Plagiogranites as late-stage immiscible liquids in ophiolite and mid-ocean ridge suites: An experimental study

Chemical studies of two ophiolite suites and of selected mid-oceanic rift (MOR) regions indicate the presence of certain magmatic compositions: basalt, Fe-enriched basalt, and sodium granite (plagiogranite). There is a notable lack of evidence for melts of intermediate composition (i.e. 50–60 wt.% SiO₂). To determine possible relationships between basic rocks (basalts and gabbros) and acidic rocks (plagiogranites) a primitive basalt was fractionated at low pressure, under anhydrous conditions, and at different oxygen fugacities near the iron-wustite buffer and slightly above the quartz-fayalite-magnetite buffer. Samples of this basalt were taken to slightly above liquidus temperatures and then cooled at rates ranging from 1 to 2°C/hr. A liquid line of descent characterized by an Fe enrichment was delineated by quenching these experiments from a final temperature in the range of 1200 to 1000°C and analyzing the residual liquid (glass). After 95% crystallization of olivine, plagioclase, calcium pyroxene, and ilmenite, the residual liquid was an Fe-enriched basalt. This Fe-enriched basalt became immiscible at a temperature of about 1010°C. The immiscible phases produced were a more Fe-enriched basaltic liquid and a granitic liquid. The granitic liquid is similar in composition to the naturally occurring plagiogranites found in small volumes in ophiolites and in certain MOR regions. It is therefore concluded that silicate liquid immiscibility could be the petrogenetic process responsible for producing plagiogranite in some MOR regions and in some ophiolites. On the other hand, plagiogranites in ophiolites and MOR rock suites having andesitic and dacitic composition rocks may have evolved under conditions more closely approximating equilibrium crystallization and/or they may have evolved at high water pressures. The available experimental data suggest that amphibole would crystallize early and yield SiO₂-enriched liquids at depths greater than 4.5 km for P_H2O's in the range 0.6–1.0 P_total.The major problem in interpreting any of the natural plagiogranites as products of silicate liquid immiscibility is the fact that neither the Fe-enriched conjugate liquid or its crystalline equivalent has been described in the ophiolite or MOR literature. The identification of this Fe-rich conjugate magma is essential in any rock suite if a completely convincing case for silicate liquid immiscibility is to be made.     

Plagioclase buoyancy in basaltic liquids as determined with a centrifuge furnace

Plagioclase buoyancy experiments have been carried out in a high-temperature centrifuge furnace using seventeen basaltic liquids and plagioclase crystals of three compositions: An₈₉, An⁷⁶ and An₅₅. The results show that the floating tendency of plagioclase in basaltic liquids is at least 0.03 g/cm³ greater than indicated by the calculations. If this correction factor is applied to calculations of plagioclase buoyancy in the Skaergaard Intrusion, it is found that the plagioclase crystals in the lower and middle zones were less dense than the coexisting liquids.Other phenomena relevant to crystal transport in basaltic liquids were observed in the centrifuge experiments. These included crystal flotation by rising bubbles, plagioclase sinking because of the formation of plagioclase-magnetite composite grains, graded bedding of olivine and magnetite, and more than 60% intercumulus basaltic liquid between settled olivine crystals.     

Plate Tectonics and mineralization in the Tethyan region

The association between mineralization and plate movements is becoming evident in the American Cordillera and the Pacific Island Arcs. This paper attempts to show how these ideas apply to the zone along which the Eurasian and Afro-Arabia plates collided during the late Mesozoic, between the Alps and the Himalayas. The study highlights some of the outstanding problems concerned with plate collision and the important question of the origin of ophiolite complexes. Some of the major mineralized districts in Greece, Turkey and Iran can be related to active plate edges, to island arc development and to related tectonism around pre-Mesozoic crystalline massifs. The study reinforces the view that the younger tectonic zones traversing the Middle East will become major mineral-producing areas in the future.     

