Formation of hydrocarbon reservoirs in the deep Earth's crust

Formation of reservoirs in crystalline rocks is associated with the development of rifts, with the periodic axial plunge of the rift floor during the extension of the Earth's crust, and with the elevation of consolidated basement masses during a compression phase.     

Thermobarometry and P–T–t paths: the granulite to eclogite transition in lower crustal xenoliths from eastern Australia

‘Lower crustal’ suite xenoliths in basaltic and kimberlitic magmas are dominated by mafic granulites and may also include eclogites and garnet pyroxenites. Pressures of up to 25 kbar obtained from such xenoliths are well in excess of an upper value of c. 12 kbar for exposed granulite terranes. Palaeogeotherms constructed from xenoliths for the lower crust beneath the Phanerozoic fold belts of eastern Australia (SEA) and beneath the eastern margin of the Australian craton (EMAC) indicate two distinct thermal regimes. The two geotherms have similar form, with the EMAC curve displaced c. 150°C to lower temperatures. Reaction microstructures show the partial re-equilibration of primary igneous assemblages to granulite and eclogite assemblages and are interpreted to reflect the cooling from magmatic temperatures. Variations in mineral compositions and zoning are used to constrain further the history of several EMAC xenoliths to near-isobaric trajectories. Detailed graphical models are constructed to predict compositional changes for isobaric P–T paths (at 7, 14 & 21 kbar) to transform an SEA-type geotherm to a cratonic geotherm. The models show that for the assemblage grt + cpx ± ky + plag + qtz, the changes associated with falling temperature in X_gr, X_jd (increase) and X_an (decrease) will be greater at higher pressures. These results indicate that discernible zoning is more likely to be preserved in the higher pressure xenoliths. The zoning recorded in clinopyroxene from mafic granulite xenoliths over the pressure range c. 12–22 kbar suggests isobaric cooling of a large crustal thickness (30–35 km). An isobaric cooling path is consistent with magma accretion models for the transition of a crust–mantle boundary from an SEA-type geotherm to a cratonic geotherm. The coexistence of granulite and eclogite over the depth range 35–75 km beneath the EMAC indicates that the granulite to eclogite transition in the lower crust is controlled by P–T conditions, bulk chemistry and kinetic factors. At shallower crustal levels, typified by exposed granulite terranes, isobaric cooling may not result in the transition to eclogite.     

The behavior of iron,manganese and silicon in the Peconic River estuary,New York

The behavior of dissolved and particulate iron and manganese and dissolved silicon has been studied as a function of chlorinity in the Peconic River estuary, New York. This study sought to identify important geochemical processes in a relatively pristine estuary facing increasing anthropogenic impact.Dissolved iron behaved in the classical non-conservative manner exhibiting removal of nearly 80% at very low chlorinities, while particulate iron increased by a corresponding amount over the same chlorinity range. Dissolved manganese was enriched by up to 200% over its predicted concentration at low and intermediate chlorinities by desorption from suspended particulates and by a probable benthic flux. Dissolved silicon was enriched by up to 100% at low and intermediate chlorinities also from a probable benthic flux. These fluxes were estimated to be 5 μg cm^?2 day^?1 for dissolved manganese and 70 μg cm^?2 day^?1 for dissolved silicon.The quantity of both particulate iron and manganese increased at high chlorinities due to an influx of suspended inorganic particulates. In the intermediate to high chlorinity region, oxidation of sediment-derived manganese is believed to contribute to the observed increase in particulate manganese.Total iron was essentially conservative throughout most of the estuary, while total manganese was non-conservative presumably due to extensive remobilization of dissolved manganese from the sediments.     

Lower Crustal Magma Genesis and Preservation: a Stochastic Framework for the Evaluation of Basalt-Crust Interaction

We present a quantitative assessment of the thermal and dynamicresponse of an amphibolitic lower crust to the intrusion ofbasaltic dike swarms in an arc setting. We consider the effectof variable intrusion geometry, depth of intrusion, and basaltflux on the production, persistence, and interaction of basalticand crustal melt in a stochastic computational framework. Distinctmelting and mixing environments are predicted as a result ofthe crustal thickness and age of the arc system. Shallow crustal(30 km) environments and arc settings with low fluxes of mantle-derivedbasalt are likely repositories of isolated pods of mantle andcrustal melts in the lower crust, both converging on daciticto rhyodacitic composition. These may be preferentially rejuvenatedin subsequent intrusive episodes. Mature arc systems with thickercrust (50 km) produce higher crustal and residual basaltic meltfractions, reaching 0·4 for geologically reasonable basaltfluxes. The basaltic to basaltic andesite composition of bothcrustal and mantle melts will facilitate mixing as the networkof dikes collapses, and Reynolds numbers reach 10^–4–1·0in the interiors of dikes that have been breached by ascendingcrustal melts. This may provide one mechanism for melting, assimilation,storage and homogenization (MASH)-like processes. Residual mineralassemblages of crust thickened by repeated intrusion are predictedto be garnet pyroxenitic, which are denser than mantle peridotiteand also generate convective instabilities where some of thecrustal material is lost to the mantle. This reconciles thethinner than predicted crust in regions that have undergonea large flux of mantle basalt for a prolonged period of time,and helps explain the enrichment of incompatible elements suchas K₂O, typical of mature arc settings, without the associatedmass balance problem. KEY WORDS: crustal anatexis; delamination; lower crust; magma mixing; thermal model     

“十五”福建省地壳形变数字化监测网简介

该文主要对福建省地形变监测数字化监测网的特色 ,监测网的内容 ,布网目的 ,主要技术指标以及数据处理中心建设作了简介 ,最后对地壳形变数字化监测网在“数字福建”工程中的作用作了阐述     

骑田岭花岗岩体的地球化学特征及其对成矿的制约

骑田岭花岗岩体位于湖南省南部 ,在其东北部接触带有骑田岭大型夕卡岩型锡矿 ,南部接触带有新探明的芙蓉超大型锡多金属矿床。本文采用ICP_AES和ICP_MS分析了骑田岭花岗岩全岩的主量、微量和稀土元素含量。其主量元素特征表明 ,骑田岭花岗岩具有富硅富碱富铝、贫镁铁的特点 ,经历了较大程度的结晶分异。稀土元素总量较高 ,轻稀土元素富集 ,重稀土元素亏损。岩石富集大离子亲石元素 ,特别是富集Rb、Th ,推测源岩可能来源于陆壳物质。与相邻的千里山花岗岩比较 ,虽然没有显著的四分组效应 ,但它们的地球化学性质具有很大的相似性。另对岩体的成因、形成时代和成矿作用进行了讨论 ,认为骑田岭花岗岩超大型锡多金属矿床的形成是侏罗纪岩石圈在伸展环境下引起地幔物质上涌使地壳物质发生重熔 ,同时在热液作用参与下金属元素重新富集的结果