We present U–Pb dates from peridotitic pyrope-rich garnet from four mantle xenoliths entrained in a kimberlite from Bultfontein, South Africa. Garnet dates magmatic emplacement due to the high mantle residence temperatures of the source material prior to eruption, which were most likely above the closure temperature for the pyrope U–Pb system. We determine a U–Pb date of 84.0 ± 8.1 Ma for the emplacement of the Bultfontein kimberlite from garnet in our four xenolith samples. The date reproduces previous dates obtained from other mineral-isotope systems (chiefly Rb–Sr in phlogopite). Garnet can be dated despite extremely low concentrations of U (median ∼0.05 μg/g), because concentrations of common Pb are often low or non-detectable. This means that sub-concordant garnets can be dated with moderate precision using very large laser-ablation spots (130 μm) measured by quadrupole inductively coupled plasma – mass spectrometry (LA-Q-ICP-MS). Our strategy demonstrates successful U–Pb dating of a U-poor mineral due to high initial ratios of U to common Pb in some grains, and the wide spread of isotopic compositions of grains on a concordia diagram. In addition, the analytical protocol is not complex and uses widely available analytical methods and strategies. This new methodology has some advantages and disadvantages for dating kimberlite emplacement versus established methods (U-based decay systems in perovskite and zircon, or Rb- or K-based systems in phlogopite). However, this method has unique promise for its potential application to detrital diamond prospecting and, more speculatively, to the dating of pyrope inclusions in diamond.

The reconstruction of the trade routes along which garnets reached Europe in the early Middle Ages demonstrates the persistence of long-distance trade after the fall of the Western Roman Empire. Early medieval garnet jewellery from Italy and the presence of lapidary workshops are important evidence for understanding the dynamics of this commerce but are systematically overlooked. Chemical trace-element analysis (LA-ICP-MS) of loose and inset garnets and glass inlays from burials in sixth–seventh-century Lombardy has identified South Asian and Bohemian garnets together with Egyptian glass. This combination shows that the long-distance trade routes crossing the Peninsula and the Alpine passes played a key role in the European market for garnets, significantly modifying the current model of the Mediterranean garnet trade and shedding new light on the character of the elites who emerged in Italy during the Migration period.

The response of garnet and zircon to prograde amphibolite-facies metamorphism in late Proterozoic mica schists from the Scottish Highlands has been investigated. Spatial analysis of zircon populations using scanning electron microscopy was undertaken in Dalradian Schists that have undergone a sequence of prograde garnet growth and localised breakdown reactions involving coupled dissolution–reprecipitation. Fluid availability and matrix permeability strongly control this metamorphic response and different generations of garnet contain radically different populations of metamorphic micro-zircon and associated changes in the detrital zircon population. Micro-zircon abundance increases during garnet growth, whereas that of detrital zircon decreases. The mineralogy of the matrix influences zircon abundance in porphyroblast phases, where garnet overgrows a micaceous matrix zircon-rich garnet forms and where it overgrows a quartzofeldspathic matrix the result is zircon-poor garnet. Following garnet growth, micro-zircon abundance decreases at each stage of the prograde reaction history, with sillimanite-zone schists containing the lowest abundance, suggesting micro-zircons are texturally less stable at staurolite- and sillimanite-grade metamorphism. Micro-zircons are distributed evenly across host minerals in the matrix, with the exception of retrograde chlorite where micro-zircons are absent due to fluids removing Zr before new zircon can precipitate. There is an overall decrease in the mode of zircon at each stage of the reaction history, indicating that zircon is a highly reactive phase during amphibolite-facies metamorphism and is very sensitive to individual prograde and retrograde reactions.

