Using estimated thermodynamic properties to model accessory phases: the case of tourmaline

Accessory phases and minor components in minerals are commonly ignored in thermodynamic modelling. Such an approach seems unwarranted, as accessory phases can represent a significant element reservoir and minor components can substantially change their host mineral's stability field. However, a lack of thermodynamic data prohibits assessment of these effects. In this contribution, the polyhedron method is used to estimate the thermodynamic properties of tourmaline, a common and widespread accessory phase, stable over a range of P–T–X conditions. The polyhedron method allows Δ H , S , V , C _P and V _m ( T , P ) properties to be estimated from a linear stoichiometric summation over the fractional properties of its polyhedron constituents. To allow for estimates of tourmaline, fractional thermodynamic properties for B^III and B^IV polyhedra were derived. Mixing contributions to molar volume were evaluated and symmetrical mixing parameters derived for Al-Mg, Al-Fe and Al-Li interaction on tourmaline's Y-site and T-site Al-Si interaction. Evaluation of the estimated properties using experimental and natural equilibria between tourmaline and melts, minerals and hydrothermal fluids, shows that reliable semi-quantitative results are obtained. The boron contents in fluids coexisting with tourmaline are calculated to within an order of magnitude of measured content, and where anchor-points are available, agreement improves to within a factor of 2. Including tourmaline in petrogenetic modelling of metamorphic rocks indicates that its presence leads to disappearance of staurolite and garnet, among others, and modifies the X _Mg of coexisting phases, in line with observations on natural rocks.     

Relict Majoritic Garnet Microstructures from Ultra-Deep Orogenic Peridotites in Western Norway

Protogranular, porphyroclastic and equigranular (or equant-polygonal)garnet microstructures from Mg–Cr type orogenic garnetperidotites, Otrøy, Western Gneiss Region, Norway, havebeen studied using naked eye, light-optical, electron-opticaland confocal laser (fluorescence) microscopy techniques. Protogranularand porphyroclastic garnets contain microstructural evidencefor the former existence of majoritic (or super-silicic) garnet.The microstructural evidence consists of exsolution texturesinvolving pyroxene. Two types of exsolution microstructuresoccur—needles parallel to <111>_grt and interstitialgrains. The maximum volume percentage for intra-crystallinepyroxene exsolution is 2·7, and 3·6 for inter-crystallinepyroxene exsolution. The maximum pyroxene total volume percentagemeasured in one single protogranular or porphyroclastic garnetis 4·0. This value, at 1200°C, corresponds to minimumpressures of 6·4 GPa (     

Mineralogy, Petrology, and Geochemistry of an Ultrapotassic Basaltic Suite, Central Sierra Nevada, California, U.S.A.

Ultrapotassic basaltic lavas erupted 3.4–3.6 m.y. ago(K/Ar) in the central Sierra Nevada and originated by partialmelting of a phlogopite-enriched, garnet-bearing upper mantlesource. Ultrapotassic basanites (K₂O: 5–9 per cent), whichare spatially related to contemporaneous potassic olivine basalts(K₂O: 3–5 per cent) and alkali olivine basalts (K₂O: 1–3per cent), contain the K₂O-bearing minerals phlogopite, sanidine,and leucite as well as olivine, diopside, apatite, magnetite,and pseudobrookite. The presence and modal abundance of theK₂O-bearing minerals closely reflects the east to west increasein K₂O throughout the basaltic suite. Many lines of evidence support the derivation of the ultrapotassicbasanites and the related basalts from an upper mantle source:TiO₂ in phlogopite phenocrysts and groundmass crystals, 2–3and 7–9 per cent respectively, support phlogopite phenocrystcrystallization at high pressure, whole rock Mg values (100Mg/Mg + 0.85 Fe) range from 66–78, phlogopite-rich pyroxeniticand periodotitic nodules are included in some flows, and geobarometriccalculations indicate depths of generation at 100–125km. Also, model calculations show that the major, rare earth,and trace elements, except for Ba, Rb, and Sr, can be accuratelygenerated by 1.0–2.5 per cent melting of a phiogopite-and garnet-bearing clinopyroxene-rich upper mantle source. Partialmelting occurred after a general upper mantle enrichment beneaththe Sierra Nevada, the phlogopite- and clinopyroxene-rich sourceof the ultrapotassic lavas being the extreme result of the enrichmentprocess. Clinopyroxene enrichment of the upper mantle probablyoccurred by introduction of a partial melting fraction intothe upper mantle source areas. Enrichment of the upper mantlein the alkali and alkali-earth elements was not accomplishedby a partial melt, but resulted from influx of a fluid phaserich in Ba, K, Rb, Sr, and, probably, H₂O The continuous rangein K₂O of the erupted lavas implies that the upper mantle enrichmentis a cumulative process. The inverse relationship in the SierraNevada between uplift and the K₂O content of the erupted basaltsimplies that a critical relationship may exist between upliftand upper mantle enrichment.     

