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| Titel |
QEMSCAN+LAICPMS: a new tool for petrochronology and sedimentary provenance analysis |
| VerfasserIn |
Pieter Vermeesch, Martin Rittner, Jenny Omma |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250152383
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| Publikation (Nr.) |
 EGU/EGU2017-17215.pdf |
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| Zusammenfassung |
| Only a relatively small number of rock-forming minerals contain sufficiently high
concentrations of naturally occurring radioactive parent isotopes and sufficiently low
background concentrations of the corresponding daughter isotopes to be suitable for
geochronology. The first step in most geochronological studies is to extract these datable
minerals from the host rock using a combination of magnetic and density separation
techniques, a process that is tedious and time consuming. We here present a new method to
avoid mineral separation by coupling a QEMSCAN electron microscope to an LA-ICP-MS
instrument. Given a polished hand specimen, a petrographic thin section, or a grain mount,
the QEMSCAN+LAICPMS method produces chemical and mineralogical maps from
which the X-Y coordinates of the datable mineral are extracted. These coordinates
are subsequently passed on to the laser ablation system for isotopic and, hence,
geochronological analysis. QEMSCAN+LAICPMS can be applied to a wide range of
problems in igneous, metamorphic and sedimentary geology, as is illustrated with
three case studies. In the first case study, a 3 × 4 cm slab of polished granite from
the L’Erée pluton in Guernsey is scanned for zircon. This yields 23 U-Pb ages
resulting in a concordia age of 615 ± 2 Ma. The second case study re-investigates a
paragneiss from an ultra-high pressure terrane in the Qaidam Basin (Qinghai, China) that
was previously analysed by conventional petrography, electron microscopy and
SIMS zircon U-Pb analysis. In this example, the QEMSCAN revealed 107 small
(20 μm) metamorphic zircons that were analysed by LA-ICP-MS to constrain the
430 Ma age of peak metamorphism. The third and final case study investigates
the mineralogy and geochronology of sedimentary rocks of the Ordovician Sarah
Formation (Saudi Arabia). We analysed 44 outcrop samples and a further 35 subsurface
samples, resulting in a dataset comprising 10,000 detrital zircon U-Pb ages and 79
heavy mineral compositions and petrographic analyses. Jointly considering this
enormous dataset by 3-way multidimensional scaling paints a remarkably simple picture
of sandstone compositions that are determined by a combination of provenance
and diagenesis. In conclusion, the QEMSCAN+LAICPMS method represents an
efficient and cost effective way to obtain chemical and isotopic compositions on the
same sample, which improves both the quantity and quality of geological data. |
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