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[1] We present new experimental partitioning data for a range of petrogenetically important elements at pressures up to 3.4 GPa. The experiments are designed to mimic low degrees of anhydrous melting beneath mid-ocean ridges. The available data indicate that the partition coefficients are pressure, temperature, and composition dependent. Therefore partitioning behavior over the appropriate range of pressure, temperature, and composition must be quantified, in order to model continuous extraction of melt during the adiabatic rise of mantle material. For this purpose, we have parameterized the partitioning behavior of the REE, Hf, Zr, U, and Th based on a simple thermodynamic model. Although these parameterizations cannot be used for retrieving thermodynamic constants yet, they do yield accurate descriptions of the partitioning behavior that are useful for modeling decompression melting. Our parameterizations show that the partitioning of trace elements is strongly dependent on the Ca and Alcontent of the clinopyroxene (cpx) and REE are always incompatible in cpx on the peridotite solidus at pressures up to 3.4 GPa. For garnet the data indicate that the heavy REE partition coefficients decrease with increasing pressure. Our data also indicates that Pb is more incompatible than Ce in clinopyroxene; Ce and Pb have similar partition coefficients in garnet. Therefore the presence of a residual phase with high Pb partition coefficients is required to produce the near-constant Ce/Pb ratios in MORB and OIB. Sulfides are the most likely phase to buffer the Pb content in the melt. Except at small porosities (<0.3%), clinopyroxene on the peridotite solidus is unable to fractionate U from Th significantly (15% 230 Th-excess), whereas garnet can fractionate U from Th effectively at porosities up to 1%. Therefore if the 230 Th-excesses in midocean ridge basalts are melting phenomena, then melting with garnet residual is required in order to be compatible with physical observations on porosities and upwelling rate at mid-ocean ridges. New model calculations that include the compositional dependent partitioning of the trace elements show that the predicted physical characteristics (depth and extent of melting, upwelling rate, porosity) of the MORB melting regime are similar for the Lu/Hf, Sm/Nd, and U-Th systems.

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[1] New piston-cylinder experiments constrain the compositions of a series of synthetic picritic liquids that are in equilibrium with forsteritic olivine, orthopyroxene, clinopyroxene, and garnet or spinel from 2.4 to 3.4 Gpa. Mass balance calculations show that two of the liquid + crystal assemblages are consistent with those expected by 4.4 and 1.6 wt % anhydrous partial melting of a peridotite generally similar in composition to estimates of depleted upper mantle (DPUM). The liquids in these runs contain 2.0 wt % Na 2 O. Lherzolitic liquids with higher concentrations of Na 2 O have negative mass balance coefficients, regardless of Mg 0 , implying that there is a limit of $2 wt % Na 2 O in anhydrous partial melts of peridotites with $0.3 wt % bulk Na 2 O in the upper garnet-lherzolite stability field. Examination of liquidus equilibria in the NCMAS system demonstrates that coupling of Na 2 O and SiO 2 concentrations in liquids saturated with lherzolite assemblages permits high-Na 2 O, high-SiO 2 melts at pressures $1.0 GPa, whereas only high-Na 2 O, low-SiO 2 melts are possible in the garnet-lherzolite stability field. Because the bulk partition coefficient for Na 2 O increases with pressure, the concentration of Na 2 O in batch melts of the same percent will necessarily decrease with pressure. Calculations of low-degree anhydrous melting of DPUM with a revised melting model, BATCH, indicate that the Na 2 O concentration decreases with increasing pressure more rapidly than in previous models. Thus, for example, 1% melting of lherzolite with Na 2 O bulk concentration typical of estimated terrestrial mantle ($0.3 wt %), can produce a liquid with $6 wt % Na 2 O at 1.0 GPa but only $2% Na 2 O at 3.0 Gpa. In calculated melts of the DPUM and PUM compositions at 1.0 Gpa, the TiO 2 concentration decreases between 10 and 1% melting in response to an increase in D TiO2 cpx , consistent reported experimental observations. The increase in D TiO2 cpx appears to be a response to increasing alkalis in the melt. However, TiO 2 concentration does not decrease with lower degrees of melting of DPUM and PUM at 3.0 GPa in part because the increase in alkali concentration with decreasing melting percentage is smaller and also because the effect of increasing alkalis in the liquid superposes on lower values of D TiO2 cpx at higher pressures. These lower values are the result of a decrease in the wollastonite component in clinopyroxene coexisting with orthopyroxene with increasing pressure. Calculations of lherzolite melting also yield coefficients for the solidus reactions that are generally consistent with earlier studies: olivine is in reaction with the melt in the lower pressure portion of the spinel field (i.e., it crystallizes during melting) but changes sign at pressures approaching the spinel to garnet transition and remains a cotectic phase in the garnet field, whereas orthopyroxene becomes a reaction phase at pressures approaching the spinel to garnet transition and remains a reaction phase well into the garnet stability fi...

