Non-matrix-matched analysis of U-Th-Pb ages of accessory minerals by LA-ICP-MS - IAG Young Scientist Medal Lecture

Geochemistry of schists of Obudu area was carried out using ICP-MS and ICP-ES techniques in order to determine the geochemical evolution of the area. 40 samples were analyzed for their major, trace and REE composition. Field mapping revealed that gneisses, amphibolites and schists comprising migmatitic schists (MS), quartz-mica schists (QMS), garnet-mica schists (GMS), and hornblende biotite schists (HBS), intruded by granites, granodiorites, quartzofeldspathic rocks and dolerites occur in the area. Structural studies revealed that the schists trend approximately NE–SW (5 – 30o ) indicating the Pan-African event. Modal analysis revealed that the schists have average concentration of quartz (15vol.%), plagioclase (An45-19 vol.%), biotite (15vol.%), garnet (9.0vol.%) and muscovite (6vol.%), the remaining consists of accessory minerals. Geochemistry showed that all the schists have molecular Al2O3 > CaO+K2O+Na2O, indicating they are peraluminous metasedimentary pelites. Trace and REE element results show that all the analyzed schist samples are depleted in Hg, Ag, Be, Bi, and Sb below < 1.0ppm, but relatively enriched in Ba, Sr and Zr with average concentration of 996, 675.73, 243.13 ppm respective. The HREE are depleted with ΣHREE < 10.2, but the LREE are relatively enriched with ΣLREE > 289.54. The ΣLREE/ΣHREE ratio ranges from 9.17 to 33.4, with a large positive delta V at Eu. These findings indicate that the schists of Northwest Obudu area are highly fractionated and had attained at least the uppermost amphibolite metamorphic grade. The schists had contributed to the development of the Pan-African continent.

Download Full-text

Two-Stage Late Jurassic to Early Cretaceous Hydrothermal Activity in the Sakar Unit of Southeastern Bulgaria

Minerals ◽

10.3390/min10030266 ◽

2020 ◽

Vol 10 (3) ◽

pp. 266

Author(s):

Krzysztof Szopa ◽

Anna Sałacińska ◽

Ashley P. Gumsley ◽

David Chew ◽

Petko Petrov ◽

...

Keyword(s):

Early Cretaceous ◽

Late Jurassic ◽

Hydrothermal Activity ◽

Two Stage ◽

Accessory Minerals ◽

Icp Ms ◽

Two Stages ◽

Age Constraints ◽

Hydrothermal Veins

Southeastern Bulgaria is composed of a variety of rocks from pre-Variscan (ca. 0.3 Ga) to pre-Alpine sensu lato (ca. 0.15 Ga) time. The Sakar Unit in this region comprises a series of granitoids and gneisses formed or metamorphosed during these events. It is cut by a series of post-Variscan hydrothermal veins, yet lacks pervasive Alpine deformation. It thus represents a key unit for detecting potential tectonism associated with the enigmatic Cimmerian Orogenic episode, but limited geochronology has been undertaken on this unit. Here we report age constraints on hydrothermal activity in the Sakar Pluton. The investigated veins contain mainly albite–actinolite–chlorite–apatite–titanite–quartz–tourmaline–epidote and accessory minerals. The most common accessory minerals are rutile and molybdenite. Apatite and titanite from the same vein were dated by U–Pb LA–ICP-MS geochronology. These dates are interpreted as crystallization ages and are 149 ± 7 Ma on apatite and 114 ± 1 Ma on titanite, respectively. These crystallization ages are the first to document two stages of hydrothermal activity during the late Jurassic to early Cretaceous, using U–Pb geochronology, and its association with the Cimmerian orogenesis. The Cimmerian tectono-thermal episode is well-documented further to the east in the Eastern Strandja Massif granitoids. However, these are the first documented ages from the western parts of the Strandja Massif, in the Sakar Unit. These ages also temporally overlap with previously published Ar–Ar and K–Ar cooling ages, and firmly establish that the Cimmerian orogeny in the studied area included both tectonic and hydrothermal activity. Such hydrothermal activity likely accounted for the intense albitization found in the Sakar Unit.

