A New Albite Microanalytical Reference Material from Piz Beverin for Na, Al and Si Determination, and the Potential for New K-Feldspar Reference Materials

Determination of alkali elements is important to Earth scientists, yet suitable and reliable microanalytical reference materials are lacking. This paper proposes a new albite reference material and evaluates the potential for future K-feldspar reference materials. The proposed Piz Beverin albite reference material from Switzerland yields a homogeneous composition at the centimetre- to micrometre-scale for Si, Al and Na with \u3c 2000 μg g-1 total trace elements (mostly heterogeneously distributed Ca, K and Sr). EPMA and LA-ICP-MS measurements confirm a composition of 99.5(2)% albite component, which is supported further by bulk XRF measurements. A round robin evaluation involving nine independent EPMA laboratories confirms its composition and homogeneity for Si, Al and Na. In addition, a set of five distinct clear K-feldspar samples was evaluated as possible reference materials. The first two crystals of adular and orthoclase yield unacceptable inhomogeneities with \u3e 2% relative local variations of Na, K and Ba contents. The three other investigated sets of K-feldspar crystals are yellow sanidine crystals from Itrongay (Madagascar). Despite distinct compositions, EPMA confirms they are each homogeneous at the centimetre to micrometre scale for Si, Al and K and have no apparent inclusions; further investigation to find larger amounts of these materials is therefore justified

Temporal and petrological constraints of ultrahigh-pressure metamorphism and exhumation of crustal material from mantle depths to Earth's surface: Insights from a large and small ultrahigh-pressure terrane

