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Aral I. Okay - One of the best experts on this subject based on the ideXlab platform.

  • Lawsonite composition and zoning as tracers of subduction processes: A global review
    Lithos, 2020
    Co-Authors: Donna L. Whitney, Laure Martin, Aral I. Okay, Katherine F. Fornash, Patricia Kang, Edward D. Ghent, Alberto Vitale Brovarone
    Abstract:

    Abstract Lawsonite is an abundant hydrous mineral in oceanic crust, sediments, and metasomatic rocks at depths of ~45 to 300 km in most subduction zones, but it is rarely preserved in the geologic record because it commonly transforms to epidote and other minerals during prograde or retrograde metamorphism. Owing to the significance of Lawsonite for water and element cycling in subduction zones, occurrences of fresh Lawsonite in blueschist and, more rarely, eclogite provide important opportunities to determine Lawsonite composition, zoning, and inclusion suites and to use this information to reconstruct reaction history during subduction and exhumation. In this review, we use new and published data to document Lawsonite composition in eight of the nine known Lawsonite eclogite localities in which fresh Lawsonite coexists with garnet + omphacite in the rock matrix, as well as the composition of Lawsonite inclusions in six of seven known sites in which Lawsonite occurs only as inclusions in garnet in eclogite-facies rocks that lack matrix Lawsonite. As Lawsonite blueschist is much more common than Lawsonite eclogite, we survey the composition of Lawsonite in representative localities of blueschist, including blueschist associated with eclogite (Lawsonite-bearing, epidote-bearing), and blueschist not associated with eclogite at current exposure levels. Included in this review are metabasaltic rocks, silica- and carbonate-rich metasedimentary rocks, metasomatic rocks, and Lawsonite-rich veins. This dataset demonstrates that Lawsonite composition is a sensitive indicator of reaction history during subduction and exhumation, and specifically of fluid–rock interaction, with implications for element cycling in subduction zones. Furthermore, most exhumed Lawsonite eclogite records slab-surface conditions that correspond to the location where the slab-mantle interface transitions from decoupled to coupled, and therefore provides key insights into the thermal history and dynamics of subduction zones.

  • Halogen (F, Cl, Br, I) behaviour in subducting slabs: a study of Lawsonite blueschists in western Turkey
    Earth and Planetary Science Letters, 2016
    Co-Authors: Lilianne Pagé, Keiko Hattori, Jan C.m. De Hoog, Aral I. Okay
    Abstract:

    Abstract We examined the F, Cl, Br and I abundance of minimally retrogressed Lawsonite blueschists from the Tavsanli Zone in northwest Turkey to evaluate the behaviour of halogens in subduction zones, and to determine the role coexisting high pressure minerals may play in transporting the halogens to the Earth's mantle. The blueschists contain sodic amphibole and Lawsonite, with variable amounts of phengite and chlorite, and minor apatite. A positive correlation between Cl, Br and I contents in bulk rocks suggests their overall coherent behaviour in subduction zones, although high ratios of I/Cl and Br/Cl compared to altered oceanic crust indicate that Cl is preferentially lost relative to Br and I before or during blueschist metamorphism. Iodine and F are enriched relative to altered oceanic crust, suggesting incorporation from marine sediments. In situ analyses of minerals in thin sections reveal F preferentially concentrates in apatite (avg. 3.13 wt%), over phengite (482 ppm), Lawsonite (avg. 413 ppm) and Na-amphibole (257 ppm). Chlorine also preferentially resides in apatite (138 ppm), followed by equal partitioning between phengite (59 ppm) and Na-amphibole (56 ppm), and lower concentrations in Lawsonite (27 ppm). Upon apatite decomposition at a depth of ∼200 km, F may redistribute into Lawsonite and phengite in slabs, whilst Cl is likely expelled to the overlying mantle wedge. Given the stability of Lawsonite and phengite to a depth of 280–300 km in cold subduction zones, they may transport F beyond subarc depths, contributing to the high F in magmas derived from the deep mantle.

  • jadeite chloritoid glaucophane Lawsonite blueschists in north west turkey unusually high p t ratios in continental crust
    Journal of Metamorphic Geology, 2002
    Co-Authors: Aral I. Okay
    Abstract:

    Sodic metapelites with jadeite, chloritoid, glaucophane and Lawsonite form a coherent regional metamorphic sequence, several tens of square kilometres in size, and over a kilometre thick, in the Orhaneli region of northwest Turkey. The low-variance mineral assemblage in the sodic metapelites is quartz +phengite +jadeite +glaucophane +chloritoid +Lawsonite. The associated metabasites are characterized by sodic amphibole +Lawsonite ± garnet paragenesis. The stable coexistence of jadeite +chloritoid +glaucophane +Lawsonite, not reported before, indicates metamorphic pres- sures of 24 ± 3 kbar and temperatures of 430 ± 30 � C for the peak blueschist facies conditions. These P-T conditions correspond to a geotherm of 5 � Ck m )1 , one of the lowest recorded in continental crustal rocks. The low geotherm, and the known rate of convergence during the Cretaceous subduction suggest low shear stresses at the top of the downgoing continental slab.

  • Jadeite–chloritoid–glaucophane–Lawsonite blueschists in north-west Turkey: unusually high P/T ratios in continental crust
    Journal of Metamorphic Geology, 2002
    Co-Authors: Aral I. Okay
    Abstract:

    Sodic metapelites with jadeite, chloritoid, glaucophane and Lawsonite form a coherent regional metamorphic sequence, several tens of square kilometres in size, and over a kilometre thick, in the Orhaneli region of northwest Turkey. The low-variance mineral assemblage in the sodic metapelites is quartz +phengite +jadeite +glaucophane +chloritoid +Lawsonite. The associated metabasites are characterized by sodic amphibole +Lawsonite ± garnet paragenesis. The stable coexistence of jadeite +chloritoid +glaucophane +Lawsonite, not reported before, indicates metamorphic pres- sures of 24 ± 3 kbar and temperatures of 430 ± 30 � C for the peak blueschist facies conditions. These P-T conditions correspond to a geotherm of 5 � Ck m )1 , one of the lowest recorded in continental crustal rocks. The low geotherm, and the known rate of convergence during the Cretaceous subduction suggest low shear stresses at the top of the downgoing continental slab.

