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Application of environmental isotopes to understand recharge mechanisms to a sub-basaltic deep lead system in western Victoria, Australia
Goldschmidt Conference Abstracts 2006
Application of environmental isotopes to understand recharge mechanisms to a sub-basaltic deep lead system in western Victoria, Australia M. RAIBER, J.A. WEBB Department of Environmental Geosciences, La Trobe University, Bundoora, Vic. 3086, Australia (mraiber@students. latrobe.edu.au) Radiocarbon, tritium and stable isotopes (d2H, d13C, d18O, and d87Sr) were used to determine aquifer interactions within and recharge to a deep lead/palaeodrainage system beneath the basalt plains of western Victoria. The deep lead aquifer (Eastern View Formation) covers an area of more than 2000 km2 and consists of several sequences of Tertiary fluvial sediments with an overall thickness of up to 140 m. Groundwater quality is highly variable ranging from drinking water quality to highly saline groundwaters. The isotope data indicate that the quality of the groundwater is controlled by the location relative to eruption points: in close proximity to volcanoes groundwaters are very fresh (<0.5 mS/ cm) and the groundwater dating gives comparatively young ages. With increasing distance away from the eruptive centres (down flow), the groundwater becomes progressively older and more saline (>10 mS/cm). The application of radiogenic and stable isotopes in combination with major ion chemistry and hydraulic data shows that the Eastern View Formation and overlying basalt aquifer are hydraulically separated by an aquitard, except where it is penetrated by eruption points. The deep lead palaeodrainage system is therefore predominantly recharged through these eruption points which are associated with rocky outcrops and thin soil cover. There is no or only limited interaquifer leakage (and thus, recharge to the deep lead system) in most areas. doi:10.1016/j.gca.2006.06.948
A515
Re-evaluating the mantle structure underlying the southwestern US F.C. RAMOS1, M.R. REID2, K.W.W. SIMS3 1
Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926, USA (ramos@geology. cwu.edu) 2 Department of Geology, Northern Arizona University, Flagstaff, AZ 86001, USA (mary.reid@nau) 3 Department of Geology and Geophysics, Woods Hole Oceanographic Institute, Woods Hole, MA 02543, USA (ksims@ whoi.edu) Previous assessments of the mantle structure underlying the southwestern US suggest the presence of highly geochemically heterogeneous mantle including depleted asthenosphere (P100 km) overlain by mantle lithosphere which grades from isotopically enriched at deep levels (>70 km) to isotopically depleted at shallow levels (<70 km). Along the Colorado Plateau-Rio Grande Rift-Basin and Range transition zone, alkalic basalts are thought to result from melting of isotopically depleted asthenospheric mantle at deep levels while tholeiites originate from melting overlying, isotopically enriched lithospheric mantle. Further work argued that the apparent absence of Th enrichment (i.e., melting occurring outside the presence of garnet) in tholeiitic basalts indicates that lithospheric mantle melting occurred at depths of <70 km. In contrast, alkalic basalts, characterized by Th enrichment up to 20%, have been attributed to melting during asthenospheric mantle upwelling. We present results from young (6150 km) alkalic and tholeiitic basalts from the Zuni Bandera, Lucero, and Cat Hills volcanic fields (New Mexico, USA) that are characterized by Th enrichments. Results demonstrate that melting of both alkalic and tholeiitic basalt sources occurs in the presence of garnet at depths P70 km, undermining previous structural and melting assessments of the mantle in this region of the US. In addition, the common presence of plagioclase in both alkalic and tholeiitic transition zone basalts, which implies staging and crystallization at crustal depths, suggests the possibility that enriched (and potentially depleted) isotope signatures in these basalts may indeed result from crustal contamination. Isotopic trends seen in tholeiites are similar to those observed in Mojave Desert basalts (e.g., Pisgah Crater, California), basalts that have assimilated mafic crustal components during residence at crustal depths. These results are inconsistent with current models and, if confirmed, undermine any assessments characterizing the melting regime and mantle structure underlying the mantle in this region of the western US. doi:10.1016/j.gca.2006.06.949
