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6. 1-Atm Melting Study of a Leg 142 Basalt

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6. 1-Atm Melting Study of a Leg 142 Basalt Batiza, R., Storms, M.A., and Allan, J.F. (Eds.), 1995 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 142 6.1-ATM MELTING STUDY OF A LEG 142 BASALT: IMPLICATIONS FOR FRACTIONATION PROCESSES BENEATH THE EAST PACIFIC RISE AT 9°30 N1 James G. Brophy2 ABSTRACT A series of 1 -atm phase equilibrium experiments was conducted on a moderately evolved normal mid-ocean ridge basalt sampled during drilling of the East Pacific Rise (EPR) ridge crest at 9°30'N. The compositions of olivine-plagioclase-clinopy- roxene-saturated liquids are used to define the position of a 1-atm cotectic boundary curve on an olivine-clinopyroxene-quartz pseudo-ternary projection diagram. Comparison of this boundary curve with the projected positions of natural lavas erupted along this same portion of the EPR suggests a fractional crystallization origin at somewhat elevated pressures. The actual frac- tionation pressure is estimated at 1.9 ± 0.3 kb based on (1) a comparison of observed and predicted CaO-MgO variation slopes as a function of pressure, and (2) an estimation of the pressure at which the liquidus of the starting basalt composition is multi- ply saturated with olivine, Plagioclase, and clinopyroxene. The above pressure estimate places the location of fractional crystal- lization within the lower crust and well beneath the axial magma chamber that exists about 1.5 km beneath the seafloor. Given that little to no fractional crystallization occurs within the axial magma chamber, it is suggested that fractional crystallization occurs during upward melt migration as rising melts pass through and interact with solid and partially crystalline material in the roots of the magmatic system INTRODUCTION Fast-Spreading Recent geologic, petrologic, and geophysical studies of both fast- and slow-spreading mid-ocean ridges have led to a modern view of mid-ocean ridge magmatic plumbing systems that is quite different 1 A than the time-honored notion of a large (several kilometer thick), long lived, continuously replenished magma chamber beneath the en- tire length of the ridge axis (e.g., Bryan and Moore, 1977; Pallister and Hopson, 1981). Instead, it is now believed that the region beneath the ridge crest consists largely of a crystal-rich mush that grades 3 H: downward and outward through increasingly crystalline material into D. solid gabbro and/or mantle material (e.g., Sinton and Detrick, 1992; Q Fig. 1). Beneath fast-spreading ridge systems such as the East Pacific Rise (EPR), this mush zone is commonly, though not necessarily, capped by a thin, sill-like axial magma chamber that lies approxi- s -i mately 1.5 km beneath the seafloor at a pressure of about 1 kb. In con- trast, evidence for similar magma chambers beneath slow-spreading ridges such as the Mid-Atlantic Ridge (MAR) is absent and the entire magmatic plumbing system is believed to consist of mushy rocks of Slow-Spreading varying degrees of crystallinity. An important aspect of this new view of mid-ocean ridge magmatic plumbing systems is that it has forced a reevaluation of how and where fractional crystallization and other differentiation processes operate beneath spreading centers in gener- al, and slow- and fast-spreading ridge systems in particular. A comparison of erupted lava compositions from slow- and fast- 2 A spreading ridge systems has shown that fast-spreading ridges are characterized by lavas that are more evolved than those erupted along α. slow-spreading ridges (e.g., Sinton and Detrick, 1992). This differ- α> ence in erupted lava compositions must be a reflection of differences Q in the nature of the magmatic plumbing system at depth, and may or 4 H may not reflect differences in the depth of fractional crystallization as batiza, R., Storms, M.A., and Allan, J.F. (Eds.), 1995. Proc. ODP, Sci. Results, 142: College Station, TX (Ocean Drilling Program). Figure 1. Schematic cross-axis diagrams showing an interpretive model of Department of Geological Sciences, Indiana University, Bloomington, IN 47405, magmatic plumbing systems beneath fast- and slow-spreading mid-ocean U.S.A. ridge systems (after Sinton and Detrick, 1992). 