. The current paradigm is that the majority of inclusions in diamond are syngenetic but recently this assumption has been questioned. This study presents an integrated cathodoluminescence (CL) and electron backscatter diffraction (EBSD) study of 8 diamonds containing eclogitic inclusions: 19 pyrope-almandine garnets, 12 omphacitic clinopyroxenes, 4 sulphides, 1 coesite and 1 rutile from the Jwaneng diamond mine, Botswana. Diamond plates were sequentially polished to expose inclusions at different levels and CL imaging and EBSD were performed to constrain the relationship between diamond and inclusion growth. Despite complex growth and resorption, individual diamonds are single crystals with a homogeneous crystallographic orientation. All individual inclusions have homogeneous crystallographic orientation and no resolvable compositional zonation. The combined CL and EBSD data suggest that epitaxial inclusion-diamond growth is rare (none of 24 inclusions) and that the imposition of cubo-octahedral faces on inclusions does not necessarily result in epitaxy. Individual diamonds contain inclusions that record evidence of both syngentic and protogenetic relationships with the host diamond and in one case an inclusion appears syngenetic to the diamond core but protogenetic to the growth zone that surrounds 70% of the inclusion. These findings emphasise that inclusions in diamonds have multiple modes of origin and that in order to validate the significance of geochronological studies, further work is needed to establish that there is rapid chemical equilibration of protogenetic inclusions with diamond forming agents at mantle temperatures.

Peridotite-derived detrital pyropes versus high-pressure felsic granulite-derived pyrope-almandine garnets from the Lower Triassic deposits of the NE foreland of the Bohemian Massif (S Poland, Central Europe)

Numerous pyrope grains (group I; Prp62–72Alm14–24Grs(<1)4–14Uv(1)2–14Sps<1–1Adr<1–2) have been found in the heavy mineral assemblages from the Older Buntsandstein arenites (Lower Triassic) cropping out in S Poland (NE foreland of the Bohemian Massif). This is the first find of the abundant detrital pyropes outside, but near to, the Bohemian Massif, and the first from the Lower Triassic deposits. In the heavy mineral assemblages from the younger wackes and marly limestones (Older Buntsandstein and Röt deposits), from the same area, Ca-low pyrope-almandines of subgroups IVA (Prp28–52Alm44–68Grs1–4Uv<1Sps1–2Adr<1) and IVB (Prp27–50Alm41–65Grs>5–15Uv<1Sps<1–2Adr<1–1) are the most numerous. The main aims of this study were to determine the primary and secondary source rocks for the garnets, and to identify a cause for the change of the garnet types in the older and younger deposits. Pyropes contained in the Bohemian Massif peridotites, presently exposed in the Moldanubian Zone, and the Kutná Hora Unit, show most comparable chemical compositions to group I pyropes. Some group I pyropes have no chemical counterparts within the comparative materials, which suggests that some other pyrope-bearing peridotites may have been exposed within the Bohemian Massif, but destroyed in the past. Garnets contained in the high-pressure felsic granulites known from several subunits of the Moldanubian Zone show similar chemical compositions to the garnets from subgroup IVA. The group I grains reveal features pointing to (possibly multiple) reworking and likely longer transport distance, while the group IV grains reveal signs of not advanced abrasion and possible short transport distance. However, no older or coeval sedimentary rocks with the similar heavy mineral assemblages, nor the garnet groups, are recognized within the Bohemian Massif or the adjacent areas, which suggests that the garnets may have been recycled from deposits largely destroyed in the past. The deposition of group IV garnets was a normal occurrence for a long time span. In contrast, the deposition of the detrital pyropes was an unusual event. These pyropes must have been redeposited from pyrope-bearing ‘horizons’ present in Carboniferous-Lower Triassic sedimentary rocks. Their availability was limited to the time span necessary for erosion of these horizons.

Shrilankite Inclusions in Garnets from Kimberlite Bodies and Diamondiferous Volcanic–Sedimentary Rocks of the Yakutian Kimberlite Province, Russia

Pyrope–almandine garnets (Mg# = 28.3–44.9, Ca# = 15.5–21.3) from a heavy mineral concentrate of diamondiferous kimberlites of the largest diamond deposit, the Yubileinaya pipe, along with kimberlite- like rocks and diamondiferous volcano–sediments of the Laptev Sea coast, have been found to contain polymineral, predominantly acicular inclusions, composed of aggregates of shrilankite (Ti2ZrO6), rutile, ilmenite, clinopyroxene, and apatite. The presence of shrilankite as an inclusion in garnets from assumed garnet–pyroxene rocks of the lower crust, lifted up by diamond-bearing kimberlite, allows it to be considered as an indicator mineral of kimberlite, which expands the possibilities when searching for kimberlite in the Arctic.

