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Enriched isotopic composition of the NW central American volcanic arc: Crustal contamination or a sediment slab melt?
Goldschmidt Conference Abstracts 2006
Enriched isotopic composition of the NW central American volcanic arc: Crustal contamination or a sediment slab melt? K. HEYDOLPH1, K. HOERNLE1,2, P. VAN DEN F. HAUFF2, S. SADOFSKY1
BOGAARD
1
,
1
SFB 574 University of Kiel, Wischhof Straße 1-3, 24148 Kiel, Germany ([email protected]) 2 Leibniz Institute for Marine Sciences (IFM-GEOMAR), Wischhof Straße 1-3, 24148 Kiel, Germany New Sr–Nd–Pb isotope data on samples from mafic volcanic centers along and behind the volcanic front in Guatemala, El Salvador and Honduras further constrain the source components for the Central American Volcanic Arc (CAVA). The subduction input (consisting of subducting sediments, seawater altered and unaltered igneous crust and serpentinites), mantle wedge and overlying lithosphere (crust and mantle) influence the composition of the erupted arc volcanic rocks. Samples from Guatemala and Honduras show negative correlation of Sr and Nd isotope ratios, whereas data from El Salvador varies significantly in 143Nd/144Nd (0.513001–0.512868) with little variation in 87Sr/86Sr (0.70361–0.70387). When compared with isotope data from the Nicaraguan volcanic front (VF), El Salvador, Guatemala and Honduras volcanics extend to much less radiogenic Nd with behind the volcanic front (BVF) samples having the least radiogenic Nd isotopic compositions. Neodymium isotope ratios correlate negatively with La/Sm and La/Yb ratios. Lead isotope data for all samples form overlapping slightly positive arrays with the BVF samples from a given arc segment being generally more radiogenic than samples from the VF. Back arc samples from Honduras have the most mid-oceanridge basalt (MORB)-like compositions and are believed to represent the composition of the mantle wedge. Samples from the Nicaraguan VF have similar Nd but higher Sr isotope compositions most likely reflecting enrichment with slab fluids containing subducted sediment or seawater Sr. Samples from BVF in El Salvador, Guatemala and Honduras define a third end member with the least radiogenic Nd, most radiogenic Sr and Pb isotopic compositions, and highest La/Sm and La/Yb ratios most likely containing a sediment melt component from the underlying continental crust or subducted sediments. doi:10.1016/j.gca.2006.06.501
A249
Nucleosynthetic barium isotopic anomalies in carbonaceous chondrites H. HIDAKA1, S. YONEDA2 1
Department of Earth Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan (hidaka@ hiroshima-u.ac.jp) 2 Department of Science and Engineering, National Science Museum, Tokyo 169-0073, Japan ([email protected]) Isotopic anomalies of carbonaceous chondrites provide hints to consider the nucleosynthetic sources of solar system materials. Nucleosynthetic isotope anomalies of several elements such as Zr, Mo and Ba have been found in carbonaceous chondrites (Schonbachler, 2005; Dauphas et al., 2002; Hidaka et al., 2003). Barium is one of promising elements to address the details of nucleothentesis and presolar grain formation of the solar system (Hidaka et al., 2003). In addition, possible detection of 135Ba isotopic excesses from the decay of 135Cs (t1/2 = 2.3 Ma) may lead to develop a new chronometer for early solar system. Here, we report barium isotopic compositions from acid leachates of carbonaceous chondrites. Twelve carbonaceous chondrites were used in this study: Orgueil (CI), Mighei (CM2), Murchison (CM2), Murray (CM2), Sayama (CM2), NWA801 (CR2), Allende (CV3), Efremovka (CV3), Isuna (CO3), Kainsaz (CO3), Karronda (CK4), and Maralinga (CK4). A mass of 0.2–0.3 g of each powdered sample was sequentially leached by 0.1 M acetic acid, 0.1 M HCl and 2 M HCl. The residue was finally decomposed by HF-HClO4. Ba isotopic compositions and elemental concentrations of Ba and Cs in individual leachates were determined by thermal ionization mass spectrometry and ICP-MS, respectively. The barium isotopic data in most of leachates show variable isotopic deviations of 135Ba correlated with 137Ba, suggesting the presence of nucleosynthetic components for s- and r-processes in the solar system. The isotopic anomalies treated in this study are generally small ( 3 < e < +3) except in the case of acid residues of CI and CM samples. Large deviations of 135Ba (e = 18.9 5.0) and 137Ba (e = 8.6 1.2) observed in the acid residues from Orgueil and four CM2 meteorites show evidence for enrichments of s-process isotopes derived from presolar grains. The leachates from acetic acid in CI and CM samples show higher Cs/Ba elemental ratios (0.05 0.13) and larger 135 Ba deviations than other fractions, which suggest isotopic excesses decayed from 135Cs.