41 J.G. BROPHY well as the nature of the actual fractionation processes themselves. Table 1. Analyses of Unit I Sample 142-864A-1M-3, 0-150 cm, starting Recent experimental phase equilibrium studies of selected basalt la- material. vas from slow-spreading ridge systems (e.g., Tormey et al., 1987; Grove et al., 1990, 1992) have suggested that fractionation occurs Unitl Unitl glassy rock crystalline rock over a pressure range of 3 to 6 kb, which places much, if not all of the fragment fragment fractionation within the upper mantle. Given the apparent lack of any Unit 1 142-864A-1M-3, 142-864A-1M-3, 1-atm fused glass long-lived magma chamber at such depths, it may be inferred that (avg)a 0-35 cma 100-150 cma 142-864A-lMb fractional crystallization occurs within individual bodies of magma SiO2 49.91 49.8 49.96 50.65 as they traverse the upper mantle on their way to the surface (e.g., TiO2 1.64 1.64 1.66 1.73 Grove et al., 1992). In contrast, the greater degrees of compositional A12O3 14.3 14.28 14.27 14.23 FeO* 10.55 10.54 10.62 10.1 evolution along fast-spreading ridge systems, coupled with the com- MnO 0.2 0.2 0.2 0.2 mon presence of actual magma chambers, have led many workers to MgO 7.3 7.21 7.35 7.55 suggest that fractionation processes within the low-pressure axial CaO 11.72 11.68 11.7 11.8 Na2O 2.55 2.59 2.53 2.83 magma chamber (P 1 kb) may still play an important role in deter- K2O 0.14 0.13 0.13 0.13 0.11 0.1 0.1 0.13 mining the final composition of erupted lavas. The purpose of this P2O5 98.42 98.17 98.52 99.35 study is to address the latter possibility. Total One way of addressing this important question is to determine the Note: FeO* = total iron. fractionation pressure of a typical suite of fractionation-related lavas a Shipboard X-ray fluorescence analysis. erupted along a fast-spreading mid-ocean ridge. One of the best stud- bMicroprobe analysis. ied portions of a fast-spreading ridge system is the EPR between 9° and 12°N (e.g., Langmuir et al., 1986; Thompson et al., 1989; Rey- nolds et al., 1992; Batiza and Niu, 1992). This study describes the re- All experiments were carried out at 1-atm in the University of Or- sults of low-pressure (1-atm) phase equilibrium experiments egon experimental petrology laboratory using techniques routinely conducted on a moderately evolved normal mid-ocean ridge basalt employed there. A small amount of the starting powder (approxi- (N-MORB) recovered during Ocean Drilling Program Leg 142 at the mately 0.15 to 0.2 grams) was mixed with poly vinyl alcohol to create fast-spreading EPR ridge crest at 9°30'N. The experiments quantify a viscous slurry which was then applied to a thin (0.004-inch diame- the 1-atm liquid line of descent as well as the phase appearance se- ter) platinum wire and allowed to dry. The sample was placed in the quence and phase proportions expected from low-pressure crystalli- isothermal zone of a Deltech DT31VT quenching furnace with a zation of N-MORB magma beneath this segment of the EPR. Batiza CO2-H2 gas atmosphere maintained at the quartz-fayalite-magnetite and Niu (1992) have presented a large data set containing well over a buffer. At the end of the run, the sample was quenched in a water hundred analyses of erupted lavas along this same segment of the bath. Oxygen fugacity was monitored using a zirconia-based solid EPR and have concluded that the compositional variation among la- electrolyte cell calibrated at the Fe-FeO buffer. Temperature was vas is best explained by fractional crystallization. The experimental monitored using a Pt-lORh thermocouple calibrated against the melt- data derived from this study have been compared with the observed ing point of Au. Experimental conditions and phase appearance tem- compositional characteristics of these fractionation-related lavas to peratures are reported in Table 2 and shown in Figure 2. Based upon assess the pressure at which fractional crystallization occurred. Based subsequent mass balance calculations, Fe loss to the platinum wire on these results, the role of fractionation processes within the low- holder was minimal. However, Na2O loss was found to be significant pressure axial magma chamber as well as the actual nature of the frac- in many of the lower temperature and/or longer duration runs (Table tionation processes themselves have been addressed. 2). All glass analyses used in subsequent data analysis have been cor- rected for Na2O loss. All run products with the exception of one (142-QFM-4, 1185°C) EXPERIMENTAL AND ANALYTICAL METHODS were mounted in epoxy, polished to a smooth finish, and then exam- ined by standard reflected light petrography, electron back-scatter Drilling at Hole 864A retrieved rock fragments from two N- imaging, and wave-length dispersive microprobe analysis. Micro- MORB units (Unit I and Unit II). Each unit is believed to represent a probe analyses of Plagioclase were performed on a four-spectrometer single eruptive event (Batiza et al, this volume). The starting compo- CAMECA SX-50 microprobe located at Indiana University while all sition used in this study is derived from several fragments of the Unit glass, olivine, and clinopyroxene analyses were performed on a sim- I basalt.
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