Richard A. Stern

Assessment of errors in SIMS zircon U–Pb geochronology using a natural zircon standard and NIST SRM 610 glass

Analytical errors calculated for individual spot 206Pb/238U measurements of zircon analyzed using high mass resolution secondary ion mass spectrometry (HR-SIMS, e.g., SHRIMP II) were assessed using natural zircon (z6266) and synthetic glass standards (NIST SRM 610). Evidence for U/Pb homogeneity of these materials includes new thermal ionization mass spectrometry (TIMS) U–Pb analyses of 22 fragments of z6266 zircon from two laboratories, which are identical within error and yield a weighted mean 206Pb/238U age of 559.0±0.2 Ma. TIMS U–Pb analyses of the SRM 610 yielded homogeneous 206Pb/238U=0.2566. Errors in HR-SIMS 206Pb/238U measurements are distinguished in increasing hierarchy as within-spot, within-session (internal), and external. Replicate SHRIMP measurements of 208Pb+/206Pb+ and 248[ThO]+/254[UO]+ in SRM 610 demonstrate that within-spot analytical errors are sufficient (i.e., MSWDs∼1) to account for the dispersion of data. In contrast, it was not possible with SRM 610 to reproduce the 206Pb+/238U[Ox=0,1,2]+ ratios from spot-to-spot within the limits of the within-spot errors, after correcting the data for obvious systematic variations in the discrimination of the Pb and U isotopes. Identical findings were made following replicate 206Pb+/238U[Ox]+ analyses of z6266 zircon. That is, for both glass and zircon standards, there remained on average about ±1% (1σ) unaccounted variation per 206Pb+/238U[Ox]+ analysis (i.e., MSWDs≫1). Thus, unlike for 248[ThO]+/254[UO]+, the within-session errors in 206Pb+/238U[Ox]+ were necessarily higher than the within-spot errors. In contrast to 208Pb+/206Pb+ and 248[ThO]+/254[UO]+, the 206Pb+/238U[Ox]+ ratios in glass and zircon are significantly fractionated relative to accepted values, and it seems probable that this difference underlies the larger within-session errors for 206Pb+/238U[Ox]+. The usual methods (e.g., using 254[UO]+/238U+) to correct for the inherent discrimination are imperfect and sensitive to small differences in analytical conditions between spots. Matrix variations are ruled out, leaving the precise cause(s) of the additional variation in 206Pb+/238U[Ox]+ currently undetermined, and a hindrance to decreasing the uncertainties in spot 206Pb/238U measurements. The minimum within-session error for an individual 206Pb/238U measurement is the standard deviation (dispersion) of the discrimination-corrected 206Pb+/238U[Ox]+ values for a homogeneous reference material (e.g., SRM 610). The external error in 206Pb/238U additionally incorporates the standard deviation of the mean of the discrimination-corrected 206Pb+/238U[Ox]+ for a natural zircon standard. The analysis of 206Pb+/270[UO2]+ offers an improvement over measuring 206Pb+/238U+ because variable discrimination is reduced or eliminated with respect to 254[UO]+/238U+ and counting statistics are superior.

U-Pb geochronology of Riphean sandstone and gabbro from southeast Siberia and its bearing on the Laurentia-Siberia connection

