Mid-ocean ridge basalts (MORB) are a consequence of pressure-release melting beneath ocean ridges, and contain much information concerning melt formation, melt migration and heterogeneity within the upper mantle. MORB major element chemical systematics can be divided into global and local aspects, once they have been corrected for low pressure fractionation and interlaboratory biases. Regional average compositions for ridges unaffected by hot spots ("normal" ridges) can be used to define the global correlations among normalized Na20, FeO, TiO2 and Si02 contents, CaO/Al 203 ratios, axial depth and crustal thickness. Back-arc basins show similar correlations, but are offset to lower FeO and TiO2 contents. Some hot spots, such as the Azores and Galapagos, disrupt the systematics of nearby ridges and have the opposite relationships between FeO, Na 20 and depth over distances of 1000 km. Local variations in basalt chemistry from slowand fast-spreading ridges are distinct from one another. On slow-spreading ridges, correlations among the elements cross the global vector of variability at a high angle. On the fast-spreading East Pacific Rise (EPR), correlations among the elements are distinct from both global and slow-spreading compositional vectors, and involve two components of variation. Spreading rate does not control the global correlations, but influences the standard deviations of axial depth, crustal thickness, and MgO contents of basalts. Global correlations are not found in very incompatible trace elements, even for samples far from hot spots. Moderately compatible trace elements for normal ridges, however, correlate with the major elements. Trace element systematics are significantly different for the EPR and the mid-Atlantic Ridge (MAR). Normal portions of the MAR are very depleted in REE, with little variability; hot spots cause large long wavelength variations in REE abundances. Normal EPR basalts are significantly more enriched than MAR basalts from normal ridges, and still more enriched basalts can erupt sporadically along the entire length of the EPR. This leads to very different histograms of distribution for the data sets as a whole, and a very different distribution of chemistry along strike for the two ridges. Despite these differences, the mean Ce/Sm ratios from the two ridges are identical. Existing methods for calculating the major element compositions of mantle melts [Klein and Langmuir, 1987; McKenzie and Bickle, 1988; Niu and Batiza, 1991] are critically examined. New quantitative methods for mantle melting and high pressure fractionation are developed to evaluate the chemical consequences of melting and fractionation processes and mantle heterogeneity. The new methods rely on new equations for partition coefficients for the major elements between mantle minerals and melts. The melting calculations can be used to investigate the chemical compositions produced by small extents of melting or high pressures of melting that cannot yet be determined experimentally. Application of the new models to the observations described above leads to two major conclusions: (1) The global correlations for normal ridges are caused by variations in mantle temperature, as suggested by Klein and Langmuir [1987] and not by mantle heterogeneity. (2) Local variations are caused by melting processes, but are not yet quantitatively accounted for. On slower spreading ridges, local variations are controlled by the melting regime in the mantle. On the EPR, local variations are predominantly controlled by ubiquitous, small scale heterogeneites. Volatile content may be an important and as yet undetermined factor in affecting the observed variations in major elements. We propose a hypothesis, similar to one proposed by Allegre et al [1984] for isotopic data, to explain the differences between the Atlantic and Pacific local trends, and the trace element systematics of the two ocean basins, as consequences of spreading rate and a different distribution of enriched components from hot spots in the two ocean basins. In the Atlantic, the hot spot influence is in discrete areas, and produces clear depth and chemical anomalies. Ridge segments far from hot spots do not contain enriched

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Petrological systematics of mid-ocean ridge basalts: Constraints on melt generation beneath ocean ridges

One-atmosphere melting experiments are used to assess the role of clinopyroxene in producing the compositional variations observed in mid-ocean-ridge basalts (MORBs) from the North Atlantic. Analog models of natural glasses and associated phenocrysts show that several possible parental magmas may undergo low pressure fractional crystallization involving olivine and spinel, followed by plagioclase, and then by augite. The phenocryst phase assemblages in natural deep-sea basalts are closely correlated with the major element compositions of their associated quenched glasses, and the projections of these glasses on the Oliv-Cpx-Qtz pseudoternary correspond to the 1-atmosphere phase boundaries and reaction points defined by laboratory experiments. Comparison of natural phenocryst's with experimental phases indicates that the augites preserved in moderately fractionated MORB from the FAMOUS area may have formed at or near the ocean floor and need not be relics of high pressure processes.

