The deformation and compaction of partial molten zones

Summary. The segregation of melt from a partially molten source region requires a corresponding deformation of the unmelted residue ('matrix'). The role of matrix deformation during melt segregation is examined using simple one-dimensional models, for which the deformation consists only of bulk compression or 'compaction'. In model I, a volume fraction φ₀ of ascending mantle material undergoes pressure-release melting at a depth z = 0 (localized melting). Compaction of the matrix occurs in a boundary layer whose thickness (reduced compaction length δ_R) is proportional to the square root of the matrix viscosity. In the Earth's mantle, δ_R∼ 10–100 m, indicating that compaction cannot be important over large distances. Model II examines the case in which melting occurs over a depth range of order h (distributed melting). In the limit h ≪δ_R, the solution is the same as for the case of localized melting, except in a 'melting layer' of thickness ∼ h near z = 0. In the more realistic limit h ≫δ_R, compaction makes a negligible contribution to the balance of forces associated with melt segregation. This result is also valid for the more general case of two-dimensional flow. Compaction is therefore likely to be of negligible importance in the Earth's mantle, with the consequence that melt segregation can be accurately described by Darcy's law.     

Diapirism in the earth's mantle: Experiments on the motion of a hot sphere in a fluid with temperature-dependent viscosity

The ascent of magma diapirs through the earth's mantle is modelled experimentally by the motion of a hot metal sphere through a fluid whose viscosity varies strongly with temperature. The dimensionless drag on the sphere (drag number D) and the heat transfer from it (Nusselt number Nu) are found as functions of the dimensionless velocity of the sphere (Peclet number Pe) and the viscosity contrast μ_∞/μ₀ = 10^γ, where μ_∞ and μ₀ are the viscosities of the fluid far from the sphere and at its surface. The drag D = D(Pe, γ) has two limits. For large Pe and small γ (“Stokes” limit), the drag approaches the Stokes' Law result. For small Pe and large γ (“lubrication” limit), the drag is orders of magnitude less than that predicted by Stokes' Law. Nu is a function of Pe alone. For reasonable values of the diapir radius and the viscosity contrast, the dimensionless scale height Pe/3 Nu may exceed a critical value, resulting in progressive melting during ascent. This suggests that diapirs may ascend great distances through the mantle while remaining largely molten. Lamont-Doherty Geological Observatory Contribution No. 3414.     

Accuracy characteristics of the electronics of waverider buoys used in the ARSLOE

During the Atlantic Remote Sensing Land Ocean Experiment (ARSLOE), October-November, 1980, a large number of meteorological and oceanographic instruments, especially wave-measurement instruments, were deployed in the ocean near the Army Corps of Engineers Pier, Duck, NC. About nine Waverider buoys were deployed in an approximately rectangular pattern, about 30 km wide by 40 km seaward. The Waveriders were an accepted reference for study of other wave-measurement instrument systems. Almost all of the Waverider buoys were calibrated on the rotating-arm facility of the NOAA, Engineering Support Office. Pre- and post-deployment calibrations formed the basis for tables and equations for correction of the Waverider measurements. The calibrations discussed here established that the Waveriders were measuring lower than specification, averaging about 5 percent low (10 percent for variance spectra coefficients.) Corrections for effects of fluctuations in water temperature on Waverider sensitivity were required and are provided. The manufacturer's modification to prevent future drift in sensitivity is also described.     

On the interpretation of frequency response functions for oceanic gravity and bathymetry

Summary. In 1972–78, the late N. D. Watkins and others carried out ajoint field programme of geological mapping in the Mio-Pliocene flood basalts of Iceland, including sampling for K-Ar dating and palaeomagnetic research. The major part of the palaeomagnetic sampling is represented by 2462 lavas in five long composite sections through the lava pile. This paper deals with various statistical properties of this data set.
It is concluded that geomagnetic reversals occur more frequently than is assumed in the current ocean-floor polarity time-scale. There is no evidence for significant asymmetries between normal and reverse polarity states of the field, neither as regards chron lengths, secular variation, or virtual dipole moment magnitude. Intensities of remanence in these lavas are shown to be well approximated in terms of a hyperbolic distribution. The latitude distribution of virtual magnetic poles can be fitted with a Bingham function having k' ∼ 4.5, and low-latitude poles do not occur preferentially in any particular longitude interval.