IODP Site U1309 was drilled at Atlantis Massif (western rift flank of Mid-Atlantic Ridge (MAR) 30°N; Expeditions 304 and 305), a 1.5-2 Myr old oceanic core complex. The main hole, Hole U1309D, is the second deepest hole drilled into an intrusive slow-spread oceanic lithosphere: it penetrated 1415.5 mbsf (75% recovery). We present here the results of a bulk rock geochemical study (major and trace elements – ICPMS -) carried out on 234 samples representative of the different lithologies sampled at Site U1309. Over 96% of Hole U1309D is made up of gabbroic rocks, cross-cut by late diabases and basaltic dykes in the upper part of the section. Diabases and basalts have depleted MORB compositions (La/Yb ~0.8 and Yb ~ 3.5 ppm) similar to basalts sampled at MAR 30°N. Relics of mantle were recovered at shallow depth. Mantle peridotites show petrographic evidence of melt impregnation. They have relatively fertile compositions, similar to MARK peridotites, with Mg# (100xMg/(Mg+Fe)) of 89-90, Ni>2400ppm and Yb 0.03-0.11 ppm. Gabbroic rocks span a wide range of lithologies and geochemical compositions. They comprise olivine-rich troctolites (>70% modal olivine), troctolites, olivine and troctolitic gabbros (5 to 50% modal olivine), gabbros (including gabbronorites) and oxide gabbros (>2% modal Fe-Ti oxides), which represent respectively 5.4 %, 2.7%, 25.5%, 55.7% and 7% of the core recovered at Hole U1309D. Minor felsic ("leucocratic") dikes cross- cutting gabbros were also sampled. Troctolites and olivine-rich troctolites have high Mg# (82-89), high Ni (up to 2300 ppm) and low trace element contents (Yb 0.06-0.8 ppm). They overlap in composition with peridotites sampled at Atlantis Massif and with impregnated peridotites drilled along the MAR (e.g., ODP Site 1271 (MAR 15°20'N)). Gabbros and olivine gabbros have high Mg# (60-86) and low trace element contents (Yb 0.125-2.5 ppm – (La/Yb)CN ~ 0.4-0.7); these gabbros are among the most primitive and depleted yet sampled along slow spreading ridges (e.g., MAR 23°N and 15°20'N and ODP Hole 735B on the Southwest Indian Ridge). Oxide gabbros and leucocratic dykes represent the most evolved end-members of the gabbroic suite with low Mg# (<50), low Ni (~65 ppm) and high trace element contents (Yb up to 26 ppm). Troctolites and gabbros have Eu positive anomalies (Eu/Eu* ~1.4) whereas oxide gabbros and leucocratic dykes have Eu negative anomalies (Eu/Eu* ~0.75). The former are interpreted as cumulates of a common parental magma, while the later precipitated after the differentiated melts. We note that gabbros and gabbronorites overlap in composition, thus precluding precipitation after a significantly more evolved melt for orthopyroxene crystallization. The bulk composition of gabbroic rocks from Hole 1309D shows no Sr or Eu anomalies; although there may have been local separation of melt and solids, there was no large scale removal of melts from this gabbro section. It is somewhat depleted in the most incompatible elements compared to parental MORB with Ba (40%), Nb (40%) and Rare Earth Elements (REE: Ce 70%, Yb 95%), but enriched in compatible and refractory elements such as Mg (120%) or Ni (200%) and Cr (200%). This may indicate strong chemical interaction between the parental melt and the intruded (assimilated?) shallow depleted mantle, probably during the earliest stages of the formation of Site U1309 gabbroic body. Olivine-rich troctolites may represent the ultimate residue of this reaction- accumulation process.

Geochemistry of a long in-situ section of intrusive slow-spread crust: Results from IODP Site U1309 (Atlantis Massif, 30°N Mid-Atlantic-Ridge) / Godard, M; Abratis, M; Awaji, S; Brunelli, Daniele; Christie, D; Hansen, H; Hellebrand, E; Johnson, K; Maeda, J; Yamasaki, T; Kato, Y.. - In: EOS. - ISSN 0096-3941. - ELETTRONICO. - 88 (52):(2007), pp. Abstract T53B-1297-Abstract T53B-1297. (Intervento presentato al convegno AGU Fall Meeting tenutosi a S. Francisco CA, USA nel Dec. 2008).