Ultrahigh-pressure (UHP) terranes expose continental material that has subducted to mantle depths and then returned to Earth’s surface. These terranes typically consist mainly of migmatitic host ortho- and paragneiss, with minor (~5%) layers and lenses of UHP eclogite; at least twenty terranes have been described and identified by the preservation of UHP minerals (i.e., coesite and diamond) mainly within eclogite. These terranes fall into two broad categories: 1) large (30,000 km2) coherent terranes characterized by slow (>20 Myr) subduction–exhumation histories, and 2) small (~4,000 km2) terranes that have undergone rapid (<10 Myr) subduction–exhumation histories. To better understand the geodynamic processes involved in deep continental subduction and subsequent exhumation of buoyant crustal material within these two types of UHP terranes, it is important to constrain the timing and conditions of: 1) peak UHP metamorphism recorded in the eclogites; 2) the subsequent eclogite retrograde metamorphism; and 3) the host-rock migmatization which likely occurs during exhumation from the mantle to the upper crust.This study investigates the Western Gneiss Region (WGR) of Norway, a giant UHP terrane, and the gneiss domes exposed in the D’Entrecasteaux Islands in eastern Papua New Guinea (PNG), a small UHP terrane, in order to understand the similarities and differences among the subduction and exhumation of these end-member terranes. To better understand the maximum pressure and temperature the rocks reached within the mantle, thermobarometry and phase-diagram modeling are applied to the PNG (U)HP eclogites, as previous work suggested a wide range of results, many of which were not at UHP. In order to understand the timing and rates of subduction and exhumation events, high-precision zircon U-Pb isotope dilution–thermal ionization mass spectrometry geochronology and trace-element analyses (ID-TIMS-TEA) of the (U)HP rocks exposed in the two terranes are used, as these tools can decipher tectonic events that occur on a sub-million year timescale. In the WGR, UHP rocks are exposed within three domains that have been interpreted to have undergone a similar tectonic history, with a long duration of (U)HP metamorphism from ca. 425–400 Ma associated with the Scandian-phase of the Caledonian orogeny. In order to test if UHP metamorphism was a single ~25 Myr event, eclogite was collected from two of the three UHP domains for high-precision ID-TIMS-TEA analysis. Zircon was extracted from the bulk rock, mounted in an epoxy mount, and screened for Scandian ages using high-spatial resolution laser ablation split-stream inductively coupled plasma mass spectrometry (LASS-ICP-MS). These Scandian-aged zircons were subsequently analyzed by ID-TIMS-TEA. The LASS analyses reveal a spread in results from both samples, with ages between 414 and 397 Ma. In comparison, the ID-TIMS analyses from the exact same zircons analyzed by LASS reveal two age populations of ca. 409 Ma and ca. 402 Ma from a garnet–quartz layer within the Saltaneset eclogite of the southern UHP domain. In comparison, the Ulsteinvik eclogite collected from the central UHP domain also yields two ID-TIMS age populations: ca. 409–407 Ma and ca. 402 Ma. Thus, two eclogites from different regions within the giant WGR UHP terrane reveal the same two age populations. Zircon trace-element data collected via laser-ablation and solution analyses for the two-age populations yield depleted heavy rare earth element (HREE) patterns and flat-to-positive Eu anomalies. The combined age and trace-element data indicate that the WGR terrane underwent two distinct eclogite-facies zircon (re)crystallization events at ca. 409–407 Ma and ca. 402 Ma at localities ~40 km apart. These results support the interpretation that the UHP terrane was subducted and exhumed as a large (30,000 km2) coherent slab of crustal material, but that as this terrane was subducted, there were two different events that affected this giant terrane while it was at eclogite-facies conditions. The Pliocene PNG UHP terrane exposes a series of east–west gneiss domes, Normanby, Oiatabu, Mailolo, and Goodenough that contain eclogites within mainly highly migmatitic quartzofeldspathic gneiss. This UHP terrane is unique in that it is the only one on Earth that is actively exhuming, in this case within the Woodlark Rift. To better understand the pressure-temperature-time-deformation path taken by this young UHP terrane, a suite of fresh to nearly-completely retrogressed eclogites were sampled from Oiatabu, Mailolo, and Goodenough Domes for thermobarometry, pseudosection modeling, and high-precision ID-TIMS-TEA zircon analyses. A kyanite-phengite eclogite from Oiatabu Dome records equilibration at UHP conditions of ~30–31 kbar and ~635–660 °C, whereas a fresh phengite eclogite from the central Mailolo Dome yields peak conditions of ~27–30 kbar and ~510–560 °C. Zircons extracted from the Mailolo Dome eclogite reveal UHP metamorphism occurred from ca. 6.0 to 5.2 Ma, based on zircon that contain inclusions of the peak metamorphic assemblage. Following UHP recrystallization, the crustal material ascended along a near-isothermal decompression path, accompanied by partial melting and retrogression, to the base of the crust. The host ortho- and paragneiss from the eastern Normanby and Oiatabu Domes record the early stages of this retrogression, with ID-TIMS zircon dates that document retrogression-related metamorphism in the structurally higher portions of the domes at ca. 5.7–4.5 Ma. Zircons from retrogressed eclogites collected within the Oiatabu and Mailolo Domes, also record initial exhumation at ca. 4.6–4.3 Ma; the zircon from these samples are associated with garnet- and omphacite-breakdown reactions. Continued exhumation and near-complete retrogression in the lower crust occurred at ca. 2.8–2.6 Ma, based on zircons from heavily retrogressed eclogites collected in the westernmost Goodenough Dome. Zircon trace-element data from all the eclogites show depleted HREE and absent negative Eu anomalies suggesting eclogite-facies zircon (re)crystallization, although some grains are clearly in textural equilibrium with lower-pressure phases (i.e., amphibole and plagioclase).To further track the exhumation history of the PNG UHP terrane, samples representing different melt generations (e.g., strongly-deformed leucosomes versus nondeformed dikes) that formed during exhumation are used to record different parts of the deformation history. The crystallization of strongly-deformed sills and leucosomes likely associated with cooling and amphibolite-facies retrogression suggests the terrane reached neutral buoyancy near the base of the crust first in the east by ca. 4.1 Ma in Normanby Dome, by ca. 3.5–3.0 in the central Mailolo Dome, and by ca. 3.9–2.8 in the far west in Goodenough Dome. Further exhumation and continued amphibolite-facies metamorphism within the mid-crust are marked by weakly deformed dikes recording melt crystallization at ca. 3.0–2.9 Ma in Oiatabu Dome and ca. 2.4–2.3 Ma in Mailolo and Goodenough Domes. Final extension-related exhumation of the entire terrane within the upper crust occurred by ca. 1.8 Ma, as recorded by the crystallization of non-deformed plutons, pegmatites, and dikes. Taking into account all of the petrologic, structural, geophysical, and geochronologic constraints, a model for the exhumation of the PNG UHP terrane involves (re)crystallization of the previously subducted continental material during a flux of hot asthenospheric fluids related to westward seafloor spreading in the Woodlark Rift from ca. 6.0 to 5.2 Ma. Subsequently, the partially-molten (U)HP rocks rose as diapirs that underwent rapid near-vertical exhumation to the base of the crust, resulting in the generation of abundant partial melting and the initial retrogression of the crustal material. Upon reaching neutral buoyancy at the base of the crust, the (U)HP body laterally flowed, but also cooled, allowing the crystallization of the strongly deformed leucosomes. Further exhumation to the upper crust was assisted by enhanced buoyancy due to later injection of partial melt and concurrent extension within the active Woodlark Rift. Eclogites from the WGR and eastern PNG were both subjected to upper mantle depths, but they preserve different metamorphic and exhumation histories. The ID-TIMS zircon results from the small PNG UHP terrane document rapid (≥1.5 cm/yr) exhumation from peak metamorphism in the upper mantle at ca. 6.0–5.2 Ma to emplacement within the brittle upper crust in ~3 Myr, some of the fastest rates of UHP exhumation documented on Earth. In comparison, studies based on multiple geochronological techniques have suggested that the WGR likely resided at mantle depths for tens of millions of years, from ca. 425–400 Ma. However, the new results from ID-TIMS U-Pb zircon dates suggest eclogite-facies metamorphism occurred during at least two distinct (re)crystallization events at ca. 409–407 Ma and ca. 402 Ma, at the youngest end of the previously proposed timescale of UHP metamorphism. Thus, these new results suggest that the interpretation that giant UHP terranes undergo a long continuous duration of UHP metamorphism may need to be reevaluated, as there may be distinct events hidden within this metamorphic window that can only be deciphered via a high-precision geochronometric technique. This is supported by the results from the PNG UHP terrain, where mantle to crustal exhumation occurs in only ~3 Myr. The high-precision results from the two UHP terranes that differ in size, age, and in their exhumation history provide important constraints on the timing and duration of UHP metamorphism and exhumation to the upper crust. Understanding tectonic events on such short timescales has drastic implications for geodynamic models attempting to characterize the switch from subduction to exhumation and the transfer of continental material through the lower crust to Earth’s surface via coupled buoyancy-driven and extension-related exhumation processes

