ALBERT

Notes: An optical configuration, utilizing a two wavelength source is presented. Its use as both a normal heterodyne-interferometric sensor yielding nanometer resolution, and as a subcarrier system with a much enhanced unambiguous range, but reduced resolution, are discussed. A major attribute of this system is that heterodyne signal processing has been achieved without the use of a frequency or phase shifter. Preliminary results indicate the resolution of the first configuration is comparable to that achievable in interferometric systems, while the latter shows a displacement resolution of greater than 1 mm, and vibration amplitude resolution of greater than 10 μm, over a 22-cm dynamic range.

Notes: An optical configuration for a current sensor utilizing the Faraday effect is described. A common path optical fiber heterodyne interferometer is used to greatly reduce noise produced by environmental effects. A miniature sensing element at the distal end of the fiber allows remote magnetic field or current measurements to be made. Solenoid generated magnetic fields over the range 0.7 to 10 μT, corresponding to solenoid currents from 10 to 150 mA, have been detected over a frequency range from 25 to 125 Hz with a linear response.

Notes: A fiber optic sensor probe, with two low-finesse Fabry–Perot cavities enabling the determination of displacement (in the range of a typical diaphragm pressure sensor) and temperature, is described. The sensors are connected in a reflective array network and their status is read using coherence tuned sensing based upon a multimode laser diode at 1.3 μm wavelength. Data are presented which show the resolution and linearity of the sensors for the measurement of displacement and temperature.

Notes: A low coherence interferometric system with large tracking range and self-initialization to be used for remote signal processing of fiber optic accelerometers is investigated. The tracking range of the low coherence system is 1.1 mm at low frequency. Phase resolution better than 1 mrad/(square root of)Hz at frequencies below 600 Hz is obtained, with a dynamic range of ≈90 dB. Data are presented which show that if the system were used to process the output from a compliant cylinder-type accelerometer, resolutions of ∼10−7 g/(square root of)Hz could be achieved.

Notes: A prototype single-mode fiber-based ultrahigh pressure remote sensor based upon a Fizeau cavity using a dual-wavelength coherence reading technique with built-in temperature compensation has been constructed capable of remote operation at distances up to 10 km. A separate fiber-based temperature sensor with similar cavity length was incorporated into the pressure probe to allow the pressure measurement to be corrected for the temperature dependence of the pressure sensor. A range to resolution of ∼1.67×104:1 and an overall measurement accuracy of ∼±0.69% over a pressure range of ∼1000 bar have been achieved. This system represents a practical approach for industrial applications, such as measuring the pressure of an oil well.

Notes: We describe an experimental demonstration of a novel technique for liquid refractometry. A channeled spectrum is produced from an optical beam generated by a diode laser operating below threshold by intercepting half of the beam with a liquid cell. The spectrum is analyzed using a grating and a linear CCD array and provides information on the refractive index of the liquid. The experimental results show that accuracies of better than 0.3% in the index may be obtained with the present method.

Notes: Granulite facies anorthosites on Holsenøy Island in the Bergen Arcs region of western Norway are transected by shear zones 0.1–100 m wide characterized by eclogite facies assemblages. Eclogite formation is related to influx of fluid along the shears at temperatures of c. 700d̀C and pressures in excess of 1.7 GPa. Combined carbon and nitrogen stable isotope, 40Ar/36Ar, trace-element and petrological data have been used to determine the nature and distribution of fluids across the anorthosite-eclogite transition.A metre-wide drilled section traverses the eclogitic centre of the shear into undeformed granulite facies garnet-clinopyroxene anorthosite. Clinozoisite occurs along grain boundaries and microcracks in undeformed anorthosite up to 1 m from the centre of the shear and clinozoisite increases in abundance as the edge of the shear zone is approached. The eclogite-granulite transition, marked by the appearance of sodic pyroxene and loss of albite, occurs within the most highly sheared section of the traverse. The jadeite-in reaction coincides with increased paragonite activity in mica. The separation between paragonite and clinozoisite reaction fronts can be semiquantitatively modelled assuming advective fluid flow perpendicular to the shear zone. The inner section of the traverse (0.25 m wide) is marked by retrogressive replacement of omphacite by plagioclase + paragonite accompanied by veins of quartz-phengite-plagioclase.C-N-Ar characteristics of fluid inclusions in garnet show that fluids associated with precursor granulite, eclogite and retrogressed eclogite are isotopically distinct. The granulite-eclogite transition coincides with a marked change in CO2 abundance and δ13C (〈36ppm, δ13C=-2% in the granulite; 〈180 ppm, δ13C=-10% in the eclogite). The distribution of Ar indicates mixing between influxed fluid (40Ar/36Ar 〉 25 times 103) and pre-existing Ar in the granulite (40Ar/36Ar 〈 8 times 103). δ15N values decrease from +6% in the anorthosite to +3% within the eclogite shear. The central zone of retrogressed eclogite post-dates shearing and is characterised by substantial enrichment of Si, K, Ba and Rb. Fluids are CO2-rich (δ13C ∼ -5%) with variable N2 and Ar abundances and isotopic compositions.Both Ar and H2O have penetrated the underformed granulite fabric more than 0.5m beyond the granulite/eclogite transition during eclogite formation. Argon isotopes show a mixing profile consistent with diffusion through an interconnecting H2O-rich fluid network. In contrast, a carbon-isotope front coincides with the deformation boundary layer, indicating that the underformed anorthosite was impervious to CO2-rich fluids. This is consistent with the high dihedral angle of carbonic fluids, and may be interpreted in terms of evolving fluid compositions within the shear zone.