We have compiled petrological and geochemical data from 71 ophiolite suites and greenstone belts, which range in age from 15 to 3760 Ma. We have selected those rocks whose compositions indicate that they are either normal mid‐ocean ridge basalts (MORBs) or hotspot‐type MORBs. Then we used the data base to calculate the most primitive liquidus temperature for each rock suite. The results show that the liquidus temperature of the Phanerozoic ophiolites ranges from a low of 1212°C to a high of 1417°C. Using these data and two exponential curves bracketing the maximum and minimum temperatures versus time, we infer that the Phanerozoic suites had a mean liquidus temperature of 1272±7°C and a mean temperature range of 1218° to 1425°C. The liquidus temperatures of Archean MORBlike greenstones range from 1305° to 1576°C. Using these data and two exponential curves bracketing the maximum and minimum temperatures versus time, we infer that Archean melts at 2.8 Ga had a mean liquidus temperature of 1399±13°C and a temperature range from 1301° to 1533°C. Using two different methods, we show that the change in the mean liquidus temperature since the late Archean is from 96±13°C (from temperature ranges) to 127±20°C (from temperature means). When we convert these liquidus temperatures to potential temperature of the mantle, we find that the change in the mean upper mantle potential temperature since the late Archean is from 137±8°C (from temperature ranges) to 187±42°C (from temperature means). This change is less than that which was previously thought to have occurred. We compared the liquidus temperatures calculated from our data set with an independent data set from the modern day Pacific plate. The resulting histograms have the same shape and the same temperature range, showing that our method for calculating mantle temperatures from MORBlike rocks in ophiolite suites is valid. When our calculated liquidus temperatures for all time intervals are plotted in histograms, the resulting distributions are not bimodal, but skewed unimodal. That is, the distributions show a high‐T tail which results from the presence of hotspot magmas in the data set. The Archean temperature distribution is also skewed unimodal, and the high‐temperature Archean rocks, such as komatiites, plot in the hotspot area of the distribution. This strongly supports the contention that komatiites do not represent “normal” Archean mantle but rather were probably erupted by hotspots. Our data suggest that the relative proportion of hotspot magmas in oceanic lithosphere has remained nearly constant over geologic time.

[1] New melting experiments in the range of 0.1 MPa to 4.7 GPa provide the basis for calculations of polybaric crystallization which show that both a >300 km deep magma ocean (MO) enriched in refractory elements and a >600 km deep magma ocean with refractory element concentrations similar to the Earth's upper mantle produce sufficient amounts of differentiated, residual liquid, capable of crystallizing the troctolitic-noritic mineral assemblages that are prevalent in lunar ferroan anorthosites (LFA). However, the both oceans become saturated with augite after $20-30% crystallization of the plagioclasesaturated residuum and thereafter are no longer capable of producing noritic assemblages. A second set of melting experiments reveals the existence of a previously unrecognized equilibrium (augite + liquid = pigeonite + plagioclase) and a thermal minimum on the pigeonite + plagioclase liquidus surface at 0.6 GPa. These equilibria will produce melts of mafic cumulates with lower Wo content than those from which the cumulates originally formed in a postmagma ocean environment of overturning mafic cumulates. Melt thus formed and entrained in a buoyant segregation of plagioclase would then form diapiric masses that would crystallize troctolitic-noritic assemblages after intruding upward. Diapiric intrusions could also provide the local deformational environment needed to produce the apparently high (!95%) plagioclase modes of the LFA. An unstable density profile in the cumulate pile following crystallization of most of the MO should provide the impetus for overturn and the heat for partially remelting mafic cumulates.