Garnet chemistry Research Papers - Academia.edu (original) (raw)

Beach sediments of the Kerala coast contain rich economically important heavy mineral deposits. Most previous studies have traced the source of heavy minerals to the Precambrian crystalline formations in the hinterland based on comparative mineral occurrences. No previous study has attempted to utilize petrological and geochemical characteristics and mineral chemistry of source rocks to effectively compare and determine sediment provenance. A clear knowledge on the composition of minerals from source and sink is important in precise recognition of source rock. Present study consists the geological, geomorphologic setting and mineralogical characteristics of hinterland rocks to trace their connection to placer deposits. It is recognized that garnet as the abundant heavy mineral in the placer sediments and in source rocks of different ages and petrogenetic affinity. Garnet composition varies between different source rock types due to its strong dependency on the bulk rock composition. In order to decipher the provenance we have compared major element composition of garnet in the source rocks and placer deposits. Compelling similarities in mineralogical and mineral chemical characteristics of garnets (Alm68Prp28Grs3Sps1) is noted between khondalites and placer sediments suggesting latter as major source rock for placer deposits of southern Kerala. The study demonstrates excellent potential of garnets in identifying placer mineral source.

- by and +1
- •
- Earth Sciences, Sedimentary provenance, Heavy Mineral studies;, Beach Placer Deposits

Zusammenfassung:
Lange Zeit haben Wissenschaftler über die Funktion und Datierung der Goldkette von Isenbüttel, einem Altfund aus der Sammlung des Niedersächsischen Landesmuseums debattiert. Bislang waren keine Parallelen zu dieser Fuchsschwanzkette mit Tierkopfenden und Granatcloisonné bekannt. Ausgrabungen von angelsächsischen Friedhöfen der letzten Jahre haben einige Neufunde an Nadelpaaren mit Verbindungskette hervorgebracht, einem Objekttyp welcher vor allem in der zweiten Hälfte des 7. Jahrhunderts in England populär war. Diese Nadelpaare mit Verbindungskette stellen die besten Parallelen zur Goldkette von Isenbüttel dar und demonstrieren weitreichende Verbindungen innerhalb der frühmittelalterlichen Eliten.

- by Alexandra Hilgner
- •
- Archaeology, Early Medieval Archaeology, Monasticism, Archaeometallurgy

Researchers from different fields like archaeology, history, philology and natural sciences present their studies on ancient gemstones. Using precious minerals as an example, trade flows and production methods, but also utilisation and perception are discussed in a cross-cultural and diachronic approach.
The present volume aims at three main questions concerning gemstones in archaeological and historical contexts: »Mines and Trade«, »Gemstone Working« as well as »The Value and the Symbolic Meaning(s) of Gemstones«. This volume contains the proceedings of the conference »Gemstones in the first Millennium AD« held in autumn 2015 in Mainz, Germany, within the scope of the BMBF-funded project »Weltweites Zellwerk – Universal Framework«.

- by RGZM Verlag / publishing house and +1
- •
- Archaeology, Silk Road Studies, Garnets, Garnet chemistry
- by Marion Sorg
- •
- Merovingian period, Garnets, Garnet chemistry, Garnet jewellery
- by Sonja Behrendt
- •
- Archaeometry, Garnet chemistry, Anglo Saxon, Merovingian garnet