Notes: Abstract Incipient charnockite formation within amphibolite facies gneisses is observed in South India and Sri Lanka both as isolated sheets, associated with brittle fracture, and as patches forming interconnected networks. For each mode of formation, closely spaced drilled samples across charnockite/gneiss boundaries have been obtained and δ13C and CO2 abundances determined from fluid inclusions by stepped-heating mass spectrometry.Isolated sheets of charnockite (c.50 mm wide) within biotite–garnet gneiss at Kalanjur (Kerala, South India) have developed on either side of a fracture zone. Phase equilibria indicate low-pressure charnockite formation at pressures of 3.4 ± 1.0 kbar and temperatures of about 700°C (for XH2O= 0.2). Fluid inclusions from the charnockite are characterized by δ13C values of −8% and from the gneiss, 2 m from the charnockite, by values of −15%. The large CO2 abundances and relatively heavy carbon-isotope signature of the charnockite can be traced into the gneiss over a distance of at least 280 mm from the centre of the charnockite, whereas the reaction front has moved only 30 mm. This suggests that fluid advection has driven the carbon-isotope front through the rock more rapidly than the reaction front. The carbon-front/reaction-front separation at Kalanjur is significantly larger than the value determined from a graphite-bearing incipient charnockite nearby, consistent with the predictions of one-dimensional advection models.Incipient charnockites from Kurunegala (Sri Lanka) have developed as a patchy network within hornblende–biotite gneiss. CO2 abundances rise to a peak near one limb of the charnockite, and isotopic values vary from δ13C of c.−5.5% in the gneiss to −9.5% in the charnockite. The shift to lighter values in the charnockite can be ascribed to the formation of a CO2-saturated partial melt in response to influx of an isotopically light carbonic fluid.Thus, incipient charnockites from the high-grade terranes of South India and Sri Lanka reflect a range of mechanisms. At shallower structural levels non-pervasive CO2 influxed along zones of brittle fracture, possibly associated with the intrusion of charnockitic dykes. At deeper levels, in situ melting occurred under conditions of ductile deformation, leading to the development of patchy charnockites.

Notes: The field relations from a quarry at Nuliyam, South India, illustrate dehydration of an amphibolite facies gneiss to granulite facies charnockite by CO2 influx, over a scale of 30 m. Both the calc-silicate source of the fluids and the full extent of their penetration into the gneiss are preserved in a continuous section. Fluid flow is by a hydraulic fracture mechanism, but is thought to be pervasive. The sharp reaction front predicted by the continuum mechanical theory for advective fluid transport is not observed. The front spreading is on too large a scale for either diffusive or dispersive control and is due to local kinetic disequilibrium between the fluid and rock, although the divariant nature of the reaction may also have a limited effect. The time-integrated fluid flux varies from the instantaneous porosity at the fluid front to 20 vol. % adjacent to the calc-silicate. Carbon isotope budgets suggest that decarbonation of the calc-silicate by a Rayleigh fractionation process provides a sufficient source for the CO2 influxing into the gneiss. Graphite abundances vary from 0.01 to 0.1% (by weight), it is principally derived by precipitation from the fluid and may be modelled from phase equilibria. Carbon isotope fronts coincide with the reaction front on the scale of sampling, although isotopic disequilibrium between graphite and inclusion-CO2 also implies local fluid-rock disequilibrium.

Notes: Two configurations for a fiber-optic torsional vibrometer are described, which can be classified as (i) differential and (ii) reference. Common features of both techniques are noncontact measurement, immunity to nontorsional vibration, large dynamic range coupled with passive, miniature, remote sensor heads. Measurements of rotation speed and torsional jitter were made with a total illuminating laser power of 130 μW (reference mode) and 1.5 mW (differential mode). Rotation speeds in the range of 270–19 000 RPM were measured.