Melting depths and mantle heterogeneity beneath Hawaii and the East Pacific Rise: Constraints from Na/Ti and rare earth element ratios

Abstract

Abstract. Mantle melting calculations are presented that place constraints on the mineralogy of the basalt source region and partial melting depths for oceanic basalts. Melting depths are obtained from pressure-sensitive mineral-melt partition coefficients for Na, Ti, Hf, and the rare earth elements (REE). Melting depths are estimated by comparing model aggregate melt compositions to natural basalts from Hawaii and the East Pacific Rise (EPR). Variations in melting depths in a peridotite mantle are sufficient to yield observed differences in Na/Ti, Lu/Hf, and Sm/Yb between Hawaii and the EPR.Initial melting depths of 95-120 km are calculated for EPR basalts, while melting depths of 200-400 km are calculated for Hawaii, indicating a mantle that is 300øC hotter at Hawaii. Some isotope ratios at Hawaii are correlated with Na/Ti, indicating vertical stratification to isotopic heterogeneity in the mantle; similar comparisons involving EPR lavas support a layered mantle model. Abundances of Na, Ti, and REE indicate that garnet pyroxenite and eclogite are unlikely source components at Hawaii and may be unnecessary at the EPR. The result that some geochemical features of oceanic lavas appear to require only minor variations in mantle mineral proportions (2% or less) may have important implications regarding the efficiency of mantle mixing. Heterogeneity required by isotopic studies might be accompanied by only subtle differences in bulk composition, and material that is recycled at subduction zones might not persist as mineralogically distinct mantle components. IntroductionFundamental questions in the Earth sciences concern the mineralogy of the basalt source region and the temperatures and depths at which basalts form. It has been proposed that isotopic and other compositional differences between basalts reflect mineralogical heterogeneity differentiate between heterogeneity with respect to mineralogy, isotope ratios, or differences in major or minor element composition. To examine issues of heterogeneity and the conditions of partial melting, alkalic and tholeiitic basalts from Hawaii and tholeiites from the EPR are compared. Hawaii is the type example for a mantle plume, while EPR basalts are representative of mid-ocean ridges [Langmuir, 1988]

Petrological constraints on melt generation beneath the Asal Rift (Djibouti) using quaternary basalts

“…Because the K Na of cpx/melt increases with increased P, while clinopyroxene and garnet K Ti min/melt remain constant or decrease, Na/Ti ranges in the same way as Lu/Hf ratio [Putirka, 1999]. Figure 7 shows the variations of Zr/Y, (Dy/ Yb) N , Lu/Hf, and Na à /Ti à ratios across Asal Rift.…”

“…[Fram et al, 1998;Putirka, 1999;Wang et al, 2002]. Heavy REE and HFSE favor garnet structure, because their partition coefficients change during melting of spinel versus garnet peridotites.…”

Ambient and excess mantle temperatures, olivine thermometry, and active vs. passive upwelling

“…A source of excess thermal energy seems inescapable. These excess temperatures are favored by us because they explain the coincidence at some hot spots of high excess topography (Sleep, 1990;Schilling, 1991;Watson and McKenzie, 1991), and primitive liquids that contain (a) high TiO 2 /Na 2 O and high Sm/Yb ratios (Putirka, 1999), and (b) high FeO, and low SiO 2 contents (Langmuir et al, 1992;Albarede, 1992). What is particularly compelling is that Ti/Na ratios are controlled by clinopyroxene-, and Sm/ Yb ratios by garnet-liquid equilibria.…”

“…This value of 1280°C has been much utilized since 1988, and Presnall et al (2002) have recently estimated that T p MOR is between 1260-1310°C. However, many independent analyses of T p MOR undertaken since 1988, based on crustal thickness and new experiments and thermodynamic models of mantle melting, yield much higher values for T p MOR : 1340-1475°C (Kinzler and Grove, 1992b;Herzberg and O'Hara, 1998;Putirka, 1999;Asimow et al, 2001;Green et al, 2001;Wang et al, 2002;Herzberg, 2004;Putirka, 2005). The value for the average potential temperature beneath MORs is thus almost certainly much greater than 1280°C, and more likely in the range of 1450 ± 50°C (Wang et al, 2002;Putirka, 2005).…”

“…Albarede (1992), Herzberg and O'Hara (1998), Herzberg (2004) and Putirka (1999Putirka ( , 2005 have also estimated melting depths and temperatures. Their work shows that major oxide compositions are sufficiently different between MORBs and ocean-island basalts (OIBs) so as to require that at least some OIBs are derived at greater pressures and temperatures.…”