The late Palaeozoic Yong’an–Meizhou depression belt is an important iron (Fe) and polymetallic metallogenic belt in southern China. It has undergone a transformation from Tethys to the circum-Pacific tectonic domain. The Luoyang deposit is one of the typical Fe skarn deposits in the Yong’an–Meizhou depression belt of eastern China. Garnet is a characteristic mineral in the deposit. Two generations of garnets are detected in the deposit based on their textural characteristics and trace-element contents, and are represented by Fe-enriched andradite. The first generation of garnets (Grt1) have two types of garnets (Grt1-A and Grt1-B). Type A garnets of the first generation (Grt1-A) (Adr80-88) replaced by massive diopside-magnetite assemblage exhibit distinct oscillatory zonings and display patterns of enriched light rare earth elements (LREE) to weak heavy rare earth elements (HREE), with weak negative to positive Eu anomalies, and highest U, ΣREE and Sn contents. Type B garnets of the first generation (Grt1-B) are irregular zones (Adr94-96) coexisting with magnetite, in which Grt1-A is generally dissolved, and have obviously LREE-enriched and HREE-depleted patterns, with weak negative to positive Eu anomalies, and moderate U, ΣREE and Zn contents. Garnets of the second generation (Grt2) (Adr96-99) that replaced massive magnetite together with sphalerite show unzoned patterns, with a flat REE pattern and pronounced negative Eu anomalies as well as contents of lowest U and ΣREE, and highest W. The substitution of REEs in garnets occurs as [X2+]VIII –1[REE3+]VIII +1[Si4+]IV –1[Z3+]IV +1in an Al-enriched environment. Luoyang hydrothermal fluids shifted from reducing conditions with relatively high-U and -ΣREE characteristics to oxidizing conditions with relatively low-U and -ΣREE characteristics. The reduced siderophile elements and increased fO2 in fluid during Grt1-B formation caused magnetite mineralization and reduced Zn contents during Grt2 formation, causing the deposition of sphalerite. All garnets formed from magmatic fluid and were controlled by infiltrative metasomatism in an opened system.

The crystal structure of nikmelnikovite, Ca12Fe2+Fe3+3Al3(SiO4)6(OH)20, a new member of the garnet supergroup from Kovdor massif, Kola Peninsula, Russia (R$\bar{3}$, a = 17.2072(6), c = 10.5689(4) Å, V = 2710.1(2) Å3 and Z = 3) has been refined to R1 = 0.046 on the basis of 1184 unique observed reflections. Nikmelnikovite is the first mineral species in the garnet supergroup that has a trigonal (rhombohedral) symmetry. The relationship between its unit cell and the pseudocubic (ideal garnet) unit cell can be described by the transformation matrix [1$\bar{1}$0 | 01$\bar{1}$ | ½½½]. The crystal-chemical relations between the ideal Ia$\bar{3}$d garnet and the nikmelnikovite structure type can be described by the following series of imaginary modifications: (1) the symmetry is lowered according to the Ia$\bar{3}$d → R$\bar{3}$ group–subgroup relationship; (2) the cation sites are split according to the following sequences: X → {X1, X2}; Y → {Y1, Y2, Y3, Y4}; Z → {Z1, Z2}; (3) the X sites remain fully occupied by Ca; (4) each Y site is occupied predominantly by a distinct chemical species: Y1 → Al (Al site), Y2 → Fe2+ (Fe1 site), Y3 → Fe3+ (Fe2 site), Y4 → vacancy (Mn site); (5) one of the Z sites (Z1) is occupied by Si, whereas the other site (Z2) is predominantly vacant. The crystal-chemical formula that takes into account the transition between the archetype and the nikmelnikovite structure type can be described as X{Ca12}Y[Fe2+Al4Fe3+2□]Z(Si6□6)O24(OH)20□4. The structural complexity of nikmelnikovite (4.529 bit/atom and 434.431 bit/cell, after H-correction) is higher than those for andradite, grossular and katoite, which is typical for low-temperature minerals formed after primary minerals with simpler structures.

Detrital coesite-bearing garnet is the final product of a complex geological cycle including coesite entrapment at ultra-high-pressure conditions, exhumation to Earth’s surface, erosion and sedimentary transport. In contrast to the usual enrichment of high-grade metamorphic garnet in medium- to coarse-sand fractions, coesite-bearing grains are often enriched in the very-fine-sand fraction. To understand this imbalance, we analyse the role of source-rock lithology, inclusion size, inclusion frequency and fluid infiltration on the grain-size heterogeneity of coesite-bearing garnet based on a dataset of 2100 inclusion-bearing grains, of which 93 contain coesite, from the Saxonian Erzgebirge, Germany. By combining inclusion assemblages and garnet chemistry, we show that (1) mafic garnet contains a low number of coesite inclusions per grain and is enriched in the coarse fraction, and (2) felsic garnet contains variable amounts of coesite inclusions per grain, whereby coesite-poor grains are enriched in the coarse fraction and coesite-rich grains extensively disintegrated into smaller fragments resulting in an enrichment in the fine fraction. Raman images reveal that: small coesite inclusions of dimension < 9 µm are primarily monomineralic, whereas larger inclusions partially transformed to quartz; and garnet fracturing, fluid infiltration and the coesite-to-quartz transformation is a late process during exhumation taking place at c. 330°C. A model for the disintegration of coesite-bearing garnet enables the heterogeneous grain-size distribution to be explained by inclusion frequency. High abundances of coesite inclusions cause a high degree of fracturing and fracture connections to smaller inclusions, allowing fluid infiltration and the transformation to quartz, which in turn further promotes garnet disintegration.