Fractionation of pyroxene-phyric MORB at low pressure: An experimental study

Fractionation of Mid-Ocean Ridge Basalt (MORB)

Chlorine and major elements in >400 mid-ocean ridge basalt (MORB) glasses from 20 suites are used to examine how spreading rate, magma flux, tectonics, and hydrothermal activity influences assimilation and crystallization beneath MOR. Crystallization depths were determined for fractionated glasses using published models that describe liquids saturated with olivine+ clinopyroxene+ plagioclase. Calculated depths are minima for the onset of crystallization and maxima for the completion of crystallization for each liquid. Glasses from fast spreading ridges and from medium and slow spreading ridges with low Na8.0 define low-pressure liquid lines of descent (LLDs). Higher crystallization pressures and greater variability are obtained from slow and medium spreading ridges with high Na8.0. Crystallization pressures do not vary regularly along individual segments. The correlation between average crystallization pressure and Na8.0 suggests that magma supply (and perhaps mantle temperature) plays an important role in determining magma ascent and crystallization depths. Cl/K in glasses is an indicator of assimilation of hydrothermally influenced material. Suites of MORB with high crystallization pressures have low Cl/K: from below detection limits (≈0.01) in normal MORB (NMORB) to about 0.05–0.08 in enriched MORB (EMORB). We propose that this trend defines the mantle limit of Cl/K and that higher values are related to assimilation. Cl/K is highest (up to 1.1) and is negatively correlated with MgO along the superfast spreading southern East Pacific Rise (EPR) and the propagating, low-Na8.0 Galapagos Spreading Center (GSC) at 85°W. Cl/K is also above mantle values, but is not well correlated with MgO, in MORB from fast and medium spreading ridges and from slow spreading ridges that have low Na8.0 and low crystallization pressures, e.g., Reykjanes Ridge. Cl/K is not correlated with crystallization pressure for individual samples within any suite. We propose that the spreading rate and the extent of melting act together to determine the total magma flux to a ridge, which influences crustal temperatures and determines how magmas ascend. At the highest magma fluxes, Cl/K is correlated with MgO, consistent with continuous assimilation of material that has a uniform Cl content: crystallization and assimilation are steady state processes that occur in crustal magma bodies that are larger than the scale of crustal heterogeneity in Cl. The lowest magma fluxes occur on slow spreading ridges that have formed by small extents of melting. In this cooler environment, magma crystallizes at the base of the strong lithosphere, below the level of alteration, and then ascends rapidly with little crystallization at shallow levels, so Cl contamination is avoided. On slow and medium spreading ridges with high extents of melting, magma flux is intermediate and forms small or transient crustal magma bodies: whether a magma batch becomes enriched in Cl depends upon the particular crust that it encounters.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JB00791

Influence of spreading rate and magma supply on crystallization and assimilation beneath mid‐ocean ridges: Evidence from chlorine and major element chemistry of mid‐ocean ridge basalts