Characterizing low-temperature aqueous alteration of Mars-analog basalts from Mauna Kea at multiple scales

We performed a multi-scale characterization of aqueous alteration of Mars analog basaltic rock from a Mauna Kea drill core using high-resolution visible and short-wave infrared (VIS-SWIR) spectral imaging, scanning electron microscopy, X-ray diffraction, and point VIS-SWIR spectra. Several types of smectites, zeolites, and primary minerals were identified. Mineral classes were mapped in cut sections extracted from the drill core and used to represent the range of alteration products seen in field data collected over 1000 m depth (Calvin et al. 2020). Ten distinct spectral end-members identified in the cut sections were used to map the field point spectra. Trioctahedral Fe- and Mg-rich smectites were present toward the top of the zone of analysis (972 m below the surface) and increased in abundance toward the bottom of the drill core (1763 m depth). The mineralogy demonstrates a general trend of discontinuous alteration that increases in intensity with depth, with less pervasive phyllosilicate alteration at the top, several zones of different mixtures of zeolites toward the center, followed by more abundant phyllosilicates in the lowest sections. Distinctly absent are Fe-Mg phyllosilicates other than smectites, as well as carbonates, sulfates, and Al phyllosilicates such as kaolinite or illite. Furthermore, hematite was only detected in two of 24 samples. The suite of assemblages points to aqueous alteration at low-to-moderate temperatures at neutral to basic pH in low-oxygen conditions, with little evidence of extensive surface interaction, presenting a possible analog for an early Mars subsurface environment. We also present a library of VIS-SWIR spectra of the analyzed cut sections, including both spatial averages (i.e., unweighted linear mixtures) of spectral images of each cut section and single point spectra of the cut sections. This will allow for consideration of nonlinear mixing effects in point spectra of these assemblages from natural surfaces in future terrestrial or planetary work