Application of environmental isotopes to understand recharge mechanisms to a sub-basaltic deep lead system in western Victoria, Australia M. RAIBER, J.A. WEBB Department of Environmental Geosciences, La Trobe University, Bundoora, Vic. 3086, Australia (mraiber@students. latrobe.edu.au) Radiocarbon, tritium and stable isotopes (d2H, d13C, d18O, and d87Sr) were used to determine aquifer interactions within and recharge to a deep lead/palaeodrainage system beneath the basalt plains of western Victoria. The deep lead aquifer (Eastern View Formation) covers an area of more than 2000 km2 and consists of several sequences of Tertiary fluvial sediments with an overall thickness of up to 140 m. Groundwater quality is highly variable ranging from drinking water quality to highly saline groundwaters. The isotope data indicate that the quality of the groundwater is controlled by the location relative to eruption points: in close proximity to volcanoes groundwaters are very fresh (<0.5 mS/ cm) and the groundwater dating gives comparatively young ages. With increasing distance away from the eruptive centres (down flow), the groundwater becomes progressively older and more saline (>10 mS/cm). The application of radiogenic and stable isotopes in combination with major ion chemistry and hydraulic data shows that the Eastern View Formation and overlying basalt aquifer are hydraulically separated by an aquitard, except where it is penetrated by eruption points. The deep lead palaeodrainage system is therefore predominantly recharged through these eruption points which are associated with rocky outcrops and thin soil cover. There is no or only limited interaquifer leakage (and thus, recharge to the deep lead system) in most areas. doi:10.1016/j.gca.2006.06.948
A515
Re-evaluating the mantle structure underlying the southwestern US F.C. RAMOS1, M.R. REID2, K.W.W. SIMS3 1
Department of Geological Sciences, Central Washington University, Ellensburg, WA 98926, USA (ramos@geology. cwu.edu) 2 Department of Geology, Northern Arizona University, Flagstaff, AZ 86001, USA (mary.reid@nau) 3 Department of Geology and Geophysics, Woods Hole Oceanographic Institute, Woods Hole, MA 02543, USA (ksims@ whoi.edu) Previous assessments of the mantle structure underlying the southwestern US suggest the presence of highly geochemically heterogeneous mantle including depleted asthenosphere (P100 km) overlain by mantle lithosphere which grades from isotopically enriched at deep levels (>70 km) to isotopically depleted at shallow levels (<70 km). Along the Colorado Plateau-Rio Grande Rift-Basin and Range transition zone, alkalic basalts are thought to result from melting of isotopically depleted asthenospheric mantle at deep levels while tholeiites originate from melting overlying, isotopically enriched lithospheric mantle. Further work argued that the apparent absence of Th enrichment (i.e., melting occurring outside the presence of garnet) in tholeiitic basalts indicates that lithospheric mantle melting occurred at depths of <70 km. In contrast, alkalic basalts, characterized by Th enrichment up to 20%, have been attributed to melting during asthenospheric mantle upwelling. We present results from young (6150 km) alkalic and tholeiitic basalts from the Zuni Bandera, Lucero, and Cat Hills volcanic fields (New Mexico, USA) that are characterized by Th enrichments. Results demonstrate that melting of both alkalic and tholeiitic basalt sources occurs in the presence of garnet at depths P70 km, undermining previous structural and melting assessments of the mantle in this region of the US. In addition, the common presence of plagioclase in both alkalic and tholeiitic transition zone basalts, which implies staging and crystallization at crustal depths, suggests the possibility that enriched (and potentially depleted) isotope signatures in these basalts may indeed result from crustal contamination. Isotopic trends seen in tholeiites are similar to those observed in Mojave Desert basalts (e.g., Pisgah Crater, California), basalts that have assimilated mafic crustal components during residence at crustal depths. These results are inconsistent with current models and, if confirmed, undermine any assessments characterizing the melting regime and mantle structure underlying the mantle in this region of the western US. doi:10.1016/j.gca.2006.06.949