  • Oscillatory zoned chrome Lawsonite in the Tavsanli Zone, northwest Turkey
    Mineralogical Magazine, 1999
    Co-Authors: Sarah C. Sherlock, Aral I. Okay
    Abstract:

    Blueschist-facies metabasite rocks from the Tavs∞ anl| Zone of northwest Turkey have been found to contain an abundance of Lawsonite displaying oscillatory zoning. Lawsonite normally adheres to the ideal composition of CaAl2[Si2O7](OH)2.H2O. In two samples from the Tavs∞ anl| Zone, Al 3+ -Cr 3+ substitution has occurred. The Cr 3+ was probably present in the protolith as magmatic chromite, became incorporated into Lawsonite during subduction, and is a metamorphic feature resulting from quantities of Cr in the protolith and local fluid conditions.

Matthias Gottschalk - One of the best experts on this subject based on the ideXlab platform.

  • ca sr fractionation between zoisite Lawsonite and aqueous fluids an experimental study at 2 0 and 4 0 gpa 400 to 800 c
    American Mineralogist, 2013
    Co-Authors: Axel Liebscher, Bernd Wunder, Glenn Dorsam, Gerhard Franz, Matthias Gottschalk
    Abstract:

    The Ca-Sr fractionation between zoisite and, respectively, Lawsonite and an aqueous fluid has been determined by synthesis experiments in the presence of a 1 M (Ca,Sr)Cl 2 aqueous fluid at 2.0 GPa/550, 600, and 700 °C and 4.0 GPa/800 °C for zoisite and 2.0 GPa/400 °C and 4.0 GPa/600 °C for Lawsonite. Solid run products were characterized by EMP, SEM, and XRD with Rietveld refinement and fluids were analyzed by ICP-OES. Zoisite exhibits notable intracrystalline Ca-Sr fractionation between the A1 and A2 sites and calculated intracrystalline exchange coefficients K D (Sr-Ca) A1-A2 = 1.5 to 26 show strong preference of Sr over Ca for the slightly larger A2 site. Calculated individual site-dependent zoisite/aqueous fluid (af, in superscripts)-exchange coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are K (Sr-Ca) zo A1-af = 3.38 to 41.08 for the A1 site and K (Sr-Ca) zo A2-af = 0.45 to 6.51 for the A2 site. Assuming γ Ca af = γ Sr af and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca 2+(af) + Sr A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + Ca A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] and Ca 2+(af) + (M 2+ ) A1 Sr A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + (M 2+ ) A1 Ca A2 Al 3 [Si 3 O 11 (O/OH)] yields Δμ 0 = −29 kJ/mol and W Sr-Ca zo A1 = 5.5 kJ/mol for the A1 site and Δμ 0 = −1.1 kJ/mol and W Sr-Ca zo A2 = 0 kJ/mol for the A2 site at P and T of the experiments. The data indicates ideal Ca-Sr substitution on the A2 site. Lawsonite formed in both the orthorhombic Cmcm and the monoclinic P 2 1 /m form. Calculated Lawsonite-aqueous fluid-exchange coefficients indicate overall preference of Ca over Sr in the solid and are K D (Sr-Ca) law Cmcm -af = 1.12 to 11.32 for orthorhombic and K D (Sr-Ca) law P 21 m -af = 1.67 to 4.34 for monoclinic Lawsonite. Thermodynamic evaluation of the exchange reaction Ca 2+(af) + SrAl 2 Si 2 O 7 (OH) 2 ·H 2 O = Sr 2+(af) + CaAl 2 Si 2 O 7 (OH) 2 ·H 2 O assuming γ Ca af = γ Sr af and a symmetric mixing model yields similar values of Δμ 0 = −9 kJ/mol and W Sr-Ca law Cmcm = 10 kJ/mol for orthorhombic and Δμ 0 = −10 kJ/mol and W Sr-Ca law P 21 /m = 11 kJ/mol for monoclinic Lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are D Sr zo-af = 2.8 ± 0.7 for zoisite at 2 GPa/600 °C and D Sr law Cmcm -af = 0.6 ± 0.2 for orthorhombic Lawsonite at 4 GPa/600 °C and show Sr to be compatible in zoisite but incompatible in Lawsonite. This opposite mineral-aqueous fluid-fractionation behavior of Sr with respect to zoisite and Lawsonite on the one hand and the ideal Ca-Sr substitution on the zoisite A2 site in combination with the strong intracrystalline Ca-Sr fractionation in zoisite on the other hand, make Sr a potential tracer for fluid-rock interactions in zoisite- and Lawsonite-bearing rocks. For low Sr-concentrations, x Sr zo directly reflects x Sr af and allows us to calculate Sr-concentrations in a metamorphic aqueous fluid. During high-pressure aqueous fluid-rock interactions in subduction zone settings the opposite mineral-aqueous fluid-fractionation behavior of Sr results in different aqueous fluid characteristics for Lawsonite- vs. zoisite-bearing rocks. Ultimately, subduction zone magmas may trace these different aqueous fluid characteristics and allow distinguishing between cold, Lawsonite-bearing vs. warm, zoisite-bearing thermal regimes of the underlying subduction zone.

  • Ca-Sr fractionation between zoisite, Lawsonite, and aqueous fluids: An experimental study at 2.0 and 4.0 GPa/400 to 800 °C
    American Mineralogist, 2013
    Co-Authors: Axel Liebscher, Bernd Wunder, Glenn Dorsam, Gerhard Franz, Matthias Gottschalk
    Abstract:

    The Ca-Sr fractionation between zoisite and, respectively, Lawsonite and an aqueous fluid has been determined by synthesis experiments in the presence of a 1 M (Ca,Sr)Cl 2 aqueous fluid at 2.0 GPa/550, 600, and 700 °C and 4.0 GPa/800 °C for zoisite and 2.0 GPa/400 °C and 4.0 GPa/600 °C for Lawsonite. Solid run products were characterized by EMP, SEM, and XRD with Rietveld refinement and fluids were analyzed by ICP-OES. Zoisite exhibits notable intracrystalline Ca-Sr fractionation between the A1 and A2 sites and calculated intracrystalline exchange coefficients K D (Sr-Ca) A1-A2 = 1.5 to 26 show strong preference of Sr over Ca for the slightly larger A2 site. Calculated individual site-dependent zoisite/aqueous fluid (af, in superscripts)-exchange coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are K (Sr-Ca) zo A1-af = 3.38 to 41.08 for the A1 site and K (Sr-Ca) zo A2-af = 0.45 to 6.51 for the A2 site. Assuming γ Ca af = γ Sr af and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca 2+(af) + Sr A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + Ca A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] and Ca 2+(af) + (M 2+ ) A1 Sr A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + (M 2+ ) A1 Ca A2 Al 3 [Si 3 O 11 (O/OH)] yields Δμ 0 = −29 kJ/mol and W Sr-Ca zo A1 = 5.5 kJ/mol for the A1 site and Δμ 0 = −1.1 kJ/mol and W Sr-Ca zo A2 = 0 kJ/mol for the A2 site at P and T of the experiments. The data indicates ideal Ca-Sr substitution on the A2 site. Lawsonite formed in both the orthorhombic Cmcm and the monoclinic P 2 1 /m form. Calculated Lawsonite-aqueous fluid-exchange coefficients indicate overall preference of Ca over Sr in the solid and are K D (Sr-Ca) law Cmcm -af = 1.12 to 11.32 for orthorhombic and K D (Sr-Ca) law P 21 m -af = 1.67 to 4.34 for monoclinic Lawsonite. Thermodynamic evaluation of the exchange reaction Ca 2+(af) + SrAl 2 Si 2 O 7 (OH) 2 ·H 2 O = Sr 2+(af) + CaAl 2 Si 2 O 7 (OH) 2 ·H 2 O assuming γ Ca af = γ Sr af and a symmetric mixing model yields similar values of Δμ 0 = −9 kJ/mol and W Sr-Ca law Cmcm = 10 kJ/mol for orthorhombic and Δμ 0 = −10 kJ/mol and W Sr-Ca law P 21 /m = 11 kJ/mol for monoclinic Lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are D Sr zo-af = 2.8 ± 0.7 for zoisite at 2 GPa/600 °C and D Sr law Cmcm -af = 0.6 ± 0.2 for orthorhombic Lawsonite at 4 GPa/600 °C and show Sr to be compatible in zoisite but incompatible in Lawsonite. This opposite mineral-aqueous fluid-fractionation behavior of Sr with respect to zoisite and Lawsonite on the one hand and the ideal Ca-Sr substitution on the zoisite A2 site in combination with the strong intracrystalline Ca-Sr fractionation in zoisite on the other hand, make Sr a potential tracer for fluid-rock interactions in zoisite- and Lawsonite-bearing rocks. For low Sr-concentrations, x Sr zo directly reflects x Sr af and allows us to calculate Sr-concentrations in a metamorphic aqueous fluid. During high-pressure aqueous fluid-rock interactions in subduction zone settings the opposite mineral-aqueous fluid-fractionation behavior of Sr results in different aqueous fluid characteristics for Lawsonite- vs. zoisite-bearing rocks. Ultimately, subduction zone magmas may trace these different aqueous fluid characteristics and allow distinguishing between cold, Lawsonite-bearing vs. warm, zoisite-bearing thermal regimes of the underlying subduction zone.

  • synthesis of pb zoisite and pb Lawsonite
    Neues Jahrbuch Fur Mineralogie-abhandlungen, 2011
    Co-Authors: Glenn Dorsam, Bernd Wunder, Axel Liebscher, Gerhard Franz, Matthias Gottschalk
    Abstract:

    Hydrothermal syntheses of Pb-zoisite Pb2Al3(SiO4|Si2O7|O|OH) and Pb-Lawsonite PbAl2(Si2O7|(OH)2)•H2O were per- formed at high pressure and temperature conditions with standard piston cylinder press experiments. Starting materials were mix- tures of PbAl2O4, SiO2, PbO and H2O. The run products were characterized by single-crystal and powder X-ray diffraction, scanning electron microscopy and electron microprobe analyses. Idiomorphic colourless Pb-zoisite crystals with sizes of 60 × 50 × 120 µm were obtained at 2 GPa and 600 °C, together with Pb- Lawsonite and traces of Pb-margarite and plumbotsumite Pb5Si4O8(OH)10. Single-crystal diffraction studies and structure solution of Pb-zoisite yielded space group Pnma (62), Z = 4, a = 16.4529(7) A, b = 5.6432(2) A, c = 10.3631(5) A, V = 962.18 A 3 , R1 = 0.067. Pb-Lawsonite was obtained at 3 GPa/600 °C and at 2 GPa/400 °C. Powder-XRD pattern of Pb-Lawsonite shows an orthorhombic unit cell. Peaks at (101), (103), (121), (211), (212), (213), (231), (301), (233) suggest space group Pbnm (62) with Z = 4, a = c 5.85 A, b = 9.03 A, c = 13.31 A, V = 703 A 3 , instead of space group Cmcn for Ca-Lawsonite and P21/m for Sr-Lawsonite. Group-subgroup relations of the Lawsonite structure family are presented.

  • Crystal chemistry of synthetic Lawsonite solid-solution series CaAl2[(OH)2/Si2O7]·H2O–SrAl2[(OH)2/Si2O7]·H2O and the Cmcm–P21/m phase transition
    American Mineralogist, 2010
    Co-Authors: Axel Liebscher, Bernd Wunder, Glenn Dorsam, Gerhard Franz, Matthias Gottschalk
    Abstract:

    Crystals of the solid-solution series of (Ca,Sr)-Lawsonite were synthesized hydrothermally at 4 GPa and 600 and 800 °C in piston-cylinder experiments. Synthesis products were analyzed with SEM, EMP, and powder-XRD. Lawsonite was observed in both the orthorhombic space group Cmcm and in the monoclinic space group P 2 1 / m . It is exclusively orthorhombic at low x Sr bulk but monoclinic at high x Sr bulk ; in the range x Sr bulk = 0.18 to 0.4 both polymorphs coexist and the data suggest a two-phase field between x Sr ortho ~0.1–0.2 and x Sr mono ~0.3–0.4 at 4 GPa/600 °C. Linear regression to the refined lattice parameters yields a = 0.017· x Sr + 5.841 (A), b = 0.197· x Sr + 8.787 (A), c = 0.263· x Sr + 13.130 (A), and v = 4.62· x Sr + 101.46 (cm 3 /mol) for orthorhombic Lawsonite and a = 0.119· x Sr + 5.306 (A), b = 0.118· x Sr + 13.160 (A), c = 0.025· x Sr + 5.833 (A), β = 0.38· x Sr + 124.07 (°), and v = 3.20· x Sr + 101.59 (cm 3 /mol) for monoclinic Lawsonite. The data suggest an increasingly negative Δ v ortho-mono with increasing x Sr . In monoclinic Lawsonite, structural expansion due to the incorporation of Sr is primarily accomplished by tilting and rotation within the Si 2 O 7 -group, whereas in orthorhombic Lawsonite this tilting and rotation is prohibited by symmetry restrictions and expansion is mostly accomplished by an increase in lattice parameters. Combining the extrapolated Ca end-member volume for monoclinic Lawsonite with published high- P data yields K 0 mono = 137(3) GPa ( K ′ = 4.4). Contrary to the Ca end-member system, the Cmcm–P 2 1 / m phase transition is quenchable within the Sr-bearing system. A tentative phase diagram for (Ca,Sr)-Lawsonite at 600 °C indicates a narrow orthorhombic-monoclinic two-phase field that shifts significantly to lower pressure with increasing x Sr . The Cmcm–P 2 1 / m phase transition in the Sr end-member system is located at ≤1 GPa at ~400 to 600 °C, 6 to 9 GPa below the transition in the Ca-system, and has a negative P - T slope.

