Determining mineral weathering rates based on solid and solute weathering gradients and velocities: application to biotite weathering in saprolites

Chemical weathering gradients are defined by the changes in the measured elemental concentrations in solids and pore waters with depth in soils and regoliths. An increase in the mineral weathering rate increases the change in these concentrations with depth while increases in the weathering velocity decrease the change. The solid-state weathering velocity is the rate at which the weathering front propagates through the regolith and the solute weathering velocity is equivalent to the rate of pore water infiltration. These relationships provide a unifying approach to calculating both solid and solute weathering rates from the respective ratios of the weathering velocities and gradients. Contemporary weathering rates based on solute residence times can be directly compared to long-term past weathering based on changes in regolith composition. Both rates incorporate identical parameters describing mineral abundance, stoichiometry, and surface area.

Weathering gradients were used to calculate biotite weathering rates in saprolitic regoliths in the Piedmont of Northern Georgia, USA and in Luquillo Mountains of Puerto Rico. Solid-state weathering gradients for Mg and K at Panola produced reaction rates of 3 to 6×10⁻¹⁷ mol m⁻² s⁻¹ for biotite. Faster weathering rates of 1.8 to 3.6×10⁻¹⁶ mol m⁻² s⁻¹ are calculated based on Mg and K pore water gradients in the Rio Icacos regolith. The relative rates are in agreement with a warmer and wetter tropical climate in Puerto Rico. Both natural rates are three to six orders of magnitude slower than reported experimental rates of biotite weathering.     

Searching for the perfect surface area normalizing term—a comparison of BET surface area-, geometric surface area- and mass-normalized dissolution rates of anorthite and biotite

Dissolution rates were calculated for a range of grain sizes of anorthite and biotite dissolved under far from equilibrium conditions at pH 3, T = 20 °C. Dissolution rates were normalized to initial and final BET surface area, geometric surface area, mass and (for biotite only) geometric edge surface area. Constant (within error) dissolution rates were only obtained by normalizing to initial BET surface area for biotite. The normalizing term that gave the smallest variation about the mean for anorthite was initial BET surface area. In field studies, only current (final) surface area is measurable. In this study, final geometric surface area gave the smallest variation for anorthite dissolution rates and final geometric edge surface area for biotite dissolution rates.     

Geology,geochemistry and genesis of the Makou magnetite-apatite deposit in the Luzong volcanic basin,Middle-Lower Yangtze River Valley Metallogenic Belt,Eastern China

The Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB) is located on the northern margin of the Yangtze Plate (Eastern China). Ore deposits in the belt are mainly clustered in seven ore districts, and are closely associated with Mesozoic intermediate-felsic magmatic rock. Among the seven ore districts, the Luzong and Ningwu districts host large-scale iron resources in volcanic basins. The Makou magnetite-apatite deposit in the southern Luzong Basin was previously interpreted to be related to a quartz syenite porphyry. In this study, we conducted field geological studies and determined the age and geochemistry of the Makou intrusive rocks. Petrography and electron probe micro analysis (EPMA) indicated that the Makou ore-hosting rocks have intense albite alteration. The wallrock alteration is spatially restricted, and comprises albite alteration (Stage I), magnetite mineralization (Stage II), quartz-sulfide alteration (Stage III) and carbonate alteration (Stage IV) stages. Fluid inclusions in syn-mineralization apatite homogenized at 252.2–322.6 °C, which slightly lower than is typical for magnetite-apatite deposits in the region. Field study revealed that the quartz syenite porphyry at Makou disrupted the orebodies along clear-cut intrusive contacts, and that the quartz syenite porphyry does not contain iron mineralization, suggesting it has no direct genetic relationship with the iron mineralization. The ore-hosting albitite and ore-forming biotite diorite have LA-ICP-MS zircon U-Pb ages of 129.6 ± 1.2 Ma and 131.2 ± 3.3 Ma, respectively, and the iron mineralization was dated by mass spectrometer phlogopite ⁴⁰Ar-³⁹Ar at 130.76 ± 0.77 Ma. We propose that the Makou magnetite-apatite deposit is genetically related to the biotite diorite, rather than to the quartz syenite porphyry in the mine pit. The biotite diorite closely resembles intrusions related to magnetite-apatite deposits elsewhere in the region.     

