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Quantifying micro-niche behaviour in sediments: Precise and accurate measurements of DGT gels by laser ablation high resolution laser ablation ICPMS
A478
Goldschmidt Conference Abstracts 2006
Quantifying micro-niche behaviour in sediments: Precise and accurate measurements of DGT gels by laser ablation high resolution laser ablation ICPMS D.G. PEARSON1, G.M. NOWELL1, A. WIDERLUND2, W. DAVISON2
Diamond geochronology—A record of continental lithosphere evolution D.G. PEARSON1, J.W. HARRIS2 1
Department of Earth Sciences, University of Durham, UK University of Glasgow, Department of Earth Sciences, Glasgow, UK
2
New developments in micron-scale measurements of trace metals and sulphur contents within sediments by DGT (diffusive gradients in thin-films) have revealed micro-niche structures. The origin of micro-niches is unclear and it is important to understand the relative roles played by inorganic and biological processes in the localised remobilisation of metals. We have undertaken a S and Fe isotope study of micro-niches, to better understand their mechanisms of formation. With their inbuilt pre-concentration, element selection and simple matrix, DGTs are an ideal medium for laser ablation ICPMS. In contrast to analysis of mineral phases, fully matrix-matched standards can be easily made. We have initially concentrated on the measurement of S isotopes and will also present experiments where Fe isotopes have been measured in DGT gels. S and Fe isotopes were measured on a Neptune at mass resolutions of >9000. A New-Wave UP-213 nm laser was used with Ar as the carrier gas. Instrumental mass bias (IMB) for S was corrected for using sample–standard bracketing. Ablation experiments on two S DGT gel standards, 10 Hz, 40% power, 170 lm wide line, scan speed 70 lms 1, show in-run d34S precisions for DGT gels with 2.1 lmol/cm2 S (3 V of 32S) of <0.2& (2SE). There is no observed correlation between d34S and 32S intensity. Reproducibility on d34S for individual analytical sessions (4–8 standards) is better than 0.3& while the reproducibility over 4 sessions is 0.19&. Further tests to mimic ablation over micro-niches (continuous ablation across a gel with two isotopic compositions) show similar internal precisions and only slightly worse reproducibility—0.48& (2SD; n = 6). The accuracy of the LA d34S measurements on a BaSO4 gel relative to the reference value of the starting BaSO4, bracketed by a AgI gel of known isotopic composition, is better than 1&, showing negligible S isotopic fractionation during gel deployment. d34S variations in microniches can be easily resolved at the 1& level and 100 lm scale whereas variations in natural micro-niches are expected to be 10& or more. Fe measurements, using external normalisation to a 60Ni/61Ni ratio and an exponential law to correct for IMB, suggest internal precisions and external reproducibility of <0.15& (2SE) and 0.07& (2SD) should be possible.
The development of a technique for the precise determination of Re–Os isotopes in single sulfide inclusions in diamonds has allowed a rapid expansion in the number of ages available for diamond suites around the world. A combination of the capability to analyse individual inclusions to construct isochrons, together with the relatively low blocking temperature of the Re–Os isotope system means that Re–Os isochron ages are a robust way to date sulfide-bearing diamonds. The relatively large database allows us to look in more detail at the linkage between mantle and crustal/ lithospheric events for diamond genesis. A detailed analysis of the data available for southern Africa reveals numerous interesting features. Diamonds crystallized in the lithosphere beneath southern Africa over a period of greater than 3 Ga, beginning in the Archean and continuing to the Mesozoic. Diamond formation in the continental lithosphere appears to occur in pulses rather than continuously. Most diamonds formed in the Archean and Proterozoic Eons. There is some correspondence between major events recorded in crustal rocks and some pulses of diamond formation. These events also correspond with events in the lithospheric mantle defined by whole rock and/or peridotite-derived sulfide Re–Os model ages. Other diamond crystallization events, of Mesoproterozoic age, such as those recorded from Jwaneng, do not correlate with recognizable crustal events but coincide with lithospheric mantle disturbances observed within peridotites. All eclogitic diamond forming events are reflected in the sulfide populations of peridotite xenoliths and indicate widespread infiltration of sulfide into the lithospheric mantle accompanying eclogitic diamond formation. This observation could reflect the general mobility of sulfur, as sulfides, during subduction around cratons, or may hint at a possible metasomatic origin, from fluids with a crustal origin, for eclogitic diamonds in general. The chronology of formation of some eclogitic diamonds can be clearly linked to craton assembly events, e.g., Argyle—assembly of cratonic nuclei during the Barramundi Orogeny; Kimberley, S. Africa—docking of the Kimberley and Wits blocks. In other regions such as Siberia, it is the silicate peridoditic diamonds that are linked to the strong signature of craton assembly at 2 Ga recorded in the crust and in other mantle assemblages while Siberian sulfide peridotitic diamonds are Mesoarchean. Hence, diamond dating is revealing an intricate pattern of diamond formation that is consistent with other isotope tracers in requiring that eclogitic diamonds result from tectonic processes that were initiated in the Archean.