  • crystal chemistry of synthetic Lawsonite solid solution series caal2 oh 2 si2o7 h2o sral2 oh 2 si2o7 h2o and the cmcm p21 m phase transition
    American Mineralogist, 2010
    Co-Authors: Axel Liebscher, Bernd Wunder, Glenn Dorsam, Gerhard Franz, Matthias Gottschalk
    Abstract:

    Crystals of the solid-solution series of (Ca,Sr)-Lawsonite were synthesized hydrothermally at 4 GPa and 600 and 800 °C in piston-cylinder experiments. Synthesis products were analyzed with SEM, EMP, and powder-XRD. Lawsonite was observed in both the orthorhombic space group Cmcm and in the monoclinic space group P 2 1 / m . It is exclusively orthorhombic at low x Sr bulk but monoclinic at high x Sr bulk ; in the range x Sr bulk = 0.18 to 0.4 both polymorphs coexist and the data suggest a two-phase field between x Sr ortho ~0.1–0.2 and x Sr mono ~0.3–0.4 at 4 GPa/600 °C. Linear regression to the refined lattice parameters yields a = 0.017· x Sr + 5.841 (A), b = 0.197· x Sr + 8.787 (A), c = 0.263· x Sr + 13.130 (A), and v = 4.62· x Sr + 101.46 (cm 3 /mol) for orthorhombic Lawsonite and a = 0.119· x Sr + 5.306 (A), b = 0.118· x Sr + 13.160 (A), c = 0.025· x Sr + 5.833 (A), β = 0.38· x Sr + 124.07 (°), and v = 3.20· x Sr + 101.59 (cm 3 /mol) for monoclinic Lawsonite. The data suggest an increasingly negative Δ v ortho-mono with increasing x Sr . In monoclinic Lawsonite, structural expansion due to the incorporation of Sr is primarily accomplished by tilting and rotation within the Si 2 O 7 -group, whereas in orthorhombic Lawsonite this tilting and rotation is prohibited by symmetry restrictions and expansion is mostly accomplished by an increase in lattice parameters. Combining the extrapolated Ca end-member volume for monoclinic Lawsonite with published high- P data yields K 0 mono = 137(3) GPa ( K ′ = 4.4). Contrary to the Ca end-member system, the Cmcm–P 2 1 / m phase transition is quenchable within the Sr-bearing system. A tentative phase diagram for (Ca,Sr)-Lawsonite at 600 °C indicates a narrow orthorhombic-monoclinic two-phase field that shifts significantly to lower pressure with increasing x Sr . The Cmcm–P 2 1 / m phase transition in the Sr end-member system is located at ≤1 GPa at ~400 to 600 °C, 6 to 9 GPa below the transition in the Ca-system, and has a negative P - T slope.

Laure Martin - One of the best experts on this subject based on the ideXlab platform.

  • Lawsonite composition and zoning as tracers of subduction processes: A global review
    Lithos, 2020
    Co-Authors: Donna L. Whitney, Laure Martin, Aral I. Okay, Katherine F. Fornash, Patricia Kang, Edward D. Ghent, Alberto Vitale Brovarone
    Abstract:

    Abstract Lawsonite is an abundant hydrous mineral in oceanic crust, sediments, and metasomatic rocks at depths of ~45 to 300 km in most subduction zones, but it is rarely preserved in the geologic record because it commonly transforms to epidote and other minerals during prograde or retrograde metamorphism. Owing to the significance of Lawsonite for water and element cycling in subduction zones, occurrences of fresh Lawsonite in blueschist and, more rarely, eclogite provide important opportunities to determine Lawsonite composition, zoning, and inclusion suites and to use this information to reconstruct reaction history during subduction and exhumation. In this review, we use new and published data to document Lawsonite composition in eight of the nine known Lawsonite eclogite localities in which fresh Lawsonite coexists with garnet + omphacite in the rock matrix, as well as the composition of Lawsonite inclusions in six of seven known sites in which Lawsonite occurs only as inclusions in garnet in eclogite-facies rocks that lack matrix Lawsonite. As Lawsonite blueschist is much more common than Lawsonite eclogite, we survey the composition of Lawsonite in representative localities of blueschist, including blueschist associated with eclogite (Lawsonite-bearing, epidote-bearing), and blueschist not associated with eclogite at current exposure levels. Included in this review are metabasaltic rocks, silica- and carbonate-rich metasedimentary rocks, metasomatic rocks, and Lawsonite-rich veins. This dataset demonstrates that Lawsonite composition is a sensitive indicator of reaction history during subduction and exhumation, and specifically of fluid–rock interaction, with implications for element cycling in subduction zones. Furthermore, most exhumed Lawsonite eclogite records slab-surface conditions that correspond to the location where the slab-mantle interface transitions from decoupled to coupled, and therefore provides key insights into the thermal history and dynamics of subduction zones.

