Search

In situ garnet Lu‐Hf geochronology has the potential to revolutionise the chronology of petrological and tectonic processes, yet there is a paucity of well‐characterised reference materials to account for laser‐induced matrix‐dependant elemental fractionation. Here, we characterise two reference garnets GWA‐1 (Lu ~ 7.0 μg g−1) and GWA‐2 (Lu ~ 8.5 μg g−1) for in situ garnet Lu‐Hf geochronology. Isochron ages from isotope dilution Lu‐Hf analyses yield crystallisation ages of 1267.0 ± 3.0 Ma with initial 176Hf/177Hfi of 0.281415 ± 0.000012 (GWA‐1), and 934.7 ± 1.4 Ma with 176Hf/177Hfi of 0.281386 ± 0.000013 (GWA‐2). In situ Lu‐Hf analyses yield inverse isochron ages up to 10% older than the known crystallisation age due to matrix effects between garnet and reference glass (NIST SRM 610) under different instrument tuning conditions. This apparent age offset is reproducible for both materials within the same session and can be readily corrected to obtain accurate ages. Our results demonstrate that GWA‐1 and GWA‐2 are robust reference materials that can be used to correct for matrix‐analytical effects and also to assess the accuracy of in situ Lu‐Hf garnet analyses across a range of commonly encountered garnet compositions. [ABSTRACT FROM AUTHOR]

Logging of tropical primary forests is a widely acknowledged global issue threatening biodiversity hotspots and indigenous communities leading to significant land erosion and decreased soil stability. The downstream effects of logging on human coastal communities include poor water quality and increased sedimentation. Quantifying the impacts of historical deforestation within a watershed requires accurate data from river discharge or satellite images, which are rarely available prior to the 1980s. In the absence of these in situ measurements, proxies have successfully produced accurate, long-range, historical records of temperature, hydrological balance, and sediment discharge in coastal and oceanic environments. We present a 30-year, monthly resolved Ba/Ca proxy record of sediment in river discharge as measured from the skeletal remains of massive corals Porites sp. from northern Malaysian Borneo. We make the comparison with local instrumental hydrology data, river discharge and rainfall, to test the reliability of the Ba/Cacoral proxy. Our results show that averaging five records into two composites results in significant positive annual correlations with river discharge (r = 0.5 and r = 0.59) as well as a difference in correlation strength coherent with distance from the river mouth, with the composite closer to the river mouth displaying a higher correlation. More importantly, Ba/Cacoral records from this region showed a very similar upward trend to that of river discharge on multi-decennial timescales. The lack of similar increase and overall stability in the precipitation record suggests that the river discharge's trend recorded by corals is linked to the increasing land use associated with ever-growing deforestation. We argue that massive corals in this region are therefore valuable archives of past hydrological conditions and accurately reflect changes in land use patterns. [ABSTRACT FROM AUTHOR]

U‐Th disequilibrium dating uses the ratio of the intermediate 230Th daughter isotope to the 238U parent isotope to date zircon crystallisation for volcanic and plutonic rocks in Pleistocene–Holocene deposits. It is frequently used to constrain the duration and rate of magma recharge in volcanic systems. While ≲ 350 ka zircon is not in secular equilibrium in the 230Th system, the current U‐Th disequilibrium methodology uses reference materials that are > 350 ka. No reference material ≲ 350 ka has been available to validate the accuracy of the approach and ensure methods are repeatable across laboratories. This study presents zircon SS14‐28 from Jeju Island (South Korea) as a suitable reference material for U‐Th disequilibrium dating. Zircon SS14‐28 was analysed using two analytical approaches (SIMS and LA‐ICP‐MS) and four instruments: CAMECA IMS 1280, ASI SHRIMP II, sector field high‐resolution LA‐ICP‐MS and multi‐collector LA‐ICP‐MS, in four laboratories. These methods each individually result in isochrons gradients within uncertainty (2s) of each other (CAMECA: 0.532 ± 0.051 (MSWD = 0.64); SF‐HR‐LA‐ICP‐MS: 0.536 ± 0.054 (MSWD = 1.3); MC‐LA‐ICP‐MS: 0.533 ± 0.041 (MSWD = 0.67); SHRIMP II: 0.68 ± 0.22 (MSWD = 0.3)). The age proposed in this study is 82 ± 6 ka calculated from a combined isochron gradient of 0.529 ± 0.025 (MSWD = 0.87, n = 132). [ABSTRACT FROM AUTHOR]

The P-T evolution (and particularly the prograde path segment) of ultrahigh-temperature (UHT) granulites is commonly ambiguous, hampering our understanding of deep crustal processes. Here, we establish the P-T path by distinguishing garnet origin (peritectic or retrograde) based on the combined Ca, Ti, Zr, and Y+REE chemical signatures, using the residual UHT granulites of the Khondalite Belt, North China Craton, as a test case. In these rocks, peritectic garnet is characterized by rare inclusions, whereas retrograde garnet has overprinted the main foliation and is characterized by abundant biotite and sillimanite inclusions, which are interpreted to have grown together with retrograde garnet during cooling. Zirconium in peritectic garnet increases from 10 to 50 ppm with garnet growth. In contrast, Zr in retrograde garnet generally decreases from 60 to 10 ppm with garnet growth. A similar trend is observed for Ti. Temperatures calculated from the Ti-in-garnet geothermometer increase from 830 to 980 °C based on Ti in peritectic garnet, indicating prograde partial melting, whereas decrease from 900 to 700 °C based on Ti in retrograde garnet, indicating post-peak cooling. Peritectic and retrograde garnets show distinct Eu/Eu* (0.2-0.5 vs. 0.05-0.2, respectively) and Ca contents (6000-12 000 vs. 4000-6000 ppm, respectively), which generally decrease with progressive garnet crystallization. The pressures calculated from the Ca-in-garnet geobarometer in peritectic and retrograde garnet are 9-11 and 7-9 kbar, respectively. Peritectic garnet shows a bell-shaped Y (80-340 ppm) pattern, whereas retrograde garnet shows an increase in Y content (20-100 ppm) toward rims. Taken together, these results establish a P-T path comprised of an earlier high-pressure peritectic garnet formation during prograde partial melting before the UHT peak and a late abundant retrograde formation during post-peak cooling stage. We conclude that change of Zr and other elements (e.g., Ti, Ca, Y, and Eu/Eu*) can well distinguish different garnet formation events in UHT granulites, which is critical for the P-T path establishment, and further sheds light on the cause of UHT metamorphism and the geodynamic evolution. [ABSTRACT FROM AUTHOR]