References Dauphas, N., Marty, B., Reisberg, L., 2002. Astrophys. J. 569, L139– L142. Hidaka, H., Ohta, Y., Yoneda, S., 2003. Earth Planet. Sci. Lett. 214, 455–466. Schonbachler, M. et al., 2005. Geochim. Cosmochim. Acta 69, 5113–5122. doi:10.1016/j.gca.2006.06.502
Enriched isotopic composition of the NW central American volcanic arc: Crustal contamination or a sediment slab melt? K. HEYDOLPH1, K. HOERNLE1,2, P. VAN DEN F. HAUFF2, S. SADOFSKY1
BOGAARD
1
,
1
SFB 574 University of Kiel, Wischhof Straße 1-3, 24148 Kiel, Germany ([email protected]) 2 Leibniz Institute for Marine Sciences (IFM-GEOMAR), Wischhof Straße 1-3, 24148 Kiel, Germany New Sr–Nd–Pb isotope data on samples from mafic volcanic centers along and behind the volcanic front in Guatemala, El Salvador and Honduras further constrain the source components for the Central American Volcanic Arc (CAVA). The subduction input (consisting of subducting sediments, seawater altered and unaltered igneous crust and serpentinites), mantle wedge and overlying lithosphere (crust and mantle) influence the composition of the erupted arc volcanic rocks. Samples from Guatemala and Honduras show negative correlation of Sr and Nd isotope ratios, whereas data from El Salvador varies significantly in 143Nd/144Nd (0.513001–0.512868) with little variation in 87Sr/86Sr (0.70361–0.70387). When compared with isotope data from the Nicaraguan volcanic front (VF), El Salvador, Guatemala and Honduras volcanics extend to much less radiogenic Nd with behind the volcanic front (BVF) samples having the least radiogenic Nd isotopic compositions. Neodymium isotope ratios correlate negatively with La/Sm and La/Yb ratios. Lead isotope data for all samples form overlapping slightly positive arrays with the BVF samples from a given arc segment being generally more radiogenic than samples from the VF. Back arc samples from Honduras have the most mid-oceanridge basalt (MORB)-like compositions and are believed to represent the composition of the mantle wedge. Samples from the Nicaraguan VF have similar Nd but higher Sr isotope compositions most likely reflecting enrichment with slab fluids containing subducted sediment or seawater Sr. Samples from BVF in El Salvador, Guatemala and Honduras define a third end member with the least radiogenic Nd, most radiogenic Sr and Pb isotopic compositions, and highest La/Sm and La/Yb ratios most likely containing a sediment melt component from the underlying continental crust or subducted sediments. doi:10.1016/j.gca.2006.06.501
A249
Nucleosynthetic barium isotopic anomalies in carbonaceous chondrites H. HIDAKA1, S. YONEDA2 1
Department of Earth Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan (hidaka@ hiroshima-u.ac.jp) 2 Department of Science and Engineering, National Science Museum, Tokyo 169-0073, Japan ([email protected]) Isotopic anomalies of carbonaceous chondrites provide hints to consider the nucleosynthetic sources of solar system materials. Nucleosynthetic isotope anomalies of several elements such as Zr, Mo and Ba have been found in carbonaceous chondrites (Schonbachler, 2005; Dauphas et al., 2002; Hidaka et al., 2003). Barium is one of promising elements to address the details of nucleothentesis and presolar grain formation of the solar system (Hidaka et al., 2003). In addition, possible detection of 135Ba isotopic excesses from the decay of 135Cs (t1/2 = 2.3 Ma) may lead to develop a new chronometer for early solar system. Here, we report barium isotopic compositions from acid leachates of carbonaceous chondrites. Twelve carbonaceous chondrites were used in this study: Orgueil (CI), Mighei (CM2), Murchison (CM2), Murray (CM2), Sayama (CM2), NWA801 (CR2), Allende (CV3), Efremovka (CV3), Isuna (CO3), Kainsaz (CO3), Karronda (CK4), and Maralinga (CK4). A mass of 0.2–0.3 g of each powdered sample was sequentially leached by 0.1 M acetic acid, 0.1 M HCl and 2 M HCl. The residue was finally decomposed by HF-HClO4. Ba isotopic compositions and elemental concentrations of Ba and Cs in individual leachates were determined by thermal ionization mass spectrometry and ICP-MS, respectively. The barium isotopic data in most of leachates show variable isotopic deviations of 135Ba correlated with 137Ba, suggesting the presence of nucleosynthetic components for s- and r-processes in the solar system. The isotopic anomalies treated in this study are generally small ( 3 < e < +3) except in the case of acid residues of CI and CM samples. Large deviations of 135Ba (e = 18.9 5.0) and 137Ba (e = 8.6 1.2) observed in the acid residues from Orgueil and four CM2 meteorites show evidence for enrichments of s-process isotopes derived from presolar grains. The leachates from acetic acid in CI and CM samples show higher Cs/Ba elemental ratios (0.05 0.13) and larger 135 Ba deviations than other fractions, which suggest isotopic excesses decayed from 135Cs.
References Dauphas, N., Marty, B., Reisberg, L., 2002. Astrophys. J. 569, L139– L142. Hidaka, H., Ohta, Y., Yoneda, S., 2003. Earth Planet. Sci. Lett. 214, 455–466. Schonbachler, M. et al., 2005. Geochim. Cosmochim. Acta 69, 5113–5122. doi:10.1016/j.gca.2006.06.502