Goldmanite, the vanadium analogue of grossular and andradite, was found as subrounded green porphyroblasts ranging from 0.1 to 1.7 mm in size in the carbonaceous black slates from the Deokpyeong area of the Ogcheon belt in Korea. A radiating aggregate of slightly curved blades of goldmanite crystals displays birefringence and replaces the fine-grained black matrix. The V2O3 content ranges from 21.9 to 26.6 wt.% (24.0 wt.% average), higher than previously reported values. The calculated mole % of the goldmanite end-member ranges from 72 to 91. Its cell dimension was calculated to be 12.04 Å from X-ray diffraction. The black slate in the Deokpyeong area is the richest known accessible source of relatively pure goldmanite.

There is an association between the development of cleavage domes, a texture reflecting the displacement of insoluble matrix grains by porphyroblasts growing under a bulk hydrostatic stress, and textural sector-zoning. This has been found in garnet, staurolite, chiastolite, pyrite and possibly emerald porphyroblasts. Sector-zoned porphyroblasts form by lineage growth normal to the crystal faces. This causes several distinctive textures (type 1 inclusions and type 2 intergrowths, inclusion bands, growth prongs), all of which are directly or indirectly related to displacement growth. Graphite or other carbonaceous material is ubiquitous in samples showing textural sector-zoning.

The early P-T evolution of a garnet-kyanite gneiss from the Variscan Ulten Zone has been defined by detailed petrographic observations leading to the detection of chlorite-epidote- and staurolite-bearing assemblages enclosed in kyanite porphyroblasts. Calculations of P-T pseudosections in the system NaCaKFeMgAlSiHO allowed us to constrain the evolution of these relics to the earliest metamorphic stages. The overall path shows a P-T increase to a peak of 11–12 kbar and 600–650°C followed by decompressional heating to 720°C and 9–10 kbar and final cooling at 7 kbar, 550–600°C. This clockwise P-T path reflects crustal thickening and subsequent thermal decay related to the continental collision of the Variscan orogeny 330–340 Ma ago. Our study demonstrates that large kyanite porphyroblasts may preserve assemblages related to prograde metamorphic stages. As a result, the detection of mineral inclusions in kyanite can complement many similar studies on mineral suites hosted in garnet and zircon.

I will consider mainly geothermobarometry in medium-grade pelitic rocks, including the garnet-biotite (GB) geothermometer, the Grossular-Al silicate-plagioclase (GASP) geobarometer, and the muscovite-almandine-biotite-sillimanite (MABS) geobarometer. For GB (Holdaway, 2000) experimental data and estimated biotite ΔWTi were used to optimize two exchange parameters and four biotite Margules parameters. Using stepwise linear regression, experimental vs. calculated T were constrained to lie on a line with slope of one and intercept of zero, maximizing r2. The best model involves experiments by Ferry and Spear (1978) and Perchuk and Lavrent’eva (1983), suggesting minimal viAl in the Ferry and Spear product biotite. For GASP (Holdaway, 2001), end-member experimental data do not adequately constrain the equilibrium. I used the GB model above, and allowed the end-member curve to rotate about the best-constrained part of the GASP end-member data. The end-member curve was further constrained with the kyanite-sillimanite (K-S) boundary using published chemical data on 76 pelitic schist samples from 11 localities, rejecting Low-Grs and low-An samples. The Fuhrman and Lindsley (1988) plagioclase model gives the best results. For MABS, work in progress involves 61 samples from the 11 localities which have muscovite analyses. Biotite Margules parameters were based on the GB model and McMullin et al. (1991). The MABS end-member curve was calibrated by comparison of P values determined using trial MABS data and GASP results. The P values for the 61 samples agree well with the K-S boundary, and sillimanite-bearing rocks of west-central Maine all fall in the sillimanite field. Preliminary biotite values are: GAnn = –5149198 – 412.05 T, WAlFe = –14023 + 28.14 T, WAlMg = –259582 + 308.44 T, WTiFe = 124842 – 98.67 T, WTiMg = –186148 + 271.72 T. For geobarometry, the Berman (1988, revised 1992) database was used with adjustable H and S of grossular for GASP and H and S of annite for MABS. The accuracy of currently available databases, activity models and mole fraction models is not adequate for good geothermobarometry, without further refinement. Adjustable parameters tend to compensate for error in activity models, mole fraction models and databases.