基于循环加卸载的矿物定向排列致各向异性岩石损伤演化规律——以黑云母石英片岩为例

黑云母石英片岩是一种典型的各向异性岩石,为研究其损伤演化规律及各向异性表现特征,针对含0°,45°,90°3种片理角度的试样,开展了等塑性应变循环加卸载试验。结果表明：不同片理面角度试样的破坏模式有所不同,0°片理面试样中的张拉破裂现象与45°片理面中的剪切破坏现象尤为明显。随着塑性应变增加,黑云母石英片岩的弹性模量表现出先强化、后弱化的现象,这一现象在高围压下更为明显。在弱化阶段中,弹性模量演化的转折点与裂纹起裂强度σ_ci稳定时对应塑性应变一致。弹性模量作为岩石损伤劣化过程的评价指标较完整性系数受片理面角度的影响更小。在岩石内部能量演化过程中,耗散能大小与片理面角度的关系为0°>90°>45°,耗散能、弹性能下降的速率大小关系为45°>90°>0°,其降至稳定时与岩石损伤强度σ_cd稳定时对应塑性应变一致。该研究借助弹性模量和能量演化规律分析,探究了黑云母石英片岩的损伤演化力学行为。

     

Biotite stability in peraluminous granitic melts: Compositional dependence and application to the generation of two-mica granites in the South Bohemian batholith (Bohemian Massif, Czech Republic)

Crystallization experiments have been conducted in the system Na₂O–K₂O–MgO–FeO–Al₂O₃–SiO₂–H₂O (with 4% normative corundum) in order to constrain the stability of biotite as a function of water activity and the Mg# of biotite [Mg/(Mg +Fe_total)] in equilibrium with peraluminous granitic melts. The temperature at which biotite breakdown starts is strongly dependent on the Mg# of biotite. At 500 MPa, the temperature of biotite breakdown to form orthopyroxene increases from 750 °C to 830 °C, as the Mg# of biotite increases from 0.4 to 0.5. Considering that the system investigated is relevant for Ca-poor peraluminous biotite-bearing rocks (metapelites), the biotite dehydration curves obtained are used to discuss the melting reactions and the temperatures that lead to the formation of two distinct types of two-mica granites found in the South Bohemian batholith (specifically the Eisgarn and Deštná granites). The phase relationships were determined experimentally for the composition of these two granites in order to constrain the composition of the biotite in equilibrium with the melt in the protoliths. We demonstrate that Eisgarn granitic melts may have been generated at temperatures in the range 830–850 °C from melting reactions involving biotite with a Mg# up to 0.5 as a reactant. In contrast, Deštná granitic melts cannot have been generated from dehydration melting reactions involving biotite.     

热水条件下黑云母断层泥的摩擦强度与稳定性

黑云母是自然界常见的层状硅酸盐矿物,其摩擦系数不高且化学稳定性好,对其摩擦性质的关注可能会对弱断层的研究有所帮助.本次工作选取的实验温度条件对应于典型地壳强度模型中脆塑性转化带的范围,为300 ℃和400 ℃.有效正应力为200 MPa,孔隙水压包括10 MPa和30 MPa,在此条件下对黑云母模拟断层泥进行摩擦实验研究.实验得出黑云母的摩擦系数平均在0.36左右.速度依赖性随温度升高速度弱化的程度增强,表现为300 ℃为十分微弱的速度弱化,而在400 ℃出现了黏滑行为,代表了更强的速度弱化.显微结构中同时出现了脆性剪切变形和塑性扭折变形,但决定宏观力学性质的显然是脆性剪切变形.在黑云母存在的情况下,本研究的实验结果有助于理解大陆地壳脆塑性转化带中地震的可能性和弱断层深部的变形机制、宏观力学行为以及地震活动.     

赣杭构造带相山火山侵入杂岩的岩浆演化特征——来自斜长石和黑云母的化学成分研究

黑云母、斜长石等矿物可作为寄主岩石的岩浆演化的指示剂。本文借助电子探针微区分析技术,通过对赣杭构造带中相山盆地火山侵入杂岩中这些矿物的化学组成进行定量分析发现:碎斑熔岩、花岗斑岩及石英二长斑岩的斜长石环带不明显,并且主要是中长石;石英二长斑岩中黑云母矿物成分为镁质黑云母和铁质黑云母,化学成分更为接近研究区镁铁质微粒包体中黑云母的成分,而碎斑熔岩及花岗斑岩中黑云母的镁含量较低。结合黑云母、斜长石矿物的化学特征及前人的工作研究,相山碎斑熔岩和花岗斑岩的物质来源主要是壳源,没有明显地幔物质的加入,而石英二长斑岩有一定量的地幔物质的加入,并且这个地幔物质的加入发生在石英二长斑岩的黑云母开始结晶之前。     