The crust of actively opening back arc basins has close chemical and mineralogic similarities to normal mid-ocean ridge basalts (N-MORB) but some subtle, but distinctive, differences exist. These are largely due to differences in volatiles, Fe oxidation ratio, abundances of large ion lithophile elements (LIL), and some high field strength (HFS) elements. Isotope data for Sr, Pb, Nd, He, and O are variable but broadly are MORB-like. We present data contrasting Mariana Trough (MTB) and Lau Basin basalts (LBB) showing how they differ in certain trace element and isotopic signatures and compare both to N-MORB. Relative to N-MORB and LBB, the MTB are enriched in alkalis, alkaline earths, light rare earth elements (REE), H2O, and Fe'''/Fe'', even for “primitive” samples, while having MORB-type He (3/4 = 7.8 to 8.4 × RA), Sr (87/86 mean = 0.70293) and Nd (e = +10.2). The Lau Basin is zoned with a central area having N-MORB characteristics for H2O, LIL, REE, Nd (e = +8.1 to 9.0), and O but a depletion in HFS elements. Sr is more radiogenic (mean 87/86 = 0.70347) and 3He/4He (3/4 = 7.2 to 11.0 × RA) is high relative to N-MORB. The Lau Basin has a zone, bordering the adjacent island arcs, which is like MTB. In both back arc basins, relatively primitive (unfractionated) basalts with Mg/100(Mg + Fe″) > 62 are found which resemble postulated primitive N-MORB. Calculated parental magmas (obtained by adding xenocrystic olivine) plot near or on 3 and 4 phase boundaries at 1–1.5 GPa for the OL-DI-SI system. Low P fractional crystallization and magma mixing explain most of the observed rock types. Possible explanations for differences between MTB and LBB include differences in the extent of previous melting episodes of their mantle sources and effects of source metasomatism through dehydration and partial melting of subducted lithosphere. The long and complex history of Mariana Trough mantle sources led to a metasomatized mantle which gives rise to the distinctive MTB. In contrast, the Lau Basin is dominated by basalts derived from a source depleted in volatiles, LIL, and HFS elements. A previous melting history is implied. Mixing with a mantle component enriched in 87Sr and 3He may be due to mantle counterflow driven by the subduction process.

Petrology of Mariana Trough and Lau Basin basalts

Tholeiite basalts from 60° N to 65° N on the Mid-Atlantic Ridge were melted and recrystallized at atmospheric pressure in a CO2-H2 gas mixture. Seven basalts are from the Langjokull-Thingvellir volcanic zone and the Reykjanes Peninsula of Iceland and nine are from the Reykjanes Ridge. The crystallization sequence in both Iceland and Reykjanes Ridge basalts with (Total Fe as FeO)/(Total Fe as FeO+ MgO) [F/F + M] less than 0.6 is olivine, plagioclase, clinopyroxene. Chromian spinel crystallizes before plagioclase in one Iceland and one Reykjanes Ridge basalt with F/F+M less than 0.57. Chemical differences of the two groups of basalts (lower SiO2 and higher alkalis in Iceland basalts) can not simply be a result of low pressure fractional crystallization. Liquidus temperatures of the seven Iceland basalts decreases from 1,230° C to 1,170° C as the F/F+M of the rock increases from 0.52 to 0.70. The liquidus temperatures of the Reykjanes Ridge basalts are about 10° C lower than those of the Iceland basalts for the same F/F+M value. The profile of measured liquidus temperatures from 65° N on Iceland to 60° N on the Reykjanes Ridge has a minimum value at 63.2° N on the Reykjanes Ridge just south of Iceland. Model calculations of the pressure of phenocryst crystallization indicate that olivine and plagioclase in Langjokull basalts could have equilibrated between 2.0 and 6.2 kb (200 to 620 MPa). Phenocryst assemblages in Reykjanes Ridge basalts at 60° N could have crystallized together at greater than 2 kb (200 MPa) and probably less than 8 kb (800 MPa). A minimum in the equilibrium pressure of phenocryst crystallization occurs between 62.9° and 64° N and coincides with the minimum in the experimentally determined liquidus temperatures. The more extensive fractionation at low pressure in this area could be related to the shift of the Mid-Atlantic Ridge axis along the leaky transform fault from the Reykjanes Ridge to the Thingvellir volcanic zone.

M. R. Fisk

Papers

An experimental investigation of Iceland and Reykjanes Ridge tholeiites: I. Phase relations