Apatite compositions from oceanic cumulates with implications for the evolution of mid-ocean ridge magmatic systems

Apatite distributions and compositions from cumulates from the EPR at Hess Deep, the MAR at the Kane Transform, and the SWIR at ODP Hole 735B were determined to assess the variability at each setting and to evaluate the potential utility of apatite in understanding the evolution of the lower ocean crust. Apatite in cumulates with low P₂O₅ contents are heterogeneously distributed along linear arrays or in tight clusters suggesting they crystallized from planar or pipe-like channels of evolved liquid. Most of the variation in the apatite composition is in the halogen site. The X_Ap^F and X_Ap^Cl are inversely correlated defining trends at near constant X_Ap^OH with EPR X_Ap^OH=0.25, MAR X_Ap^OH=0.45, and SWIR X_Ap^OH=0.65. These trends are defined both by sample averages and by the range of individual analyses in samples with large ranges in their F/Cl ratio. These trends are interpreted to reflect the reequilibration of apatite from variably evolved liquids/fluids that moved through the crystal mush. Comparison of the F–Cl–OH contents of apatite with the F–Cl–OH contents of glasses recovered from the same ridges show approximate correspondence: apatite and glasses from the EPR have the highest halogen contents while apatite and glasses from the MAR and SWIR have more OH. Some of the higher Cl contents in the apatite are interpreted to be produced by degassing of the cumulus pile while others reflect the assimilation of a seawater component. We suggest that systematic analysis of apatite from oceanic cumulates might allow cumulates that crystallized at shallow depths and assimilated seawater-derived components to be distinguished from those that crystallized below the level of seawater interaction.     

板块构造理论研究新进展

本文评述了80年代以来板块构造理论的重要研究成果,就板块划分与板块运动学研究,大洋中脊与板块的扩张,活动边缘与板块的俯冲,板块再造和造山研究等方面的进展,展示板块构造的许多概念已更趋复杂化,板块构造理论变得更加成熟了。     

Ice-collapse induced waves and graded gravel ridges on a Lake Beach

Ice collapsing into the waters of proglacial Briksdal Lake, western Norway, generates waves which rework shoreline sediments. Swash establishes graded gravel ridges sub-perpendicular to the shoreline, derived from material eroded by backwash. A terminal gravel bar at the lakeward limit of wave-induced transport marks the maximum depth of water surface waves.     

洋脊分段研究进展

概述了近１０多年来大洋中脊构造分段研究方面的某些成果,重点介绍洋脊分段现象、分段结构和层次、分段机制和洋脊分段逐步扩展过程,并以大西洋中脊的宏观分段现象和分段现象和分段扩展过程为例,提出洋脊超１级分段的初步构思,进而简要探讨了洋脊分段研究的构造意义。     

苏北岸外辐射沙洲王港西洋潮流通道稳定性研究*

在粉砂淤泥海岸,利用潮流通道建港,稳定性是一个首要问题。该文首先分析了西洋潮流通道的环境背景,然后根据实测资料,利用多种方法,包括地理息信系统、遥感研究、沉积物粒度、矿物、微体及水文泥沙资料分析等对西洋的稳定性作了综合研究,认为,西洋通道特别是其西水道是一条稳定性好的潮流通道。     

Rheological Properties of Partially Molten Lherzolite

Lherzolite samples synthesized from fine-grained powders preparedfrom a natural xenolith were deformed at P = 300 MPa and 1373     

Diffusive fractionation of noble gases and helium isotopes during mantle melting

The large differences in He and Ar diffusivities in silicate minerals could result in fractionation of the He/Ar ratio during melting of the mantle, producing He/Ar ratios in the primary mantle melts that are higher than those of the bulk mantle. Modeling noble gas diffusion out of the bulk mantle into fast diffusion pathways (such as fractures or melt channels) suggests that significant (order of magnitude) He/Ar fractionation will occur if the fast diffusion channels are spaced several meters apart and the noble gas residence in these diffusion channels is of the order days to weeks. In addition, the 15% difference in ³He and ⁴He diffusivities could also produce isotopic fractionation between the melt and its solid source. Modeling the behavior of He and Ar during melting shows that small increases (few %) in ³He/⁴He should be correlated with larger variations (factor of 5) in ⁴He/⁴⁰Ar. However, in order to test this hypothesis the effects of subsequent He–Ar fractionation that occur during degassing have to be corrected. I describe a scheme that can separate He/Ar variations in the primary melt from overprinted fractionation during magmatic degassing. Using the degassing-corrected data, there is a correlation between the primary melt’s ⁴He/⁴⁰Ar and ³He/⁴He in mid-ocean ridge basalts (MORBs). The slope of the correlation is consistent with the models of preferential diffusion of ³He relative to ⁴He and of ⁴He relative to ⁴⁰Ar from the solid mantle into the melt. Diffusive fractionation of noble gases during melting of the mantle can also account for low ⁴He/⁴⁰Ar ratios commonly found in residual mantle xenoliths: preferential diffusion of He relative to Ar will produce some regions of the mantle with low ⁴He/⁴⁰Ar, the complement of the high ⁴He/⁴⁰Ar ratios in basalts. Diffusive fractionation cannot, however, account for differences between the He and Ne isotopic compositions of MORBs compared with ocean island basalts (OIBs); not only are the extremely high ³He/⁴He ratios of OIBs (up to 50 Ra) difficult to produce at reasonable mantle time and lengthscales, but also the Ne isotopic compositions of MORBs and OIBs do not lie on a single mass fractionation line, therefore cannot result from diffusive fractionation of a single mantle Ne source. If preferential diffusion of He from the solid mantle into primary melts is a significant process during generation of MORBs, then it is difficult to constrain the He concentration of the mantle: He concentrations in basalts and the He flux to the ocean essentially result from extraction of He from a larger (and unknown) volume of mantle than that that produced the basalts themselves. The He concentration of the mantle cannot be constrained until more accurate estimates of the diffusion contribution are available.     

Salt tectonics,a controlling factor on the development of the Marseilles and Grand-Rhone sedimentary ridges,Gulf of Lions,western Mediterranean Sea

The Marseilles (also called Marseilles/Planier) and Grand Rhone sedimentary ridges, offshore the Gulf of Lions, western Mediterranean Sea, are generated by sediment spillover funnelled by the Marseilles and Grand-Rhone canyons, respectively. We show that the Marseilles and Grand-Rhone canyon directions are tectonically conditioned by Plio-Quaternary salt-induced sea-floor relief. Overburden extension in the area created an assemblage of listric faults running parallel to the strike of the continental margin. This salt-induced topography resulted in accentuated fault scarps impacting the sea-floor morphology and forcing submarine valleys to deflect to the east. Gravity-induced extension equally influenced the morphological pattern of the valley overbanking deposits. Sediment spillover occurred on both flanks of the canyons, but because active faulting generated space accommodation for overflow deposits, it inhibited the development of typical levee morphology along the canyons' left flanks. To cite this article: A.T. dos Reis, C. R. Geoscience 336 (2004).