Plesovice zircon : A new natural reference material for U-Pb and Hf isotopic microanalysis

We present the first finding of continental crust-derived Precambrian zircons in garnet/spinel pyroxenite veins within mantle xenoliths carried by the Neogene Hannuoba basalt in the central zone of the North China Craton (NCC). Petrological and geochemical features indicate that these mantle-derived composite xenoliths were formed by silicic melt^lherzolite interaction. The Precambrian zircon ages can be classified into three age groups of 2·4^2·5 Ga, 1·6^2·2 Ga and 0·6^1·2 Ga, coinciding with major geological events in the NCC. These Precambrian zircons fall in the field of continental granitoid rocks in plots of U/Yb vs Hf and Y. Their igneous-type REE patterns and metamorphic zircon type CL images indicate that they were not crystallized during melt^peridotite interaction and subsequent high-pressure metamorphism.The 2·5 Ga zircons have positive eHf(t) values (2·9^10·6), whereas the younger Precambrian zircons are dominated by negative eHf(t) values, indicating an ancient continental crustal origin.These observations suggest that the Precambrian zircons were xenocrysts that survived melting of recycled continental crustal rocks and were then injected with silicate melt into the host peridotite. In addition to the Precambrian zircons, igneous zircons of 315 3 Ma (2 ), 80^170 Ma and 48^64 Ma were separated from the garnet/spinel pyroxenite veins; these provide evidence for lower continental crust and oceanic crust recycling-induced multi-episodic melt^peridotite interactions in the central zone of the NCC. The combination of the positive eHf(t) values (2·91^24·6) of the 315 Ma zircons with the rare occurrence of 302^324 Ma subduction-related diorite^granite plutons in the northern margin of the NCC implies that the 315 Ma igneous zircons might record melt^peridotite interactions in the lithospheric mantle induced by Palaeo-Asian oceanic crust subduction. Igneous zircons of age 80^170 Ma generally coexist with the Precambrian metamorphic zircons and have lower Ce/Yb and Th/U ratios, higher U/Yb ratios and greater negative Eu anomalies.The eHf(t) values of these zircons vary greatly from ^47·6 to 24·6.The 170^110 Ma zircons are generally characterized by negative eHf(t) values, whereas the 110^100 Ma zircons have positive eHf(t) values.These observations suggest that melt^peridotite interactions at 80^170 Ma were induced by partial melting of recycled continental crust. The 48^64 Ma igneous zircons are characterized by negligible Ce anomalies, unusually high REE, U and Th contents, and positive eHf(t) values.These features imply that the melt^peridotite interactions at 48^64 Ma could be associated with a depleted mantle-derived carbonate melt or fluid.

Continental and Oceanic Crust Recycling-induced Melt^Peridotite Interactions in the Trans-North China Orogen: U^Pb Dating, Hf Isotopes and Trace Elements in Zircons from Mantle Xenoliths

Hf isotopic compositions of the standard zircons and baddeleyites used in U-Pb geochronology

The transition from Laramide syntectonic sedimentation of the lower Eocene Willwood Formation to the post-Laramide volcanogenic sedimentation of the middle Eocene Wapiti Formation was studied in the upper South Fork Shoshone River Valley, Wyoming. To better understand the regional age, paleogeography, and provenance of volcaniclastic sandstones in the lower stratified member of the Wapiti Formation, we sampled three units for detrital zircon U/Pb geochronology (n=241). The maximum depositional age for the sandstone units within the lower-most, middle, and upper-most units are 49.03 Ma, 49.44 Ma, and 48.99 Ma, respectively, which is consistent with previous geochronologic and paleontologic studies. These ages also are consistent with rocks deposited immediately prior to the emplacement of the Heart Mountain slide. Detrital-zircon age spectra show a transition from a mixed (recycled?) provenance, consistent with drainage from the west, composed of minor primary Eocene volcanic contributions to one dominated by primary Eocene and Archean contributions from the northern Absaroka volcanoes and the Laramide Beartooth Uplift. Thus, uplift and unroofing of the Beartooth Plateau was occurring during the deposition of the oldest members of the Wapiti Formation.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007GC001805

Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb ages by laser ablation-multicollector-inductively coupled plasma-mass spectrometry

Elemental fractionation effects during analysis are the most significant impediment to obtaining precise and accurate U-Pb ages by laser ablation ICPMS. Several methods have been proposed to minimize the degree of downhole fractionation, typically by rastering or limiting acquisition to relatively short intervals of time, but these compromise minimum target size or the temporal resolution of data. Alternatively, other methods have been developed which attempt to correct for the effects of downhole elemental fractionation. A common feature of all these techniques, however, is that they impose an expected model of elemental fractionation behavior; thus, any variance in actual fractionation response between laboratories, mineral types, or matrix types cannot be easily accommodated. Here we investigate an alternate approach that aims to reverse the problem by first observing the elemental fractionation response and then applying an appropriate (and often unique) model to the data. This approach has the versatility to treat data from any laboratory, regardless of the expression of downhole fractionation under any given set of analytical conditions. We demonstrate that the use of more complex models of elemental fractionation such as exponential curves and smoothed cubic splines can efficiently correct complex fractionation trends, allowing detection of spatial heterogeneities, while simultaneously maintaining data quality. We present a data reduction module for use with the lolite software package that implements this methodology and which may provide the means for simpler interlaboratory comparisons and, perhaps most importantly, enable the rapid reduction of large quantities of data with maximum feedback to the user at each stage. Copyright 2010 by the American Geophysical Union.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009GC002618

