Eutectic and Peritectic Systems

Eutectic Systems Example: Diopside - Anorthite No solid solution Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915), Amer. J. Sci. 40, 161-185.

Cooling from composition a: bulk composition = An70 At a: phi = 1 F = C - phi + 1 = 2 - 1 + 1 = 2 Good choice is T and X(An)Liq

First crystal forms at 1455oC (point b) with a compositon of pure An What happens at b? Pure anorthite forms (point c) phi = 2 F = 2 - 2 + 1 = 1 composition of all phases determined by T Xliq is really the only compositional variable in this particular system

Cooling continues as Xliq varies along the liquidus Continuous reaction: liqA ® anorthite + liqB Lever rule shows less liquid and more anorthite as cooling progresses

At 1274oC f = 3 (three phases co-exist) Therefore, F = 2 - 3 + 1 = 0 This is an invariant point (P) T and the composition of all phases is fixed Must remain at 1274oC as a discontinuous reaction proceeds until a phase is lost What happens at 1274oC? get new phase Pure diopside g joins liquid d and anorthite h d is called the eutectic point: MUST reach it in all cases

A discontinuous reaction occurs at d Temperature remains constant at 1274 Use geometry to determine liquid crystal ratios d between g and h, so reaction must be: diopside + anorthite = liquid must run right -> left as cool (toward low entropy) therefore first phase lost is liquid Below 1274oC have diopside + anorthite phi = 2 and F = 2 - 2 + 1 = 1 Only logical variable in this case is T (since the composition of both solids is fixed)

Left of the eutectic there is a similar relationship Cool An20 1350oC (e) and diopside forms first at f Continue cooling: continuous reaction with F = 1 as liqA -> liqB + diopside At 1274oC anorthite (h) joins diopside (g) + liquid (d) Stay at 1274oC until liquid consumed by discontinuous reaction: liquid -> diopside + anorthite

The melt crystallizes over a T range up to ~280oC A sequence of minerals forms over this interval - And the number of minerals increases as T drops The minerals that crystallize depend upon T - The sequence changes with the bulk composition

Augite Forms Before Plagioclase Gabbro of the Stillwater Complex, Montana This forms on the left side of the eutectic

Plagioclase Forms Before Augite Ophitic texture Diabase dike This forms on the right side of the eutectic

The last melt to crystallize in any binary eutectic The last melt to crystallize in any binary eutectic mixture is the eutectic composition Equilibrium melting is the opposite of equilibrium crystallization Thus the first melt of any mixture of Di and An must be the eutectic composition as well

Fractional Crystallization Since solids are not reactants in eutectic-type continuous reactions, the liquid path is not changed Only the final rock will differ: = eutectic X, and not bulk X Isobaric T-X phase diagram at atmospheric pressure. After Bowen (1915), Amer. J. Sci. 40, 161-185.

Partial Melting: if remove liquid perfectly as soon as it forms: melt Di + An to 1274oC discontinuous reaction: Di + An -> eutectic liquid d consume either Di or An first depending on bulk X then melting solid = pure Di or An and jump to 1- C system Thus heat 118 or 279oC before next melt

Binary Peritectic Systems Three phases involved: enstatite = forsterite + SiO2 Figure 6-12. Isobaric T-X phase diagram of the system Fo-Silica at 0.1 MPa. After Bowen and Anderson (1914) and Grieg (1927). Amer. J. Sci. Cool bulk composition a (42 %) At a phi = 1 (liquid) F = 2 - 1 + 1 = 2 Cool to 1625oC Cristobalite forms at b phi = 2 F = 2 - 2 + 1 = 1 Xliq = f(T)

Binary Peritectic Systems Figure 6-12. Isobaric T-X phase diagram of the system Fo-Silica at 0.1 MPa. After Bowen and Anderson (1914) and Grieg (1927). Amer. J. Sci. As T lowered Xliq follows path to c: the eutectic At 1543oC, enstatite forms: d Now f = 3 and F = 2 - 3 + 1 = 0 invariant Discontinuous reaction: liq = En + Crst colinear Stay at this T until liq is consumed Then have En + Crst phi = 2 F = 2 - 2 + 1 = 1 univariant At 1470oC get polymorphic transition Crst -> Trid Another invariant discontinuous rxn

At 1557oC get opx (En) forming phi = 3 F = 2 - 2 + 1 = 0 invariant Figure 6-12. Isobaric T-X phase diagram of the system Fo-Silica at 0.1 MPa. After Bowen and Anderson (1914) and Grieg (1927). Amer. J. Sci. Next cool f = 13 wt. % at 1800oC get olivine (Fo) forming phi = 2 F = 2 - 2 + 1 = 1 univariant Xliq = f(T) At 1557oC get opx (En) forming phi = 3 F = 2 - 2 + 1 = 0 invariant

i m k d c i = “peritectic” point At 1557oC there is colinear equilibrium of Fo-En-liq geometry indicates a reaction: Fo + liq = En consumes olivine (and liquid) ® resorbed textures c d i k m Fo En 1557 When is the reaction finished? Expanded view of the Fo-SiO2 diagram: When is the reaction finished?? When a phase is used up Which phase will it be? Since the bulk composition lies between En and Fo, liq must be used up first Then have En + Fo: f = 2 F = 1 Bulk X

Olivine with orthopyroxene rims, Mt McLoughlin, OR

Same photo with olivine at extinction to make Opx more visible

1543 c d i k m Fo En 1557 bulk X x y Cr Now begin with bulk composition shown, hi T: F = 2 olivine y forms at 1590oC liq = x phi = 2 F = 1 1557oC En joins Fo and liq phi = 3 F = 0 again discontinuous reaction: Fo + liq = En this time olivine consumed before liquid olivine appears then disappears Xliq follows liquidus -> c below m: En + liq phi = 2 F = 1 At 1543oC get En + Crist + liq Eutectic invariant liq -> En + Cr

Incongruent Melting of Enstatite Melt of En does not produce a melt of same composition Rather En goes to Fo + Liq i at the peritectic Partial Melting of Fo + En (harzburgite) mantle En + Fo also ® firsl liq = i Remove i and cool Result = ? 1543 c d i Fo En 1557 bulk X Cr

Consequence For Basalt Magma The Fo-En-Q system causes compositions to migrate from alkali basalt toward tholeiite as the degree of crystallication or melting progresses