Thirty-one detrital zircons from the mid–late Riphean Mayamkan Formation sandstone (Uy Group) of the Sette–Daban fold belt, southeast Siberia yielded SHRIMP 207Pb/206Pb ages ranging between 1500 and 1050 Ma. Other grains yielded ages between 2.7 and 1.8 Ga. There is no known source region for the Mesoproterozoic zircons in Siberia; however, this range of ages closely matches those of detrital zircons from Neoproterozoic sandstones from northwest Canada, which are considered to have been derived from the Grenville Province of southeast Laurentia (all directions cited are with reference to present-day coordinates). These data suggest a formerly close connection between southeast Siberia and northwest Laurentia prior to their separation in the Neoproterozoic. However, two gabbro sills which intrude the Riphean sedimentary succession of the Sette–Daban fold belt are dated here at 1005±4 Ma and 974±7 Ma (U–Pb baddeleyite), an unknown age in northern Laurentia and unlike the widespread and well characterized 723 Ma Franklin and 1267 Ma Mackenzie mafic magmatic events. These somewhat incongruous results cast uncertainty on existing continental reconstructions, which link Siberia to Laurentia from about 1900 to 700 Ma. Our data can be reconciled with existing data by proposing an alternative continental configuration based on former continuity of the following tectonic entities: Archean Tungus Province (Siberia) with Archean Slave Province (Laurentia); Paleoproterozoic Angara fold belt (Siberia) with Paleoproterozoic Wopmay orogen and Great Bear magmatic zone (Laurentia); and Paleoproterozoic Akitkan fold belt (Siberia) with Paleoproterozoic Thelon–Taltson magmatic zone (Laurentia). Our reconstruction also considers the proposed northern extension of the Grenville orogen to be a potential source for Mesoproterozoic detrital zircons from the Mayamkan Formation. Such an orientation also is required to explain the apparent absence of Franklin and/or Mackenzie mafic magmatic rocks and the lack of distinctive Neoproterozoic lithofacies in the Sette–Daban fold belt. An additional conclusion of our study is that the lowermost Uy Group can be no younger than ca. 1010 Ma because it is intruded by a diabase sill dated at 1005±4 Ma. Previous work indicated that the Uy Group and underlying Lakhanda Group are of late Riphean age (1000–650 Ma). The youngest detrital zircon from the Mayamkan Formation provides a maximum U–Pb age of 1070±40 Ma for the upper Uy Group.

Monazite U–Pb and Th–Pb geochronology by ion microprobe, with an application to in situ dating of an Archean metasedimentary rock

Abstract Monazite grains from nine different Precambrian rock samples and having concordant isotope dilution U–Pb and Th–Pb ages were used to develop and evaluate techniques for U–Pb, Th–Pb, and Pb–Pb isotopic dating using an ion microprobe operating at high mass resolution ( R =5500, 1%). An unidentified isobar at ∼203.960 amu complicated the determination of 204 Pb in the monazites with higher Th, but it appeared to have been eliminated when using a slight energy offset. The 207 Pb*/ 206 Pb* ratios of individual spots were determined in 10 min analyses to ±0.3–0.5% (1 σ ) of the true values using a 20-μm-wide O 2 − primary beam, whereas the uncertainty in the mean of several spots on a single generation of monazite was typically ±0.1–0.4% (2 σ ). Instrumental bias in the measured Pb + /UO + and Pb + /ThO + values of individual analyses was monitored using linear relationships between 206 Pb + /UO + vs. UO 2 + /UO + and 208 Pb + /ThO + vs. UO 2 + /UO + . The bias appears to vary somewhat with monazite Th content, requiring the use of compositionally matched standards. Individual spot measurements of the 206 Pb/ 238 U and 208 Pb/ 232 Th ratios in unknown monazites of low to moderate Th can be determined to ±2% (1 σ ). The dating technique was applied to in situ analysis of small monazite inclusions hosted in an amphibolite-grade metapelite of Archean origin. Both the contextual setting and the morphology of the monazite grains were found to be important factors in interpreting the timing of growth of the host metamorphic mineral (e.g., garnet, staurolite, plagioclase). Euhedral monazite grains armoured in low-CaO garnet and matrix plagioclase yielded an age of 2548±17 Ma, interpreted as dating an early episode of low-pressure metamorphism. Partially resorbed monazite within a high-CaO garnet rim yielded ages of ca. 2500 Ma, but are interpreted as inherited from the first event rather than syn-genetic with respect to the growth of this garnet. An age of 1755±30 Ma was obtained for anhedral monazite associated with late, chlorite-filled fractures, apparently reflecting post-tectonic retrogression.

