chemical extractions
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2021 ◽  
Author(s):  
Zena Severin ◽  
Jessica L. Till ◽  
Oman Drilling Project Phase 1 Science Party

<p>Within the Samail Ophiolite, Oman, there are intervals of listvenite outcrops between layers of serpentinite zones above the basal thrust zone, atop the metamorphic sole. Near the base of the ophiolite mantle section, some peridotites underwent 100% carbonation from metasomatic introduction of CO<sub>2</sub>-bearing fluids <200°C to form listvenites during the time of emplacement (97 ± 29 Ma, Falk and Kelemen, 2015). The carbonate rocks comprise mostly magnesite and/or dolomite, quartz, Cr-spinel, and Fe-(hydr)oxides; with carbonates as the sole Mg-minerals and quartz as the only silicate phase. The aim of this study is to chemically and petrographically investigate the Fe-bearing minerals within the fluid-altered mantle rocks in drill core samples from hole BT1B of the ICDP Oman Drilling Project. Sequential chemical extractions are useful for recognizing iron pools based on the minerology. We investigated the quantities of Fe-oxide/hydroxide phases through a series of chemical extractions (Poulton and Canfield, 2005) via atomic absorption spectroscopy in addition to optical microscope, SEM/EDS, EPMA/WDS and ICP analysis. Extractions performed at room temperature and one at 50°C included: carbonate-associated Fe (sodium acetate) targeting siderite, HCl-extractable Fe(II), reducible oxides (citrate-dithionite) targeting hematite and possible goethite, and magnetite (oxalate). Carbonate-based Fe in the listvenites from a sodium acetate extraction ranges from 12-28 mg/g, while the same extraction performed at 50°C for twice as long resulted in higher proportions of carbonate-associated Fe (15-35 mg/g). Easily reducible iron quantities from the diluted HCl solution extraction display the lowest overall Fe fractions (0.75-5.5 mg/g) following the room temperature acetate and 0.63-1.7 mg/g after the 50°C acetate extraction. Fe in reducible oxides extracted by dithionite ranged from 1.4-15 mg/g with similar result after both a room-temperature acetate and a 50°C acetate step. Oxalate extraction succeeding the room-temperature acetate yielded magnetite concentrations of 1.9-8.0 mg/g, while the increased temperature and time in the first step (acetate extraction) were followed by significantly lower amounts of Fe extracted by oxalate (0.47- 3.6 mg/g). Additionally, the same extractions were performed on a pure siderite sample from Greenland. For siderite samples crushed a week prior to analysis, the carbonate-associated Fe in sodium acetate extract was 165±17 mg/g; the sidenote yielded 42 wt% of overall extracted Fe (392±33 mg/g). This is only slightly lower than the expected 48.2 wt% of Fe for a pure siderite sample. Dilute HCl extractions display results of 126±5.4 mg/g, dithionite solution extracted 25±0.5 mg/g and oxalate proportions were 76±9 mg/g. Due to possible oxidation of siderite to magnetite occurring during the time between powdering the samples and analysis, the full dissolution of siderite may not be fully represented in only the acetate. Microprobe data shows a total amount of FeO in carbonates as 1.3-10.8 wt%. This is more than or similar to the acetate and HCl proportions of Fe which represent carbonate associated minerals in the listvenites. Data obtained from EMPA and ICP will additionally be discussed in relation to the Fe-oxide phases with relation to the listvenites minerology.</p>


2020 ◽  
Vol 183 (4) ◽  
pp. 416-428
Author(s):  
Nina Høj Christiansen ◽  
Peter Sørensen ◽  
Rodrigo Labouriau ◽  
Bent T. Christensen ◽  
Gitte Holton Rubaek

2020 ◽  
Author(s):  
Zena Severin ◽  
Jessica L. Till ◽  
Oman Drilling Project Phase 1 Science Party