  • Lawsonite geochemistry and stability implication for trace element and water cycles in subduction zones
    Journal of Metamorphic Geology, 2014
    Co-Authors: Laure Martin, Donna L. Whitney, Katherine F. Fornash, Jörg Hermann, Laure Gauthiezputallaz, Vitale A Brovarone, Noreen J. Evans
    Abstract:

    This contribution reviews the existing data on Lawsonite stability and trace element geochemistry, and provides new data for metabasaltic and metasedimentary (quartzite) rocks from New Caledonia, Turkey and California. Lawsonite is a major host of REE, Sr, U, Th and Pb in basaltic compositions. Trace element-rich Lawsonite also occurs in metasedimentary rocks, in which comparatively fewer phases compete for trace elements than in metabasaltic rocks. Trace element patterns in Lawsonite are influenced by the coexistence or breakdown of allanite, titanite, apatite and garnet that compete for these elements in high-P metamorphic rocks. Lawsonite is restricted to cool geotherms and therefore is an indicator mineral for subduction-zone metamorphism. The Lawsonite stability field shows a strong dependence on temperature and composition and it is largest in rocks with a high normative anorthite content and, in basaltic systems, carbon content. Along cold geotherms, Lawsonite can transport water and trace elements to great depths, providing a source for these elements in the deep mantle. Along warmer geotherms, Lawsonite disappears on a continuous reaction, gradually releasing water over a temperature interval of several tens of degrees. During Lawsonite breakdown in complex systems, Th and LREE remain trapped in newly formed accessory allanite. However, owing to extreme LREE content, allanite has lower Pb/Ce and Sr/Nd than Lawsonite, resulting in a relative enrichment of Sr and Pb compared with Ce and Nd in the fluids produced during Lawsonite breakdown. Existing experimental data on the solidus of altered oceanic crust suggest that the Lawsonite-breakdown reaction is within 50 °C of the solidus at sub-arc pressures of 3–4 GPa.

  • Lawsonite geochemistry and stability – implication for trace element and water cycles in subduction zones
    Journal of Metamorphic Geology, 2014
    Co-Authors: Laure Martin, Donna L. Whitney, Katherine F. Fornash, Jörg Hermann, Laure Gauthiez-putallaz, A. Vitale Brovarone, Noreen J. Evans
    Abstract:

    This contribution reviews the existing data on Lawsonite stability and trace element geochemistry, and provides new data for metabasaltic and metasedimentary (quartzite) rocks from New Caledonia, Turkey and California. Lawsonite is a major host of REE, Sr, U, Th and Pb in basaltic compositions. Trace element-rich Lawsonite also occurs in metasedimentary rocks, in which comparatively fewer phases compete for trace elements than in metabasaltic rocks. Trace element patterns in Lawsonite are influenced by the coexistence or breakdown of allanite, titanite, apatite and garnet that compete for these elements in high-P metamorphic rocks. Lawsonite is restricted to cool geotherms and therefore is an indicator mineral for subduction-zone metamorphism. The Lawsonite stability field shows a strong dependence on temperature and composition and it is largest in rocks with a high normative anorthite content and, in basaltic systems, carbon content. Along cold geotherms, Lawsonite can transport water and trace elements to great depths, providing a source for these elements in the deep mantle. Along warmer geotherms, Lawsonite disappears on a continuous reaction, gradually releasing water over a temperature interval of several tens of degrees. During Lawsonite breakdown in complex systems, Th and LREE remain trapped in newly formed accessory allanite. However, owing to extreme LREE content, allanite has lower Pb/Ce and Sr/Nd than Lawsonite, resulting in a relative enrichment of Sr and Pb compared with Ce and Nd in the fluids produced during Lawsonite breakdown. Existing experimental data on the solidus of altered oceanic crust suggest that the Lawsonite-breakdown reaction is within 50 °C of the solidus at sub-arc pressures of 3–4 GPa.

  • Lawsonite metasomatism and trace element recycling in subduction zones
    Journal of Metamorphic Geology, 2014
    Co-Authors: A. Vitale Brovarone, Laure Martin, Olivier Alard, Olivier Beyssac, M. Picatto
    Abstract:

    Although Lawsonite-bearing rocks are rare in exhumed high-pressure (HP) terranes, they are considered to exert a primary role in subduction dynamics. Recent observations in natural settings have shown that fluid–rock interaction at HP conditions, including metasomatism, may lead to unusually high Lawsonite amounts even in rocks that originally contained little or no Lawsonite. This process may therefore bear important implications for element recycling in subduction zones. A detailed characterization of the geochemical fingerprints associated with Lawsonite metasomatism is presented in this contribution. The studied rocks belong to the HP terranes of Alpine Corsica (France), which is the largest documented exposure for Lawsonite metasomatism. Metasomatic Lawsonite displays complex compositional zoning, including high trace element, Cr and Ti content. The trace element content is much higher compared with the average of non-metasomatic Lawsonite, and is in line with the re-incorporation of large amounts of trace elements (e.g. REE, Sr, Pb, Th) in the rock during metasomatism, as shown by mass transfer calculations. Our data suggest that serpentinites represented the main fluid source for the metasomatism, with concurrent contribution of other, possibly Ca-rich lithologies, such as mafic or meta-sedimentary rocks. We propose that the breakdown of metasomatic Lawsonite may contribute to the genesis of magmas and their characteristic geochemical signatures.

  • late eocene Lawsonite eclogite facies metasomatism of a granulite sliver associated to ophiolites in alpine corsica
    Lithos, 2011
    Co-Authors: Laure Martin, Alberto Vitale Brovarone, Daniela Rubatto, Jörg Hermann
    Abstract:

    Abstract The metamorphic history of two samples of continental slivers from Alpine Corsica is reconstructed on the basis of detailed petrology, trace and major element geochemistry of the bulk rock and of metamorphic minerals, and U–Pb dating of zircon. A Lawsonite fels with a Ca-rich and Si, Na, K-poor bulk composition, and a garnet-bearing schist were investigated. They both exhibit strongly zoned garnet crystals characterised by cores with high Mg# (0.25–0.33), low grossular component (0.03–0.09) and REE patterns with a negative Eu anomaly. U–Pb dating of zircons from the Lawsonite fels yielded scattered inherited core ages from 912 to 575 Ma and a first metamorphic rim that is dated at 292.5 ± 3.3 Ma. Ti-in zircon thermometry, the trace element pattern of the zircon and the composition of the garnet cores indicate that both samples experienced Permian upper amphibolite- to granulite-facies metamorphism. These high-grade metamorphic rocks are associated to serpentinites, metabasalts and metasediments that are typical of the Alpine Tethys, indicating that the continental slivers were incorporated into an ocean–continent transition during rifting. Alpine eclogite-facies metamorphism is characterised by low temperature conditions as demonstrated by Lawsonite crystallisation in both samples. The Lawsonite fels preserved the most complex history with two stages of garnet growth corresponding to a first prograde path, possibly in the Lawsonite-blueschist facies, and a second stage at Lawsonite-eclogite facies conditions. This second stage was marked by strong Ca-metasomatism that is responsible for the peculiar composition of the sample and the massive crystallisation of Lawsonite. The strong metasomatism is also responsible for the partial recrystallisation of zircon and the formation of Alpine rims. The age of these zircon rims (34.4 ± 0.8 Ma) represents the first U–Pb age for Alpine high-pressure metamorphism in Corsica.

