17:15 〜 18:45
[SCG42-P04] Mantle source and segmentation in the Oman ophiolite revealed by mantle chemistry in Wadi Tayin massif
キーワード：オマーンオフィオライト、カンラン岩、マントルソース
The Oman ophiolite is the largest ophiolite in the world, extending on more than 400 km. Its size is
comparable to several ridge axis segments and the presence of several segment has been shown using
crustal structure and mantle chemistry.
On the other hand, Magmatic dykes present within the mantle section show that several type of magma
may have circulated in the Oman ophiolite: a MORB-like end-member melt and more a more depleted
end-member melt, richer in Si with possible andesitic affinity. Only little mixing between these two end-
members is observed in the mantle dykes and the geographical distribution is non random with MORB-
like melts concentrating in the South-eastern massifs (Maqsad, Wadi tayin, etc.) while andesitic melts are
dominant in the north-western massifs. The same distribution can be observed for other ophiolitic
features, like the chemistry of chromitite pods, the chemistry of chromian-spinels in pyroxenite dykes
(richer in Ti and lower in Cr in the south-eastern massifs for both features), and the presence or absence of
pyroxene-rich concordant layers in the mantle (abundant only in the north-western massifs). All these data
together strongly suggest that, in addition to segmentation, mantle source played a role in the various unit
chemistry in the Oman ophiolite.
In this study, we put an additional piece to this edifice by showing that the mantle chemistry, in particular
its Cr-spinel chemistry show basically the same type of variation. The silicate chemistry was mostly
buffered by mantle processes and do not show any significant difference all along the ophiolite, but Cr# in
Cr-pinels is on average 15% lower in the Wadi Tayin, Maqsad and Samad massifs, than in the north-
western ones. This could be caused by a significantly lower melting degree in the Wadi Tayin massif
compared to the northern peridotite, however the structure of the Wadi Tayin massif is in contradiction
with this hypothesis. The crustal section is particularly thick in the Wadi Tayin, suggesting that it was
generated by a large amount of magma, which is not compatible with a low melting degree.
Cr# in mantle Cr-spinel was then influenced by other processes than only partial melting and possibly
reflects other petrographical properties. It is reasonable to think that the Wadi Tayin mantle melted at the
same degree than in the other massifs of the Oman ophiolite and the lower Cr# can be explained by
mantle heterogeneities and mantle sources variations. The mantle dykes, chromitite pods, etc. showed that
melts were richer in Al, Ti and lower in Cr, Si in the south-eastern massifs compared to the north-western
part of the ophiolite. This is in accordance with the mantle chemistry which shows lower Cr and Si in
Wadi tayin compared to the nothwestern massifs. We conclude then that the mantle below the ophiolite
presented large scale (70 to 150 km wide) heterogeneities and that mantle sources were basically different
in the northern and in the southern parts of the ophiolite. Crust and mantle dykes chemistry was strongly
influenced by their mantle sources and cannot be interpreted only in the frame of tectonic settings. Mantle
chemistry was also influenced by heterogeneities preceding melting and cannot be interpreted only in the
frame of melting degree.
Our study showed that the mantle chemistry variation reflect the superposition of mantle source to
segmentation.
comparable to several ridge axis segments and the presence of several segment has been shown using
crustal structure and mantle chemistry.
On the other hand, Magmatic dykes present within the mantle section show that several type of magma
may have circulated in the Oman ophiolite: a MORB-like end-member melt and more a more depleted
end-member melt, richer in Si with possible andesitic affinity. Only little mixing between these two end-
members is observed in the mantle dykes and the geographical distribution is non random with MORB-
like melts concentrating in the South-eastern massifs (Maqsad, Wadi tayin, etc.) while andesitic melts are
dominant in the north-western massifs. The same distribution can be observed for other ophiolitic
features, like the chemistry of chromitite pods, the chemistry of chromian-spinels in pyroxenite dykes
(richer in Ti and lower in Cr in the south-eastern massifs for both features), and the presence or absence of
pyroxene-rich concordant layers in the mantle (abundant only in the north-western massifs). All these data
together strongly suggest that, in addition to segmentation, mantle source played a role in the various unit
chemistry in the Oman ophiolite.
In this study, we put an additional piece to this edifice by showing that the mantle chemistry, in particular
its Cr-spinel chemistry show basically the same type of variation. The silicate chemistry was mostly
buffered by mantle processes and do not show any significant difference all along the ophiolite, but Cr# in
Cr-pinels is on average 15% lower in the Wadi Tayin, Maqsad and Samad massifs, than in the north-
western ones. This could be caused by a significantly lower melting degree in the Wadi Tayin massif
compared to the northern peridotite, however the structure of the Wadi Tayin massif is in contradiction
with this hypothesis. The crustal section is particularly thick in the Wadi Tayin, suggesting that it was
generated by a large amount of magma, which is not compatible with a low melting degree.
Cr# in mantle Cr-spinel was then influenced by other processes than only partial melting and possibly
reflects other petrographical properties. It is reasonable to think that the Wadi Tayin mantle melted at the
same degree than in the other massifs of the Oman ophiolite and the lower Cr# can be explained by
mantle heterogeneities and mantle sources variations. The mantle dykes, chromitite pods, etc. showed that
melts were richer in Al, Ti and lower in Cr, Si in the south-eastern massifs compared to the north-western
part of the ophiolite. This is in accordance with the mantle chemistry which shows lower Cr and Si in
Wadi tayin compared to the nothwestern massifs. We conclude then that the mantle below the ophiolite
presented large scale (70 to 150 km wide) heterogeneities and that mantle sources were basically different
in the northern and in the southern parts of the ophiolite. Crust and mantle dykes chemistry was strongly
influenced by their mantle sources and cannot be interpreted only in the frame of tectonic settings. Mantle
chemistry was also influenced by heterogeneities preceding melting and cannot be interpreted only in the
frame of melting degree.
Our study showed that the mantle chemistry variation reflect the superposition of mantle source to
segmentation.