A new integrated stratigraphic, sedimentological and geochemical study was carried out on the Gabal El Urf deposits to clarify their facies architecture, sequence stratigraphic framework, and evolutionary stages. The Gabal El Urf in the Northeastern Desert of Egypt is a well-preserved example of the Neoproterozoic volcanosedimentary deposits. It comprises inter-and syn-eruptive facies that were generated by diverse depositional processes including debris flows, sheet floods, lacustrine flood flows, and pyroclastic ash falls/density currents. These facies correspond to alluvial fan, aeolian-fluvial, lacustrine, and pyroclastic facies associations that are arranged in four depositional cycles. They reveal variations in accommodation to sediment supply (A/S) ratio. Unconformities and lithofacies assemblages define six stages of volcanism and sedimentation and three unconformity-bounded tectonic sequences that should be allied to tectonic subsidence and basin uplift. These sequences may be subdivided in Low-Accommodation Systems Tracts (LAST) having negative A/S (A/S ratio close to zero) and High-Accommodation Systems Tracts (HAST) with fluctuation in A/S ratio between zero and +1. Sequences 1 and 3 are characterized by clast-supported conglomerates and pyroclastics, overlain by fluvial sand bodies and debris flow deposits, respectively denoting a LAST (positive A/S ratio close to zero). Sequence 2 is marked by pyroclastic ash fall facies at the base, passing to lacustrine deposits that are interlayered with fandelta conglomerates at the top, defining a HAST (A/S ratio > 1). The defined sequences are superimposed by thick fissure-fed ignimbrites which are capped by a co-ignimbritic breccia lag deposits with concentrated pyroclastic fountains. Clastic petrofacies of the El Urf succession indicate an adjacent miscellaneous rock sources involving volcanic, plutonic, and metamorphic terrains during the chief sedimentation processes in rift-related paleotectonic regime. The low to moderate magnitude of the CIA and PIA together with low SiO 2 /Al 2 O 3 ratio indicate sediment recycling and moderate weathering of the protolith near uplifted basin flank(s) under dominant semi-arid to arid conditions. The development of the El Urf basin is akin to the post-amalgamation Ediacaran volcano-sedimentary basins that are widely distributed in the northern part of the Arabo-Nubian Shield, and is a key tectono-stratigraphic marker for the Cryogenian-Ediacaran shift along a major unconformity surface.

- by Hesham mokhtar and +1
- •
- Geology, Mineralogy, Volcanic Geology, Mineral Processing

The H.U. Sverdrupfjella is part of the high-grade Maud Belt in Dronning Maud Land (East Antarctica), which was located in a central position of the Gondwana supercontinent. Here we study high-pressure granulites from the eastern H.U. Sverdrupfjella and present a detailed reconstruction of the P–T–t history based on a combination of Zr-in-rutile and Ti-in-zircon thermometry, zircon U–Pb dating, monazite chemical dating, garnet diffusion modelling and petrological modelling. Peak metamorphic conditions of 930Cand1Aˊ45GPapersistedforlessthan6Myrandwereattainedat57067Ma,embeddedinawell−documentedclockwiseloading,heating,anddecom−pressionpath.Therockshadalreadybeenrapidlyexhumedtoacrustaldepthof930 C and 1Á45 GPa persisted for less than 6 Myr and were attained at 570 6 7 Ma, embedded in a well-documented clockwise loading, heating, and decom-pression path. The rocks had already been rapidly exhumed to a crustal depth of 930Cand1Aˊ45GPapersistedforlessthan6Myrandwereattainedat57067Ma,embeddedinawell−documentedclockwiseloading,heating,anddecom−pressionpath.Therockshadalreadybeenrapidlyexhumedtoacrustaldepthof30 km at 556 6 7 Ma. In addition to the very short-lived ultrahigh-temperature peak, zircon preserves evidence for protracted granulite-facies conditions with temperatures above 800 C from as early as c. 590 Ma, persisting for c. 40 Myr. Constraints on prograde metamorphism are recorded in zircon and in rutile inclusions in garnet. Zr-in-rutile thermometry using rutile included in different generations of garnet is used to reconstruct the prograde P–T path, documenting burial followed by heating to ultrahigh temperatures at peak pressures. Complementary Ti zonation in prograde cores of zircon grains documents and dates heating and peak temperatures, whereas younger zircon rims show lower Ti-in-zircon temperatures and date the retrograde stages of metamorphism. Our results provide the first evidence for Neoproterozoic high-pressure granulite-facies metamorphism and ultrahigh-temperature conditions for this region. The clockwise loading–heating path and the peak P–T conditions strongly indicate that the rocks preserved in Dronning Maud Land were part of the lower plate during a continent–continent collision event related to Gondwana assembly at c. 570 Ma. The metamorphic evolution determined in this study and the correlation with similar P–T evolutions documented in adjacent terranes favour the continuation of the c. 580–560 Ma Mozambique Belt into Dronning Maud Land. Furthermore, the striking contemporaneity of the metamorphism in the different parts of central Gondwana suggests that the Coats Land Block was part of greater India prior to this collision.