<p>Within the Samail Ophiolite, Oman, there are intervals of listvenite outcrops between layers of serpentinite zones above the basal thrust zone, atop the metamorphic sole. Near the base of the ophiolite mantle section, some peridotites underwent 100% carbonation from metasomatic introduction of CO<sub>2</sub>-bearing fluids (~100°C) to form listvenites during the time of emplacement (97 ± 29 Ma, Falk and Kelemen, 2015). The carbonate rocks comprise mostly magnesite and/or dolomite, quartz, spinel, and Fe-(hydr)oxides; with carbonates as the sole Mg-minerals and quartz as the only silicate phase. The aim of this study is to chemically and petrographically investigate the Fe-bearing minerals within the fluid-altered mantle rocks in drill core samples from hole BT1B of the ICDP Oman Drilling Project. We investigated the quantities of Fe-oxide/hydroxide phases through a series of chemical extractions (Poulton and Canfield, 2005) via atomic absorption spectroscopy in addition to optical microscope/ SEM/EDS analysis. Sequential chemical extractions are useful for recognizing iron pools based on the minerology. Extractions preformed at room temperature show varying proportions of carbonate-associated Fe (sodium acetate), reducible oxides (citrate-dithionite), magnetite (oxalate), and HCl-extractable Fe(II). The amount of Fe in carbonates based on sodium acetate extraction ranges from 17-54% of the overall extracted iron (12-28 ‰) in the samples. The same extraction performed at 50°C for twice as long resulted in higher proportions of carbonate-associated Fe extracted with a range of 44-85% of the total extracted iron (15-35 ‰). Easily reducible iron quantities from a diluted HCl solution extraction display the lowest overall Fe fractions at 6.2-25% following the room temperature acetate and 2.6-6.2% after the 50°C acetate extraction. Reducible oxides extracted by dithionite were wide ranging (8.3-49%) as a proportion of the overall extracted iron, with similar results following the 50°C acetate step (5.3-48%). Oxalate extraction succeeding the room temperature acetate revealed magnetite proportions of 13-28%, while the increased temperature and time in the first step (acetate extraction) resulted in significantly lower proportions of Fe extracted by oxalate (3.1-10%). Falk and Kelemen (2015) suggest significant amounts of poorly crystalline Fe-phases or amorphous oxides within the listvenites not detected by X-ray diffraction, but we do not see evidence of this based on the relatively small HCl fractions. Further examination of the total elemental compositions of the individual solutions and electron microprobe analyses will reveal more details about the Fe-minerals dissolved in each extract and weather they represent separate Fe-oxide/hydroxide phases.    </p><p> </p><p>Falk, E. S., & Kelemen, P. B. (2015). Geochemistry and petrology of listvenite in the Samail ophiolite, Sultanate of Oman: Complete carbonation of peridotite during ophiolite emplacement.<em> Geochimica et Cosmochimica Acta</em>, 160, 70-90.</p><p>Poulton, S. W., & Canfield, D. E. (2005). Development of a sequential extraction procedure for iron: implications for iron partitioning in continentally derived particulates. <em>Chemical Geology</em>, 214(3-4), 209-221.</p><p> </p>


Geoderma ◽  
2019 ◽  
Vol 353 ◽  
pp. 423-434 ◽  
Author(s):  
Beatrice Giannetta ◽  
César Plaza ◽  
Claudio Zaccone ◽  
Costantino Vischetti ◽  
Pere Rovira

Soil Research ◽  
2019 ◽  
Vol 57 (1) ◽  
pp. 1 ◽  
Author(s):  
Thilo Rennert

Wet-chemical extraction of soil is a standard procedure to characterise pedogenic aluminium (Al) and iron (Fe) species, especially oxides, allophanic minerals and metal–organic associations. This article critically reviews the suitability of commonly used extractants (e.g. dithionite, oxalate and pyrophosphate) and the potentials and restrictions in their use for species identification and in soil classification. None of the commonly used extractants is completely selective and quantitative. The degree of completeness differs between the extractants and depends on soil composition. Dithionite-based methods provide a ‘pseudo-total’ content of pedogenic Fe oxides, as they are not always completely dissolved. Oxalate may attack further non-target species, releasing additional Al and Fe. Therefore, the extraction of Al and Fe exclusively from poorly crystalline species is not always guaranteed. As a consequence of dispersion of aggregates, pyrophosphate solubilises both mineral particles and metals from organic associations. Thus, quantification of species based on these extractions and their implementation in pedogenic thresholds may be questionable. Alternative extractants such as citrate–ascorbate and dithionite–citrate–oxalate could be used in addition, as applicable and reliable wet-chemical extractions will be still demanded for research and practical applications. The examination of the effectiveness and selectivity of wet-chemical extraction methods by spectroscopic techniques is recommended.


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