Marine phosphatization events cause episodic carbonate fluorapatite (CFA) precipitation on seamounts, and are commonly linked to growth hiatuses in ferromanganese (Fe-Mn) crusts. However, the complete record of these events and their paleoenvironmental significance remains poorly understood, in large part due to poor age constraints. Here, we apply U-Pb dating to CFA in Fe-Mn crusts from Western Pacific seamounts. These data exhibit good alignment with Sr isotope ages, revealing six potential phosphatization events. This established CFA chronology tightens the timespan of phosphatization events and refines the age framework of Fe-Mn crusts. We subsequently utilize a multiproxy approach to demonstrate that the phosphatization events occurred coeval with the expansion of oceanic oxygen minimum zones. The Western Pacific Fe-Mn crusts thus document major perturbations in global oceanic phosphorus cycling, which appear to have been driven by climate-induced increases in primary productivity linked to changes in global ocean circulation.
The giant Beiya Au skarn deposit and Machangqing porphyry Cu-Mo-(Au) deposit are located in the middle part of the Jinshajiang–Ailaoshan alkaline porphyry metallogenic belt. The Beiya deposit is the largest Au skarn deposit in China, whilst the Machangqing deposit comprises a well-developed porphyry-skarn-epithermal Cu-Mo-(Au) mineral system. In this paper, we present new allanite U-Th-Pb ages and trace element geochemical data from the two deposits and discuss their respective skarn metallogenesis. Based on the mineral assemblage, texture and Th/U ratio, the allanite from the Beiya and Machangqing deposits are likely hydrothermal rather than magmatic. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) allanite U-Th-Pb dating has yielded Th-Pb isochron ages of 33.4 ± 4.6 Ma (MSWD = 0.22) (Beiya) and 35.4 ± 9.8 Ma (MSWD = 0.26) (Machangqing), representing the retrograde alteration and magnetite skarn mineralization age of the two deposits. The Beiya and Machangqing alkali porphyry-related mineralization are synchronous and genetically linked to the magmatic hydrothermal activities of the Himalayan orogenic event. Major and trace element compositions reveal that the Beiya allanite has higher Fe3+/(Fe3+ + Fe2+) ratios, U content and Th content than the Machangqing allanite, which indicate a higher oxygen fugacity and F content for the ore-forming fluids at Beiya. Such differences in the ore-forming fluids may have contributed to the different metallogenic scales and metal types in the Beiya and Machangqing deposit.
Summary Seismic frequency characters analysis for recognition of hydrocarbon reservoirs has become more popular. In this abstract, we focus on explaining frequency-dependent amplitude versus offset of thin sand-shale interbedded reservoirs, by quantifying the effect of anisotropy arising in a thin interlayer reservoirs and fluid-properties saturated in sandstones on seismic responses. A set of physical modelling were conducted and rock physical theoretical experiments were carried out based on the materials properties of physical model, in order to facilitate understanding of the underlying physical reason. The result of our analysis shows that the frequency-dependent amplitude versus offset properties calculated using the singular value decomposition (SVD) can be used to identify the fluid properties of thin sand-shale interlayer reservoirs. The fluid property of porous layers does change the frequency-dependent amplitude versus offset. The agreement between the physical modelling and theoretical results is quite well, demonstrates that frequency-dependent amplitude versus offset is a reliable seismic indictor for detecting fluid properties in thin interlayer reservoirs, especially in gas-bearing reservoirs.