Download Full-text

Garnet-amphibole miaskites of the Ilmenogorsky miaskite massif (Southern Urals): Mineralogy and geochemistry

LITOSFERA ◽

10.24930/1681-9004-2020-20-5-652-667 ◽

2020 ◽

Vol 20 (5) ◽

pp. 652-667

Author(s):

A. B. Nemov

Keyword(s):

Mineral Composition ◽

Microprobe Analysis ◽

Southern Urals ◽

Crustal Material ◽

Research Subject ◽

Accessory Minerals ◽

Low Concentrations ◽

Garnet Composition ◽

Icp Ms ◽

High Concentrations

Research subject. This paper presents original findings about textural-structural, mineralogical, petrological, and geochemical features of the garnet-amphibole miaskites (firstes) of the Ilmenogorsky miaskite massif.Materials and methods. The microprobe analysis of mineral composition was performed using Tescan Vega3 sbu and REMMA202M scanning microscopes equipped with microanalyzers. The content of major, trace and rareearth elements (REE) in rock samples was determined by the methods of AAS and ICP-MS.Results. The garnet-amphibole miaskites under study are characterized by a rare mineral paragenesis, i.e. garnet-amphibole-pyroxene-nepheline-plagioclase. The mafic minerals exhibit a high ferruginosity (f = 70–99), while the accessory minerals have high Al, F and low REE contents. The garnetamphibole miaskites contains high concentrations of Al, Fe3+, Ca, Na, Be, Rb, Mo, Tl and low concentrations of LILE, HFSE, REE and transit elements.Conclusions. According to the garnet composition and its ferruginosity (f = 95– 99), high contents of Al and F in accessory minerals, the prevalence of Fe3+, as well as negative Eu/Eu* and positive Ce/ Ce* anomalies, the garnet-amphibole miaskites under study are assumed to be the product of acid-alkaline metasomatism occurring under the oxidizing conditions of petrogenesis. The low ratios of Cr/V and Ni/Co indicate the immobility of transit elements during metasomatism, and their clarke of concentration corresponds to the content in metaterrigenous and metacarbonate rocks, which suggests crustal substratum for garnet–amphibole miaskites. Garnet-amphibole miaskites are the markers of the interaction of crustal material with deep fluids, which occurred during the stage of shear tectonics development (270–240 Ma) due to the broad permeability of the rocks composing the Ilmenogorsky miaskite massif.

Download Full-text

Understanding the role of accessory minerals in the Sm-Nd isotopic evolution of ancient rocks: An in-situ LA-(MC)-ICP-MS approach

10.5194/egusphere-egu2020-9103 ◽

2020 ◽

Author(s):

Johannes Hammerli

Keyword(s):

Hf Isotope ◽

Nd Isotope ◽

Accessory Minerals ◽

Icp Ms ◽

Isotopic Evolution ◽

Magma Sources ◽

Mantle Reservoir ◽

Isotope System

The long-lived radiogenic isotope systems Lu-Hf and Sm-Nd have been widely used by geochemists to study magma sources and crustal residential times of (igneous) rocks in order to understand how early crust formed and to model the production rate and volume of continental crust on global and regional-scales during the last ~4.4 Ga. However, while throughout most of Earth&#8217;s history Nd and Hf isotope signatures in terrestrial rocks are well correlated due to their very similar geochemical behavior, some of Earth&#8217;s oldest rocks show an apparent inconsistency in their Nd and Hf isotope signatures. While Hf isotopes in early Archean rocks are generally (near) chondritic, Nd isotope signatures can be distinctly super- or sub-chondritic. The super-chondritic Nd isotope values in Eoarchean samples would suggest that these rocks are derived from a mantle reservoir depleted by prior crust extraction. The chondritic Hf isotope values, on the other hand, support a mantle source from which no significant volume of crust had been extracted. While a range of different processes, some of them speculative, might explain this Hf-Nd isotope paradox, recent research [1, 2] has shown that relatively simple, post-magmatic, open-system processes can explain decoupling of the typically correlative Hf-Nd isotope signatures. This talk will focus on the importance of identifying Nd-bearing accessory minerals in (Archean) rocks to understand how the Sm-Nd isotope system is controlled and how in situ isotope and trace element analyses by LA-(MC)-ICP-MS in combination with detailed petrographic observations help to understand when and via which processes the two isotope systems become decoupled. Reconstructing the isotopic evolution of the different isotope systems since formation of the protoliths has important implications for our understanding of early crust formation and questions some of the proposed current models for early crust extraction from the mantle.&#160;[1] Hammerli et al. (2019) Chem. Geol 2; [2] Fisher et al. (2020) EPSL

Download Full-text