Incomplete retention of radiation damage in zircon from Sri Lanka

Abstract A suite of 18 zircon gemstones from placers in the Highland/Southwestern Complex, Sri Lanka, were subjected to a comprehensive study of their radiation damages and ages. The investigation included X-ray diffraction, Raman and PL spectroscopy, electron microprobe, PIXE and HRTEM analysis, as well as (U-Th)/He and SHRIMP U-Th-Pb age determinations. Zircon samples described in this study are virtually homogeneous. They cover the range from slightly metamict to nearly amorphous. Generally concordant U-Th-Pb ages averaging 555 ± 11 Ma were obtained. Late Ordovician zircon (U-Th)/He ages scattering around 443 ± 9 Ma correspond reasonably well with previously determined biotite Rb-Sr ages for rocks from the HSWC. Slightly to moderately metamict zircon has retained the radiogenic He whereas only strongly radiation-damaged zircon (calculated total fluences exceeding ~3.5 × 1018 α-events/g) has experienced significant He loss. When compared to unannealed zircon from other localities, Sri Lanka zircon is about half as metamict as would correspond to complete damage accumulation over a ~555 m.y. lasting self-irradiation period, suggesting significant annealing of the structural radiation damage. Insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon. We suggest to estimate “effective α-doses” for Sri Lanka zircon by multiplying total α-fluences, which were calculated using the zircon U-Th-Pb age, by a correction factor of 0.55. This conversion may be applied to literature data as well, because all gem-zircon samples from Sri Lanka (this work and previous studies) seem to reveal the same general trends of property changes depending on the radiation damage. The use of “effective α-doses” for Sri Lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation-damaged zircon.

Intraoceanic tectonics and the development of continental crust: 1.92–1.84 Ga evolution of the Flin Flon Belt, Canada

The Flin Flon Belt is a complex (“Amisk collage”) of distinct tectono-stratigraphic assemblages that was brought together at a relatively early stage in the tectonic evolution of the Paleoproterozoic Trans-Hudson orogen. Four main tectono-stratigraphic assemblage types are recognized: isotopically juvenile oceanic arc (1.90–1.88 Ga), ocean floor (ca. 1.90 Ga), oceanic plateau/ocean island, and isotopically evolved arc (1.92–1.90 Ga). Oceanic arc assemblages include tholeiitic, calc-alkaline, and lesser shoshonitic volcanic and volcaniclastic rocks, as well as turbidites and arc-rift volcanic rocks. The tectono-stratigraphic assemblages were juxtaposed in an accretionary complex (Amisk collage) at ca. 1.88–1.87 Ga, probably as a result of arc-arc collision/collisions. Accretionary collage structures are largely obliterated by subsequent deformation and metamorphic events, but can be inferred where cut by calc-alkaline plutons related to a 1.866–1.838 Ga arc. Coeval subaerial volcanism is recorded in ca. 1.87–1.85 Ga calc-alkaline to shoshonitic volcaniclastic sequences. Unroofing of the accretionary collage, development of a paleosol, and deposition of continental (alluvial-fluvial) sedimentary rocks occurred ca. 1.85–1.84 Ga, coeval with the waning stages of post-accretion arc magmatism. Stabilization of the Flin Flon Belt by 1.85–1.84 Ga as part of a Philippines- or Japan-like microcontinent is attributed to both tectonic and magmatic thickening of the lithosphere.

Minto block, Superior province: Missing link in deciphering assembly of the craton at 2.7 Ga

Plate-tectonic models of the Superior province are rooted in the granite-greenstone and metasedimentary belts of the southern part of the craton. North-striking domains of the Minto block in the northeastern part of the province evolved at similar times, in 3.1-2.8, 2.725, and 2.69 Ga events, requiring expansion of models of late Archean assembly to accommodate Minto geology. Western and eastern protocratons (∼3.1-2.8 Ga) rifted at ∼2.79 Ga to produce an ocean basin that was mostly consumed by subduction at 2.725 Ga. The Leaf River plutonic suite of cale-alkalic hornblende + biotite ± orthopyroxene ±clinopyroxene granodiorite represents magmatic arcs built on the protocratons, whereas the intervening Goudalie domain—containing fault-bounded fragments of rifted continental crust, rift volcanics, primitive oceanic crust, 2724 Ma island-arc rocks, and a <2718 Ma back-arc assemblage—marks the suture. Terminal collision at ∼2.7 Ga led to thickening and crustally derived granitoid magmatism. The southern Superior province also experienced vigorous activity between 2.725 and 2.69 Ga as island arcs, oceanic plateaus, continental fragments, and accretionary prisms were amalgamated progressively from north to south in a regime of dextral transpression then stitched by granites. A northern proto-Superior craton had continental magmatic arcs built on its eastern and southern flanks in response to west-northwest-directed subduction; orthogonal convergence in the east produced wide plutonic arcs, in contrast to terrane-accretion tectonics in the more oblique regime along the southern margin.

