Details
| Original language | English |
|---|---|
| Pages (from-to) | 558-570 |
| Number of pages | 13 |
| Journal | American Mineralogist |
| Volume | 102 |
| Issue number | 3 |
| Publication status | Published - 1 Mar 2017 |
Abstract
We present textures and halogen and trace-element compositions of apatites in intrusive rocks from the dike-gabbro transition zone of a fast-spreading mid-ocean ridge, which was formed at the East Pacific Rise and recently drilled by IODP Hole 1256D. These data are used to discuss the properties of parental magmas and seawater-derived hydrothermal fluids at the roof of the axial melt lens during the formation of oceanic crust. In general, zoning of apatites from three different lithologies, tonalites, diorites, and gabbros, is common and shows a consistent evolution trend with depletion in Cl and REEs from core to rim. The cores are usually homogenous in composition and interpreted as magmatic origin, whereas zones with lower Cl and REEs are disseminated with heterogeneous concentrations, indicating exchanges with hydrothermal fluids. The apatite cores in tonalites are rich in both F and Cl, with XAp F (proportion of fluorapatite end-member) up to 0.5 and XAp Cl (proportion of chlorapatite end-member) up to 0.4. In contrast, the apatite cores in gabbros have high XAp Cl (up to 0.85) and very low XAp F (<0.05). The two contrasting types of apatite cores are both observed in diorites, implying that magma mixing processes may have controlled the formation of the dioritic intrusives. The strong depletions in Cl and REES in some parts of the apatite crystals (mainly rim) can be explained by removal of these components via hydrothermal fluids. Based on available F-Cl-OH exchange coefficients for apatite-melt, the very high Cl/OH and Cl/F ratios and high Cl contents calculated for tonalitic melts cannot be reconciled with a formation of these felsic melts by partial melting of amphibole-bearing metabasalts, but indicate that an assimilation of high-Cl brines must have occurred. Similarly, the low-F chlorapatites in gabbros also imply an assimilation of high-Cl brines. The source of high-Cl fluids in the axial magmatic system may result from seawater-derived fluids, which may form immiscible vapor and brine at high temperatures as a result of hydrothermal boiling.
Keywords
- Apatite, axial melt lens, brine, chlorine, IODP, mid-ocean ridge
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)
- Geophysics
- Earth and Planetary Sciences(all)
- Geochemistry and Petrology
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In: American Mineralogist, Vol. 102, No. 3, 01.03.2017, p. 558-570.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - Apatite in the dike-gabbro transition zone of mid-ocean ridge
T2 - Evidence for brine assimilation by axial melt lens
AU - Zhang, Chao
AU - Koepke, Juergen
AU - Albrecht, Moritz
AU - Horn, Ingo
AU - Holtz, Francois
N1 - Funding information: We gratefully acknowledge the shipboard crew and Scientific Party of IODP Expeditions 312 and 335 for their assistance in data collection. The samples used in this study were provided by the Integrated Ocean Drilling Program. We thank Michael A.W. Marks (University of Tübingen) for sharing the reference Durango apatite. The manuscript was benefited significantly from comments of Jim Webster, Romain Tartese, and other three anonymous reviewers. This research was funded by the DFG (German Research Foundation) project KO 1723/17.
PY - 2017/3/1
Y1 - 2017/3/1
N2 - We present textures and halogen and trace-element compositions of apatites in intrusive rocks from the dike-gabbro transition zone of a fast-spreading mid-ocean ridge, which was formed at the East Pacific Rise and recently drilled by IODP Hole 1256D. These data are used to discuss the properties of parental magmas and seawater-derived hydrothermal fluids at the roof of the axial melt lens during the formation of oceanic crust. In general, zoning of apatites from three different lithologies, tonalites, diorites, and gabbros, is common and shows a consistent evolution trend with depletion in Cl and REEs from core to rim. The cores are usually homogenous in composition and interpreted as magmatic origin, whereas zones with lower Cl and REEs are disseminated with heterogeneous concentrations, indicating exchanges with hydrothermal fluids. The apatite cores in tonalites are rich in both F and Cl, with XAp F (proportion of fluorapatite end-member) up to 0.5 and XAp Cl (proportion of chlorapatite end-member) up to 0.4. In contrast, the apatite cores in gabbros have high XAp Cl (up to 0.85) and very low XAp F (<0.05). The two contrasting types of apatite cores are both observed in diorites, implying that magma mixing processes may have controlled the formation of the dioritic intrusives. The strong depletions in Cl and REES in some parts of the apatite crystals (mainly rim) can be explained by removal of these components via hydrothermal fluids. Based on available F-Cl-OH exchange coefficients for apatite-melt, the very high Cl/OH and Cl/F ratios and high Cl contents calculated for tonalitic melts cannot be reconciled with a formation of these felsic melts by partial melting of amphibole-bearing metabasalts, but indicate that an assimilation of high-Cl brines must have occurred. Similarly, the low-F chlorapatites in gabbros also imply an assimilation of high-Cl brines. The source of high-Cl fluids in the axial magmatic system may result from seawater-derived fluids, which may form immiscible vapor and brine at high temperatures as a result of hydrothermal boiling.
AB - We present textures and halogen and trace-element compositions of apatites in intrusive rocks from the dike-gabbro transition zone of a fast-spreading mid-ocean ridge, which was formed at the East Pacific Rise and recently drilled by IODP Hole 1256D. These data are used to discuss the properties of parental magmas and seawater-derived hydrothermal fluids at the roof of the axial melt lens during the formation of oceanic crust. In general, zoning of apatites from three different lithologies, tonalites, diorites, and gabbros, is common and shows a consistent evolution trend with depletion in Cl and REEs from core to rim. The cores are usually homogenous in composition and interpreted as magmatic origin, whereas zones with lower Cl and REEs are disseminated with heterogeneous concentrations, indicating exchanges with hydrothermal fluids. The apatite cores in tonalites are rich in both F and Cl, with XAp F (proportion of fluorapatite end-member) up to 0.5 and XAp Cl (proportion of chlorapatite end-member) up to 0.4. In contrast, the apatite cores in gabbros have high XAp Cl (up to 0.85) and very low XAp F (<0.05). The two contrasting types of apatite cores are both observed in diorites, implying that magma mixing processes may have controlled the formation of the dioritic intrusives. The strong depletions in Cl and REES in some parts of the apatite crystals (mainly rim) can be explained by removal of these components via hydrothermal fluids. Based on available F-Cl-OH exchange coefficients for apatite-melt, the very high Cl/OH and Cl/F ratios and high Cl contents calculated for tonalitic melts cannot be reconciled with a formation of these felsic melts by partial melting of amphibole-bearing metabasalts, but indicate that an assimilation of high-Cl brines must have occurred. Similarly, the low-F chlorapatites in gabbros also imply an assimilation of high-Cl brines. The source of high-Cl fluids in the axial magmatic system may result from seawater-derived fluids, which may form immiscible vapor and brine at high temperatures as a result of hydrothermal boiling.
KW - Apatite
KW - axial melt lens
KW - brine
KW - chlorine
KW - IODP
KW - mid-ocean ridge
UR - http://www.scopus.com/inward/record.url?scp=85015300802&partnerID=8YFLogxK
U2 - 10.2138/am-2017-5906
DO - 10.2138/am-2017-5906
M3 - Article
AN - SCOPUS:85015300802
VL - 102
SP - 558
EP - 570
JO - American Mineralogist
JF - American Mineralogist
SN - 0003-004X
IS - 3
ER -