- by Horst R Marschall
- •
- Zircon U-Pb Geochronology, Granulites, High-pressure rocks, Antarctic Geology
- by Christine Siddoway
- •
- Geology, Metamorphic Petrology, Garnet chemistry, Metamorphic Geology
- by Horst R Marschall
- •
- Geology, Geochemistry, Petrology, Zircon U-Pb Geochronology
- by Paul Giesting
- •
- Earth Sciences, Raman Spectroscopy, Mantle Petrology, Entropy
- by Bowen Dong
- •
- Chemical Engineering, Geology, Mineralogy, Chemistry
- by Bowen Dong
- •
- Environmental Engineering, Chemical Engineering, Mathematical Physics, Geology

Linking mineral growth and time is required to unravel the evolution of metamorphic rocks. However, dating early metamorphic stages is a challenge, due to subsequent retrograde overprinting. A perfectly fresh eclogite and a former eclogite retrogressed under amphibolite facies from the southern French Massif Central (Lévézou massif, Variscan belt) were investigated with a large panel of geochronometers (U-Pb on zircon, rutile and apatite, Lu-Hf and Sm-Nd on garnet) in a petrological context tightly constrained by petrographic observation, trace element analyses and phase equilibrium modelling. Both samples recorded similar HP conditions at 18-23 kbar and 680-800°C, while the retrogressed eclogite later equilibrated at 8-9.5 kbar and ca. 600°C. In the retrogressed sample, most of the zircon grains are characterized by negative Eu anomalies and HREE enrichment, and yield an Ordovician U-Pb date of 472.3 ± 1.7 Ma, interpreted as the emplacement age of the mafic protolith. In agreement with other data available for the Variscan belt, and based on zircon trace elements record and whole rock geochemistry, this age is considered to represent the magmatism associated with the extreme thinning of the continental margins during the Ordovician. In the same sample, few zircon rims show a weaker HREE enrichment and yield a date of 378 ± 5.7 Ma, interpreted as a prograde pre-eclogitic age. Lu-Hf garnet dating from both samples yields identical dates of 357 ± 13 Ma and 358.0 ± 1.5 Ma inferred to approximate the age of the high-pressure metamorphic peak. Fresh and retrogressed samples yield respectively 350.4 ± 7.7 Ma and 352 ± 20 Ma dates for Sm-Nd garnet dating, and 367.8 ± 9.1 Ma and 354.9 ± 9.5 Ma for U-Pb rutile dating. Apatite grains from the retrogressed sample give a mean age of 351.8 ± 2.8 Ma. The similarity between all recorded ages from distinct chronometers and radiometric methods (U-Pb – rutile, apatite; Lu-Hf – garnet; Sm-Nd – garnet) combined with P–T estimations from high-pressure metamorphic rocks equilibrated under different conditions testifies to very fast processes that occurred during the Variscan orogeny, highlighting a major decompression of 15-8.5 kbar in less than 7 Myr, and suggesting mean exhumation rates in excess of 6.3 mm/yr.

- by Steve Mittwede
- •
- Mineralogy, Metasomatism, Garnet chemistry, Serpentine

Linking mineral growth and time is required to unravel the evolution of metamorphic rocks. However, dating early metamorphic stages is a challenge due to subsequent retrograde overprinting. A fresh eclogite and a former eclogite retrogressed under amphibolite facies from the southern French Massif Central (Lévézou massif, Variscan belt) were investigated with a large panel of geochronometers (U–Pb in zircon, rutile and apatite, Lu–Hf and Sm–Nd in garnet) in a petrological context tightly constrained by petrographic observations, trace element analyses and phase equilibrium modelling. Both samples recorded similar HP conditions at 18–23 kbar and 680–800°C, whereas the retrogressed eclogite later equilibrated at 8–9·5 kbar and c.600°C. In the retrogressed sample, most of the zircon grains are characterized by negative Eu anomalies and HREE enrichment, and yield an Ordovician U–Pb date of 472·3 ± 1·7 Ma, interpreted as the emplacement age of the mafic protolith. In agreement with other...

