Geological applications of PGAA
Numerous geological rock samples, mainly volcanic-, metamorphic rocks and sediments were measured in the past 5 years with PGAA at the Budapest Research Reactor. Geological applications started with measuring international geological standards (GSJ etc.), in order to check the precision, accuracy and the capabilities of this relatively new method, when applied in geochemical analysis.
The results for the GSJ standards have proven the reliability of PGAA in measuring the boron, chlorine, other trace elements (Sc, Nd, Sm, and Gd) and the major element contents of the samples. Interlaboratory comparisons were also carried out with volcanic and metamorphic rocks, which were already analyzed with other techniques (XRF, ICP-MS, ICP-AES, TIMS, SIMS etc) at different laboratories.
With PGAA we can usually measure all major elements and some trace elements: B, Sm, Gd, Nd, Cl and H. Other elements (e.g. S, Sc, V, Cr, Co, Ni, Cu, Cd, Eu, Dy, Er) occuring at trace quantity in rock samples, can be determined only if their quantities are above the detection limit of PGAA. On the other hand, due to the low sensitivity of the method, usually we are not able to quantify the concentrations of some light elements, such as Li, Be, N, F and O in geological samples.
The characteristics and advantages of PGAA for whole rock measurements
- The sample preparation is relatively easy, in some cases not even needed. Heterogeneous samples have to be powdered (the grain size is irrelevant, it can be fine or coarse). Since most of the geological samples are heterogeneous, and the neutrons can penetrate deep into the matter, we would determine the average composition of the illuminated volume (the largest cross-section of the neutron beam is 2cm x 2 cm). For example, having a slice of basalt with a huge olivine in the middle which is hidden, the measured concentration would represent the analysed volume (with higher Mg or Fe content), rather than the general composition of the sample. In these cases the samples should be homogenized before the measurement.
- PGAA is uniquely applicable to determine whole-rock boron concentration. In contrast to other geoanalytical methods, the risk of contamination is low, thanks to simple sample preparation procedure.
- PGAA can provide the H content of the whole rocks, from which the H2O content of the samples can be determined according to stoichiometry.
The analytical procedure
The powdered rock samples are generally dried at 105 °C for 8 h (to remove H2O-) and 2-3 g of each are heat-sealed in fluorinated ethylene propylene (FEP) bags (2.5×3.5 cm). The acquisition lasts typically for 60-120 minutes.The relative uncertainties of the major oxide data typically range between 1.5 and 3%, except for MnO and MgO, where it can reach 4.5% and 10% respectively, depending on the concentration. The uncertainty in general, depends on the elements' sensitivities, the absolute concentrations in the sample and the experimental conditions. The precision for the B concentrations is rather good, about 1-1.5%.
Geological importance of the boron content of volcanic and metamorphic rocks
Boron, relative to other incompatible and fluidimmobile elements, is a highly sensitive indicator of fluid addition to the sub-arc mantle that occurs as a result of slab devolatilization. B and Cl progressively decrease with ongoing subduction. The average B content of subduction related arc volcanics is between 1 and 35 μg/g. The B abundance declines across volcanic arcs towards the back-arc region, approaching MORB-OIB values (0.6-2 and 1-20 μg/g, respectively). Sources of intraplate lavas appear to be systematically depleted in B relative to arc volcanics. Mantle-derived basaltic rocks have low B values (in the mantle the average B is 0.1 μg/g). Since the B content of the mantle is negligible (<0.1 μg/g), the higher B content of volcanics at intraplate settings can either be due to the greater role of subduction components or the effect of ground and surface water on the magma, or both.Because absolute trace element concentrations reflect differing degrees of magma evolution, it is useful to monitor B enrichment relative to other highly incompatible and relatively immobile trace elements (e.g. La, Ce, Nb, Sm, Gd, Yb, Zr). Variations of B with respect to other incompatible elements during normal fractional crystallization are not significant, but B versus incompatible element variations do provide information about fluid enrichment.
Geological projects using PGAA at the Budapest Research Reactor
- Mio-, Pliocene volcanic rock samples from the Carpathian-Pannonian Region were measured with PGAA to determine the concentration of major elements, and especially the B content of the whole rocks.
Miocene
calc-alkaline volcanic rocks following
the Carpathian Mountains chain show more or less subduction-
related geochemistry. As boron is a highly sensitive indicator of fluid
addition to the sub-arc mantle - which occurs as a result of slab
devolatilization -, by looking at the boron contents of the whole rock
samples we experienced different fluid enrichment features at the
western-, central-, and eastern part of the volcanic arc.
At the western part no typical correlation can be seen between the changes in the B content and the distance of the location of the volcanic samples from the supposed subduction suture zone. However, as the rocks are getting younger, their B content dramatically decreases. At the central segment the B content of the samples were decreasing parallel with the direction of the subduction towards the back arc basin. While in the eastern part, where the location of the volcanic chain is parallel with the subduction zone, the boron content is increasing as the volcanic formations are getting younger and closer to the subduction zone. Interesting feature at the southern end of the eastern segment is that south from a deep tectonic fault the B content of the volcanics reaches lower value; implying the effect of the deep fault in making a change in the source region.
At the western part no typical correlation can be seen between the changes in the B content and the distance of the location of the volcanic samples from the supposed subduction suture zone. However, as the rocks are getting younger, their B content dramatically decreases. At the central segment the B content of the samples were decreasing parallel with the direction of the subduction towards the back arc basin. While in the eastern part, where the location of the volcanic chain is parallel with the subduction zone, the boron content is increasing as the volcanic formations are getting younger and closer to the subduction zone. Interesting feature at the southern end of the eastern segment is that south from a deep tectonic fault the B content of the volcanics reaches lower value; implying the effect of the deep fault in making a change in the source region.