A modern oceanic hardground on the Carnegie Ridge in the eastern Equatorial Pacific

Fossil surfaces of erosion and non-deposition are common in limestone sequences from the Mesozoic in western Europe and in the Tethys and have been described under the name ‘hardgrounds’. They are of shallow water as well as of oceanic origin. A modern example in the Pacific is described in this paper. The Carnegie Ridge, an east-west trending shallow ridge between South America and the Galapagos Islands, has a central, deeper saddle where erosion has removed most of the sediment cover down to a hard chalk and chert bed (acoustic basement), and has cut intricate channel patterns on the south flank and two deep canyons on the north. The erosion has produced a karst-like relief of steep-walled channels, cliffs, and corroded chalk remnants. The floors of the channels are covered with ferromanganese oxide crusts or crust fragments over which loose sediment is being transported. In the two canyons on the north flank, this sediment consists of foraminiferal sand travelling downslope in the form of barchan dunes. All sediment down to acoustic basement has been stripped from the Carnegie Ridge crest except where it is protected behind basement ridges and pinnacles. Surface features of the eroded chalk are strongly reminiscent of features observed in Mesozoic hardgrounds. Current measurements over several days indicate a net northward movement, slow but possibly adequate to keep the sea floor free of fresh deposits. The rates, however, seem inadequate to explain the formation of the deep channels, and there is no evidence for the southward flow which is implied by the southern channel system. A process of combined carbonate dissolution and removal by the current of fresh sediment and dissolution residues can account for the required erosion in about two million years. Regional unconformities identifiable in seismic reflection profiles and dated in cores are of middle to late Pliocene age, suggesting that the formation of the erosion surface began 2-3 million years ago. Buried Miocene unconformities of local extent show that the present erosion period had minor precursors possibly related to short-lived increases in current action and carbonate dissolution.     

Contrasting Sodic and Potassie Glassy Inclusions in Apatite Crystals from an Ijolite

Electron probe analysis of isotropic to weakly birefringentglassy inclusions in apatite crystals within the Usaki ijoliteof western Kenya, indicates that two contrasting compositionsexist. These inclusions are thought to represent samples oforiginal silicate melts. One is rich in K₂O(6 weight per cent),poor in Na₂O(0.3 weight per cent), and oversaturated with respectto silica; and the other is rich in Na₂O(6–14 weight percent), poor in K₂O(0.2 weight per cent), has an antipatheticrelation between Na₂O and CaO (together they usually total 15weight per cent) and is undersaturated with respect to silica.One inclusion shows these two compositions co-existing, apparentlyin an immiscible relationship. Other inclusions show compositionsintermediate between the Na-rich and K-rich types, and theyare interpreted as the result of reduced immiscibility. Thepresence of halides and calcium phosphate is considered to enhancethe immiscibility process. The parental composition, estimatedon a volatile-free basis is: SiO₂ 54.9, Al₂O₃, 27.4, CaO 7.3,Na₂O 3.4, K₂O 6.9, which corresponds to a lime-rich aluminoussyenite.     

