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Isochrones for in situ cosmogenic 3He measurement
Goldschmidt Conference Abstracts 2006
Isochrones for in situ cosmogenic 3 He measurement
Dating young basalt eruptions by (U–Th)/He on xenolithic zircons
P.H. BLARD1,2, R. PIK1
M.S. BLONDES1, P.W. REINERS1, B.R. EDWARDS2, A. BISCONTINI2
A55
1
CRPG, Vandoeuvre-le`s-Nancy, France ([email protected]) CEREGE, Aix en Provence, France
2
The standard routine used to measure in situ cosmogenic 3He ( Hecos) in mafic phenocrysts (olivines and clinopyroxenes) is usually based on a 2-step analytical protocol: (i) preliminary in vacuo crushing followed by (ii) subsequent fusion of the phenocrysts. This method is particularly suited for young (<100 kyr) and U– Th poor lava flows because of its efficiency to unravel properly the magmatic 3He contribution. However, the preliminary crushing step may trigger 3Hecos release (Blard et al.) and consequently lead to underestimate the actual cosmogenic concentration. Moreover, the 3Hecos determination may be drastically complicated for rocks characterized by older ages and/or high U and Th content, which can lead to an unsolved system with three He components (cosmogenic, radiogenic, and magmatic). A cosmogenic isochrone can be defined as the regression line obtained plotting (3He/4He)fusion against (1/4He)fusion for several aliquots that have the same cosmogenic 3He concentration. The array of this plot can be directly used to address several undetermined problems, such as (3He/4He) magmatic heterogeneities or variable radiogenic 4He* concentrations between aliquots. Some calculations simulating the respective influence of the independent key parameters (age of the rock, magmatic He content, U–Th content) were performed and confronted to new datasets. This comparison allowed to define several geochemical and geological configurations for which the use of isochrones (two He components) or pseudo-isochrones (three He components) may be considered as a pertinent technique to determine properly the cosmogenic 3He concentrations. In summary, whatever the additional corrections to apply, this approach basically avoids the crushing step and thereby potential 3 Hecos loss. It may also help to check the homogeneity of helium components (magmatic, radiogenic) in a population of few aliquots, which is a crucial point to validate the non-cosmogenic corrections. Moreover, skipping the crushing step may also provide a significant gain of time, especially for noble gases laboratories equipped with automated furnaces. 3
Reference Blard, P.-H. et al., 2006. EPSL, in press. doi:10.1016/j.gca.2006.06.216
1
Yale University, Department of Geology and Geophysics, 210 Whitney Ave., New Haven, CT 06511, USA (madalyn. [email protected]) 2 Dickinson College, Department of Geology, James Center, Carlisle, PA 17013, USA Accurate ages for young (e.g., Pleistocene) volcanic eruptions are important for geomorphic, tectonic, climatic, and hazard studies. Existing techniques can be time-consuming and expensive when many ages are needed, and in the case of K/Ar and 40 Ar/39Ar dating, extraneous Ar often can limit precision, especially for continental basalts erupted through old lithosphere. We present a new technique for dating young basaltic eruptions by (U–Th)/He dating of zircons (ZHe) from crustal xenoliths. Single-crystal ZHe dates generally have lower precision than typical 40 Ar/39Ar dates, but can be determined on multiple replicate grain aliquots relatively easily. We dated zircons from xenoliths from four volcanic centers in western North America: Little Bear Mountain, BC (157 ± 3.5 [2.2%] 95% conf., MSWD = 1.7) and Prindle Volcano, AK (176 ± 16 [8.9%], MSWD = 13) , in the northern Cordilleran volcanic province, and Fish Springs (273 ± 23 [8.6%], MSWD = 43) and Oak Creek (179 ± 8.1 [4.5%], MSWD = 12), in the Big Pine Volcanic Field, CA. All ZHe ages are either equivalent to or younger than previously determined K–Ar or 40Ar/39Ar ages, indicating the possibility of inherited 40Ar in some of the previous measurements. Zircons from upper crustal xenoliths in the Oak Creek and Fish Springs vents show poorer reproducibility and multiple apparent age distribution peaks, consistent with either intracrystalline U–Th zonation or <99.99% He degassing (assuming 100-Ma pre-entrainment ZHe ages) of some zircons during magmatic entrainment. Removal of clear outliers in the older age-distribution peaks of the upper crustal xenoliths, most of which have extremely high U compared to other zircons of the same xenolith, improve the reproducibilities of Fish Springs to 4.7% (95% conf., MSWD = 4.8) and Oak Creek to 3.4% (95% conf., MSWD = 6.2). Coupled thermal and He diffusion modeling using appropriate xenolith sizes and magma temperatures and assuming published diffusion kinetics for zircon indicates that incomplete He degassing would require entrainment times less than 1 h. However, the observation of extremely high U in most zircons with older ages raises the possibility that zircons with high radiation dosages and significant pre-eruptive He may have more retentive He diffusion characteristics. doi:10.1016/j.gca.2006.06.217
