Mineralogy, Petrology, and Magmatic Conditions from the Fish Canyon Tuff, Central San Juan Volcanic Field, Colorado

The Fish Canyon Tuff is one of the largest currently recognizedash-flow tuffs (> 3000 km³). It is a crystal-rich quartzlatite containing about 40 per cent phenocrysts of plagioclase,sanidine, biotite, hornblende, quartz, magnetite, sphene, andilmenite. Pyrrhotite occurs as inclusions in magnetite, sphene,and hornblende. The consistency of mineralogy and whole rockchemistry confirms that the Fish Canyon tuff is remarkably homogeneous.Most chemical variations can be accounted for by phenocryst-matrixfractionation, probably due to glass winnowing during eruptionand emplacement. The composition of the parent magma, correctedfor such winnowing, is very similar to that of calc-alkalinebatholiths such as the Boulder and the Sierra Nevada batholiths. Fe-Ti oxide geothermometers indicate temperatures of 800 ? 30?C for most of the outflow tuff. No evidence for a regular thermalgradient in the magma chamber could be detected. Two feldsparand Fe-Ti oxide equilibria indicate that the magma developedat depths of 25 to 30 km (about 9 kb pressure), and was eruptedwithout time for phenocryst re-equilibration. The reconstructedcomposition of the liquid in equilibrium with the phenocrystsalso suggests a deep source for this ash flow. A late, upperpackage of flow units have mineralogical characteristics whichmay reflect partial re-equilibration in a shallower environment. Oxygen fugacities are moderately high (log fO₂ = — 11.5?0.3) but are similar to those obtained from other continentalcalc-alkaline ash-flow tuffs. The water fugacity is limitedby calculations using biotite equilibria and experimental workrelating to the stability of the phenocryst assemblage. Bestestimates are that water fugacity was 2000 ? 1000 bars. Theactivities of sulphurous gases are estimated at fSO₂ = 2 to4 bars, fso₂ = 150 to 200 bars, fH₂S = 70 to 80 bars. The Fish Canyon Tuff therefore came from a deep, homogeneous,granitic magma body of batholithic proportions. Calculationsof its probable viscosity, density, and size indicate that thesystem should convect with any reasonable thermal gradient.Convective mixing may account for the homogeneity of the parentmagma body.     

Ocean urea fertilization for carbon credits poses high ecological risks

The proposed plan for enrichment of the Sulu Sea, Philippines, a region of rich marine biodiversity, with thousands of tonnes of urea in order to stimulate algal blooms and sequester carbon is flawed for multiple reasons. Urea is preferentially used as a nitrogen source by some cyanobacteria and dinoflagellates, many of which are neutrally or positively buoyant. Biological pumps to the deep sea are classically leaky, and the inefficient burial of new biomass makes the estimation of a net loss of carbon from the atmosphere questionable at best. The potential for growth of toxic dinoflagellates is also high, as many grow well on urea and some even increase their toxicity when grown on urea. Many toxic dinoflagellates form cysts which can settle to the sediment and germinate in subsequent years, forming new blooms even without further fertilization. If large-scale blooms do occur, it is likely that they will contribute to hypoxia in the bottom waters upon decomposition. Lastly, urea production requires fossil fuel usage, further limiting the potential for net carbon sequestration. The environmental and economic impacts are potentially great and need to be rigorously assessed.     

WMO

ABSTRACT

Long term changes in streamflow following forest cutting are presented for three experimental basins at Coweeta Hydrologic Laboratory, North Carolina, USA. Previous analyses have shown that, following forest cutting, streamflow increases and then declines with the logarithm of time as the forest regrows. Recent data indicate that the streamflow decline following cutting is related to vegetation regrowth, but the relationship is not a consistent function of simple stand measurements. The mixed hardwood forest of one basin was clearcut twice in the past 40 years. During the second regrowth period, streamflow increases after the first year were about one-half the increases at the same points in time after the first treatment. Concurrent with the second cutting, two other basins were cut whose mid-elevations are 400 m higher. The increases for the upper basins were similar, even though basal area was reduced by only 65% on one. Both streamflow increases were less than for either cutting on the lower basin. Variability of flow increases for the three concurrent treatments is similar and appears partly related to precipitation.     

