Partial Melt Distributions from Inversion of Rare Earth Element Concentrations

Inverse theory is used to calculate the melt distribution requiredto produce the rare earth element concentrations in a wide varietyof terrestrial and extra-terrestrial magmas. The concentrationsof the major and minor elements in the source regions are assumedto be the same as those for the bulk Earth, and the peridotitemineralogy calculated from the mineral compositions by leastsquares. Rare earth element partition coefficients are thenused for inversion, assuming the melt generation is by fractionalmelting. The mean composition of the magmas is taken to be anestimate of the average composition of the melt. For n-typcand e-type MORB the results agree well with the adiabatic decompressioncalculations if the potential temperatures are 1300 and 1500?Crespectively. The major and minor element compositions calculatedfrom the melt distribution obtained from the inversion alsoagree well with those observed. The observations are consistentwith a melt fraction that increases monotonically towards thesurface, starting at 80 km and producing 9 km of melt in thecase of n-type MORB, and at 120 km to produce 23 km in thecase of e-type MORB. The inversion calculations show that the melt fractions producedbeneath an intact plate by a plume like that beneath Hawaiiare smaller, and are also in agreement with the adiabatic calculationsif the potential temperature of the plume is 1500?C. Much ofthe melt is produced in the depth and temperature range of thetransition from garnet to spinel peridotite, in agreement withlaboratory experiments and with the full convective models ofthe Hawaiian plume. The inversion calculations show that thesource region for Hawaiian tholeiites changes with time fromprimitive to depleted mantle. This behaviour is likely to resultfrom percolation, and the processes involved can be understoodwith the help of a simple analytic model. The last, post-erosional,magmas produced on Oahu come from a source that has been uniformlyenriched in all rare earth elements by a factor of about two.Magmas associated with island arcs come from two sources. Oneresembles that of n-type MORB, and probably is produced by adiabaticupwelling. The other generates calc-alkaline basalt stronglyenriched in light rare earth elements, but with a smaller constantenrichment between Gd and Lu. This composition is consistentwith the extraction of a melt fraction of 1% from a source containing9% of amphibole. Such a source region can also account for thelow values of Ti and Nb, and perhaps also of Ta, observed inisland arc magmas. Basaltic andesites and andesites from islandarcs show the same amphibole signature, and can be producedfrom the calc-alkaline basalts by fractional crystallizationif amphibole separates with olivine and orthopyroxene. The percolationof a small melt fraction through a mantle wedge that containsconsiderable amounts of amphibole can only transport very incompatibleelements, such as He, U, Th, and Rb, towards the Earth's surface.Sr and Nd are likely to be too compatible to move against thematrix flow, but Pb may do so locally. These results have importantimplications for the isotopic systematics of the upper mantle. The melt distributions obtained from ophiolites are like thosefor island arc tholeiites, though a potential temperature of1400 ?C fits the results better than does one of 1300?C. Archaeantholeiites and basaltic komatiites give melt distributions similarto that of e-type MORB from Iceland, and can be produced byadiabatic decompression if the mantle potential temperatureis 1500cC, with tholeiites having lost more material by fractionalcrystallization. The melt distribution obtained from komatiitesrequires the melt fraction to reach 60% at the surface. Thoughthe calculated compositions agree with those observed, decompressionis unable to generate such large melt fractions. Inversion shows that plateau basalts can be produced from theupper mantle beneath the plates by adiabatic upwelling beneatha mechanical boundary layer 60 km thick. Many of the variedalkali-rich continental magmas are generated by melting an enrichedsource in the stability field of garnet peridotite. The averageenrichment required, by a factor of between two and five, canbe produced by the addition of a small melt fraction. Carbonatitesshow no evidence of amphibole involvement at any stage, a resultthat is consistent with their formation by liquid immiscibility.Inversion of the rare earth element concentrations in shalesgives a melt distribution similar to that from calc-alkalinebasalts from island arcs, with a strong amphibole signature.Generation of the continental crust by separation of calc-alkalinemagma from 40% of the mantle can account for the differencebetween primitive and depleted mantle. Low-K highland basalts from the Moon can be produced directlyfrom the average primitive lunar mantle if the melt fractioninvolved is ?0-5%, and if they were generated in the stabilityfield of plagioclase and spinel peridotite. Intermediate-K highlandbasalts come from a source that has been enriched by a factorof about two, and show no evidence of amphibole involvement.The rare earth concentrations in mare basalts require melt fractionsof up to 7% in the spinel peridotite stability field, and canbe generated by adiabatic upwelling of mantle whose potentialtemperature is 1300?C beneath a mechanical boundary layer thatis 150 km thick. Because lunar gravity is only one-sixth ofthat of the Earth, the thickness of the melting zone and thevolume of melt produced are six times greater for the Moon thanfor the Earth for the same value of Tp. Both low-Ti and high-Timare basalts may have lost as much as 70 and 85% respectivelyof their original material through crystal fractionation. Itis, however, difficult to understand how such an origin canaccount for the high magnesium concentrations. Basaltic achondritesinvolve melt fractions of 10-15%, generated in the spinel orplagioclase stability field.