MnO zoning profiles for various sizes of garnets from two pelitic schists were examined. The data from garnets of specimen A are consistent with a constant growth law, while the compositional-crystal size relationships of garnets from specimen B are best explained by heterogeneous growth rates.

Garnet is a widespread minor accessory mineral in the Late Proterozoic Elat-Quarry granite of southern Israel and is more abundant in the associated pegmatite and aplite. All garnets are dominated by almandine and spessartine end-members. Granite-hosted garnets are zoned with relative enrichment of Mn in the core and Fe in the rim. The chemistry of the garnet in the pegmatite and aplite are comparable to the rim compositions of garnets in the granite, but with a slight Fe-depletion at the rims. Geochemical parameters for the granite indicate fractional crystallization largely of an S-type source magma to a peraluminous composition.

In the highly evolved granite magma, Fe is relatively diminished, and only small amounts of biotite can crystallize. Manganese becomes a compatible element forming Mn-rich garnet (cores), and reducing the Mn content in the magma, subsequently leading to Fe-enriched rims. The greater abundance of garnet in the pegmatite and aplite (and its larger crystal-size in the former) relate to the enhanced presence of a hydrous fluid within the magma. The tendency for garnet crystals to concentrate in bands is much more developed in the pegmatite than in the granite, and is associated with the effects of hydrofracturing (fracture-filling), and the crystallization of coarse-grained alkaline feldspars.

Almandine- and grossular-rich garnet occurs as both a coronitic and non-coronitic phase in ferruginous, Grenvillian (c. 1050 Ma) granitoid rocks of the composite Potato Hill pluton of the Long Range Inlier, Newfoundland. The country rock includes garnetiferous gneiss, but garnets in the pluton are compositionally distinct (higher Ca and Mn, lower Mg), so none are interpreted as xenocrysts from the Long Range gneiss complex.

Coronal garnet, quartz and hornblende separate primary pyroxene, ilmenite and hornblende from feldspar in two-pyroxene charnockite. Balanced mass-transfer reactions based on microprobe data and modes for the pyroxene-centred corona structures suggest that corona sites gained Fe and lost Na. The flux of Fe apparently controlled corona growth in the charnockite. The corona structures are attributed to subsolidus cooling of the pluton rather than to a metamorphic overprint because the coronas are obliterated in high strain zones cutting the charnockite. Temperature of formation is constrained at c. 775–630°C by two-pyroxene and garnet-hornblende thermometry. Compositionally-similar coronas in rare, Fe-rich enderbite of the Long Range gneiss complex probably formed during cooling after early, high-grade metamorphism or following the regional emplacement of the Grenvillian plutons.

Non-coronitic garnets occur in equigranular and megacrystic hornblende-biotite granite. Garnets in the equigranular granite are large, well-formed, and in some instances are associated with compositional layering of probable igneous origin. These garnets are enriched in grossular (XCa = 0.28), and are therefore interpreted as phenocrysts crystallized at high pressure (>9 kbar?). They would nevertheless have been stabilized to lower pressures by their moderate spessartine content (XMn = 0.13).

Garnets in foliated megacrystic granite form tiny crystals depleted in Fe and Mg, and enriched in Mn and Ca, and in these respects are similar to garnet in deformed charnockite. These garnets are therefore interpreted to have formed (or re-equilibrated) during the late Grenvillian deformation. Garnet is absent in relatively magnesian Grenvillian granites (bulk XFe2+ = 0.50–0.85) elsewhere in the inlier. The restriction of garnet to the Potato Hill pluton (bulk XFe2+ = 0.88–0.94) therefore testifies to bulk compositional controls on the formation of both magmatic and subsolidus garnet in this intrusion.