Axel Liebscher - One of the best experts on this subject based on the ideXlab platform.

  • ca sr fractionation between zoisite Lawsonite and aqueous fluids an experimental study at 2 0 and 4 0 gpa 400 to 800 c
    American Mineralogist, 2013
    Co-Authors: Axel Liebscher, Bernd Wunder, Glenn Dorsam, Gerhard Franz, Matthias Gottschalk
    Abstract:

    The Ca-Sr fractionation between zoisite and, respectively, Lawsonite and an aqueous fluid has been determined by synthesis experiments in the presence of a 1 M (Ca,Sr)Cl 2 aqueous fluid at 2.0 GPa/550, 600, and 700 °C and 4.0 GPa/800 °C for zoisite and 2.0 GPa/400 °C and 4.0 GPa/600 °C for Lawsonite. Solid run products were characterized by EMP, SEM, and XRD with Rietveld refinement and fluids were analyzed by ICP-OES. Zoisite exhibits notable intracrystalline Ca-Sr fractionation between the A1 and A2 sites and calculated intracrystalline exchange coefficients K D (Sr-Ca) A1-A2 = 1.5 to 26 show strong preference of Sr over Ca for the slightly larger A2 site. Calculated individual site-dependent zoisite/aqueous fluid (af, in superscripts)-exchange coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are K (Sr-Ca) zo A1-af = 3.38 to 41.08 for the A1 site and K (Sr-Ca) zo A2-af = 0.45 to 6.51 for the A2 site. Assuming γ Ca af = γ Sr af and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca 2+(af) + Sr A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + Ca A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] and Ca 2+(af) + (M 2+ ) A1 Sr A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + (M 2+ ) A1 Ca A2 Al 3 [Si 3 O 11 (O/OH)] yields Δμ 0 = −29 kJ/mol and W Sr-Ca zo A1 = 5.5 kJ/mol for the A1 site and Δμ 0 = −1.1 kJ/mol and W Sr-Ca zo A2 = 0 kJ/mol for the A2 site at P and T of the experiments. The data indicates ideal Ca-Sr substitution on the A2 site. Lawsonite formed in both the orthorhombic Cmcm and the monoclinic P 2 1 /m form. Calculated Lawsonite-aqueous fluid-exchange coefficients indicate overall preference of Ca over Sr in the solid and are K D (Sr-Ca) law Cmcm -af = 1.12 to 11.32 for orthorhombic and K D (Sr-Ca) law P 21 m -af = 1.67 to 4.34 for monoclinic Lawsonite. Thermodynamic evaluation of the exchange reaction Ca 2+(af) + SrAl 2 Si 2 O 7 (OH) 2 ·H 2 O = Sr 2+(af) + CaAl 2 Si 2 O 7 (OH) 2 ·H 2 O assuming γ Ca af = γ Sr af and a symmetric mixing model yields similar values of Δμ 0 = −9 kJ/mol and W Sr-Ca law Cmcm = 10 kJ/mol for orthorhombic and Δμ 0 = −10 kJ/mol and W Sr-Ca law P 21 /m = 11 kJ/mol for monoclinic Lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are D Sr zo-af = 2.8 ± 0.7 for zoisite at 2 GPa/600 °C and D Sr law Cmcm -af = 0.6 ± 0.2 for orthorhombic Lawsonite at 4 GPa/600 °C and show Sr to be compatible in zoisite but incompatible in Lawsonite. This opposite mineral-aqueous fluid-fractionation behavior of Sr with respect to zoisite and Lawsonite on the one hand and the ideal Ca-Sr substitution on the zoisite A2 site in combination with the strong intracrystalline Ca-Sr fractionation in zoisite on the other hand, make Sr a potential tracer for fluid-rock interactions in zoisite- and Lawsonite-bearing rocks. For low Sr-concentrations, x Sr zo directly reflects x Sr af and allows us to calculate Sr-concentrations in a metamorphic aqueous fluid. During high-pressure aqueous fluid-rock interactions in subduction zone settings the opposite mineral-aqueous fluid-fractionation behavior of Sr results in different aqueous fluid characteristics for Lawsonite- vs. zoisite-bearing rocks. Ultimately, subduction zone magmas may trace these different aqueous fluid characteristics and allow distinguishing between cold, Lawsonite-bearing vs. warm, zoisite-bearing thermal regimes of the underlying subduction zone.

  • Ca-Sr fractionation between zoisite, Lawsonite, and aqueous fluids: An experimental study at 2.0 and 4.0 GPa/400 to 800 °C
    American Mineralogist, 2013
    Co-Authors: Axel Liebscher, Bernd Wunder, Glenn Dorsam, Gerhard Franz, Matthias Gottschalk
    Abstract:

    The Ca-Sr fractionation between zoisite and, respectively, Lawsonite and an aqueous fluid has been determined by synthesis experiments in the presence of a 1 M (Ca,Sr)Cl 2 aqueous fluid at 2.0 GPa/550, 600, and 700 °C and 4.0 GPa/800 °C for zoisite and 2.0 GPa/400 °C and 4.0 GPa/600 °C for Lawsonite. Solid run products were characterized by EMP, SEM, and XRD with Rietveld refinement and fluids were analyzed by ICP-OES. Zoisite exhibits notable intracrystalline Ca-Sr fractionation between the A1 and A2 sites and calculated intracrystalline exchange coefficients K D (Sr-Ca) A1-A2 = 1.5 to 26 show strong preference of Sr over Ca for the slightly larger A2 site. Calculated individual site-dependent zoisite/aqueous fluid (af, in superscripts)-exchange coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are K (Sr-Ca) zo A1-af = 3.38 to 41.08 for the A1 site and K (Sr-Ca) zo A2-af = 0.45 to 6.51 for the A2 site. Assuming γ Ca af = γ Sr af and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca 2+(af) + Sr A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + Ca A1 (M 2+ ) A2 Al 3 [Si 3 O 11 (O/OH)] and Ca 2+(af) + (M 2+ ) A1 Sr A2 Al 3 [Si 3 O 11 (O/OH)] = Sr 2+(af) + (M 2+ ) A1 Ca A2 Al 3 [Si 3 O 11 (O/OH)] yields Δμ 0 = −29 kJ/mol and W Sr-Ca zo A1 = 5.5 kJ/mol for the A1 site and Δμ 0 = −1.1 kJ/mol and W Sr-Ca zo A2 = 0 kJ/mol for the A2 site at P and T of the experiments. The data indicates ideal Ca-Sr substitution on the A2 site. Lawsonite formed in both the orthorhombic Cmcm and the monoclinic P 2 1 /m form. Calculated Lawsonite-aqueous fluid-exchange coefficients indicate overall preference of Ca over Sr in the solid and are K D (Sr-Ca) law Cmcm -af = 1.12 to 11.32 for orthorhombic and K D (Sr-Ca) law P 21 m -af = 1.67 to 4.34 for monoclinic Lawsonite. Thermodynamic evaluation of the exchange reaction Ca 2+(af) + SrAl 2 Si 2 O 7 (OH) 2 ·H 2 O = Sr 2+(af) + CaAl 2 Si 2 O 7 (OH) 2 ·H 2 O assuming γ Ca af = γ Sr af and a symmetric mixing model yields similar values of Δμ 0 = −9 kJ/mol and W Sr-Ca law Cmcm = 10 kJ/mol for orthorhombic and Δμ 0 = −10 kJ/mol and W Sr-Ca law P 21 /m = 11 kJ/mol for monoclinic Lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl 2 aqueous fluids are D Sr zo-af = 2.8 ± 0.7 for zoisite at 2 GPa/600 °C and D Sr law Cmcm -af = 0.6 ± 0.2 for orthorhombic Lawsonite at 4 GPa/600 °C and show Sr to be compatible in zoisite but incompatible in Lawsonite. This opposite mineral-aqueous fluid-fractionation behavior of Sr with respect to zoisite and Lawsonite on the one hand and the ideal Ca-Sr substitution on the zoisite A2 site in combination with the strong intracrystalline Ca-Sr fractionation in zoisite on the other hand, make Sr a potential tracer for fluid-rock interactions in zoisite- and Lawsonite-bearing rocks. For low Sr-concentrations, x Sr zo directly reflects x Sr af and allows us to calculate Sr-concentrations in a metamorphic aqueous fluid. During high-pressure aqueous fluid-rock interactions in subduction zone settings the opposite mineral-aqueous fluid-fractionation behavior of Sr results in different aqueous fluid characteristics for Lawsonite- vs. zoisite-bearing rocks. Ultimately, subduction zone magmas may trace these different aqueous fluid characteristics and allow distinguishing between cold, Lawsonite-bearing vs. warm, zoisite-bearing thermal regimes of the underlying subduction zone.

  • synthesis of pb zoisite and pb Lawsonite
    Neues Jahrbuch Fur Mineralogie-abhandlungen, 2011
    Co-Authors: Glenn Dorsam, Bernd Wunder, Axel Liebscher, Gerhard Franz, Matthias Gottschalk
    Abstract:

    Hydrothermal syntheses of Pb-zoisite Pb2Al3(SiO4|Si2O7|O|OH) and Pb-Lawsonite PbAl2(Si2O7|(OH)2)•H2O were per- formed at high pressure and temperature conditions with standard piston cylinder press experiments. Starting materials were mix- tures of PbAl2O4, SiO2, PbO and H2O. The run products were characterized by single-crystal and powder X-ray diffraction, scanning electron microscopy and electron microprobe analyses. Idiomorphic colourless Pb-zoisite crystals with sizes of 60 × 50 × 120 µm were obtained at 2 GPa and 600 °C, together with Pb- Lawsonite and traces of Pb-margarite and plumbotsumite Pb5Si4O8(OH)10. Single-crystal diffraction studies and structure solution of Pb-zoisite yielded space group Pnma (62), Z = 4, a = 16.4529(7) A, b = 5.6432(2) A, c = 10.3631(5) A, V = 962.18 A 3 , R1 = 0.067. Pb-Lawsonite was obtained at 3 GPa/600 °C and at 2 GPa/400 °C. Powder-XRD pattern of Pb-Lawsonite shows an orthorhombic unit cell. Peaks at (101), (103), (121), (211), (212), (213), (231), (301), (233) suggest space group Pbnm (62) with Z = 4, a = c 5.85 A, b = 9.03 A, c = 13.31 A, V = 703 A 3 , instead of space group Cmcn for Ca-Lawsonite and P21/m for Sr-Lawsonite. Group-subgroup relations of the Lawsonite structure family are presented.

  • Crystal chemistry of synthetic Lawsonite solid-solution series CaAl2[(OH)2/Si2O7]·H2O–SrAl2[(OH)2/Si2O7]·H2O and the Cmcm–P21/m phase transition
    American Mineralogist, 2010
    Co-Authors: Axel Liebscher, Bernd Wunder, Glenn Dorsam, Gerhard Franz, Matthias Gottschalk
    Abstract:

    Crystals of the solid-solution series of (Ca,Sr)-Lawsonite were synthesized hydrothermally at 4 GPa and 600 and 800 °C in piston-cylinder experiments. Synthesis products were analyzed with SEM, EMP, and powder-XRD. Lawsonite was observed in both the orthorhombic space group Cmcm and in the monoclinic space group P 2 1 / m . It is exclusively orthorhombic at low x Sr bulk but monoclinic at high x Sr bulk ; in the range x Sr bulk = 0.18 to 0.4 both polymorphs coexist and the data suggest a two-phase field between x Sr ortho ~0.1–0.2 and x Sr mono ~0.3–0.4 at 4 GPa/600 °C. Linear regression to the refined lattice parameters yields a = 0.017· x Sr + 5.841 (A), b = 0.197· x Sr + 8.787 (A), c = 0.263· x Sr + 13.130 (A), and v = 4.62· x Sr + 101.46 (cm 3 /mol) for orthorhombic Lawsonite and a = 0.119· x Sr + 5.306 (A), b = 0.118· x Sr + 13.160 (A), c = 0.025· x Sr + 5.833 (A), β = 0.38· x Sr + 124.07 (°), and v = 3.20· x Sr + 101.59 (cm 3 /mol) for monoclinic Lawsonite. The data suggest an increasingly negative Δ v ortho-mono with increasing x Sr . In monoclinic Lawsonite, structural expansion due to the incorporation of Sr is primarily accomplished by tilting and rotation within the Si 2 O 7 -group, whereas in orthorhombic Lawsonite this tilting and rotation is prohibited by symmetry restrictions and expansion is mostly accomplished by an increase in lattice parameters. Combining the extrapolated Ca end-member volume for monoclinic Lawsonite with published high- P data yields K 0 mono = 137(3) GPa ( K ′ = 4.4). Contrary to the Ca end-member system, the Cmcm–P 2 1 / m phase transition is quenchable within the Sr-bearing system. A tentative phase diagram for (Ca,Sr)-Lawsonite at 600 °C indicates a narrow orthorhombic-monoclinic two-phase field that shifts significantly to lower pressure with increasing x Sr . The Cmcm–P 2 1 / m phase transition in the Sr end-member system is located at ≤1 GPa at ~400 to 600 °C, 6 to 9 GPa below the transition in the Ca-system, and has a negative P - T slope.