Geochemistry of a long in-situ section of intrusive slow-spread crust: Results from IODP Site U1309 (Atlantis Massif, 30°N Mid-Atlantic-Ridge)

BRUNELLI, Daniele;
2007

Abstract

IODP Site U1309 was drilled at Atlantis Massif (western rift flank of Mid-Atlantic Ridge (MAR) 30°N; Expeditions 304 and 305), a 1.5-2 Myr old oceanic core complex. The main hole, Hole U1309D, is the second deepest hole drilled into an intrusive slow-spread oceanic lithosphere: it penetrated 1415.5 mbsf (75% recovery). We present here the results of a bulk rock geochemical study (major and trace elements – ICPMS -) carried out on 234 samples representative of the different lithologies sampled at Site U1309. Over 96% of Hole U1309D is made up of gabbroic rocks, cross-cut by late diabases and basaltic dykes in the upper part of the section. Diabases and basalts have depleted MORB compositions (La/Yb ~0.8 and Yb ~ 3.5 ppm) similar to basalts sampled at MAR 30°N. Relics of mantle were recovered at shallow depth. Mantle peridotites show petrographic evidence of melt impregnation. They have relatively fertile compositions, similar to MARK peridotites, with Mg# (100xMg/(Mg+Fe)) of 89-90, Ni>2400ppm and Yb 0.03-0.11 ppm. Gabbroic rocks span a wide range of lithologies and geochemical compositions. They comprise olivine-rich troctolites (>70% modal olivine), troctolites, olivine and troctolitic gabbros (5 to 50% modal olivine), gabbros (including gabbronorites) and oxide gabbros (>2% modal Fe-Ti oxides), which represent respectively 5.4 %, 2.7%, 25.5%, 55.7% and 7% of the core recovered at Hole U1309D. Minor felsic ("leucocratic") dikes cross- cutting gabbros were also sampled. Troctolites and olivine-rich troctolites have high Mg# (82-89), high Ni (up to 2300 ppm) and low trace element contents (Yb 0.06-0.8 ppm). They overlap in composition with peridotites sampled at Atlantis Massif and with impregnated peridotites drilled along the MAR (e.g., ODP Site 1271 (MAR 15°20'N)). Gabbros and olivine gabbros have high Mg# (60-86) and low trace element contents (Yb 0.125-2.5 ppm – (La/Yb)CN ~ 0.4-0.7); these gabbros are among the most primitive and depleted yet sampled along slow spreading ridges (e.g., MAR 23°N and 15°20'N and ODP Hole 735B on the Southwest Indian Ridge). Oxide gabbros and leucocratic dykes represent the most evolved end-members of the gabbroic suite with low Mg# (<50), low Ni (~65 ppm) and high trace element contents (Yb up to 26 ppm). Troctolites and gabbros have Eu positive anomalies (Eu/Eu* ~1.4) whereas oxide gabbros and leucocratic dykes have Eu negative anomalies (Eu/Eu* ~0.75). The former are interpreted as cumulates of a common parental magma, while the later precipitated after the differentiated melts. We note that gabbros and gabbronorites overlap in composition, thus precluding precipitation after a significantly more evolved melt for orthopyroxene crystallization. The bulk composition of gabbroic rocks from Hole 1309D shows no Sr or Eu anomalies; although there may have been local separation of melt and solids, there was no large scale removal of melts from this gabbro section. It is somewhat depleted in the most incompatible elements compared to parental MORB with Ba (40%), Nb (40%) and Rare Earth Elements (REE: Ce 70%, Yb 95%), but enriched in compatible and refractory elements such as Mg (120%) or Ni (200%) and Cr (200%). This may indicate strong chemical interaction between the parental melt and the intruded (assimilated?) shallow depleted mantle, probably during the earliest stages of the formation of Site U1309 gabbroic body. Olivine-rich troctolites may represent the ultimate residue of this reaction- accumulation process.
2007
EOS
88 (52)
Abstract T53B-1297
Abstract T53B-1297
Godard, M; Abratis, M; Awaji, S; Brunelli, Daniele; Christie, D; Hansen, H; Hellebrand, E; Johnson, K; Maeda, J; Yamasaki, T; Kato, Y.
Geochemistry of a long in-situ section of intrusive slow-spread crust: Results from IODP Site U1309 (Atlantis Massif, 30°N Mid-Atlantic-Ridge) / Godard, M; Abratis, M; Awaji, S; Brunelli, Daniele; Christie, D; Hansen, H; Hellebrand, E; Johnson, K; Maeda, J; Yamasaki, T; Kato, Y.. - In: EOS. - ISSN 0096-3941. - ELETTRONICO. - 88 (52):(2007), pp. Abstract T53B-1297-Abstract T53B-1297. (Intervento presentato al convegno AGU Fall Meeting tenutosi a S. Francisco CA, USA nel Dec. 2008).
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