On the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling

Abstract Finite difference models of Fe-Mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet-biotite Fe-Mg exchange thermometry. For rocks cooled from high grades where the garnet was initially homogeneous, the calculated temperature (T_calc) using garnet core and matrix biotite depends on the size of the garnet, the ratio of garnet to biotite in the rock (V_garnet/V_biotite) and the cooling rate. For garnets with radii of 1 mm and V_garnet/V_biotite<1, T_calc is 633, 700 and 777°C for cooling rates of 1, 10 and 100°C/Ma. For V_garnet/V_biotite= 1 and 4 and a cooling rate of 10° C/Ma, T_calc is approximately 660 and 610° C, respectively. Smaller and larger garnets have lower and higher T_calc, respectively. These results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below T_calc of the garnet core, provided that V_garnet/V_biotite is sufficiently small (<0.1) and that the composition of the biotite at the metamorphic peak has not been altered during cooling. Numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550–600° C generate a ‘well’in Fe/(Fe + Mg) near the rim during cooling. Maximum calculated temperatures for the assemblage garnet + chlorite + biotite + muscovite + plagioclase + quartz using the Fe/(Fe + Mg) at the bottom of the ‘well’with matrix biotite range from 23–43° C to 5–12° C below the peak metamorphic temperature for cooling rates of 1 and 100° C/Ma, respectively. Maximum calculated temperatures for the assemblage garnet + staurolite + biotite + muscovite + plagioclase + quartz are approximately 70° C below the peak metamorphic temperature and are not strongly dependent on cooling rate. The results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet-biotite Fe-Mg exchange thermometry on amphibolite facies rocks (T_max > 550° C) because the rim composition of the garnet, which is required to calculate the peak temperature, is that most easily destroyed by diffusion.     

A SEARCH TECHNIQUE FOR CHOOSING OPTIMUM PARAMETERS IN NUMERICAL WEATHER PREDICTION

The steepest descent(or ascent)search is employed for finding optimum diffusion coefficients in T42L9Gmodel,with a view to improving the model's computational stability or prediction accuracy.The method of thesteepest descent search is first described,in which the golden section search is chosen as the fundamental one-dimensional search used in the multi-dimentional steepest descent search,and then the optimization of the dif-fusion coefficients is described.     

Evaluation and application of dialysis porewater samplers for microbiological studies at sediment-water interfaces

The equilibrium diffusion technique has become a valuable tool for ecological and biogeochemical studies in aquatic environments. In sediment ecosystems, changes in concentration of microbial metabolites with increasing depth can be determined dependably and reproducibly with this technique. Since the permeation characteristics of the membranes employed are crucial, selecting the proper membrane requires knowledge about its behavior under conditions which prevail in the natural environment. Thirteen polymer sheets were evaluated comparing permeation terms for biogeochemically relevant solutes, biodegradability, and mechanical strength. Cellulose-based dialysis membranes are most satisfactory when employed in low temperature anoxic environments. For this membrane, correction terms were calculated to account for diffusion losses during retrieval and sampling. Optimal incubation times can now be predicted from experimentally determined permeation coefficients for several porewater solutes. Dialysis porewater samplers (DPS) have been successfully applied during more than 100 independent experiments for the collection of interstitial water from surface sediments. DPS were used for water depths as deep as 290 meters.     

A critical analysis of processes governing the nutrient profiles in the ocean

We have theoretically considered the problem of interpretation of nutrient profiles in the upper ocean (100–2000m). We compare the experimental depth profiles of nonconservative tracers, both stable and radioactive, with solutions of one-dimensional steady state transport equations of increasing complexity including situations not encountered in the real oceans. Apart from gaining insight into the nutrient transport processes, this analysis is useful in offering a way to obtain operational estimates of depth dependent/independent eddy diffusivity and dissolution fluxes in the ocean. These parameters are essential for estimating new production, total production and burial of carbon in the sediments.     

Transport of water in frozen soil: V, Method for measuring the vapor diffusivity when ice is absent

A new experimental method is introduced for determining the relative magnitudes of liquid and vapor diffusion by using a small amount of soluble chemical as a tracer. The theoretical justification of the method is presented for the case where ice is absent. The feasibility of the method is demonstrated by an experiment using marine-deposited clay.     

对流边界层中的一个双高斯型PDF扩散模式

对流边界层（ＣＢＬ）中的污染扩散是非高斯型的。本文在下列三个假设下建立了双高斯型几率分布函数（ＰＤＦ）模式：１．对流边界层任一确定高度铅直速度Ｗ的几率分布函数ｐｗ由两个高斯分布迭加而成；２．从污染源释放的粒子具有源高的铅直速度几率分布，且其轨迹是线性的；３．粒子在地面的反射为全反射，在混合层高度Ｚｉ为全反射或有部分吸收。然后分析了三个高度上铅直速度Ｗ的一些统计特征量，比较了由ＰＤＦ模式计算的横风向积分浓度和Ｌａｍｂ的数值模拟［１－３］，Ｄｅａｒｄｏｒｆｆ的水槽模拟［４－６］结果，并用美国ＣＯＮＤＯＲＳ计划的外场试验资料［７］对ＰＤＦ模式进行了验证，结果均相当一致。     

闾阳－海城－东沟剖面深地震波动力学特征初探

本文利用国内外有关深地震波动力学特征的研究方法，结合现有原始资料的实际条件，对闾阳－海城－东沟深地震测深剖面某些地段的主要波组的动力学特征进行了定性或半定量的分析与研究。在频谱分析，速度振幅的计算与相对理论振幅的对比等方面，获得了一些有益信息。为进一步合理解释深地震测深资料提供依据。     

中小震近场地震动估计中地震动衰减关系的适用性分析

核工程场地设计基准地震动确定中涉及弥散地震的地震动估计，这导致中小震近场地震动估计成为被关注的研究课题。本文基于一些中小震近场地震动记录资料，分析和探讨了国内外多种地震动衰减关系对中小震近场地震动估计的适用性问题，并结合我国几个核工程地震问题研究工作的实践对中小震近场地震动衰减关系确定的思路和原则提出了建议。     

川西地区小震重新定位及其活动构造意义

使用双差地震定位法对川西地区1992～2002年的13367个小震进行重新定位， 初步分析了地震活动性与地表活动构造的关系及其揭示的构造信息. 重新定位后，地震活动沿活动断裂成线（带）状分布现象非常突出，呈现出与地表活动构造的密切关系：结构简单的单一走滑断层具有上宽下陡的花状结构特征，拉分盆地与逆断裂具有线性而发散的分布式结构特征，逆断裂之下还存在缺震层. 此外，沿活动断裂带地震活动还具有空间分段性，揭示出局部地段存在着隐伏活动断裂和可圈定为地震危险区的地震空区. 震源深度分布显示，川西高原在15～20km的深度范围内普遍存在厚度约5km的缺震层，以高温高压实验结果为基础，通过计算川西地区地壳强度表明，大约14～19km的深度范围花岗岩处于塑性流变状态，说明缺震层的出现具有地壳物质塑性变形基础.