Garnet hornblendite in the Meatiq Core Complex, Central Eastern Desert of Egypt: Implications for crustal thickening preceding the ∼600 Ma extensional regime in the Arabian-Nubian Shield

Dike-like bodies of garnet (Py77.04–84.66) hornblendites were recorded, for the first time, in the structurally lowest parts of the gneiss of Meatiq Core Complex in the Central Eastern Desert of Egypt. They are composed mainly of hornblende (pargasite to pargasitic hornblende), [depleted in LREE with flat HREE segments (normalized to CI-chondrite), and enriched in LILE (normalized to N-MORB)] and crystallized from sub-arc tholeiitic melt that derived from wet mantle wedge (615–600Ma ago), at P=23–27kbar and T=510–600°C. Diapirism of hot mantle due to decompression caused partial dehydration melting (P=∼12kbar and T=800–>1100°C) of hornblende into pyrope-almandine garnet and diopside (Wo49–51En32–34Fs16–17.5). The clockwise P-T path continued by isothermal decomposition (10–11kbar) of garnet into hercynite-rich spinel (XFe2+=0.46–0.64) and quartz. By continuous cooling and probably by introduction of the Abu Ziran granite-related solutions, the hornblendites followed a retrograde greenschist facies hydration path, including transformation of diopside to tremolite (T=280–420°C and pressure>5kb) and garnet to chlorite (T=190–345°C). In addition, metasomatic minerals mainly of feldspars and titanite, and minor ilmenite, rutile, topaz and calcite formed, as well as composition of the original amphibole changed having increases of FeO, TiO2 and K2O and decreases of Al2O3, MgO and Na2O. Deep source of the Meatiq hornblendites implies for a probable local crustal thickening of Meatiq Core Complex by crustal shortening, while their exhumation was most probably accompanied the NW-SE extension and thinning of the previously thickened crust that occurred during oblique island arc convergence with the closure of the Mozambique ocean and the collision of East and West Gondwanaland.

Mineral inclusions in sapphire from the basalt-related deposit in Bo Phloi, Kanchanaburi, western Thailand: indication of their genesis

The Bo Phloi gem field in Kanchanaburi Province, Western Thailand, is closely associated with Cenozoic basalts. Blue and yellow sapphire, black spinel, and minor zircon have been mined for over three decades. The mineral inclusions observed in sapphire samples are alkali feldspar, nepheline, hercynitic spinel, zircon, manganiferous ilmenite, silica-rich enstatite, almandine–pyrope garnet, monazite, calcite, sapphirine, biotite–phlogopite mica, and staurolite. Based on their geochemical affinity, these mineral inclusions can be categorized into two main groups: felsic alkaline and contact-metamorphic, which appear to have originated from different processes. These inclusions provide new evidence for proposing a bimodal genetic model. Felsic alkaline origin is evidenced by the occurrence of a felsic alkaline inclusion suite and the REE geochemistry of sapphire-associated zircon, which indicates that most of the sapphires crystallized from a high-alkali felsic melt (probably, in the lower crust). Contact-metamorphic origin is evidenced by the presence of a contact-metamorphic inclusion suite, suggesting that some of these sapphires might also have originated from metasomatized crustal rocks and a contaminated melt along the contact zone of a basaltic intrusion (probably, in the upper mantle or lower crust).

Garnet-bearing basites of the Kuvalorog massif ( Kamchatka Peninsula )

The petrographic, petrochemical, and mineralogical compositions of the Kuvalorog Ni-bearing cortlandite-norite intrusion and endocontact leucodiorites hosting pyrope-almandine garnet are considered. Lamprophyre dikes with scarce pyrope-almandine garnet, first discovered in the massif, and plagioclase-pyroxene symplectites in garnet porphyroblast areas are studied. Judging from the petrography of rocks and the composition of inclusions rich in incompatible elements in the garnet, the mineral was produced by the reaction of orthopyroxene with the anorthite component of plagioclase at the subsolidus stage of formation of endocontact diorites. This reaction was probably favored by the fluid phase abundant in the parental magma of the Kuvalorog intrusion and, especially, in the zones near its contact with relics of terrigenous rocks, where it was produced as a result of the rock dehydration under the thermal effect of the intrusion.