Erosion rather than plant dieback as the mechanism of marsh loss in an estuarine marsh

Coastal marsh loss in Louisiana is attributed to plane dieback caused by processes that stress vegetation, and a common landscape pattern is broken marsh that expands at the expense of surrounding unbroken marsh. We tested the hypothesis that vegetation is more stressed in broken marsh than in adjacent unbroken marsh, as indicated by vegetation aboveground biomass, species diversity and soil Eh, on transects that extended from broken marsh to unbroken marsh at Marsh Island, Louisiana. Soil Eh, vegetation above-ground biomass and species diversity did not differ between broken marsh and unbroken marsh, and above-ground biomass was similar to that reported from other marshes. Thus, we rejected the hypothesis that marsh loss is related to vegetation stress. Two factors were related to vegetation vigour: soil drainage and soil bulk density. Surprisingly, significant soil drainage occurred in broken marsh but not in unbroken marsh. Above-ground biomass of the dominant plant, Spartina patens (Aiton) Muhl., was lowest where soil bulk density was less than 0-08 gcm⁻³, which illustrated the importance of mineral matter accumulation in submerging coastal marshes. The mechanism of marsh loss appeared to be erosion below the living root zone, as indicated by the vertical and often undercut marsh-water interface, and by the separation of sod clasts. This is different from more rapid marsh loss associated with plant stress which we observed in other Louisiana marshes only 135 km away, indicating that marsh loss mechanisms can vary spatially even within a relatively small region.     

云南瓦渣钨矿区老君山花岗岩体的SHRIMP锆石U-Pb定年、地球化学特征及成因探讨

云南省瓦渣钨矿区位于老君山花岗岩体的东北接触带,属于滇东南-桂西白垩纪钨锡多金属成矿区中的组成部分。该矿床与矿区西部大面积分布的老君山第一期中粗粒二云二长花岗岩在时间和成因上有着密切联系。本文通过SHRIMP锆石U-Pb测年、岩石主量元素、微量元素和稀土元素的系统研究,进而探讨该花岗岩的形成年龄和动力学背景、岩石物质来源以及形成温度和压力。研究表明,与瓦渣钨矿床有关的中粗粒二云二长花岗岩属于高钾钙碱性岩系,为强过铝质S型花岗岩。岩体形成时代为83.3±1.5Ma,属于晚白垩世,形成于岩石圈伸展环境,其源岩为富粘土的泥质来源,其形成温度为875～900℃,压力约为7×108Pa,相当于深度为26km。     

新疆博格达造山带晚古生代构造-岩浆演化过程:火山岩组合及地球化学证据

最近,笔者在博格达山东段北部的西地-伊齐-小红柳峡一带的地质调查中发现,该区发育有大量晚石炭世柳树沟组双峰式火山岩及早二叠世卡拉岗组酸性火山岩建造,但双峰式火山岩性质及成因有别于其南侧七角井早石炭世双峰式火山岩。玄武岩富钠贫钾(K_2O=0.18%～0.45%,Na_2O=2.24%～3.63%),属拉斑系列;TiO_2=1.6%～1.7%,略高于MORB,较高的Al(Al_2O_3=16.2%～16.7%)、高Mg(MgO=8.12%～9.54%,Mg~#=61～64),以及低K_2O/TiO_2和K_2O/P_2O_5比值(分别为0.1～0.27、0.63～1.68),反映了在岩浆演化过程中分离结晶作用不明显;Rb/Sr比值0.01～0.02,Zr/Nb=21.6～39.7,Zr/Y=5.38～7.47,以及不相容元素Ba、Zr、Hf相对略富集、Nb-Ta和Th相对亏损,显示岩石具有板内玄武岩的特点;稀土元素球粒陨石标准化配分图上整体接近于平坦型,(La/Yb)_N=1.8～1.9,Eu无异常至轻微正异常(δEu=1.07～1.12),正ε_(Nd)(t)值(+5.63～+5.89),(~(143)Nd/~(144)Nd)_I=0.512927～0.512944,Th/Yb0.2,Ta/Yb=0.1,表明玄武岩浆源于亏损软流圈地幔,且在演化过程中不曾发生过斜长石的分离结晶作用,并暗示当时的大陆地壳可能由于拉张而变得较薄,玄武岩浆形成后快速上升至地表喷发。双峰式火山岩中的流纹岩Rb-Sr等时线年龄为296±2Ma(1σ),具高Si(SiO_2=76%～80%),富钾贫钠(K_2O=5.1%～5.7%,Na_2O=0.94%～2.03%);低Al(Al_2O_3=7.9%～10.4%);低Ti、Ca和P含量,属高钾钙碱性系列;微量元素Rb、Th、Zr、Hf、K相对富集,Ba、Sr、P、Ti、Nb、Ta为显著亏损;轻稀土元素适度富集且轻、重稀土分馏程度低,(La/Yb)_N=5.1～7.1,(La/Sm)_N=2、(Gd/Yb)_N=1.6～2.2,以及强烈的负Eu异常(δEu=0.17～0.2),(~(87)Sr/~(86)Sr)_I=0.7051～0.7052,δ~(18)O=11.6‰,指示岩石源于地壳物质的部分熔融,源区存在有斜长石残留,形成于大陆裂谷环境。早二叠世末流纹岩(Rb-sr等时线年龄为278±2Ma)具高Si(SiO_2=74%),富钾贫钠(K_2O/Na_2O2),低Al(Al_2O_3=11.0%)以及较低的Ti和P含量的特征,岩石为高钾钙碱性系列;微量元素PM标准化图解上表现为Rb、Ba、Th、U、K、La、Ce不相容元素相对富集,高场强元素Nb、Ta、P、Ti以及Sr为明显的负异常;轻稀土轻度富集,(La/Yb)_N=5～6,(La/Sm)_N=3,(Gd/Yb)_N=1.3～1.4,以及强烈的负Eu异常(δEu=0.31～0.39),(~(87)Sr/~(86)Sr)_I为0.7069,δ~(18)O=11.97‰,指示源于地壳物质部分熔融的产物,形成于伸展垮塌的构造环境。综合研究结果表明,博格达山前身裂谷岩浆作用始于早石炭世,结束于晚石炭世末期,早二叠世末进入后造山伸展的演化阶段。     