Twenty vapour-phase garnets were studied in two samples of the Topopah Spring Tuff of the Paintbrush Group from Yucca Mountain, in southern Nevada. The Miocene-age Topopah Spring Tuff is a 350 m thick, devitrified, moderately to densely welded ash-flow tuff that is zoned compositionally from high-silica rhyolite to latite. During cooling of the tuff, escaping vapour produced lithophysae (former gas cavities) lined with an assemblage of tridymite (commonly inverted to cristobalite or quartz), sanidine and locally, hematite and/or garnet. Vapour-phase topaz and economic deposits associated commonly with topaz-bearing rhyolites (characteristically enriched in F) were not found in the Topopah Spring Tuff at Yucca Mountain. Based on their occurrence only in lithophysae, the garnets are not primary igneous phenocrysts, but rather crystals that grew from a F-poor magmaderived vapour trapped during and after emplacement of the tuff. The garnets are euhedral, vitreous, reddish brown, trapezohedral, as large as 2 mm in diameter and fractured. The garnets also contain inclusions of tridymite. Electron microprobe analyses of the garnets reveal that they are almandinespessartine (48.0 and 47.9 mol.%, respectively), have an average composition of (Fe1.46Mn1.45Mg0.03Ca0.10)(Al1.93Ti0.02)Si3.01O12 and are comparatively homogeneous in Fe and Mn concentrations from core to rim. Composited garnets from each sample site have δ18O values of 7.2 and 7.4%. The associated quartz (after tridymite) has δ18O values of 17.4 and 17.6%, values indicative of reaction with later, low-temperature water. Unaltered tridymite from higher in the stratigraphic section has a δ18O of 11.1% which, when coupled with the garnet δ18O values in a quartz-garnet fractionation equation, indicates isotopic equilibration (vapour-phase crystallization) at temperatures of ∼600°C. This high-temperature mineralization, formed during cooling of the tuffs, is distinct from the later and commonly recognized low-temperature stage (generally 50−70°C) of calcite, quartz and opal secondary mineralization, formed from downward-percolating meteoric water, that locally coats fracture footwalls and lithophysal floors.

Manganiferous garnets occur in metamorphosed Mn silicate-oxide and silicate-carbonate-oxide rocks of the Sausar Group, India. The garnets of the carbonatic rock show maximum calderite content (∼50 mole%). Increased Ca content in the bulk has been observed to be sympathetically related to the concentration of calderite, rather than the expected andradite component of the garnets. This is the consequence of the preferential partitioning of Ca in coexisting pyroxmangite and/or carbonate. Petrochemical characteristics of the diverse assemblages in response to the physical conditions of metamorphism show that the calderite solubility in manganiferous garnet is not only a function of pressure. Such characteristics only indicate that the mobile components in the bulk influenced the mineralogy of the assemblages at the ambient physical conditions of metamorphism, and this in turn controlled the calderite solubility in garnet.

The garnet-orthopyroxene Al barometer specifically considers the Al content of orthopyroxene in equilibrium with garnet resulting from Mg-Tschermaks substitution. It is demonstrated that P-T calibrations of this barometer derived solely from experimental data for the MAS system, such as that favoured by Finnerty and Boyd (1984, 1987) based on the data of MacGregor (1974), cannot be expected to yield meaningful pressure estimates for natural garnet lherzolite assemblages. The presence of additional CaO, FeO and Cr2O3 components in natural garnet lherzolites can be expected to influence substantially the Al partitioning between orthopyroxene, garnet and/or spinel at any particular P and T. Thus a more comprehensive barometer formulation is required, such as the one provided by Nickel and Green (1985) that is based on experimental data for the CMAS and SMACCR systems with thermodynamic modelling and addition of an Fe correction term.

