Mantle of the Earth • The Mantle – See Jeanloz (2000) for glossary of terms and overview (we will return to this)

– Virtually the entire silicate portion of our planet – 84% of Earth’s volume, 67% of its mass • What is the mineralogy and composition? – Evidence from: • • • • •

Major and trace element, and isotopic composition of basalts Xenoliths in basalts & kimberlites Slices of thrust-faulted upper mantle & ophiolites Partial melting experiments on lherzolite produce basaltic melt Seismic velocity variations with depth

– Lead to conclusion that: upper mantle (to 670 km depth) is peridotite • Rigid lithosphere (50-100 km thick) • More “plastic” asthenosphere (to about 250 km) • Dense upper mantle, inversions of olivine to spinel structures

Mantle of the Earth

From: Jeanloz (2000) in Encyclopedia of Volcanoes, H. Sigurdsson, ed.

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Mantle of the Earth Upper mantle layers from seismic velocity variations

Wilson ( 1989)

Mantle of the Earth • Mineralogy of upper mantle – Three essential minerals: Ol+Opx+Cpx, plus an aluminous phase

Opx

Ol

Cpx Grt Cpx Grt

Garnet lherzolite xenolith, Kimberly, South Africa 5 mm

Wilson ( 1989)

Spinel lherzolite xenolith, Dish Hill basalt, California

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Mantle of the Earth From high P-T experiments

z

z

Phase diagram for aluminous 4phase lherzolite:

Al-phase: z z z z

Plagioclase: shallow (< 50 km) Spinel: 50-80 km Garnet: 80-400 km Si Æ VI coordination: > 400 km

Figure 1010-2 Phase diagram of aluminous lherzolite with melting interval (gray), subsubsolidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. Rundsch. 70, 128128-153.

Mantle of the Earth •

The mantle is chemically heterogenous Average ME composition

Jeanloz ( 2000)

Variable TE (and isotopes)

Wilson ( 1989)

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Partial Melting of the Mantle • Partial melting of the mantle – Partial fusion, anatexis, partial melting = synonyms • Production of melt from a system in some proportion less than the whole

– Equilibrium (batch) melting vs. Fractional (Rayleigh) melting • Equilibrium – Melt continuously reacts and equilibrates with crystalline residue until segregation – Constant bulk composition

• Fractional – Melt continuously removed, no reaction with crystalline residue – Changing bulk composition

• Thus, melt segregation governs type of melting – Function of permeability in partially molten lherzolite » Equilibrium melting: accumulation until permeability threshold » Fractional melting: melt extracted at low % of melting (1%)

• Reality is somewhere between these end members – Extent and type of partial melting can control diversity of primary magmas

Partial Melting of the Mantle • Partial melting of the mantle – Ternary phase equilibria

Wilson ( 1989)

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Partial Melting of the Mantle • Partial melting of the mantle – Ternary phase equilibria (continued)

Wilson ( 1989)

Partial Melting of the Mantle •

Causes of partial melting – –

As temperature of mantle is raised above its solidus, % melting increases At liquidus temperature, system is totally molten

–

Three principal mechanisms achieve 5-30% melting to create basalt:

1. Anomalous thermal perturbation of the geotherm 2. Adiabatic decompression (pressure pressure--release melting) melting – Relatively rapid rise of rock without significant heat loss 3. Lowering mantle solidus by volatile addition (fluid fluid--fluxed melting) melting – –

•

# 2 is important beneath mid ocean ridges and hot spots/plume heads # 3 is important in subduction zones

Ultimate sources of heat that drive convective upwelling – –

Radioactive decay of U, Th, K, Rb, etc., within the mantle Conductive heating from the underlying liquid outer core (original heat from accretion process)

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Adiabatic, Decompression, or Pressure-release Melting

Figure 1010-4. Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting.

• Percent melt generated increases as upwelling rock ascends above solidus T • Depth of melting and melt segregation governs composition of primary magma • Deeper melting (> 15-20 kbar, 60 km depth) produces more alkaline magma (e.g., OIB) • Shallower melting (< 15-20 kbar) produces tholeiitic magma (e.g., MORB)

Volatile addition or Fluid-fluxed melting

From: Perfit and Davidson (2000) in Encyclopedia of Volcanoes, H. Sigurdsson, ed.

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Partial melting of the Mantle • Trace elements and partial melting processes – Trace element concentrations in partial melts may vary considerably during melting • Provide information about mechanism and % of melting • Provide information about mineralogy of mantle undergoing partial melting • Simple, yet powerful mathematical equations describe the melting process:

– Batch partial melting • Concentration of TE in liquid CL is related to concentration in original solid Co by:

CL 1 = Co F + D − FD

(1)

∑

X α Dα and X" is weight fraction of phase " and D" is its crystalwhere D = α liquid partition coefficient. F is weight fraction of melt formed. D is bulk distribution coefficient for residual solids at moment melt removed from system. • But with increasing F, different minerals may be progressively consumed – D varies discontinuously – Changes when minor phases disappear

Partial Melting of the Mantle – Non-modal batch partial melting • If proportions of phases entering melt p" differ from their proportion in the rock Xo", then: D − FP

D=

where

Do =

o

1− F

∑α X α Dα o

and

• Substituting into (1) gives:

– Fractional melting Modal fractional:

non-modal fractional :

P=

∑α pα Dα

.

CL 1 = Co Do + F (1 − P)

(2)

CL 1 = (1 − F ) (1/( D −1)) Co D

(3)

CL PF (1/( P −1)) 1 = (1 − ) Co Do Do

(4)

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Partial Melting of the Mantle • Compare non-modal batch and fractional melting using REE (Ce, Yb) – System: Forsterite+Diopside+Enstatite – Bulk Xo" = 0.4 + 0.4 + 0.2 – p" initial = eutectic proportions: = 0.1 + 0.7 + 0.2

Non-modal melting models

– Calculate F at which each phase used up, find only CPX consumed • Verify using ternary and lever rule

batch

– Know Dce and Dyb for each mineral fractional

– Calculate CL/Co as F increases using equations 2 and 4.

Fraction of melt 0 to 0.5

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