Characterizing low-temperature aqueous alteration of Mars-analog basalts from Mauna Kea at multiple scales

We performed a multi-scale characterization of aqueous alteration of Mars analog basaltic rock from a Mauna Kea drill core using high-resolution visible and short-wave infrared (VIS-SWIR) spectral imaging, scanning electron microscopy, X-ray diffraction, and point VIS-SWIR spectra. Several types of smectites, zeolites, and primary minerals were identified. Mineral classes were mapped in cut sections extracted from the drill core and used to represent the range of alteration products seen in field data collected over 1000 m depth (Calvin et al. 2020). Ten distinct spectral end-members identified in the cut sections were used to map the field point spectra. Trioctahedral Fe- and Mg-rich smectites were present toward the top of the zone of analysis (972 m below the surface) and increased in abundance toward the bottom of the drill core (1763 m depth). The mineralogy demonstrates a general trend of discontinuous alteration that increases in intensity with depth, with less pervasive phyllosilicate alteration at the top, several zones of different mixtures of zeolites toward the center, followed by more abundant phyllosilicates in the lowest sections. Distinctly absent are Fe-Mg phyllosilicates other than smectites, as well as carbonates, sulfates, and Al phyllosilicates such as kaolinite or illite. Furthermore, hematite was only detected in two of 24 samples. The suite of assemblages points to aqueous alteration at low-to-moderate temperatures at neutral to basic pH in low-oxygen conditions, with little evidence of extensive surface interaction, presenting a possible analog for an early Mars subsurface environment. We also present a library of VIS-SWIR spectra of the analyzed cut sections, including both spatial averages (i.e., unweighted linear mixtures) of spectral images of each cut section and single point spectra of the cut sections. This will allow for consideration of nonlinear mixing effects in point spectra of these assemblages from natural surfaces in future terrestrial or planetary work

Insights into (U)HP metamorphism of the Western Gneiss Region, Norway: A high-spatial resolution and high-precision zircon study

Combining high-spatial resolution and high-precision geochronology and geochemistry of zircon provides constraints on the timing and duration of ultrahigh-pressure (UHP) metamorphism resulting from the collision of Baltica–Avalonia and Laurentia during the Scandian orogeny in the Western Gneiss Region of Norway. Zircons were extracted from a layered eclogite in the Saltaneset region (southern UHP domain) and from an eclogite in the Ulsteinvik region (central UHP domain). Zircons were first analyzed for U–Pb and trace element compositions by laser ablation split-stream (LASS) inductively coupled plasma mass spectrometry (ICP-MS), followed by analysis of those same zircons that yielded Scandian dates by integrated U–Pb isotope dilution–thermal ionization mass spectrometry and Trace Element Analysis (TIMS–TEA). LASS results from a garnet–quartz layer within the Saltaneset eclogite give Scandian dates of ca. 413–397 Ma, with subsequent ID–TIMS analyses ranging from 408.9 ± 0.4 Ma to 401.4 ± 0.2 Ma (2σ). An omphacite-rich layer from the same eclogite yields LASS dates of ca. 414–398 Ma and a single ID–TIMS date of 396.7 ± 1.4 Ma. In comparison, the Ulsteinvik eclogite LASS results give dates spanning ca. 413–397 Ma, whereas ID–TIMS analyses range from 409.6 ± 0.6 Ma to 401.3 ± 0.4 Ma. ID–TIMS zircon data from the eclogites reveals two age populations: 1) ca. 409–407 Ma and 2) ca. 402 Ma. Both in situ and solution trace element data show a distinct pattern for Scandian zircons, with strongly-depleted HREE and weakly-negative Eu anomalies (Eu/Eu*), whereas inherited zircon REE patterns are distinguished by steep HREE slopes and marked negative Eu/Eu*. When coupled with partition coefficients calculated for zircon and garnet, these REE patterns indicate that zircon (re)crystallized during eclogite-facies metamorphism at ca. 409–407 Ma and ca. 402 Ma at two widely separated UHP localities

Using Eclogite Retrogression to Track the Rapid Exhumation of the Pliocene Papua New Guinea UHP Terrane

