| 1. |
Ryoko Senda, Katsuhiko Suzuki, Tomoaki Morishita, Eiichi Takazawa, The Oman Drilling Project Science Party, The PGE and Os isotope variations in the mantle transition zone from the Samail Ophiolite from the Cores of the Oman Drilling Project, International Conference on Ophiolites and the Oceanic Lithosphere, 2020.01, Oceanic plates are formed at mid-ocean ridges and subducted into the mantle at convergent plate margins. This planetary process returns many surfaceial materials to Earth’s interior contributing to a global recycling system. However the characteristics of and relationships between the lower oceanic crust and the underlying upper mantle remain poor understanding. Much of our knowledge of the construction processes of the lower oceanic crusts comes from of ophiolites; ancient blocks of oceanic lithosphere exposed on land. To generate better understanding of the oceanic crust and its uppermost mantle, the Oman Drilling Project drilled a suit of diamond-coded boreholes from 2016 to 2018 into the Samail ophiolite Oman the world’s largest sequence of upper oceanic lithosphere exposed on land.
We focused on the lower crustal rocks and upper mantle peridotites from the GT and CM sites. The GT site is located at the southward from a divide near the crust-mantle transition. Our samples are from the cores of the GT site which are the intact sections of lower crust, capturing the sheeted dyke - gabbro transition (GT3), middle (foliated) gabbros (GT2) and lower (layered) gabbros (GT1). The CM site is the place of the crust-mantle transition beautifully exposed, with mantle harzburgites in the North transitioning to dunites then gabbros. The cores from the CM site are sampled the crust-mantle transition of the Samail Ophiolite and our samples are from the both sides of the core-mantle boundary.
Here we report the preliminary results of the PGE abundances and Os isotope ratios from the lower crustal rocks and upper mantle peridotites and compare to data from Oman outcrops. Similar PGE patterns in the drillcore derived gabbros and outcrop samples indicates that PGE are probably resistant to surficial weathering processes in Oman. The average of the age corrected Os isotope ratios at 96 Ma in the drillcore gabbros (0.14) show clearly higher than the mantle Os isotope ratios at the same time (CHUR: 0.127 and PUM: 0.129). These data will be help us to better understanding of the relationships between the oceanic crust and underlying upper mantle, mechanisms of making oceanic crust, and planning the future Mohole projects.
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Ryoko Senda, Elliot Carter, Jon Zaloumis, Chang Zhang, Alexander Sousa, Delphine Klaessens, Marguerite Godard, Peter Kelemen, Katsu Michibayashi, Damon Teagle, Eiichi Takazawa, Jude Coggon, Saebyul Choe, The Oman Drilling Project Phase 2 Science Party, Geochemistry of the Samail ophiolite mantle section drilled at Oman Drilling Project phase 2, Holes BA1B, BA3A and BA4A ~ preliminary results on board, International Conference on Ophiolites and the Oceanic Lithosphere, 2020.01, Cores from Holes BA1B, BA3A and BA4A – drilled from the mantle section of the Samail Ophiolite (Oman) – have been systematically sampled and analyzed during Phase 2 of the Oman Drilling Project. 82 samples were selected from Hole BA1B, 62 from Hole BA3A, and 70 from Hole BA4A. Their major, trace and volatile elements and mineralogy were determined aboard D/V Chikyu August-September 2018.
For the three holes, harzburgites and dunites are the most common rocktypes recovered with subordinate gabbro and pyroxenite dikes present in all holes, though their proportions vary in each hole. All samples are moderately to highly altered. The geochemical signatures and co-variations seen in each lithological group are very consistent across the three holes suggesting the underlying processes are common among them.
Harzburgites and dunites are generally restricted in their composition and fall on differentiation trends defined for Oman peridotites. Harzburgites are relatively depleted (Al2O3
The peridotites from the three holes are highly to completely serpentinized and have CO2 contents in excess of estimated mantle concentrations (~300 ppm). Filtering the data to exclude shallow samples gives a mean CO2 content of 0.21% with little downhole variation, suggesting minor but pervasive carbonation has taken place.
Gabbros are primitive (Mg# 80-90) and show compositions intermediate between peridotites and Oman lower crustal gabbros. Pronounced variation in all major element abundances and complete lack of differentiation trends with varying SiO2 (37-50 wt.%) and Mg# are interpreted as most likely due the result of interaction with wall-rock peridotites during emplacement of the dikes. However, the influence of alteration processes such as rodingitization cannot be ruled out.
Key outstanding questions include: the causes of background CO2 enrichment, the relationship between gabbro dikes and lower crustal gabbros elsewhere and whether the gabbro signatures represent a “missing link” - rarely sampled but naturally ubiquitous – between source peridotites and intruded crustal gabbros.
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*RYOKO SENDA, KATSUHIKO SUZUKI, TOMOAKI MORISHITA, EIICHI TAKAZAWA, KATSUYOSHI MICHIBAYASHI, PETER B KELEMEN, DAMON AH TEAGLE, JUDE A COGGON, MARGUERITE GODARD, THE OMAN DRILLING PROJECT SCIENCE PARTY, The PGE and Os isotope variations in the mantle transition zone from the Samail Ophiolite from the Oman Drilling Project Cores, Goldschmidt Conference 2019, 2019.08, Oceanic plates are formed at mid-ocean ridges and subducted into the mantle at convergent plate margins. This planetary process returns many surfacial materials to Earth’s interior contributing to a global recycling system. However the characteristics of and relationships between the oceanic crust and the underlying upper mantle remain poor understanding. Much of our knowledge of the construction processes of the lower oceanic crust comes from of ophiolites; ancient blocks of oceanic lithosphere exposed on land. To generate better understanding of the oceanic crust and its uppermost mantle, the Oman Drilling Project drilled a suit of diamond-cored boreholes from 2016 to 2018 into the Samail ophiolite Oman, the world’s largest sequence of upper oceanic lithosphere exposed on land.
Here we report the preliminary results of the PGE abundances and Os isotope ratios from lower crustal gabbros and upper mantle peridotites sampled by the OmanDP drill cores and compare to data from Oman outcrops. Similar PGE patterns in the drillcore derived gabbros and outcrop samples indicates that PGE are probably resistant to surficial weathering processes in Oman. The average of the age corrected Os isotope ratios at 96 Ma in the drillcore gabbros (0.14) shows clearly higher than the estimated mantle Os isotope ratios at that time (CHUR: 0.127 and PUM: 0.129).. |
| 4. |
Ryoko Senda, Katsuhiko Suzuki, Tomoaki Morishita, Eiichi TAKAZAWA, Katsuyoshi Michibayashi, Peter B Kelemen, Damon AH Teagle, Jude Ann Coggan, Marguerite Godard, The Oman DP Science Party, PGE and Os isotope variation in the gabbros and peridotites from the Oman DP drilling cores in the Samail ophiolite, Oman, JpGU2019, 2019.05. |
| 5. |
Ryoko Senda, Elliot Carter, Jon Zaloumis, Chang Zhang, Alexander Sousa, Delphine Klaessens, Marguerite Godard, Peter Kelemen, Katsu Michibayashi, Damon Teagle, Eiichi Takazawa, Jude Coggon, Saebyul Choe, and the Oman Drilling Project Phase 2 Science Party, Geochemistry of the Samail ophiolite mantle section drilled at Oman Drilling Project Holes BA1B, BA3A and BA4A (Batin area, Oman Drilling Project Phase 2), American Geophysical Union fall meeting 2018, 2018.12. |