Origin of Basalt Magmas: An Experimental Study of Natural and Synthetic Rock Systems

Natural basalts and eclogites were investigated experimentallyat a series of temperatures in the pressure range 1 atm to 40kb and with water pressures 1 to 10 kb. Some runs were alsomade on related synthetic systems at 10 and 33 kb. The two principal magma types recognized by field investigators—tholeiiteand alkali basalt types-appear to be separated by equilibriumthermal divides at 1 atm. The principal divides were found byexperiment at elevated pressures to give way to a new set ofequilibrium thermal divides resulting from a new mineralogy.The change of the equilibrium thermal divides with pressureleads to the derivation of the two principal magma trends fromthe same bulk composition. The melting behavior of basalts and eclogites indicates thatboth are the partial melting products of a more primitive rock(e.g. garnet peridotite). In the region of magma generation(below 60 km) the parental material, presumed to be garnet peridotite,yields an eclogitic magma and its fractionation depends on thegarnet and omphacite of the eclogite, not on plagioclase andclinopyroxene of a basaltic magma. Increase of the garnet constituentsin the magma at high pressure by effective removal of omphaciteor shift of the garnet-omphacite boundary ‘surface’will give rise to a tholeiite-type magma at low pressure. Similarly,increase of the omphacite constituents in the magma at highpressure by physical or physicochemical means will give riseto an alkali basalt-type magma at low pressure. In general,alkali basalt-type magmas are to be expected to be generatedat greater depths than tholeiite-type magmas from the same primarysource rock. Establishment of the two major basalt series takes place inthe region of generation; additional minor diversification ofeach series may come about after emplacement in or on the crustby crystal settling, oxidation or reduction, gas fluxing, contamination,and other processes. The derivative magmas are greatly restrictedby the course of liquid thermal descent imposed at generation. Pressure-temperature limits established experimentally suggestthat the basalt-eclogite transformation may be responsible forthe Mohorovii discontinuity under the continents, but not underthe oceans. The field of stability of basalt is drastically reduced in thepresence of water, and amphibolite is produced. The meltingof amphibolite takes place over a much greater range of temperaturethan basalt. At 10 kb water pressure the beginning of meltingof amphibolite closely approaches that of granite. Partial meltingof amphibolite may yield anorthositic liquids having a relativelylow anorthite content at exceptionally low temperatures. Eclogiteitself is not stable in the presence of water and gives placeto amphibolite or pyroxene hornblendite. Magmas which crystallizeto basalt, gabbro, or eclogite must have had a low water-contentat the time of crystallization. Fifteen rock and twenty-three mineral analyses as well as numerouspartial chemical analyses of experimental products were madeby J. H. Scoon in the course of the investigation. These chemicalanalyses bear on many mineralogical and petrological problems.