The D’Entrecasteaux Islands of eastern Papua New Guinea (PNG) host the youngest known ultrahigh-pressure terrane on Earth and represent the only location where ultrahigh-pressure (UHP) rocks have been exhumed in an active rift. The PNG (U)HP rocks, consisting of Pliocene eclogites, garnet amphibolites and migmatitic gneisses, are exposed in five domal structures across the Islands. Zirconium-in-rutile thermometry records peak temperatures of ∼780°C from the eastern Oiatabu and nearby central Mailolo Domes, and hotter temperatures of ∼825–865°C within the western Goodenough Dome. Uranium–lead (U–Pb) and trace element zircon compositions from a suite of eclogite, host gneiss, felsic dikes and pegmatite from three domes document the rapid exhumation history of the PNG UHP terrane. High-spatial resolution laser-ablation split-stream inductively coupled plasma-mass spectrometry (LASS ICP-MS) analyses of select eclogite zircons exhibit no resolvable age zoning within single crystals. The same eclogite zircons, combined with separate zircons extracted from additional eclogite, host gneiss and felsic intrusions, were subsequently analysed by high-precision U–Pb chemical-abrasion isotope-dilution thermal ionization mass spectrometry and solution ICP-MS trace element analysis (TIMS-TEA). The results record discrete tectonic events across the three domes at sub-million year timescales: (1) (re)crystallization of host gneiss within the lower crust exposed in the eastern Oiatabu Dome from c.5·7–4·5 Ma; (2) initial retrogression and local decompression melting of eclogites from the Oiatabu and Mailolo Domes at c.4·6–4·3 Ma; (3) melt crystallization of weakly deformed felsic dikes of the Oiatabu Dome at c.3·0–2·9 Ma; and (4) retrogression and melt crystallization within eclogite–amphibolite-facies rocks in the western Goodenough Dome at c.2·9–2·6 Ma. In comparison to Zr-in-rutile peak temperature estimates, Ti-in-zircon temperatures >800°C may reflect increased temperatures during exhumation that resulted in partial melting of the eclogites. Inclusions of crystallized hydrous melt consisting of Na-rich plagioclase ± K-feldspar + quartz within eclogite zircons document this process. The elevated temperatures and the presence of the polyphase inclusions are the first documentation of partial melting of the (U)HP eclogites within PNG during initial retrogression from c.4·6–4·3 Ma. Overall, U–Pb zircon geochronology and geochemistry track both the timing of retrogressive overprinting within the lower-to-middle crust and final upper crustal emplacement over a relatively short span of ∼2 Myr during the rapid (≥2·3 cm/yr) exhumation of the youngest known (U)HP eclogites

GHR 1 Zircon – A New Eocene Natural Reference Material for Microbeam U‐Pb Geochronology and Hf Isotopic Analysis of Zircon

We present multitechnique U‐Pb geochronology and Hf isotopic data from zircon separated from rapakivi biotite granite within the Eocene Golden Horn batholith in Washington, USA. A weighted mean of twenty‐five Th‐corrected 206Pb/238U zircon dates produced at two independent laboratories using chemical abrasion‐isotope dilution‐thermal ionisation mass spectrometry (CA‐ID‐TIMS) is 48.106 ± 0.023 Ma (2s analytical including tracer uncertainties, MSWD = 1.53) and is our recommended date for GHR1 zircon. Microbeam 206Pb/238U dates from laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) and secondary ion mass spectrometry (SIMS) laboratories are reproducible and in agreement with the CA‐ID‐TIMS date to within < 1.5%. Solution multi‐collector ICP‐MS (MC‐ICP‐MS) measurements of Hf isotopes from chemically purified aliquots of GHR1 yield a mean 176Hf/177Hf of 0.283050 ± 17 (2s, n = 10), corresponding to a εHf0 of +9.3. Hafnium isotopic measurements from two LA‐ICP‐MS laboratories are in agreement with the solution MC‐ICP‐MS value. The reproducibility of 206Pb/238U and 176Hf/177Hf ratios from GHR1 zircon across a variety of measurement techniques demonstrates their homogeneity in most grains. Additionally, the effectively limitless reserves of GHR1 material from an accessible exposure suggest that GHR1 can provide a useful reference material for U‐Pb geochronology of Cenozoic zircon and Hf isotopic measurements of zircon with radiogenic 176Hf/177Hf