Consequences of a crystal mush-dominated magma plumbing system: a mid-ocean ridge perspective

Crystal mush is rapidly emerging as a new paradigm for the evolution of igneous systems. Mid-ocean ridges provide a unique opportunity to study mush processes: geophysical data indicate that, even at the most magmatically robust fast-spreading ridges, the magma plumbing system typically comprises crystal mush. In this paper, we describe some of the consequences of crystal mush for the evolution of the mid-ocean ridge magmatic system. One of these is that melt migration by porous flow plays an important role, in addition to rapid, channelized flow. Facilitated by both buoyancy and (deformation-enhanced) compaction, porous flow leads to reactions between the mush and migrating melts. Reactions between melt and the surrounding crystal framework are also likely to occur upon emplacement of primitive melts into the mush. Furthermore, replenishment facilitates mixing between the replenishing melt and interstitial melts of the mush. Hence, crystal mushes facilitate reaction and mixing, which leads to significant homogenization, and which may account for the geochemical systematics of mid-ocean ridge basalt (MORB). A second consequence is cryptic fractionation. At mid-ocean ridges, a plagioclase framework may already have formed when clinopyroxene saturates. As a result, clinopyroxene phenocrysts are rare, despite the fact that the vast majority of MORB records clinopyroxene fractionation. Hence, melts extracted from crystal mush may show a cryptic fractionation signature. Another consequence of a mush-dominated plumbing system is that channelized flow of melts through the crystal mush leads to the occurrence of vertical magmatic fabrics in oceanic gabbros, as well as the entrainment of diverse populations of phenocrysts. Overall, we conclude that the occurrence of crystal mush has a number of fundamental implications for the behaviour and evolution of magmatic systems, and that mid-ocean ridges can serve as a useful template for trans-crustal mush columns elsewhere. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.

Estimating range to a vocalizing fin whale using the timing and amplitude of multipath arrivals

A semi-automated method is described to range to vocalizing fin whales using the timing and amplitude of multipath arrivals measured on seafloor receivers. Calls are detected and multipath arrivals identified with a matched filter. Multipath times and relative amplitudes are predicted as a function of range by ray tracing. Because the direct and first water-column multiple arrivals are not always observed, different hypotheses for the observed arrival paths must be considered. For two arrivals, an amplitude threshold is used to determine if the first arrival is the direct path and if not, the call is disregarded as distant. When three or more arrivals are detected, three hypotheses for the paths of arrivals are considered; the solution is the hypothesis and range that minimizes the timing and optionally, amplitude ratio or absolute amplitude residual. The method is tested with data from two ocean bottom seismometers, one on the Juan de Fuca Ridge and the other in the Cascadia Basin. Solutions obtained by minimizing a combined residual from timing and an empirical absolute amplitude model extracted from the data yield reliable ranges up to 5 km at both sites, and are sufficient to estimate call density using point transect distance sampling.

Linkages between mineralogy, fluid chemistry, and microbial communities within hydrothermal chimneys from the Endeavour Segment, Juan de Fuca Ridge

Rock and fluid samples were collected from three hydrothermal chimneys at the Endeavour Segment, Juan de Fuca Ridge to evaluate linkages among mineralogy, fluid chemistry, and microbial community composition within the chimneys. Mössbauer, midinfrared thermal emission, and visible-near infrared spectroscopies were utilized for the first time to characterize vent mineralogy, in addition to thin-section petrography, X-ray diffraction, and elemental analyses. A 282°C venting chimney from the Bastille edifice was composed primarily of sulfide minerals such as chalcopyrite, marcasite, and sphalerite. In contrast, samples from a 300°C venting chimney from the Dante edifice and a 321°C venting chimney from the Hot Harold edifice contained a high abundance of the sulfate mineral anhydrite. Geochemical modeling of mixed vent fluids suggested the oxic-anoxic transition zone was above 100°C at all three vents, and that the thermodynamic energy available for autotrophic microbial redox reactions favored aerobic sulfide and methane oxidation. As predicted, microbes within the Dante and Hot Harold chimneys were most closely related to mesophilic and thermophilic aerobes of the Betaproteobacteria and Gammaproteobacteria and sulfide-oxidizing autotrophic Epsilonproteobacteria. However, most of the microbes within the Bastille chimney were most closely related to mesophilic and thermophilic anaerobes of the Deltaproteobacteria, especially sulfate reducers, and anaerobic hyperthermophilic archaea. The predominance of anaerobes in the Bastille chimney indicated that other environmental factors promote anoxic conditions. Possibilities include the maturity or fluid flow characteristics of the chimney, abiotic Fe²⁺ and S^2- oxidation in the vent fluids, or O₂ depletion by aerobic respiration on the chimney outer wall.

Source levels of fin whale 20 Hz pulses measured in the Northeast Pacific Ocean

Source levels of fin whale calls can be used to determine range to recorded vocalizations and to model maximum communication range between animals. In this study, source levels of fin whale calls were estimated using data collected on a network of eight ocean bottom seismometers in the Northeast Pacific Ocean. The acoustic pressure levels measured at the instruments were adjusted for the propagation path between the calling whales and the instruments using the call location and estimating losses along the acoustic travel path. A total of 1241 calls were used to estimate an average source level of 189 ± 5.8 dB re 1μPa at 1 m. This variability is largely attributed to uncertainties in the horizontal and vertical position of the fin whale at the time of each call and the effect of these uncertainties on subsequent calculations. Variability may also arise from station to station differences within the network. For call sequences produced by a single vocalizing whale, no consistent increase or decrease in source level was observed over the duration of a dive. Calls within these sequences that immediately followed gaps of 27 s or longer were classified as backbeat calls and were consistently lower in both frequency and amplitude.