The pre-subduction features of rocks and
their influence on subduction-zone processes
The pre-subduction alteration produces mineralogical and chemical heterogeneities
in the oceanic lithosphere, playing a key role for the genesis of subduction fluids
and for trace element recycling into the mantle. The changes are still recorded as
relics, and as mineralogic and compositional variations of the high pressure mafic
and ultramafic rocks
Increasing serpentinization
Eclogite – no change
Rodingite – Ca metasomatism
Ti-clinohumite metagabbro –
Mg metasomatism
Mineral transformations and trace element variability in
ultramafic rocks: The oceanic stage
Whole-rock/Chondrite
10
Profiles from partially
serpentinized peridotite
to serpentinite
1
Profile 1
Serpentinized lherzolite
Serpentinite mylonite
0.1
Profile 2
Serpentinized lherzolite
Serpentinite mylonite
0.01
Ce Sr Nd ZrSm Eu Gd Ti Dy
Sr (ppm)
15
Er
Yb
Mantle peridotite
Serpentinized lherzolite
(no HP overprint)
Serpentinized lherzolite
(HP overprint)
Serpentinite mylonite
10
Increase in Sr and
water
5
0
0
5
10
H2O wt%
15
Scambelluri et al., J Petrol, 2001, 42, 55-67
Trace element variability in
rocks and minerals
The oceanic stage
Profile on a rim around mantle
spinel, showing increase of alkalis in
the outer parts of the rim
Increase in alkalis, Sr, B, Cl
Scambelluri et al., EPSL, 2004, 222, 217-234
Rock forming minerals of ultramafites recording
early serpentinization, subsequent subduction
and antigorite breakdown. Formation of lowgrade serpentine after mantle is accompanied
by strong Cl and B increase.
Serpentine compositions
Hess Deep
Iberia abyssal plain
Tyrrenian Sea
The oceanic stage
Erro-Tobbio
2
4
6
8
10
δ O permil
12
Oxygen and hydrogen
isotope variability
serpentinite-forming
minerals
14
18
Clinopyroxene compositions
Bracco Massif
Mid-Cayman Rise
Nunivak Island
Galicia Margin
Samail ophiolite
SW Indian Ridge
Erro-Tobbio
2
3
4
5
6
δ O permil
7
8
18
Frueh Green et al., 2001, CMP, 141, 145-159
Influence of pre-subduction transformations in mafic rocks
1. Rodingitization: Ca metasomatism
Chemical exchange between serpentinizing
ultramafites and mafic rocks is well known in the
Ligurian Alps. It produces decrease in Si and
alkalis and increase in Ca in mafic rocks
(rodingites). High-pressure metarodingites
display such pre-subduction modification and may
still retain relics of igneous minerals overgrown
by a calc-silicate assemblage. Ti-rich gabbros are
frequently the protholiths of such rocks.
Relics of igneous
(brown) Ti-augite
and of interstitial
ilmenite are still
preserved in HP
metarodingites.
The HP assemblage
is diopside (white) +
grossular + chlorite.
The influence of pre-subduction
transformations in mafic rocks
2. Mg metasomatism and formation of Ticlinohumite assemblages:
This process is much less documented than
rodingitization. It occurs between ultramafites and
thin mafic layers during the early stages of
serpentinization. It is associated with formation of
rodingites and drives to decrease in Si and alkalis and
increase in Mg in mafic rocks (Mg-metasomatism). The
high pressure recrystallization of the Mg-metagabbros
produces Ti-clinohumite -bearing assemblages. The
Mg-metagabbros formed this way share strong
similarity with high pressure ultramafites, particularly
with metamorphic veins, as they contain HP Ticlinohumite, diopside, magnetite and chlorite.
Distinctive feature is the preservation in the
metagabbros of igneous (brown) Ti-augite, of large
ilmenite and of interstitial apatite, to demostrate
derivation from precursor Ti-gabbros.
Scambelluri and Rampone, 1999, CMP, 135, 1-17
Compositional variations of rodingites and Ticlinohumite metagabbros
Fig. 10 Bulk rock compositions of Ti-clinohumite metagabbros (zones A and B)
compared to unaltered gabbros (data after: Thompson 1973; Serri 1980;
Pognante et al. 1982; Mottana 1990) and metarodingites (data after: Honnorez
and Kirst 1975; Evans et al. 1981). Also reported are metarodingites MF14, MF16,
MF24/1, R, R2 (Table 5). Samples R and R2 (metarodingite dike with rims of Ticlinohumite) are respectively indicated by the dotted circle and by the dotted
diamond
Scambelluri and Rampone (1999) Contrib Mineral Petrol 135: 1-17