Sedimentary evolution of the Riphean–Vendian basin of southeastern Siberia

Abstract The Riphean to Vendian ( ≈1600–540 Ma) sedimentary succession of the southeastern margin of the Siberian platform is 12–14 km thick and consists of terrigenous-carbonate successions termed, from oldest to youngest, the Uchur, Aimchan, Kerpyl, Lakhanda, Uy and Yudoma Groups. Group boundaries typically are regional unconformities; local angular unconformities occur at the base of the Aimchan and Yudoma Groups. Deposition mostly occurred in terrestrial to shallow marine sedimentary environments; only the Uy Group contains evidence of deep-water sedimentation. Paleocurrent and facies trends show that provenance of the Uchur, Aimchan and most of Kerpyl Groups was from the Siberian craton to the west. This corresponds with the mineralogical and chemical composition of sandstones, which suggests continental block to recycled orogen provenance with predominance of granites in the source area. Sandstones from the Uy and Yudoma Groups were derived from both western (Siberian) and eastern (non-Siberian) sources. The Uy Group contains graywacke that implies local recycled orogen to arc orogen provenance. Trace and rare earth element geochemistry suggests provenance from post-Archean source rocks and this is supported by U–Pb detrital zircon geochronology which indicates that only 3 of the 96 grains analyzed are of Archean age. Detrital zircons ≈2050 Ma predominate at the base of the Uchur Group. At the base of the Kerpyl Group ≈2060–1880 Ma zircons predominate with youngest grains ≈1300 Ma. The latter represents an unknown source, as rocks younger ≈1700 Ma are not reported from the basement of the Siberian platform. Zircons in the uppermost part of the Uy Group range in age from 1500 to 1050 Ma suggesting a non-Siberian provenance, perhaps from the Grenville orogen of Laurentia. Conventional U–Pb analysis of a few detrital zircon grains from the Yudoma Group sandstones yielded ages ≈2200–2000. Sedimentological and stratigraphic studies indicate that the Riphean–Vendian sedimentary basin of southeastern Siberia initiated by rifting that subsequently failed, allowing the development of a long-lived intracratonic sedimentary basin. Mafic magmatism and depositional features of the Uy Group suggest that there was renewed rifting ≈1000 Ma, when the basin evolved into an aulacogen. Rifted arms spread to form the Verkhoyansk ocean, the margins of which were approximately parallel to the modern margin of Siberian platform and Okhotsk massif.

Geochemistry of 1.9 Ga MORB- and OIB-like basalts from the Amisk collage, Flin Flon Belt, Canada: Evidence for an intra-oceanic origin

Subaqueously-erupted basalts that occur in kilometre-scale allochthons within the 1.9 Ga Flin Flon Belt, Canada, appear to have been generated at oceanic ridges and possibly oceanic plateaus, remote from Archean cratons. The ocean-floor basalts fall into two categories: (1) N-type, resembling N-MORBs and Mariana-type back-arc basin basalts (depleted to flat REE patterns, high ZrNb, variable ThNb, and initial ϵNd = + 3.3 to + 5.4); (2) E-type, resembling transitional and plume MORBs (slightly enriched REE patterns, lower ZrNb, initial ϵNd = +3.1 to +4.5). In the largest and best-studied allochthon, the Elbow-Athapapuskow ‘assemblage,’ mixing between depleted (N-MORB) and enriched (OIB) sources or melts, coupled with variable addition of a subduction LILE component, can explain the chemical variations in the basalts. Zircon U-Pb dates of 1904 ± Ma for a syn-volcanic diabase sill and 1901 +6-5 Ma for a gabbro-peridotite cumulate complex demonstrate that crystallization of the ‘ocean-floor’ basalts overlapped with, in part, eruption of the tectonically juxtaposed 1.90-1.88 Ga arc volcanic rocks. The Elbow-Athapapuskow allochthon is interpreted as back-arc basin crust that developed simultaneously with Flin Flon arc magmatism. Subaerially erupted basalts that chemically resemble tholeiitic OIBs (8–14% MgO, relative HREE depletion, initial ϵNd = +2.2 to +3.4) occur in tectonic contact with the Elbow-Athapapuskow assemblage. The OIBs may have been generated by deeper (garnet residue) melting of enriched mantle tapped during extension in the Elbow-Athapapuskow back-arc basin, and were possibly erupted onto a remnant arc. Deeper mantle melting is also indicated by the presence of the LREE-enriched oceanic plateau-like basalts of the Sandy Bay assemblage. The back-arc, 01B, and plateau volcanic assemblages were jux-taposed against ca. 1.9 Ga arc assemblages in a Philippines-like intraoceanic accretionary complex by 1.87 Ga.