Garnets of crustal parageneses in alluvial deposits of the eastern Siberian Platform: genesis and search significance

We present results of study of the specific composition and parageneses of pyrope-almandine garnets from alluvial deposits of the Muna-Markha interfluve drainage in the Yakutian diamondiferous province. The data are correlated with the specific features of garnets (tentatively crustal) from Yakutian and Guinean kimberlites and their crustal xenoliths and from metamorphic rocks of the Anabar Shield. The conclusion is drawn that most of orange pyrope-almandine garnets from the alluvial deposits were generated from kimberlites with varying contents of disintegrated crustal material. This gives grounds to include the studied garnets into a complex of indicator minerals of kimberlites and use them in search for diamonds in the Yakutian diamondiferous province.

Electrical conductivities of pyrope-almandine garnets up to 19 GPa and 1700 C

Electrical conductivities of polycrystalline garnets ranging in chemical composition from almandine (Fe 3 Al 2 Si 3 O 12 ) to pyrope (Mg 3 Al 2 Si 3 O 12 ) were measured at 10 GPa and 19 GPa at temperatures ranging from 300 to 1700 °C using complex impedance spectroscopy in a multianvil device. Mössbauer spectroscopy of each sample was carried out both before and after the electrical measurements to characterize the oxidation state of Fe in the almandine bearing garnets. Similar to the behavior of other ferromagnesian silicates, the substitution of Fe for Mg along this compositional join dramatically increases electrical conductivity, but this compositional effect is reduced with increasing temperature. Conductivities increase with increasing total Fe content, as the average Fe 2+ -Fe 3+ distance decreases. At 10 GPa, activation energies for conductivity vary smoothly with composition and increase rapidly toward the pyrope end-member composition, where it reaches a value of 2.5 eV. The results are consistent with an electrical conductivity mechanism involving small polaron mobility in the Fe-bearing garnets at 10 GPa. At 19 GPa, however, there is virtually no change in the activation energy as a function of Fe-Mg substitution for the pyrope-rich garnets. These higher pressure measurements reflect a mechanism involving oxygen related point defects, as conductivities increase with pressure at constant T for each garnet, and the effect of pressure is greater for the more Mg-rich garnets. The data also allow for a more quantitative evaluation of the effect of chemical composition, specifically Fe-Mg substitution, on the electrical conductivity profile of the mantle, using a recently developed laboratory- derived model. We apply the model using these data to a portion of the transition zone between 520 and 660 km, in which we vary the garnet composition from Py 100 to Py 85 Alm 15 . Although only a minor effect on bulk mantle conductivity results, we conclude that the overall garnet composition may, however, be important in characterizing the magnitude of any EC discontinuity with respect to the above-lying mantle.

The origin of asterism in almandine-pyrope garnets from Idaho

We have shown that the asterism in garnets emanating from Emerald Creek, Latah County, Idaho, is due to the precipitation of acicular crystals in the garnet crystal host. These inclusions have been identified, using electron microscopy, as tetragonal rutile, the stable phase of TiO2. One set of rutile (r) needles was found to have the following crystallographic relationship with its garnet (g) host: \([011]_g /\!/[0\bar 10]_r,(02\bar 2)_g /\!/(100)_r,(\bar 400)_g /\!/(001)_r\). From meticulous examinations of the asteriated garnets and the angular relationships between the rutile inclusions and the garnet matrix it was possible to determine the orientations of the other sets of needles for both four and six rayed star garnets. A six rayed star garnet exhibits needles aligned along the \(\langle 111\rangle\) and \(\langle 011\rangle\) directions of the garnet while a four rayed star garnet exhibits needles aligned along the \(\langle 111\rangle\) directions only. These results are believed to be applicable to asteriated garnets emanating from all sources.

