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TitleElement and isotopic signature of re-fertilized mantle peridotite as determined by nanopowder and olivine LA-ICPMS analyses
AuthorLawley, C J MORCID logo; Pearson, D G; Waterton, P; Zagorevski, A; Bédard, J H; Jackson, S EORCID logo; Petts, D CORCID logo; Kjarsgaard, B A; Zhang, S; Wright, D
SourceChemical Geology vol. 536, 119464, 2020 p. 1-17, https://doi.org/10.1016/j.chemgeo.2020.119464
Image
Year2020
Alt SeriesNatural Resources Canada, Contribution Series 20190600
PublisherElsevier
Documentserial
Lang.English
Mediapaper; on-line; digital
File formatpdf; html
ProvinceBritish Columbia
NTS94K/12; 94K/13; 94L/09; 94L/10; 94L/11; 94L/12; 94L/13; 94L/14; 94L/15; 94L/16; 94M; 94N/04; 94N/05; 94N/12; 94N/13
Lat/Long WENS-127.7525 -125.7214 59.8019 58.6883
Subjectseconomic geology; igneous and metamorphic petrology; geochemistry; geochronology; Science and Technology; Nature and Environment; mineral deposits; base metals; precious metals; ore mineral genesis; mineralization; mineral enrichment; partial melting; lithosphere; mantle; bedrock geology; lithology; ultramafic rocks; igneous rocks; intrusive rocks; peridotites; pyroxenites; dunites; harzburgites; lithogeochemistry; whole rock geochemistry; olivine; geochemical analyses; trace element geochemistry; major element geochemistry; isotopic studies; radioisotopes; lead isotope ratios; osmium; mass spectrometer analysis; petrographic analyses; electron probe analyses; tectonic history; intrusions; dykes; deformation; metasomatism; ophiolites; field relations; inclusions; models; Middle Triassic; Lower Triassic; Upper Permian; Nahlin Ophiolite; Cache Creek Terrane; platinum group element geochemistry; Phanerozoic; Mesozoic; Triassic; Paleozoic; Permian
Illustrationslocation maps; geoscientific sketch maps; photographs; photomicrographs; plots; models; bar graphs
ProgramTargeted Geoscience Initiative (TGI-5), Gold ore systems
ProgramGEM: Geo-mapping for Energy and Minerals
Released2020 01 09
AbstractThe lithospheric mantle should be depleted in base- and precious-metals as these elements are transferred to the crust during partial melting. However, some melt-depleted mantle peridotites are enriched in these ore-forming elements. This may reflect re-fertilization of the mantle lithosphere and/or sequestering of these elements by residual mantle phase(s). Both processes remain poorly understood because of the low abundances of incompatible elements in peridotite and the nugget-like distribution of digestion-resistant mantle phases that pose analytical challenges for conventional geochemical methods. Herein we report new major and trace element concentrations for a suite of mantle peridotite and pyroxenite samples from the Late Permian to Middle Triassic Nahlin ophiolite (Cache Creek terrane, British Columbia, Canada) using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) analysis of nanoparticulate powders and olivine. Compatible to moderately incompatible element concentrations suggest that Nahlin ophiolite peridotites represent residues after greater than or equal to 20% melt extraction. Pyroxenite dykes and replacive dunite bands are folded and closely intercalated with residual harzburgite. These field relationships, coupled with the presence of intergranular base metal sulphide, clinopyroxene and Cr-spinel at the microscale, point to percolating melts that variably re-fertilized melt-depleted mantle peridotite. Radiogenic Pb (206Pb/204Pb = 15.402-19.050; 207Pb/204Pb = 15.127-15.633; 208Pb/204Pb=34.980-38.434; n=45) and Os (187Os/188Os 0.1143-0.5745; n=58) isotope compositions for a subset of melt-depleted peridotite samples further support metasomatic re-fertilization of these elements. Other ore-forming elements are also implicated in these metasomatic reactions because some melt-depleted peridotite samples are enriched relative to the primitive mantle, opposite to their expected behaviour during partial melting. New LA-ICPMS analysis of fresh olivine further demonstrates that a significant proportion of the highly incompatible element budget for the most melt-depleted rocks is either hosted by, and/or occurs as trapped inclusions within, the olivine-rich residues. Trapped phases from past melting and/or re-fertilization events are the preferred explanation for unradiogenic Pb isotope compositions and Paleozoic to Paleoproterozoic Re-depletion model ages, which predate the Nahlin ophiolite by over one billion years.
Summary(Plain Language Summary, not published)
New method is described for the determination of very low concentrations of elements within rocks. Method is applied to better understand upgrading of ore-forming elements from very low concentrations.
GEOSCAN ID321929

 
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