Improved laser ablation U‐Pb zircon geochronology through robust downhole fractionation correction

Zircon Hf-isotope analysis with an excimer laser, depth profiling, ablation of complex geometries, and concomitant age estimation

Pan-African intraplate deformation in the northern Prince Charles Mountains, east Antarctica

Tonga was unique in the prehistoric Pacific for developing a maritime state that integrated the archipelago under a centralized authority and for undertaking long-distance economic and political exchanges in the second millennium A.D. To establish the extent of Tonga's maritime polity, we geochemically analyzed stone tools excavated from the central places of the ruling paramounts, particularly lithic artifacts associated with stone-faced chiefly tombs. The lithic networks of the Tongan state focused on Samoa and Fiji, with one adze sourced to the Society Islands 2,500 km from Tongatapu. To test the hypothesis that nonlocal lithics were especially valued by Tongan elites and were an important source of political capital, we analyzed prestate lithics from Tongatapu and stone artifacts from Samoa. In the Tongan state, 66% of worked stone tools were long-distance imports, indicating that interarchipelago connections intensified with the development of the Tongan polity after A.D. 1200. In contrast, stone tools found in Samoa were from local sources, including tools associated with a monumental structure contemporary with the Tongan state. Network analysis of lithics entering the Tongan state and of the distribution of Samoan adzes in the Pacific identified a centralized polity and the products of specialized lithic workshops, respectively. These results indicate that a significant consequence of social complexity was the establishment of new types of specialized sites in distant geographic areas. Specialized sites were loci of long-distance interaction and formed important centers for the transmission of information, people, and materials in prehistoric Oceania.

https://www.pnas.org/content/pnas/111/29/10491.full.pdf

Stone tools from the ancient Tongan state reveal prehistoric interaction centers in the Central Pacific.

A primordial solar-neon enriched component in the source of EM-I-type ocean island basalts from the Pitcairn Seamounts, Polynesia

The flux of halogens into the Earth’s mantle at subduction zones is a critical yet poorly constrained parameter in the geochemical evolution of the planet. Here we report the first ever combined high-precision measurements of chlorine, bromine, and iodine for backarc basin basalt (BABB) and ocean island basalt (OIB) glasses. The measurements were undertaken in order to evaluate the depth and extent of the halogen subduction cycle by comparing: (1) melts formed in the Manus Basin (Papua New Guinea) proximal to a modern subduction zone, and (2) melts formed from enriched mantle (EM) reservoirs that have been linked to ancient subduction recycling [EM1 and EM2 sampled by the Pitcairn and Society seamounts (central Pacific Ocean), respectively]. As expected from previous studies, the BABBs are strongly enriched in chlorine relative to other trace elements and mid-oceanic ridge basalts (MORB); however, the combined Br/Cl and I/Cl data provide additional insights. The BABBs have I/Cl weight ratios of up to 5.3 × 10–4, that are up to five times higher than typical MORB; and the BABBs with the highest I/Cl have Br/Cl ratios of 2–3 × 10–3, that are lower than typical MORB, and significantly lower than either iodine-rich sediments or seawater-derived sedimentary pore fluids. The final breakdown of iodine-rich serpentine is considered the most likely source of the halogen enrichment in the BABB, suggesting that subduction of serpentinized peridotites enables transport of strongly incompatible, fluid-mobile, volatile elements, like iodine, beyond zones of arc-magma generation. The Pitcairn and Society melts exhibit a remarkable correlation between K/Cl and 87Sr/86Sr. The K/Cl ratios vary from MORB-like values of ~15 to maxima of ~40 in the isotopically most enriched EM end members. The trend reflects the lower subduction efficiency of halogens compared to K and other lithophile elements. Melts formed from EM and MORB mantle reservoirs have very similar Br/Cl and I/Cl weight ratios of 3.6 ± 0.8 × 10–3 and 85 ± 42 × 10–6 (2 ) respectively, that could indicate that subducted volatiles have been mixed throughout the mantle.

/pdf/tracking-halogens-through-the-subduction-cycle-5b9wphx85i.pdf

Jonathan Woodhead

Papers

Zircon Hf-isotope analysis with an excimer laser, depth profiling, ablation of complex geometries, and concomitant age estimation

Pan-African intraplate deformation in the northern Prince Charles Mountains, east Antarctica

Stone tools from the ancient Tongan state reveal prehistoric interaction centers in the Central Pacific.

A primordial solar-neon enriched component in the source of EM-I-type ocean island basalts from the Pitcairn Seamounts, Polynesia

Tracking halogens through the subduction cycle