It is further emphasised that for orthopyroxenes in natural garnet lherzolites the amount of Al introduced as Mg-Tschermaks substitution cannot be assessed simply as the total Al cation content since such orthopyroxenes frequently contain Al cations linked to Na substitution in M2 sites or to Cr, Ti and possibly Fe3+ in M1 sites. Revised algorithms for the calculation of specific orthopyroxene contents are presented. Application to analytical data sets for garnet lherzolite zenolith suites in the Thaba Putsoa and Mothae kimberlites generates revised upper mantle P-T arrays which refute the widely accepted advocacy by Finnerty and Boyd (1984, 1987) and Finnerty (1989) of an upper-mantle palaeogeotherm beneath northern Lesotho that is markedly inflected to a higher thermal gradient at the depths of derivation of the more chemically fertile, porphyroclastic textured, xenoliths.

Garnets with an unusual inclusion pattern of cylindrical quartz intergrowths have been found to develop exclusively in the presence of graphite. The intergrowths consist of quartz rods, 1–5 µm in diameter, originating at the sector-zone interfaces in the garnet with the long axes normal to the crystal faces. The lattice orientation and continuity of the quartz suggests that the interphase boundaries between the quartz and garnet are epitaxially related and that new material was added to the tube as the crystal face of the garnet grew. In the presence of a C-O-H fluid, at the temperatures and pressures recorded, (P = 6.5 kbar, T = 500°C), the amount of CO2 present restricts the solubility of SiO2 in the intergranular fluid phase, where the oxygen fugacity (fo2) is below the Quartz-fayalite-magnetite (QFM) buffer, and within the stability field of graphite. The reduced solubility will lower the concentration of SiO2 in solution, and hence restrict its ease of transport via the fluid, resulting in an excess of SiO2at the site of garnet growth. Under such conditions the SiO2 is incorporated in the growing garnet in the form of the cylindrical quartz intergrowths.

Fine-grained garnet (grains <50 μm across) is an accessory component of compact hematite, a rock consisting essentially of hematite; compact hematite is a variety of high-grade Fe ore. The garnet is characterized compositionally as spessartine (81–86 mol.%) with subordinate, but significant, amounts of calderite (5–11 mol.%) and “blythite” (up to ∼5 mol.%), as well as andradite (4–7 mol.%); pyrope and almandine endmembers are ≤∼2 and 1 mol.%, respectively. The recognition of spessartine in compact hematite indicates that oxidation state, rather than whole-rock chemical composition, controlled the garnet composition. The spessartine has a positive Eu anomaly, and a low Th/U ratio (0.13–0.65) compared to the average upper continental crust, Th/U = 3.9, as well as a positive linear correlation of U vs. Li. The spessartine-hosting compact-hematite rock is a high-grade hematite ore that similarly shows a low Th/U ratio, but a convex, tetrad-like segment between Gd and Dy. Decoupling of Eu from other rare-earth elements and of U from Th in the spessartine, together with the anomalous geochemical behaviour of Gd, Tb and Dy in the compact hematite, and its low Th/U ratio, could have been achieved under oxidizing conditions at greenschist-facies metamorphic temperatures. The U and Li present in the spessartine could have been sourced from metamorphic fluids of continental, possibly evaporitic, origin. These interpretations are underpinned by the regional hematitization and tourmalinization observed in the southern Serra do Espinhaço.

Garnet pyroxenite xenoliths from the Delegate nephelinitic breccia pipes, New South Wales, Australia, contain relict garnets (py40 alm39 gr21) which are replaced by dark kelyphitic rims resulting from garnet breakdown. The kelyphite is composed of a lamellar intergrowth of secondary minerals, in which the lamellae are <1 μm in width. Analyses by SEM and ICPMS reveal that the kelyphite has an identical bulk chemical composition to the primary garnet. Kelyphitic rims on garnet are well known from xenoliths and xenocrysts in kimberlite pipes and from tectonically-uplifted mafic and ultramafic rocks in some metamorphic terranes. Orthopyroxene occurs in metamorphic kelyphites and it has been assumed that orthopyroxene is also the breakdown product of garnet transported in basic-ultrabasic magmas. However, TEM study of Delegate kelyphite shows that the ultrafine lamellae do not contain orthopyroxene but are instead composed of magnesian clinoferrosilite (En45Fs55), and lesser ferroan spinel and anorthite. The clinoferrosilite is probably the inversion product of initially-formed magnesian protoferrosilite. The breakdown reaction is believed to result from a sudden change to lower temperature and pressure conditions when the xenoliths were transported in the Delegate magma from ∼40 km depth to the surface.