- by Robert Anczkiewicz
- •
- Geology, Geochemistry, Petrology, Metamorphic Petrology

In 2013, some dark red crystals were found in the southern mountains of the Negev Desert region near Eilat, Israel. This area is known to be rich in granite- and quartz-bearing rocks. Around 500 g of samples were collected during a two-month period, and a few were fashioned as cabochons, the biggest weighing 1.54 ct.
Gemmological testing was performed on seven rough samples and on one partially polished piece. They ranged from 0.38 to 0.67 g, averaging slightly over 0.4 g and measuring 4.3–7.8 mm in maximum dimension.

- by Guy Borenstein
- •
- Gemology, Geology, Mineralogy, Garnets

The chemistry of garnet can provide clues to the formation of skarn deposits. The chemical analyses of garnets from the Astamal Fe-LREE distal skarn deposit were completed using an electron probe micro-analyzer. The three types of garnet were identiﬁed in the Astamal skarn are: (I) euhedral coarse-grained isotropic garnets
(10–30 mm across), which are strongly altered to epidote, calcite and quartz in their rim and core, with intense pervasive retrograde alteration and little variation in the overall composition (Adr94.3–84.4 Grs8.5–2.7 Alm1.9–0.2) (garnet I); (II) anhedral to subhedral brecciated isotropic garnets (5–10 mm across) with minor alteration, a
narrow compositional range along the growth lines (Adr82–65.4 Grs21.9–11.7 Alm11.1–2.4) and relatively high Cu (up to 1997 ppm) and Ni (up to 1283 ppm) (garnet II); and (III) subhedral coarser grained garnets (N30 mm across) with moderate alteration, weak diffusion and irregular zoning of discrete grossular-almandine-rich do-mains (Adr84.2–48.8 Grs32.4–7.6 Alm19.9–3.5) (garnet III). In the third type, the almandine content increases with increasing grossular/andradite ratio and increasing substitutions of Al for Fe3+.
Almost all three garnet types have been replaced by ﬁne-grained, dark-brown allanite that is typically disseminated and has the same relief as andradite. The Cu content increases while Ni content decreases slightly towards the rim of garnet II and garnet III. Copper in garnet II is positively correlated with increasing almandine content and decreasing andradite content, indicating that the almandine structure, containing relatively more Fe2+,is
more suitable than andradite and grossular to host divalent cations such as Cu2+. Nickel in garnet II is positively
correlated with increasing andradite content, total Fe, and decreasing almandine content. This is because Ni
2+
substitutes for Fe3+ in the Y (octahedral) position. There are unusual discrete grossular-almandine rich domains
within andraditic garnet III, indicating the low diffusivity of Ca compared to Fe at high temperatures

- by saeid baghban
- •
- Geology, Mineralogy, Economic Geology, Mineral Chemistry
- by Jörg Drauschke and +1
- •
- Archaeology, Late Antique Archaeology, Early Medieval Archaeology, Byzantine Archaeology

Garnet from skarns exposed at Empire Mountain, Sierra Nevada (California, United
States) batholith, have variable δ18O values including the lowest known δ18O values of skarn
garnet (–4.0‰) in North America. Such values indicate that surface-derived meteoric water
was a signifi cant component of the fl uid budget of the skarn-forming hydrothermal system,
which developed in response to shallow emplacement (~3.3 km) of the 109 Ma quartz diorite
of Empire Mountain. Values of δ18O, measured in situ across single garnet crystals by secondary
ion mass spectrometry, vary considerably (up to 7‰) and sometimes abruptly, indicating
variable mixing of meteoric, magmatic, and metamorphic water. Brecciation in the skarns and
alteration of the Empire Mountain pluton suggests that fracture-enhanced permeability was a
critical control on the depth to which surface waters penetrated to form skarns and later alter
the pluton. Compared to other Sierran systems, much greater volumes of skarn rock suggest
an exceptionally vigorous hydrothermal system that saw unusually high levels of decarbonation
reaction progress, likely a consequence of the magma intruding relatively cold wallrocks
inboard of the main locus of magmatism in the Sierran arc at that time.