A Microscopy Study on the Origin of Asterism in Almandine-Pyrope Garnets

Extract HTML view is not available for this content. However, as you have access to this content, a full PDF is available via the ‘Save PDF’ action button. Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005

Garnet zoning in high pressure granulite-facies metapelites, Mozambique belt, SE-Kenya: constraints on the cooling history

Three metapelitic samples from the granulite facies Taita Hills, part of the Neoproterozoic Mozambique belt in SE-Kenya, contain nearly pure almandine-pyrope garnets. These garnets show a diffusional zoning of X Fe = Fe/(Fe+Mg) at the rim over a distance of ∼200-500 μm if in contact with biotite. Garnet-biotite Fe-Mg exchange thermometry yields closure temperatures between 530–735°C. Diffusion zoning profiles in garnets are used to estimate cooling rates using a numerical model. For the calculations a metamorphic peak temperature and pressure of 820°C and 1.15 GPa are obtained from mafic granulites. Matching of numerically modelled and observed zoning profiles indicates cooling rates between 1–3°C/my. Comparison with cooling rates estimated with the analytical approach of Ehlers & Powell (1994) and with geochronologically derived cooling rates shows that the volumetric ratio of biotite to garnet was about 0.5 during closure. This is consistent with the volumetric ratio observed in thin section, but inconsistent with microprobe analyses that indicate that only biotite in the immediate vicinity of garnet equilibrated with garnet. Conversely, significant garnet zoning only occurs where in contact with biotite. We suggest that these inconsistencies can be explained with changes in the grain boundary processes during cooling: in the thermal evolution above the closure temperature around 735°C a fast grain boundary model applied so that all biotite in the thin section equilibrated with garnet. At lower temperatures local zoning developed, but did not influence the composition of the garnet grain centers. The change in grain boundary process from fast to slow diffusing grain boundaries may correlate with the solidus temperature of the rock.

Major and trace elements in pyrope–almandine garnets as sediment provenance indicators of the Lower Carboniferous Culm sediments, Drahany Uplands, Bohemian Massif

The Lower Carboniferous Culm Basin is part of the European Variscan foreland basin. Garnet is an abundant heavy mineral at all stratigraphic levels of the Culm sediments (the Protivanov, Rozstání and Myslejovice Formation from base to top). This paper compares the major element (electron microprobe) and trace element (LA-ICP-MS) compositions of garnet for the purpose of studying the provenance of the Culm sediments. The polymict garnet assemblages ranging from the Protivanov to lower part of the Myslejovice Formation (Goniatite zones Pe γ-Go α) pass into oligomict ones dominated by pyrope–almandine garnets (uppermost part of the Myslejovice Formation; Goniatite zones Go β-Go γ). As the predominant low-grossular pyrope–almandines from oligomict garnet assemblages are very uniform and homogeneous in major element composition, we studied their trace element composition and zoning. Pyrope–almandine garnets are poor in LREE and show enrichment in HREE; the chondrite-normalized patterns are almost flat from Dy to Lu and show a significant negative Eu anomaly, typical for granulite-grade garnets. Major element compositions of detrital low-grossular pyrope–almandines can only be matched with some granulites and garnetiferous felsic gneisses of the Bohemian Massif. Garnets from granulites cropping out at the present-day erosion level of the Bohemian Massif have usually higher Ca and/or lower Mg contents. The major and trace element compositions and zoning patterns of pyrope–almandine garnets from the upper part of the Myslejovice Formation compare well with garnets from the Miroslav granulites and small Moldanubian granulite bodies west of the Třebíč Massif. The Moldanubian granulites are commonly associated with mantle-derived peridotites and eclogites. The presence of detrital Cr-spinels with high Cr# and low Mg# (single grains of Cr-spinels and fine-grained kelyphitic intergrowths of Cr-spinel with chlorite) comparable with spinel types from Moldanubian peridotites could indicate the joint occurrence of peridotites, serpentinized peridotites, and granulites at the erosion level of the Bohemian Massif during the deposition of the uppermost Culm sediments (325 Ma). While the older Culm sediments (polymict garnet assemblages) represent the accumulation of clastic material from the northern and north-western areas (Moravo-Silesian and Lugian Zone), the uppermost Culm sediments (oligomict garnet assemblages) contain material from western and south-western areas (Moldanubian Zone and Moravian Nappe); the general influx of clastic material changed as a result of clockwise rotation of the eastern part of the Variscan belt during this time span (340–325 Ma).