Petrology and chemistry of recent lavas in the northern Marianas: Implications for the origin of island arc basalts

Petrologic and chemical data are presented for samples from five volcanically active islands in the northern Marianas group, an intra-oceanic island arc. The data include microprobe analyses of phenocryst and xenolith assemblages, whole rock major and trace element chemistry including REE, and Sr isotope determinations (⁸⁷Sr/⁸⁶Sr=0.7034±0.0001). Quartz-normative basalt and basaltic andesite are the most abundant lava types. These are mineralogically and chemically similar to the mafic products of other intra-oceanic islands arcs. It is suggested, however, that they are not typical of the ‘island arc tholeiitic’ series, having Fe enrichment trends and K/Rb, for example, more typical of calc-alkaline suits. Major and trace element characteristics, and the presence of cumulate xenoliths, indicate that extensive near surface (< 3 Kb) fractionation has occurred. Thus, even least fractionated basalts have low abundances of Mg, Ni and Cr, and high abundances of K and other large cation, imcompatible elements, relative to ocean ridge tholeiites. However, abundances of REE and small cation lithophile elements, such as Ti, Zr, Nb, and Hf are lower than typical ocean ridge tholeiites. The REE data and Sr isotope compositions suggest a purely mantle origin for the Marianas island arc basalts, with negligible input from subducted crustal material. Thus, subduction of oceanic lithosphere may not be a sufficient condition for initiation of island arc magmatism. Intersection of the Benioff zone with an asthenosphere under appropriate conditions may be requisite. Element ratios and abundances, combined with isotopic data, suggest that the source for the Marianas island arc basalts is more chondritic in some respects, and less depleted in large cations than the shallow (?) mantle source for ocean ridge tholeiites.     

Early dismissals in public schools on potential severe weather days

Survey responses from 76 public school districts in Mississippi and Alabama were analyzed along with county-level demographics, tornado-path data, and tornado warning data to investigate the emerging trend of early dismissals. No two districts had identical responses, but the concern of safety was consistently mentioned. The highest number of reported dismissals would have resulted in a loss of <2 % of total annual class time. The rates of dismissals reported by districts were compared with tornado warning days to identify potential over-dismissal, which was found in 51 % of districts. Rates of dismissal and over-dismissal were analyzed with variables from survey responses, tornado warning data, tornado-path data, and county census data to determine variables that influence differences among districts. Various stepwise models indicate that districts with higher poverty levels could be less likely to dismiss. Permutation tests and stepwise models show that early dismissal and over-dismissal are more likely when a district: (1) has a higher number of killer tornadoes in the last 5 years, (2) has a higher number of recent injury-causing tornadoes, (3) has a higher number of total killer tornadoes since 1950, (4) is in a smaller county, (5) has dismissal decisions made by a team, (6) is a county district rather than a city district, and (7) has a lower level of poverty. The inverse relationship with poverty suggests that schools in poorer areas may be holding students in school because they consider them to be safer there than at home, or it could be due to a lack of resources at the school. All of these are based on a district’s location and plans, things that can be determined before a potential dismissal. These indicators can be used to highlight districts most likely to dismiss and possibly reduce rates of over-dismissal by alerting districts to their predisposed tendencies.     

Yucatan after the wind: Human and environmental impact of hurricane Gilbert in the central and Eastern Yucatan Peninsula

Hurricane Gilbert made its landfall on the Yucatan peninsula on September 14, 1988, destroying valuable resort property in Cancun, Isla Mujeres, and Cozumel. Salt flats (salinas) in northern Quintana Roo and Yucatan were flooded by the storm surge and coastal marine ecosystem were devastated. Inland, the hurricane caused damages to houses, power lines, and farming land, but increased opportunities for garden hunting on destroyed fields. Twenty percent of tropical rain forest suffered losses of canopy and in the deciduous forests of north-central Yucatan noxiuous insects populations increased. Although damaging in its ultimate effects, hurricane Gilbert tested the resilience of Maya Indians and their readiness for post-disaster accommodation.