Estimating TP – models and pit-falls Olivine-liquid equilibria Roeder & Emslie 1970 Contrib. Mineral. Petrol., 29, 275-289 Dungan & Rhodes 1978 Geophys. Res. Lett. 5, 423-425 Mg# Olivine Olivine is in equilibrium with a host liquid Composition of olivine varies systematically with temperature Based on partitioning of Fe and Mg into olivine during crystallization Mg# host-rock

Equilibrium Olivine Herdubriedatogl HBT-10 Crystallization T not TP MgO FeO Eqm Ol Temp (°C) Rock 17.5 9.6 90.5 1390 Olivine 49.1 9.2 15.1 89.3 1346 48.2 10.3 9.9 9.3 85.9 1230 45.7 13.4 8.0 10.4 79.3 1185 42.9 16.8 L+Ol Mantle olivine Fractional crystallization L+Ol+Plag L+Ol+Pl+Cpx

Equilibrium Olivine addition (e.g. Larsen & Pedersen 2000) Add or subtract olivine to or from a magma that has crystallized only olivine, until olivine in magma is in equilibrium with mantle olivine MgO a function of T FeO a function of P Fo84 Fo88 Fo92 solidus L+Ol Is the olivine in equilibrium with the whole rock or glass? Is the calculated primary magma consistent with melting of peridotite under the estimated T-P conditions? TP=1500°C TP=1400°C

Olivine compositions – West Greenland/ Baffin picrites Normal distribution Awful distribution Eqm olivine Eqm olivine CUMULATES

Accumulative olivine (disequilibrium addition) L+Ol →L+Ol+Pl+Cpx Cotectic compositions i.e more than 1 phase crystallizing Fo84 Fo88 Fo92 Add olivine Fo88 EPR Normal distribution Eqm olivine TP=1500°C TP=1400-1500°C TP=1350°C

Other major element considerations Permitted range primary magmas L+Ol  L+Ol+Pl  L+Ol+Pl+Cpx Consistent with melting of mantle peridotite at TP~1350°C Not consistent with any primary magmas from mantle peridotite Not consistent with any primary magmas from mantle peridotite

Test………… Is it possible to generate a picrite by addition of olivine to a cotectic glass to yield a high mantle potential temperature? Use cotectic MORB glasses which should have been formed of part of a fractionation series from ambient TP (~1350°C). Siqueiros Fracture Zone. Add olivine of Fo88 (and other Fo) until the composition of a Baffin Island picrite is acheived (~20.0 wt% MgO in the whole-rock)

Accumulate Fo88 Start with EPR glass samples along the cotectic and add olivine Fo88 Generate an artificial magma with same MgO as picrite (~20.0 wt%) containing accumulative olivine Calculate model primary magma compositions Large range of possible TP Examine CaO-MgO……………..

Accumulate Fo88 Model Cumulates Fe3+/FeT~0.05 West Greenland For Fo88 not all the model primary magmas are suitable candidates for derivation from mantle peridotite Those that are not are too CaO deficient A result of fractionation of Ol+Pl+Cpx 1449°C 1462°C higher Fo But – two possible solutions with variable TP of 1449-1462°C uncertainty in the determination of the peridotite solidus is ±42°C Vary Fo content of olivine Lower Fo Fe3+/FeT~0.09

Melting column considerations MgO isopleths TP °C 13 15 17 19 lithosphere 1350°C 1450°C 1550°C SQFZ Melt-fraction F ~ 0.3 harzburgite spinel peridotite Disko Olivine cumulates garnet peridotite Hawaii F=0

Mixing liquids Mixing a MORB primary magma (~12wt% MgO) formed at TP~1350°C with cotectic liquids from a MORB fractionation series over the range 5-10 wt% MgO Model Cumulates CaO deficient West Greenland 5%MgO 6% TP elevation, maximum 1458°C 9% L+Ol+Pl+Cpx OK L+Ol Primary

Conclusions For PRIMELT3, it is possible to generate elevated TP from MORB cotectic glasses by addition of disequilibrium olivine (olivine accumulation) up to a maximum of ~1460°C But - MgO contents in olivine cumulates have to be <~16 wt % otherwise CaO is too deficient to give sensible temperature estimates At <~8.5 wt% MgO in the cotectic liquid, accumulation to picritic compositions becomes untenable because of CaO deficiency in the melt Other methods (e.g. simple olivine addition to glass) have no upper limits to TP and are constrained only by assumptions as to mantle olivine Fo content. In many cases, it is not possible to generate the calculated TP and olivine- spinel equilibrium T by simple addition of olivine to a cotectic melt derived from peridotite at ambient TP

Conclusions - Additional T issues – source lithology A pyroxenite component in the upper mantle might provide an explanation for both the olivine and TP Some pyroxenite melts (experiments) can result magmas that mimic the compositions of peridotite-derived magmas – they are not necessarily CaO deficient as commonly assumed Pyroxenite has a different melting behaviour from peridotite More melt production at the same T Garnet stable to shallower depths (1.7 GPa) in pyroxenite compared to peridotite allows for LREE-enrichment at lower pressures than required by garnet peridotite (≥2.7 GPa)

Herdubreidatogl W Greenland disequilibrium Ol-Gl Olivine and glass as close to equilibrium as can be found Tcryst 0GPa Fo86.5 TP Method 1265 ~1460-1500 Ol-Gl 1240 Ol-Sp ~1250 1498 Primelt3 Gl 1492 Primelt3 WR Fe3+/FeT=0.05 Applying the Putirka Ol-Gl geothermometer gives a crystallization temperature of 1263±28°C Samples from the same site have olivine-spinel equilibration Tcryst of 1241±22°C PRIMELT3 on glasses (9.4-10.0 wt% MgO) gives TP=1498[±42]°C at Fo91.4 PRIMELT3 on whole rocks (9.4-10.1 wt% MgO) gives TP=1492[±42]°C at Fo91.4 This is equivalent to Tcryst~1250°C at Fo86.5, the same as the glasses