This contribution presents new U-Pb geochronological data and attempts to elucidate the complex evolution history of various garnet types identified from two kimberlite pipes in the Wafangdian diamond mining district, southern Liaoning Province. These calcic garnets are dominated by andradite with relatively low proportions of schorlomite, grossular and pyrope. Abundant euhedral to subhedral, highly brecciated andradite phenocrysts hosted by LN30 "carbonatite-like" kimberlite samples yield a lower-intercept age of 459.3 ± 3.4 Ma, which is in excellent agreement with the previously reported phlogopite Ar-Ar (463.9 ± 0.9 Ma) and Rb-Sr ages (461.7 ± 4.8 Ma). Based on their trace element and C-O isotopic compositions of associated groundmass carbonate, we infer that these primary magmatic andradites probably originated from kimberlitic magmas. By comparison, three compositionally and texturally distinct groups of Ti-bearing andradites from LN42 hypabyssal kimberlites separately define three well-fitted regression lines with lower intercept ages at 581 ± 12 Ma, 414.9 ± 9.3 Ma and 292.0 ± 5.7 Ma, respectively. Relict andradite xenocrysts implies that ancient lower crust of the North China Craton (NCC) might have been affected by a significant but less-known tectonothermal event to varying degrees at ∼ 0.6 Ga. By contrast, fresh grains of magmatic Ti-andradites with chemical zoning produce a relatively young age of ∼ 415 Ma, which can still provide minimum age estimates for the most recent pulses of Paleozoic kimberlite magmatism in this study area. Noteworthily, a yet unrecognized local-scale hydrothermal alteration event at ∼ 292 Ma has been recorded in the texturally distinct population of secondary hydroandradites, whose age reported here for the first time is geologically meaningful. To sum up, this study further highlights andradite U-Pb dating as a potential robust geochronometer for constraining the late-stage evolution of kimberlite magmas as well as post emplacement hydrothermal alteration.
Colloform pyrite with core-rim texture is commonly deposited in carbonate platforms associated with the sulfide ores such as the Caixiashan Pb–Zn deposit. However, the genesis of colloform pyrite in Pb–Zn deposits, its growth controls and their geological implication are insufficiently understood. Integration of in-situ trace element and SIMS sulfur isotopes has revealed geochemical variations among these pyrite layers. These colloform pyrite occur as residual phases of core-rim aggregates, the cores are made up of very fine-grained anhedral pyrite particles, with some rims being made up of fine-grained and poorly-crystallized pyrite, while the other rims were featured with euhedral cubic pyrite, which are cemented by fine-grained calcite and/or dolomite with minor quartz. Sulfur isotope analysis shows that some well-preserved rims have negative δ34S values (–28.12‰ to –0.49‰), whereas most of the cores and rims have positive δ34S values (>0 to +44.28‰; peak at +14.91‰). Integrating with the methane and sulfate were observed in previous fluid inclusion study, we suggest that the 34S depleted rims were initially formed by bacteria sulfate reduction (BSR), whereas the positive δ34S values were resulted from the sulfate reduction driven by anaerobic methane oxidation (AOM). The well-developed authigenic pyrite and calcite may also support the reaction of AOM. Combined with petrographic observations, trace element composition of the colloform pyrite reveals the incorporation and precipitation behavior of those high abundance elements in the pyrite: Pb and Zn were present as mineral inclusion and likely precipitated before Fe, as supported by the time-resolved Pb–Zn signal spikes in most of the analyzed pyrite grains. Other metals, such as Hg, Co and Ni, may have migrated as chloride complexes and entered the pyrite lattice. Arsenic and Sb, generally influenced by complex-forming reactions rather than substitution ones, could also enter the pyrite lattice, or slightly predate the precipitation of colloform pyrite as mineral inclusions, which are controlled by their hydrolysis constant in the ore fluids. The colloform pyrite may have grown inward from the rims. The successive BSR reaction process would enrich H232S in the overlying water column but reduce the metal content, the nucleation of these pyrite rims was featured by strongly negative sulfur isotopes. The following AOM process should be activated by deformation like the turbidity sediment of the mudstone as the sulfide deposition are associated with fault activities that caused the emission of methane migration upward and simultaneously replenishing the metal in the column. The higher AOM reaction rate and the higher metal supply (not only Fe, but with minor other metals such as Pb and Zn) caused by sediment movement enhanced the metal concentration within the pyrite lattice.
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