Fe2+-Mg partitioning between garnet, magnesiowüstite, and (Mg,Fe)2SiO4phases of the transition zone

The partitioning of Fe and Mg between garnet coexisting with olivine, wadsleyite, ringwoodite and magnesiowustite solid solutions has been measured between 9 and 19 GPa and 1400-1700 °C. In addition to studying exchange reactions involving the pyrope-almandine garnet solid solution, majoritic garnets forming in a natural peridotite bulk composition have been also examined. Metallic Fe was added to all bulk compositions to buffer the Fe 3 + concentration at its lowest possible level. Partitioning data between pyrope-almandine garnet and ringwoodite were combined with data for garnet-magnesiowustite and ringwoodite-magnesiowustite partitioning to refine thermodynamic mixing properties for these solid solutions at 18 GPa and 1400 °C. A multiple non-linear regression employing cation exchange data between these 3 solid solutions yielded the following well-constrained values for the differences in symmetric solid-solution interaction parameters: W m w F e M g - W r i n g F e M g = 8.8(2) kJ/mol, W m w F e M g - W g 1 F e M g = 12.8(5) kJ/mol,W r i n g F e M g - W g t F e M g = 4.0(4) kJ/mol. These differences were then solved simultaneously to give, on a single site basis: W m w F e M g = 13.2(3) kJ/mol, W r i n g F e M g = 4.4(2) kJ/mol, W g t F e M g = 0.3(3) kJ/mol. In a peridotite composition, the occurrence of majorite and grossular garnet components produced a measurable influence on the Fe-Mg partitioning, however, the combined effect was relatively small because each individual component affected the partitioning in an opposite direction. These data, combined with previously determined phase relations in the Mg 2 SiO 4 -Fe 2 SiO 4 system, were used to calculate the influence of garnet Fe-Mg partitioning on the pressure intervals of divariant (Mg,Fe) 2 SiO 4 phase transformations in the transition zone. Results show that the presence of garnet reduces the pressure interval over which the (Mg 0 . 9 ,Fe 0 . 1 ) 2 SiO 4 olivine to wadsleyite transformation occurs in a peridotite composition to 0.35 GPa (10 km) in comparison to 0.5 GPa in a garnet-free system. This width, however, is still greater than the estimated width of 4 km for the 410 km discontinuity observed in some regions of the Earth as inferred from high frequency reflected and converted seismic waves. The existence of garnet in a peridotite composition in the mantle cannot, therefore, be the only explanation as to why the 410 km discontinuity is, in some regions of the Earth, apparently sharper than experimental estimates for the (Mg 0 . 9 ,Fe 0 . 1 ) 2 SiO 4 olivine to wadsleyite transformation. The wadsleyite to ringwoodite transformation is reduced to 0.8 GPa (25 km) compared to 1 GPa in the garnet-free system.

Combined external-beam PIXE and μ-Raman characterisation of garnets used in Merovingian jewellery

Red garnets were the dominant gemstones used for jewels in Europe during the Early Middle Ages. We have studied over 350 garnets set on 12 jewels unearthed in the royal necropolis of the Saint-Denis Basilica, close to Paris. This famous collection of “cloisonné” style artefacts dates from the Merovingian period (late fifth century AD to early seventh century AD). The archaeological issue addressed is the identification of the geographical origin of these garnets, in view to establish the gem trading routes during the Dark Ages. External beam PIXE was used to determine the major constituents (Mg, Al, Si, Ca, Mn, Fe), specifying the garnet type (composition in various mineralogical end-members, e.g. almandine, pyrope, spessartite, …), and the trace element content (Cr, Y). Three sorts of garnets were identified. Ten jewels are adorned with almandine garnets (Fe-rich). One jewel has intermediate almandine-pyrope garnets (“rhodolite”). The last and most recent jewel is inlaid with pyrope (Mg-rich) garnets. Trace element content and slight differences in major composition allowed to distinguish five different sources: two sources for pyrope garnets (with and without chromium), and two sources for almandine garnets (distinctive calcium, magnesium and yttrium contents). A preliminary comparison with literature data suggested that almandine garnets may have been mined from India while the “rhodolite” garnets may have been imported from Sri Lanka. The sources of pyrope garnets could be the Bohemian deposits (Czech republic). In addition, μ-Raman spectrometry was used to identify most of the mineral inclusions (apatite, zircon, ilmenite, monazite, calcite, quartz) present in almandine garnets. Even if two specific types of inclusions were not identified, due to the lack of corresponding reference spectra in our database, the Raman spectra collected provided an interesting inclusion fingerprint.