Chemical Geology最新文献

The present study provides new petrological and geochemical data of the dredged rocks from submarine volcanoes along the Andaman arc and describes the petrogenetic evolution of the arc system in terms of mantle wedge characteristics, nature and quantitative input of subducted slab components, and fractionation processes of precursor magma. The studied rocks include basaltic andesite, andesite, dacite and rhyolite. These volcanic rocks exhibit LILE, LREE enrichments and HFSE depletion, corroborating their generation through subduction processes. High abundances of Th/Nd, La/Sm_(N), LREE/HFSE than LILE/HFSE, LILE/LREE suggest a substantial contribution of sediments from the subducting slab over slab-dehydrated aqueous fluids to the mantle wedge. The ⁸⁷Sr/⁸⁶Sr-Ba/La mixing model suggests 0.6–0.8% addition of slab fluid (90:10 AOC: sediment fluid) to account for the fluid signature, whereas the ¹⁴³Nd/¹⁴⁴Nd-La/Sm_(N) mixing model envisages ∼3–4% addition of sediment melt to the mantle source, reconciling the sediment signature in Andaman submarine volcanic rocks. The presence of N-MORB type mantle is attributed to the absence and/or inefficient convection of asthenospheric material from the Andaman back-arc basin to the mantle wedge. This ineffective convection can be equated with the flat subduction of the Indian Plate, caused by the convergence of the aseismic Ninety East Ridge. The non-modal batch-melting model suggests that 13–24% partial melting of the spinel lherzolite mantle beneath the Andaman submarine volcanic arc formed the parent magma. The crystallization model invokes up to 60–70% of fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, sanidine and magnetite in all the rock types with subordinate proportions of amphibole, biotite, apatite, ilmenite, and sanidine in rhyolites. The basaltic andesites, andesites and dacites do not show upper crustal input, while rhyolites indicate crustal contamination from an upper crust and/or arc crust.

The Banda Alta Formation (Urucum district, Mato Grosso do Sul, Brazil) comprises ∼600 Ma Fe and Mn deposits, which are among the world's youngest and largest Neoproterozoic sedimentary Fe and Mn formations (IF; MnF). These have been deposited in a redox-stratified, marine sub-basin (Jacadigo Basin), which was strongly influenced by glacial advance/retraction cycles with temporary influx of continental freshwater and upwelling of metal-enriched deep anoxic seawater. Cr and Cd isotopes measured on meticulously separated hematite mesobands from drill core samples are relatively homogenous throughout the ca. 325 m thick sequence sampled in the Banda Alta Fm., with average authigenic δ⁵³Cr values of +0.93 ± 0.24 ‰ (2σ; n = 23) and δ¹¹⁴Cd values of −0.14 ± 0.14 ‰ (2σ; n = 15). The significant enrichment of Cr, in parallel with the strong enrichments of other redox sensitive elements (U, Mo), attests for effective and efficient reduction removal processes in the surface waters during cycles where upwelling Fe²⁺-rich waters reached the oxygenated surface layer exposed to the atmosphere during episodic glacier retreat stages. Assuming a similar quantitative and efficient removal pathway of dissolved Cd by iron oxyhydroxides, the so-inferred average δ¹¹⁴Cd signature of −0.14 ± 0.14 ‰ in the Jacadigo Basin surface water is significantly lower than signatures of modern ocean surface waters with a range of δ¹¹⁴Cd of ca. ∼0.4 to ∼1 ‰ and even lower than the signature of modern ocean deep waters with δ¹¹⁴Cd of ∼0.3 ‰. It possibly attests to reduced primary production levels and lower nutrient utilization rates during deposition of the Late Neoproterozoic Jacadigo Group, compared to today. This despite the inferred oxidized surface water layer that must have prevailed during this time, as implied by the strongly positively fractionated Cr isotope signatures and pronouncedly negative Ce-anomalies recorded in the seawater-like, shale-normalized Rare Earth Element and Yttrium (REY) patterns exhibited by the hematite mesobands. Data presented herein speak for: (1) a stable, isotopically heavy Cr input to the Jacadigo Basin at the time of deposition, implying high atmospheric O₂ levels in the Late Neoproterozoic (2) likely quantitative, reductive incorporation / adsorption processes of dissolved Cr and Cd, respectively, into/onto precipitating iron oxyhydroxides, and (3) the prevalence of low nutrient concentrations and utilization rates in the Jacadigo Basin during glacier retreat cycles. Banded iron formations are considered suitable archives for reconstructing redox and bioproductivity levels in past marine depositional basin, including those prevalent in Neoproterozoic glacial conditions, via employing the CrCd isotope double tracer to iron-rich mesobands.