Isochrones for in situ cosmogenic 3 He measurement
Dating young basalt eruptions by (U–Th)/He on xenolithic zircons
P.H. BLARD1,2, R. PIK1
M.S. BLONDES1, P.W. REINERS1, B.R. EDWARDS2, A. BISCONTINI2
A55
1
CRPG, Vandoeuvre-le`s-Nancy, France ([email protected]) CEREGE, Aix en Provence, France
2
The standard routine used to measure in situ cosmogenic 3He ( Hecos) in mafic phenocrysts (olivines and clinopyroxenes) is usually based on a 2-step analytical protocol: (i) preliminary in vacuo crushing followed by (ii) subsequent fusion of the phenocrysts. This method is particularly suited for young (<100 kyr) and U– Th poor lava flows because of its efficiency to unravel properly the magmatic 3He contribution. However, the preliminary crushing step may trigger 3Hecos release (Blard et al.) and consequently lead to underestimate the actual cosmogenic concentration. Moreover, the 3Hecos determination may be drastically complicated for rocks characterized by older ages and/or high U and Th content, which can lead to an unsolved system with three He components (cosmogenic, radiogenic, and magmatic). A cosmogenic isochrone can be defined as the regression line obtained plotting (3He/4He)fusion against (1/4He)fusion for several aliquots that have the same cosmogenic 3He concentration. The array of this plot can be directly used to address several undetermined problems, such as (3He/4He) magmatic heterogeneities or variable radiogenic 4He* concentrations between aliquots. Some calculations simulating the respective influence of the independent key parameters (age of the rock, magmatic He content, U–Th content) were performed and confronted to new datasets. This comparison allowed to define several geochemical and geological configurations for which the use of isochrones (two He components) or pseudo-isochrones (three He components) may be considered as a pertinent technique to determine properly the cosmogenic 3He concentrations. In summary, whatever the additional corrections to apply, this approach basically avoids the crushing step and thereby potential 3 Hecos loss. It may also help to check the homogeneity of helium components (magmatic, radiogenic) in a population of few aliquots, which is a crucial point to validate the non-cosmogenic corrections. Moreover, skipping the crushing step may also provide a significant gain of time, especially for noble gases laboratories equipped with automated furnaces. 3
Reference Blard, P.-H. et al., 2006. EPSL, in press. doi:10.1016/j.gca.2006.06.216
1
Yale University, Department of Geology and Geophysics, 210 Whitney Ave., New Haven, CT 06511, USA (madalyn. [email protected]) 2 Dickinson College, Department of Geology, James Center, Carlisle, PA 17013, USA Accurate ages for young (e.g., Pleistocene) volcanic eruptions are important for geomorphic, tectonic, climatic, and hazard studies. Existing techniques can be time-consuming and expensive when many ages are needed, and in the case of K/Ar and 40 Ar/39Ar dating, extraneous Ar often can limit precision, especially for continental basalts erupted through old lithosphere. We present a new technique for dating young basaltic eruptions by (U–Th)/He dating of zircons (ZHe) from crustal xenoliths. Single-crystal ZHe dates generally have lower precision than typical 40 Ar/39Ar dates, but can be determined on multiple replicate grain aliquots relatively easily. We dated zircons from xenoliths from four volcanic centers in western North America: Little Bear Mountain, BC (157 ± 3.5 [2.2%] 95% conf., MSWD = 1.7) and Prindle Volcano, AK (176 ± 16 [8.9%], MSWD = 13) , in the northern Cordilleran volcanic province, and Fish Springs (273 ± 23 [8.6%], MSWD = 43) and Oak Creek (179 ± 8.1 [4.5%], MSWD = 12), in the Big Pine Volcanic Field, CA. All ZHe ages are either equivalent to or younger than previously determined K–Ar or 40Ar/39Ar ages, indicating the possibility of inherited 40Ar in some of the previous measurements. Zircons from upper crustal xenoliths in the Oak Creek and Fish Springs vents show poorer reproducibility and multiple apparent age distribution peaks, consistent with either intracrystalline U–Th zonation or <99.99% He degassing (assuming 100-Ma pre-entrainment ZHe ages) of some zircons during magmatic entrainment. Removal of clear outliers in the older age-distribution peaks of the upper crustal xenoliths, most of which have extremely high U compared to other zircons of the same xenolith, improve the reproducibilities of Fish Springs to 4.7% (95% conf., MSWD = 4.8) and Oak Creek to 3.4% (95% conf., MSWD = 6.2). Coupled thermal and He diffusion modeling using appropriate xenolith sizes and magma temperatures and assuming published diffusion kinetics for zircon indicates that incomplete He degassing would require entrainment times less than 1 h. However, the observation of extremely high U in most zircons with older ages raises the possibility that zircons with high radiation dosages and significant pre-eruptive He may have more retentive He diffusion characteristics. doi:10.1016/j.gca.2006.06.217