The Magma Ocean Concept
The lunar magma ocean crystallized sequentially, producing a
mineralogically and compositionally zoned cumulate pile.
Jeff Taylor
Strong Convection
Strong convection leads to
cycling of liquidus minerals
(mostly olivine and
orthopyroxene), and to
equilibrium crystallization. Most
models of magma ocean
crystallization have at least 50%
equilibrium crystallization,
followed by fractional
crystallization.
Jeff Taylor
Genesis of Mg-suite
2
Geochemistry of Magma Ocean
Snyder et al. (1992) PCS = percent crystallized
Jeff Taylor
Magma Oceanography
Equil X’tal
Fractional X’tal
0-40%: olivine
40-78%: opx
78-86%:
53% plag
25% oliv
22% pigeonite
86-95%:
38% cpx
36% plag
26% pigeonite
95-99.5%:
34% pigeonite
31% plag
24% cpx
11% ilmenite
3
Magma Ocean Overturn
Jeff Taylor
Magma Oceanography
4
Formation of the Mg-Suite
• Most important characteristics to satisfy:
– Generally high Mg/(Mg+Fe), especially for norites and
troctolites (up to Fo92)
– Magmas had lots of KREEP in them
– Variety of rock types
– Almost all contain quite a bit of plagioclase
– Formed over time interval of 300-400 My
Jeff Taylor
Genesis of Mg-suite
5
KREEP Basalt: Maybe Central to MgSuite Origin
Jeff Taylor
Genesis of Mg-suite
6
Formation of the Mg-Suite
All but one model assume a magma ocean first
Jeff Taylor
Genesis of Mg-suite
7
Rising Diapirs and Formation of the MgSuite
(Elardo et al., 2011)
Rising diapirs of olivine-rich, magnesian cumulate rocks stall near the
crust/mantle boundary and react with KREEP and crustal anorthosite.
Jeff Taylor
But are there two Mg-suites?
Are there Mg-suite
rocks on the far side,
but with concentrations
of trace elements much
lower than in KREEP?
Jeff Taylor
Genesis of Mg-suite
9
Magnesian Granulites imply that the farside
could contain low-KREEP Mg-suite rocks
1000
FAN Suite
Mg-Suite
KREEP Basalts
Alkali Anor
Alkali Nor+Gabb-Nor
Alkali-QMD
Alkali-Felsites
Dh 489 Clast d2
Granulites
Highlands Meteorites
Mg-granulites
La (ppm)
100
10
1
0.1
0
20
40
60
80
100
Mg/(Mg+Fe) (mol%)
Jeff Taylor
Genesis of Mg-suite
10
Magnesian Granulitic Breccias: Cannot
be mixtures of known lunar rocks
Do they reflect the presence of a magnesian, low-KREEP magma?
Jeff Taylor
Genesis of Mg-suite
11
Non-Conclusions
From lucid paper by Taylor (2009):
It seems clear that feldspathic rocks with a wide
range in Mg# and relatively low concentrations of
incompatible trace elements dominate the farside
lunar highlands. It is not clear, however, whether
these represent magma-ocean cumulates,
cumulates from trace-element-poor magnesian
mafic magmas, or complicated impact-induced
mixtures of assorted magnesian lithologies.
Jeff Taylor
Genesis of Mg-suite
12