云母:花岗岩-伟晶岩稀有金属成矿作用的重要标志矿物

云母是花岗岩、伟晶岩中的重要造岩矿物,不仅是整个岩浆阶段的结晶产物,而且也是热液过程的参与者。作为层状硅酸盐矿物,层间或八面体位置上可容纳锂、铷铯、锡、铌钽等稀有金属。本文结合前人研究积累和作者近年来的研究成果,阐述了云母作为一个重要的稀有金属成矿标志矿物的矿物学特征。铁锂云母-锂云母是稀有金属成矿作用中重要的锂矿物,同时云母中锂含量可以反映花岗岩的分异程度。铷、铯可以置换云母层间钾,在高演化花岗岩、伟晶岩中可以形成铷、铯为主的云母(既可以是锂云母系列,也可以是黑云母系列)。黑云母是稀有金属花岗岩中一个特殊的矿物。准铝质含锡花岗岩中黑云母锡含量可达100×10~(-6),其锡含量可以指示其锡成矿能力。稀有金属花岗岩中,常见的是铌钽氧化物矿物。但是最近研究发现,黑云母中铌可以超常富集(超过1000×10~(-6)),成为稀有金属花岗岩中最重要、甚至唯一的铌矿物,形成一种以富铌黑云母为特色的新类型稀有金属花岗岩,并可能代表了一种新型的潜在铌资源。基于云母在花岗岩中的重要性和结构的特殊性,今后要利用微区成分和结构分析技术,加强对云母中稀有金属晶体化学的研究,以及进一步揭示云母对稀有金属成矿的特殊重要意义。     

青藏高原玉树混杂带糜棱岩化石英闪长岩年代学和地球化学研究

玉树混杂带内含有大量金沙江西段古特提斯演化信息,对其物质组成进行详细解剖,有望厘清青藏高原东北缘的特提斯演化过程。本文以该带内糜棱岩化岩体为研究对象,研究其岩石学、构造变形特征,报道其锆石U-Pb年龄、黑云母Ar-Ar年龄、主量、微量和Sr-Nd同位素数据,结合区域岩浆事件和构造变形资料讨论大地构造意义。研究表明该岩体主要为石英闪长岩,变形强烈,发育北西西向糜棱面理,面理走向与区域构造线一致。岩石为准铝质中-高钾钙碱性系列,富集K,Rb,Ba,Th等大离子亲石元素, 亏损高场强元素,具有高Sr负Nd的同位素组成,为基性下地壳部分熔融的产物。石英闪长岩锆石U-Pb年龄为208Ma,黑云母⁴⁰Ar/³⁹Ar年龄为201Ma,讨论认为前者代表岩浆结晶年龄,形成于洋盆俯冲背景;后者代表变形年龄,与区域碰撞挤压有关。     

阿尔泰南缘克朗盆地铁木尔特花岗岩体年龄、地球化学特征及成因

铁木尔特黑云母花岗岩出露于阿尔泰造山带南缘克朗盆地北西侧,岩体主要为黑云母花岗岩。锆石的SHRIMPU-Pb定年结果表明该岩体结晶年龄为459±4.9Ma。岩石具有高的SiO_2(71.92%～76.58%)和Al_2O_3(12.92%～14.55%)含量,富碱(K_2O+Na_2O=7.09%～8.11%)、高钾(K_2O/Na_2O=1.1～1.62)、低P_2O_5(0.14%)和MgO+FeO(5%)含量,强过铝质(ACNK=1.13～1.34);富集LREE和Th、U、Pb,贫Ba、P、Sr、Ti、Nb,呈现明显的Eu负异常(δEu=0.33～0.53);并具有低的Al_2O_3/TiO_2(100)比值和高的CaO/Na_2O(0.3)比值。以上特征表明该岩体属高钾钙碱性高温型强过铝花岗岩。同时岩石具有低的Sr初始值(~(87)Sr/~(86)Sr=0.701727～0.709951)和相对高的ε_(Nd)(459Ma)(-1.43～-0.98)值,两阶段Nd模式年龄较为均一(t_(2DM)=1.2～1.3Ga)。岩体的原始岩浆可能是富含白云母和黑云母的变泥质岩在大于875℃的条件下脱水熔融的产物,在部分熔融过程中有磷灰石、斜长石、钛铁矿等矿物的残留,并混入少量的幔源物质。结合阿尔泰南缘的区域地质背景综合分析,推测岩体形成于俯冲作用的初期,可能是在挤压背景下由于局部伸展减压以及异常地幔和深部热流不均匀上升提供的热影响发生的部分熔融产生。