- Younger Pliocene alkaline volcanics were also investigated from the middle of the Pannonian Basin, from the Bakony Balaton Highland, in comparison with other maar volcanics from Mexico, New Zealand and Tenerife. These types of rocks have relatively low boron contents as their source region is not, or just slightly effected by subduction-related fluids.
- Chemical composition of lithospheric mantle- and lower crustal xenoliths and granulites were also determined with PGAA. As it was expected, their boron content is very low, usually less than 1 μg/g. That is the point where the importance of PGAA in the whole rock B content measurements comes out, as it is possible to measure as low as 0.1-0.3 μg/g boron in the whole rocks without distracting the sample.
- From different European countries numerous users were applying our PGAA facility in their geological research projects. Thus we had a possibility to measure high-pressure metamorphic rocks from Syros (Greece); silica rich volcanics from south-Poland; sediments from a Black See cores; serpentinites from core's of the Ocean Drilling Program; ophiolites from Greece; Alpine serpentinites and calcites and syenites and foyalites from Greenland.
References
- Gméling, K., Kasztovszky, Zs., Harangi, Sz., Szentmiklósi, L. and Révay, Zs. (2007): Geological use of prompt gamma activation analysis: importance of the boron concentration in volcanic rocks. Journal of Radioanalytical and Nuclear Chemistry, 271, No.2 (2007) 397–403.
- Szentmiklósi, L., Gméling, K. and Révay, Zs. (2007): Fitting the boron peak and resolving interferences in the 460-490 keV region of PGAA spectra. Journal of Radioanalytical and Nuclear Chemistry, Vol. 271, No.2 (2007) 439–445.
- Gméling, K; Németh, K; Martin, U; Eby, N; Varga, Zs. (2007): Boron concentrations of volcanic fields in different geotectonic settings. Journal of Volcanology and Geothermal Research159 (2007) 70–84
- Marschall, H.R., Altherr, R., Ludwig, T., Kalt, A., Gméling, K., Kasztovszky, Zs. (2006): Partitioning and budget of Li, Be and B in high-pressure metamorphic rocks. Geochimica et Cosmochimica Acta 70, 4750–4769.
- Hillers, M., Altherr, R., Ludwig, T., Meyer, H.P., Marschall, H.R., Kasztovszky, Zs., Gméling, K. (2007): Lithium, beryllium and boron in I-type granitoids from the Aegean Sea, Greece. Submitted to Geochimica et Cosmochimica Acta (in 29 November 2005).
- Gméling, K., Harangi, Sz., Kasztovszky, Zs. (2007): Boron variation in the volcanic rocks of the Eastern Carpathians. Submitted to NECAM 2006 conference special issue.
- Gméling, K., Pécskay, Z., Simonits, A. (2007): Variation of boron content through time and space in the Tokaj Mts. Submitted to NECAM 2006 conference special issue.
- Németh, K., Pécskay, Z., Martin, U., Gméling, K., Molnár, F., Cronin, S. (2007): Peperites and soft sediment deformation textures of a shallow subaqueous Miocene rhyolitic cryptodome and dyke complex, Pálháza, Hungary. LASI-II. Physical geology of subvolcanic systems: Laccoliths, sills and dykes. Isle of Skye April 1-3 rd 2006. Geol. Soc. London Spec. Publ. (submitted)
- Gméling, K., Harangi, Sz., Kasztovszky, Zs. (2007): A bór geokémiai szerepe szubdukciós zónákban (A bór geokémiai változékonysága a Kárpát Pannon térségben). Földtani Közlöny (beküldve 2007. február: Földtani Közlöny)
- Kasztovszky, Zs; Gméling, K. (2007): Validation of Prompt Gamma Activation Analysis on a set of geological standards. –(in preparation)
- Panczky, M; Gméling, K; Kasztovszky, Zs. (2005): Prompt Gamma Activation Ananlysis (PGAA) of the Lower Permian rhyolitic rocks from the North Sudetic Basin. Polskie Towarzysstwo Mineralogiczne – Prace Specjalne Mineralogical Soc. Poland – Spec. Papers 26,
- Gméling Katalin (2005, augusztus 5.): „Hanyatló” Apostolok – Élet és Tudomány LX, 31, 963.
- Gméling, K; Pécskay, Z. (2005): Boron content of Miocene calc-alkaline volcanic core samples from the Trans-Tisza Region (Hungary). Mineralia Slovaca 3,3 37, 2005, ISSN 0369-2086, 363-366.
- Gméling Katalin, Harangi Szabolcs, Kasztovszky Zsolt (2005, március 4): Mit üzen a bór a vulkáni hegyek keletkezéséről? – Élet és Tudomány LX, 9, 266-268. Élet és Tudomány Országos Tudományos Kutatási Alapprogramok (OTKA) és az Élet és Tudomány támogatásával kiírt pályázat tudományos kutatók számára 2004 3. díjas
- Gméling, K.; Harangi, Sz.; Kasztovszky, Zs. (2005): Boron and chlorine concentration of volcanic rocks: an application of prompt gamma activation analysis - Journal of Radioanalytical and Nuclear Chemistry, Vol. 265, No. 2; 201–212.
- Marschall, H. R.; Kasztovszky, Zs.; Gméling, K.; Altherr, R. (2005): Chemical analysis of high-pressure metamorphic rocks by PGNAA – comparison with results from XRF and solution ICP–MS - Journal of Radioanalytical and Nuclear Chemistry, Vol. 265, No. 2; 339-348.
