Matthew C. Smith

Petrology and geochemistry of basalts from the southern Juan de Fuca Ridge: Controls on the spatial and temporal evolution of mid-ocean ridge basalt

Three morphologically distinct regions within the neovolcanic zone of the Cleft segment of the southern Juan de Fuca Ridge were investigated and sampled in detail using the DSRV Alvin. Additional along-strike and off-axis samples were recovered by dredge. The southernmost region, the Southern Cleft site, is characterized by a 3-km-wide axial valley floored by ponded sheet flows and bisected by a 30- to 50-m-wide cleft. Farther north at the “Young Sheet Flow” site, the ridge axis is characterized by a distinct 500-m-wide inner graben that is largely covered by distinctly younger looking sheet flows. The northernmost of the three regions is defined by a linear series of discontinuous constructional pillow mounds that extend the trend of the Cleft segment well into the zone of overlap with the neighboring Vance segment. The pillowed lavas at the “Young Pillow Mound” site represent the most recent episode of volcanism along the Cleft segment. Strong correlations exist between degrees of fractionation, relative ages of lavas, and latitude; lavas are progressively younger looking and more mafic to the north. The compositional range of mid-ocean ridge basalts from the neovolcanic zone can generally be accounted for by 35–40% low-pressure fractional crystallization of relatively primitive, but not primary, depleted (N-type) melts. Scatter of the geochemical data about calculated liquid lines of descent is probably the result of mixing of magmas with slightly different parental compositions, generated from small-scale mantle heterogeneities. Furthermore, the chemical variability may be the result of mixing of very depleted and more enriched sources or melts that are present beneath the southern Juan de Fuca Ridge. The more primitive nature of the young pillow mound basalts and their slightly different chemical characteristics indicates they cannot be simply related to the older southernmost lavas by along-axis flow in a continuous axial magma chamber or conduit. Rather, the data suggest lavas were derived from discrete magma chambers or lenses, each in a different stage of evolution. The youngest events may be associated with a new influx of magma into the northern part of the segment and subsequent northward diking and propagation to form the new pillow mounds. The oldest stage (∼40% additional crystallization of the most mafic composition) is associated with focused hydrothermal activity and tectonic extension, whereas the youngest events are characterized by ridge inflation, diking, and dispersed hydrothermal activity. Geochemical and tectonomagmatic features observed along the Cleft segment are similar to those recently documented along the East Pacific Rise from 9°30′N–10°N suggesting the scales, processes, and stages of magmagenesis are similar along medium to fast spreading ridge segments.

Magmatic processes in developing oceanic crust revealed in a cumulate xenolith collected at the East Pacific Rise, 9°50′N

The petrology and geochemistry of a xenolith, a fragment of a melt-bearing cumulate, within a recently erupted mid-ocean ridge (MOR) lava flow provide information on petrogenetic processes occurring within the newly forming oceanic crust beneath the northern East Pacific Rise (NEPR). The xenolith reveals important petrologic information about MOR magmatic systems concerning (1) melt distribution in a crystal-dominated mush; (2) melt-crystal reactions within the mush; (3) the chemistry of melts that have contributed to the cumulate lithology; and (4) the chemistry of axial melts that enter the axial magma system. The xenolith was enclosed within a moderately primitive, normal mid-ocean ridge basalt (NMORB) erupted in 1991 within the neovolcanic zone of the NEPR, at approximately 9°50′N. The sample is a matrix-dominated, cumulate olivine anorthosite, composed of anorthite (An94-90) and bytownite (An89-70), intergranular olivine (Fo86±0.3), minor sulfide and spinel, and intergranular glass. Marginal corrosion of plagioclase, and possibly olivine, and internal remelting of plagioclase indicate syntexis. It is surmised that the pore volume was eviscerated several times with moderately primitive basaltic melts and reduced by intergranular crystallization of forsteritic olivine. The presence of anorthite as a cumulate phase in the xenolith and the observation of anorthite xenocrysts in NMORB lavas, and as a cumulate phase in ophiolite gabbros, indicate that Ca-rich melts that are not a part of the NMORB lineage play an important role in the construction of the oceanic crust.

Engaging today's Students in Earth Science 101

It is a sad fact, or perhaps a happy one, that many geoscientists in academia will find themselves in front of a classroom of 100–300 undergraduate nonscience majors, lecturing to them for three hours per week. Whether it is ‘Rocks for Jocks’ or ‘Waves for Babes,’ students often are under the impression that geoscience classes will be the least painful way to fulfill their science credit requirements. The sense of personal anonymity that can accompany large-enrollment classes often results in a different level of student engagement compared with smaller classes. Thus, if students are physically present at all, instructors often have only their much-divided attention. How can professors keep 300 students, even the ones in the back of the classroom who are barely visible, awake and engaged?

Microcomputer control of a process to regenerate saturated desiccants

Abstract This paper describes a method of data acquisition and process control for a desiccant regeneration system using an Apple IIe microcomputer. It also provides circuit diagrams for multiplexing thermocouples and activating a modutrol motor and a solenoid valve for process control. The main program for data acquisition and control is written in Fortran language and utilizes several subroutines written in Assembly language of the 6502 microprocessor for data collection and control. A flow chart for the algorithm along with some details of system control are provided. Using this technique, over 600 h of data have been collected in steps of 10 min.