Mud depocenters play an important role in the sediment source–sink system of marginal seas; however, the factors controlling the formation of mud deposits are not well understood. This study focused on Core LHSD-1, which is located on the southern margin of the Shandong Peninsula mud wedge in the northwestern part of the South Yellow Sea. Elemental ratios, such as the C/S ratio (the mass ratio of total organic carbon and total sulfur) and Sr/Ba ratio, were used to track the changes in paleosalinity during the late Quaternary and to investigate the mechanisms that influenced the formation of the mud wedge off the Shandong Peninsula. Our results indicate that Core LHSD-1 likely recorded two transgression events, corresponding to MIS3 and MIS1. Following sea-level rise after the Last Glacial Maximum (LGM), transgressive deposition began in the study area at approximately 11.5 kyr BP. During the early Holocene, fluctuating water salinity was recorded by Sr/Ba ratios, which became stable at approximately 6.8 kyr BP when the sea level reached its highstand. At the same time, the modern current system was established, and a significant amount of terrestrial material, such as refractory organic carbon and iron, has been transported to the study area by enhanced coastal currents, which was documented by elevated C/S ratios (C/S > 2.8) in mud sediments. Our findings indicate that marine transgressions controlled the initial development of the mud depocenter and that the establishment of the modern ocean current system promoted the rapid deposition of mud sediments, which corresponded to the growth of global mud depocenters.

Cenozoic mountain building in Asia has been proposed as an important control over global climate by atmospheric CO₂ drawdown through silicate weathering and burial of organic carbon (OC) offshore. Because Asian submarine fans represent the most complete record of Asian orogenic erosion and weathering over the Cenozoic, evaluation of sediment major element chemistry and OC content can be used to estimate CO₂ sequestration rates driven by silicate chemical weathering and OC burial. From deep-sea fans in the Indian Ocean and South China Sea, weathering rates are calculated by comparison of weathered sediment to initial bedrock source compositions and then integrated with deposited volumes to derive the first regional weathering budget for India-Asia collision over the last ∼25 m.y. Results indicate the Indus is more important than previously recognized before and during the middle Miocene in sequestering CO₂ (∼76% of Asian total at 16–14 Ma). This in part reflects the more reactive mafic and ultramafic bedrock sources compared to the largest and major east Himalayan drainage, the Ganga-Brahmaputra, but also greater erosional flux from the onshore Indus drainage at that time. This regional synthesis further concludes that OC burial only represented a minority (20–25%) of the regional carbon budget but became more important after 17 Ma, peaking at ∼38% after ∼3 Ma. CO₂ sequestration rates increased from 17 to 15 Ma, coinciding with the Miocene Climatic Optimum, and remained mostly steady in Asia from ∼15–5 Ma as the climate cooled. Only one of three possible sediment flux models for the Bengal Fan predicts increased CO₂ consumption rates after 15 Ma and, even then, only predicts steady rates from 11 to 5 Ma. The timing of changes in CO₂ consumption rates are not consistent with Asian orogenic silicate weathering acting as the dominant control over late Cenozoic atmospheric CO₂.

Different discretizations methods applied to models of core/mantle segregations are tested (single stage, multistage accretion, results of N-body simulations) in order to test the sensitivity of the thermo-chemical coupling to the type of discretization used. We found that while single stage and large discretization of segregation steps yield very different core temperature, multistage models of accretion and core mantle segregation, at least for the model presented in this paper, tend to yield similar results, whether it is for the light element concentrations or the final temperature of the core. As long as a magma ocean existence throughout the entire process of core/mantle segregation is posited, and that only the impactor's masses of metal and silicate are re-equilibrated at each step, the degree of discretization of continuous model does not matter as long as it encompasses more than 10 steps of calculations.