- by Megan D'Errico and +2
- •
- Garnet chemistry, Hydrothermal Ore Deposits, Oxygen Isotopes, Sierra Nevada

- by Pavel Pitra
- •
- Geology, Geochemistry, Petrology, Metamorphic Petrology
- by Ana Albuquerque
- •
- Chemical Engineering, Geology, Mineralogy, Chemistry

The chemistry of garnet can provide clues to the formation of skarn deposits. The chemical analyses of garnets from the Astamal Fe-LREE distal skarn deposit were completed using an electron probe micro-analyzer. The three types of garnet were identiﬁed in the Astamal skarn are: (I) euhedral coarse-grained isotropic garnets (10–30 mm across), which are strongly altered to epidote, calcite and quartz in their rim and core, with intense pervasive retrograde alteration and little variation in the overall composition (Adr94.3–84.4 Grs8.5–2.7 Alm1.9–0.2) (garnet I); (II) anhedral to subhedral brecciated isotropic garnets (5–10 mm across) with minor alteration, a narrow compositional range along the growth lines (Adr82–65.4 Grs21.9–11.7 Alm11.1–2.4) and relatively high Cu (up to 1997 ppm) and Ni (up to 1283 ppm) (garnet II); and (III) subhedral coarser grained garnets (N30 mm across) with moderate alteration, weak diffusion and irregular zoning of discrete grossular-almandine-rich do-mains (Adr84.2–48.8 Grs32.4–7.6 Alm19.9–3.5) (garnet III). In the third type, the almandine content increases with increasing grossular/andradite ratio and increasing substitutions of Al for Fe3+. Almost all three garnet types have been replaced by ﬁne-grained, dark-brown allanite that is typically disseminated and has the same relief as andradite. The Cu content increases while Ni content decreases slightly towards the rim of garnet II and garnet III. Copper in garnet II is positively correlated with increasing almandine content and decreasing andradite content, indicating that the almandine structure, containing relatively more Fe2+,is more suitable than andradite and grossular to host divalent cations such as Cu2+. Nickel in garnet II is positively correlated with increasing andradite content, total Fe, and decreasing almandine content. This is because Ni 2+ substitutes for Fe3+ in the Y (octahedral) position. There are unusual discrete grossular-almandine rich domains within andraditic garnet III, indicating the low diffusivity of Ca compared to Fe at high temperatures

- by Ali mokhtari
- •
- Geology, Geochemistry, Mineralogy, Economic Geology

Garnets are commonly found minerals, being important indicators of regional and local metamorphism and P-T conditions. Their zonal structure and mixed chemical composition, represented mainly by almandine, pyrope, grossular, spessartine end-members, are useful tools for geothermobarometric considerations. The almandine-pyrope is considered one of the most important geothermometers, mainly due to practically undisturbed Fe and Mg partitioning between garnets and other minerals, like biotite and pyroxenes (Koziol and Bohlen 1992).
The crystalline bedrock of central Sweden was formed in the Svecofennian (Svecokarelian) orogenic phase, under conditions of active continental margin subduction zone. Its age is Orosirian, estimated
locally on 1.96-1.75 Ga. Main geological division is here the Bergslagen lithotectonic unit, composed of large variety of metamorphic and magmatic rocks. Several metasedimentary and metavolcanic types
had been here distinguished (Bergman et al. 2012).
Nynäshamn is partially located on the bedrock consisting of migmatisised “older granitoids”, syenites of age 1.89-1.85 Ga and
complex of metagreywackes, mica schists, graphite-sulphide-bearing schists, paragneiss, migmatites, quartzites and amphibolites (Bergman et al. 2012).

- by Łukasz Maciąg
- •
- Mineralogy, Petrology, Metamorphic Petrology, Igneous petrology