doi:10.1016/j.gca.2006.06.1418
doi:10.1016/j.gca.2006.06.1419
1
Department of Earth Sciences, University of Durham, UK Lancaster University, Department of Environmental Science, Lancaster LA1 4YQ, UK
2
Goldschmidt Conference Abstracts 2006
Quantifying micro-niche behaviour in sediments: Precise and accurate measurements of DGT gels by laser ablation high resolution laser ablation ICPMS D.G. PEARSON1, G.M. NOWELL1, A. WIDERLUND2, W. DAVISON2
Diamond geochronology—A record of continental lithosphere evolution D.G. PEARSON1, J.W. HARRIS2 1
Department of Earth Sciences, University of Durham, UK University of Glasgow, Department of Earth Sciences, Glasgow, UK
2
New developments in micron-scale measurements of trace metals and sulphur contents within sediments by DGT (diffusive gradients in thin-films) have revealed micro-niche structures. The origin of micro-niches is unclear and it is important to understand the relative roles played by inorganic and biological processes in the localised remobilisation of metals. We have undertaken a S and Fe isotope study of micro-niches, to better understand their mechanisms of formation. With their inbuilt pre-concentration, element selection and simple matrix, DGTs are an ideal medium for laser ablation ICPMS. In contrast to analysis of mineral phases, fully matrix-matched standards can be easily made. We have initially concentrated on the measurement of S isotopes and will also present experiments where Fe isotopes have been measured in DGT gels. S and Fe isotopes were measured on a Neptune at mass resolutions of >9000. A New-Wave UP-213 nm laser was used with Ar as the carrier gas. Instrumental mass bias (IMB) for S was corrected for using sample–standard bracketing. Ablation experiments on two S DGT gel standards, 10 Hz, 40% power, 170 lm wide line, scan speed 70 lms 1, show in-run d34S precisions for DGT gels with 2.1 lmol/cm2 S (3 V of 32S) of <0.2& (2SE). There is no observed correlation between d34S and 32S intensity. Reproducibility on d34S for individual analytical sessions (4–8 standards) is better than 0.3& while the reproducibility over 4 sessions is 0.19&. Further tests to mimic ablation over micro-niches (continuous ablation across a gel with two isotopic compositions) show similar internal precisions and only slightly worse reproducibility—0.48& (2SD; n = 6). The accuracy of the LA d34S measurements on a BaSO4 gel relative to the reference value of the starting BaSO4, bracketed by a AgI gel of known isotopic composition, is better than 1&, showing negligible S isotopic fractionation during gel deployment. d34S variations in microniches can be easily resolved at the 1& level and 100 lm scale whereas variations in natural micro-niches are expected to be 10& or more. Fe measurements, using external normalisation to a 60Ni/61Ni ratio and an exponential law to correct for IMB, suggest internal precisions and external reproducibility of <0.15& (2SE) and 0.07& (2SD) should be possible.
The development of a technique for the precise determination of Re–Os isotopes in single sulfide inclusions in diamonds has allowed a rapid expansion in the number of ages available for diamond suites around the world. A combination of the capability to analyse individual inclusions to construct isochrons, together with the relatively low blocking temperature of the Re–Os isotope system means that Re–Os isochron ages are a robust way to date sulfide-bearing diamonds. The relatively large database allows us to look in more detail at the linkage between mantle and crustal/ lithospheric events for diamond genesis. A detailed analysis of the data available for southern Africa reveals numerous interesting features. Diamonds crystallized in the lithosphere beneath southern Africa over a period of greater than 3 Ga, beginning in the Archean and continuing to the Mesozoic. Diamond formation in the continental lithosphere appears to occur in pulses rather than continuously. Most diamonds formed in the Archean and Proterozoic Eons. There is some correspondence between major events recorded in crustal rocks and some pulses of diamond formation. These events also correspond with events in the lithospheric mantle defined by whole rock and/or peridotite-derived sulfide Re–Os model ages. Other diamond crystallization events, of Mesoproterozoic age, such as those recorded from Jwaneng, do not correlate with recognizable crustal events but coincide with lithospheric mantle disturbances observed within peridotites. All eclogitic diamond forming events are reflected in the sulfide populations of peridotite xenoliths and indicate widespread infiltration of sulfide into the lithospheric mantle accompanying eclogitic diamond formation. This observation could reflect the general mobility of sulfur, as sulfides, during subduction around cratons, or may hint at a possible metasomatic origin, from fluids with a crustal origin, for eclogitic diamonds in general. The chronology of formation of some eclogitic diamonds can be clearly linked to craton assembly events, e.g., Argyle—assembly of cratonic nuclei during the Barramundi Orogeny; Kimberley, S. Africa—docking of the Kimberley and Wits blocks. In other regions such as Siberia, it is the silicate peridoditic diamonds that are linked to the strong signature of craton assembly at 2 Ga recorded in the crust and in other mantle assemblages while Siberian sulfide peridotitic diamonds are Mesoarchean. Hence, diamond dating is revealing an intricate pattern of diamond formation that is consistent with other isotope tracers in requiring that eclogitic diamonds result from tectonic processes that were initiated in the Archean.
doi:10.1016/j.gca.2006.06.1418
doi:10.1016/j.gca.2006.06.1419
1
Department of Earth Sciences, University of Durham, UK Lancaster University, Department of Environmental Science, Lancaster LA1 4YQ, UK
2