Paleoproterozoic (1.90 1.86 Ga) arc volcanism in the Flin Flon Belt, Trans-Hudson Orogen, Canada

Geochemical and isotopic (Nd, Sr) data are reported on Paleoproterozoic (1904–1864 Ma), maficintermediate (<63% SiO2), arc metavolcanic rocks from the Flin Flon greenstone belt, Manitoba and Saskatchewan. Major element criteria permit subdivision of the rocks into tholeiitic (TH), calc-alkaline (CA), alkaline, and boninitic (BO) magma series. Subaqueously erupted, TH and related CA basalt-basaltic andesite, and rare high-Ca boninites dominated between 1904 Ma and 1890 Ma. The TH rocks are similar to modern island are tholeiites, having low high-field-strength element (HFSE) and rare earth element (REE) abundances, and chondrite-normalized light REE depletion to slight enrichment. The boninites have even lower HFSE and REE abundances (1–2X chondrites). Along with their extreme ratios of refractory incompatible elements (e.g., high Al/Ti, Ti/Zr, low Ti/V, Zr/Y), these features indicate that the arc mantle source was strongly depleted, probably residual after MORB or back-arc basin basalt extraction. Elevated Th/Yb, Ba/La, La/Nb values, and the spread in Nd isotopic compositions (initial ɛNd=−0.4 to +4.8) suggest recycling of small amounts (0–8%) of Archean and possibly older Proterozoic crust via sediment subduction and, locally, intracrustal contamination. Calcalkaline andesite-rhyolite and rare shoshonite and trachyandesite, erupted between 1890 Ma and 1864 Ma, are more strongly light REE enriched and have comparatively higher HFSE abundances, and higher Zr/Y and Nb/Y values. The rocks have strong arc trace element signatures (e.g., high Th/Nb, La/Nb), and initial ɛNd values (+2.3 to +4.6) indicate that depleted mantle contributions to the magmas continued to be dominant. The geochemistry and geology of these younger volcanic rocks suggest a mature island arc setting in which the arc lithosphere was thicker than in the previous period, and a more fertile sub-arc mantle source was tapped. The pre-1890 Ma volcanism occurred in one or more separate arcs, probably characterized by rapid subduction of oceanic lithosphere, relatively thin, tholeiitic arc crust, and extensive backarc basin formation. In contrast, post-1890 Ma volcanism is dominantly calc-alkaline to (rarely) alkaline, and is interpreted to reflect crustal thickening due to longterm growth of arc edifice(s) and tectonic thickening associated with intraoceanic arc-arc (>1870 Ma) collision and subsequent intra-arc deformation.

An investigation of artificial biasing in detrital zircon U-Pb geochronology due to magnetic separation in sample preparation

Abstract The application of detrital zircon geochronology for provenance analysis is complicated by the presence of biases induced by natural processes and sample preparation. The biasing of age distributions as a result of magnetic susceptibility is illustrated using sensitive high resolution ion microprobe dating of detrital zircon from a metaquartzite sample partitioned using a Frantz magnetic barrier separator. The relationship of paramagnetism with U content, α-dose, and discordance is demonstrated, but no relationship between grain size and discordance or age is found. The data also demonstrate that previous limits of zircon survival in sedimentary processes based on U content alone are too simplistic. Two age modes at ∼3150 and ∼2960 Ma are present in all the paramagnetic fractions; there is a bias toward the ∼3150 Ma mode being more prominent in the least-paramagnetic fractions. While the ∼2960 Ma is present in the least-paramagnetic fraction, it is argued that such fortuitous representation cannot be assumed before analysis. Such “there or not” provenance interpretations are considered simplistic, and at the very least there is no harm in broadening the range of paramagnetic fractions sampled for analysis. The results indicate a compromise between broad representation and analytical efficiency (avoiding discordant and thus unreliable results) can be made with a Frantz setting of ∼1.8 A and 10° side-slope.