Melting polybaric

Rare earth element data will also serve as the basis for a forward modeling study to better constrain melting systematics in the Galapagos. The melting model will invoke clinopyroxene-rare earth element partition coefficients, which vary with composition (Gallahan and Nielsen, 1992), and a polybaric or column melting process. 

Figure 14 shows normative mineral modes versus rock Mg for abyssal peridotite trends relative to calculated trends for polybaric, near-fractional melting. These models are constmcted... 

Figure 14 Normative spinel Iherzolite mineral abundances (wt.%) versus rock Mg for abyssal peridotite trends relative to calculated trends for polybaric, near-fractional melting. Three melt extraction models are shown over pressure ranges of 2.5-0.4 (short dashed line), 2.0-0.4 (long dashed line), and 1.5-0.4GPa (solid line). One percent melting occurs per 0.1 GPa of decompression, and 90% of the melt is extracted at each pressure. Starting mantle is fertile upper mantle (Table 1, 8).

In order to explore the role of melt extraction more fully in generating abyssal peridotite compositions, Asimow (1999) used the MELTS model to investigate a variety of melt extraction scenarios including polybaric batch and mixtures of polybaric near-fractional and batch melting. 

Figure 16 Na20 versus normative olivine content for reconstmcted abyssal peridotites and melt extraction models. Short-dashed line is 0-25% batch melt extraction at 1 GPa. Solid line is polybaric, near-fractional melting from 2.5 GPa to 0.4 GPa as described in Figure 14. Long dash and long-short dashed lines are for mixed-mode melting with 3% polybaric fractional and 8% fractional melt extraction, respectively, followed by polybaric batch melting as described in Figure 15.

Relatively young (<250 Ma) oceanic upper mantle is formed at the lowest average pressures and temperatures by melt extraction at mid-ocean ridges. Melt extraction is expected to be a polybaric process, and likely involves a combination of near-fractional and batch melt extraction. Assuming an average pressure of melt extraction of 1 GPa and 15-20% melt extraction at the ridge axis, an average temperature of 1,315 50 °C is estimated. 

Hirose K. and Kushiro 1. (1998) The effect of melt segregation on polybaric mantle melting estimation from incremental melting experiments. Pfiyi. Earth P/anct. Int. 107, 111-118. 

Baker J. A., Menzies M. A., Thirlwall M. F., and MacPherson C. G. (1997) Petrogenesis of Quaternary intraplate volcanism, Sana a, Yemen implications for plume-lithosphere interaction and polybaric melt hybridization. J. Petrol. 38, 1359-1390. 

Primary Lavas, melts, and liquids derived solely via melting of a specihc, homogeneous source. In practice, it is hard to recognize or even conceive of a truly primary melt. Strictly speaking, even mantle-derived MORBs may be mix-mres of primary melts derived from a variety of sources, including polybaric melts of variably depleted peridotites and/or basaltic veins. For brevity we have used the term primary in a few cases in this chapter. Where it is used without qualihcation, we refer to melts that are, or could be, in equilibrium with mantle olivine with Mg = 90-93. 

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