ELECTRONIC ACCELERATION-SENSITIVE GEOPHONE FOR SEISMIC PROSPECTING***

Previously ignored characteristics of the seismic recording instrument are presently experienced as limitations as more sophisticated interpretive methods using wider frequency ranges are developed to extract stratigraphic information from seismic land data for hydrocarbon and mineral exploration. Most of these limitations arise from inadequate characteristics of the first element of the seismic instrument: the geophone. A geophone does not faithfully follow the motion of the earth for higher frequencies due to poor geophone-earth coupling. This filtering effect brings about time shifts that are dependent on the frequency and the soil type. A geophone can also produce spurious outputs, brought about by the motion of the suspended part of the geophone, with a magnitude comparable to that of the desired output. The suspension is made very compliant to obtain the required sensitivity. A compliant suspension, however, gives a large sag. The geophone can therefore only be used in one position, tolerating little tilt. A compliant suspension also widens the traveling range of the movable part. Minor sensitivity changes with travel are then noticeable as nonlinearity, since the surface wave is large with respect to the reflected wave. A compliant suspension is usually realized in the form of thin, spirally shaped spring-spiders. Such suspensions exhibit transverse or rotational resonances that are in or close to the seismic frequency band. Excited by ground roll, they can produce considerable undesirable output. The novel geophone we describe is a light-weight (17 g) acceleration-sensitive transducer which gives good ground coupling and partial correction for the increasing damping in the earth with increasing frequencies. It employs internal hybrid electronics for a magnetodynamic velocity-nulling feedback system. Velocity nulling makes the movable part of the geophone virtually rigid with respect to the housing. This makes the geophone characteristics independent of the suspension. The springs used are stiff in a transverse and rotational direction so that the suspension resonances are well outside the useful frequency band. This suspension also allows the geophone to be used in any orientation while being only sensitive to the vibration component along the main axis. The feedback system makes the sensitivity flat within 1 dB from 2 Hz to 500 Hz, with a phase tolerance smaller than 5°. The geophone is robust, has no moving internal wires, employs a current output [sensitivity 1 mA/(m s^?2)] and internal gain so that the signal-to-cable-noise ratio is improved. This type of output allows parallel connection without any interaction between the geophones.     

SOME RESULTS OF THE GEO-ELECTRICAL RESISTIVITY METHOD IN GROUND WATER INVESTIGATIONS IN THE NETHERLANDS*

In the year 1958 the Service for Water Management of the ?Rijkswaterstaat” started its program of geo-electrical resistivity prospecting in the western part of the Netherlands. The aim of this program was to obtain data on the salinity distribution of the ground water. The ground water regime in this part of the Netherlands is most intricate. This is due to the geological and geo-hydrological conditions and to the low elevation of the land. Many reclaimed areas are up to several metres below mean sea level. The resistivity data obtained are closely related to the salinity of the ground water. On the basis of bore hole data it was even possible to arrive at calibration curves for the salinity of the ground water in sand deposits. Under special conditions it was also possible to draw conclusions with respect to the presence of less permeable formations as e.g. clay layers. Some remarks are given on the practical performance and the interpretation of the measurements. A review is given of the work done until now. Some results are shown by means of maps of the salinity distribution of the ground water in the western and northern parts of the Netherlands. Two examples are described of the use of the data obtained during the survey in the province of Zuid-Holland. Another two examples are presented of detailed investigations for special purposes in relatively small areas.     

西藏纳木错地区约120kaBP以来的古植被、古气候与湖面变化

西藏纳木错湖相沉积的U系和^11C测年结果表明，湖泊沿岸的6级湖岸阶地及拔湖约48～139．2m的高位湖相沉积形成于约120ka BP以来的晚第四纪。本文根据该套湖相层的发育和其中的孢粉组合特征对纳木错地区约120ka BP以来的湖面变化与古植被、古气候变迁进行了探讨。结果表明，纳木错地区晚更新世以来经历了频繁的湖面波动、气候的冷暖与干湿变化以及森林—草原与草原植被的交替演化。其总体特征是：约115．9ka BP时，纳木错湖面最高。在116～78ka BP期间，该区气候温和凉爽或温和偏湿，植被以疏林草原与森林草原或森林的交替出现为特征，湖面经历了较大幅度的波动，但基本保持在拔湖140～88m之间。在78～53ka BP期间，该区气候干冷，植被以疏林草原为主，湖面大幅度下降，并在拔湖约36～48m之间波动。约53～32ka BP期间，气候转为温暖偏湿或温暖湿润，湖面波动于拔湖约15～28m之间，波动较为频繁。与阶地的发育相对应，该时期内包含了3次明显的暖期和湖面波动过程，区域植被主要以松、蒿、桦为主，为含一定量的冷杉的森林。其中36ka BP左右气候最温暖湿润，区域内可能出现针叶林或针阔混交林。约32～12ka BP期间，该区气候最为干冷，古植被以草原和疏林草原为主，湖面再次发生较大幅度的下降，最低可至拔湖约8m处，但通常维持在拔湖约12～17m之间。约11．8～4．2ka BP期间，气候整体较为暖湿，其中在约8．4～4．2ka BP期间气候最温暖湿润，该区可能发育针叶林或针阔混交林，湖面波动于拔湖2～9m之间，整体波动幅度较小，但波动最为频繁。区域气候对比发现，纳木错地区的冷、暖气候变化过程与整个青藏高原乃至北半球的气候变化基本是一致的，特别是阶地下切所反映的湖面退缩过程与北大西洋的Henrich冷事件之间具有很好的对应关系。