  • crystal chemistry of synthetic Lawsonite solid solution series caal2 oh 2 si2o7 h2o sral2 oh 2 si2o7 h2o and the cmcm p21 m phase transition
    American Mineralogist, 2010
    Co-Authors: Axel Liebscher, Bernd Wunder, Glenn Dorsam, Gerhard Franz, Matthias Gottschalk
    Abstract:

    Crystals of the solid-solution series of (Ca,Sr)-Lawsonite were synthesized hydrothermally at 4 GPa and 600 and 800 °C in piston-cylinder experiments. Synthesis products were analyzed with SEM, EMP, and powder-XRD. Lawsonite was observed in both the orthorhombic space group Cmcm and in the monoclinic space group P 2 1 / m . It is exclusively orthorhombic at low x Sr bulk but monoclinic at high x Sr bulk ; in the range x Sr bulk = 0.18 to 0.4 both polymorphs coexist and the data suggest a two-phase field between x Sr ortho ~0.1–0.2 and x Sr mono ~0.3–0.4 at 4 GPa/600 °C. Linear regression to the refined lattice parameters yields a = 0.017· x Sr + 5.841 (A), b = 0.197· x Sr + 8.787 (A), c = 0.263· x Sr + 13.130 (A), and v = 4.62· x Sr + 101.46 (cm 3 /mol) for orthorhombic Lawsonite and a = 0.119· x Sr + 5.306 (A), b = 0.118· x Sr + 13.160 (A), c = 0.025· x Sr + 5.833 (A), β = 0.38· x Sr + 124.07 (°), and v = 3.20· x Sr + 101.59 (cm 3 /mol) for monoclinic Lawsonite. The data suggest an increasingly negative Δ v ortho-mono with increasing x Sr . In monoclinic Lawsonite, structural expansion due to the incorporation of Sr is primarily accomplished by tilting and rotation within the Si 2 O 7 -group, whereas in orthorhombic Lawsonite this tilting and rotation is prohibited by symmetry restrictions and expansion is mostly accomplished by an increase in lattice parameters. Combining the extrapolated Ca end-member volume for monoclinic Lawsonite with published high- P data yields K 0 mono = 137(3) GPa ( K ′ = 4.4). Contrary to the Ca end-member system, the Cmcm–P 2 1 / m phase transition is quenchable within the Sr-bearing system. A tentative phase diagram for (Ca,Sr)-Lawsonite at 600 °C indicates a narrow orthorhombic-monoclinic two-phase field that shifts significantly to lower pressure with increasing x Sr . The Cmcm–P 2 1 / m phase transition in the Sr end-member system is located at ≤1 GPa at ~400 to 600 °C, 6 to 9 GPa below the transition in the Ca-system, and has a negative P - T slope.

Donna L. Whitney - One of the best experts on this subject based on the ideXlab platform.

  • Lawsonite composition and zoning as tracers of subduction processes: A global review
    Lithos, 2020
    Co-Authors: Donna L. Whitney, Laure Martin, Aral I. Okay, Katherine F. Fornash, Patricia Kang, Edward D. Ghent, Alberto Vitale Brovarone
    Abstract:

    Abstract Lawsonite is an abundant hydrous mineral in oceanic crust, sediments, and metasomatic rocks at depths of ~45 to 300 km in most subduction zones, but it is rarely preserved in the geologic record because it commonly transforms to epidote and other minerals during prograde or retrograde metamorphism. Owing to the significance of Lawsonite for water and element cycling in subduction zones, occurrences of fresh Lawsonite in blueschist and, more rarely, eclogite provide important opportunities to determine Lawsonite composition, zoning, and inclusion suites and to use this information to reconstruct reaction history during subduction and exhumation. In this review, we use new and published data to document Lawsonite composition in eight of the nine known Lawsonite eclogite localities in which fresh Lawsonite coexists with garnet + omphacite in the rock matrix, as well as the composition of Lawsonite inclusions in six of seven known sites in which Lawsonite occurs only as inclusions in garnet in eclogite-facies rocks that lack matrix Lawsonite. As Lawsonite blueschist is much more common than Lawsonite eclogite, we survey the composition of Lawsonite in representative localities of blueschist, including blueschist associated with eclogite (Lawsonite-bearing, epidote-bearing), and blueschist not associated with eclogite at current exposure levels. Included in this review are metabasaltic rocks, silica- and carbonate-rich metasedimentary rocks, metasomatic rocks, and Lawsonite-rich veins. This dataset demonstrates that Lawsonite composition is a sensitive indicator of reaction history during subduction and exhumation, and specifically of fluid–rock interaction, with implications for element cycling in subduction zones. Furthermore, most exhumed Lawsonite eclogite records slab-surface conditions that correspond to the location where the slab-mantle interface transitions from decoupled to coupled, and therefore provides key insights into the thermal history and dynamics of subduction zones.

  • Lawsonite-rich layers as records of fluid and element mobility in subducted crust (Sivrihisar Massif, Turkey)
    Chemical Geology, 2020
    Co-Authors: Katherine F. Fornash, Donna L. Whitney
    Abstract:

    Abstract Lawsonite is an abundant hydrous mineral in subducted oceanic crust and sediments and is an important carrier of water (∼11.5 wt%) and trace elements (REE, U, Th, Sr, Pb) into the mantle. Despite its widespread stability at the high-pressure/low-temperature conditions of subduction, it is rarely preserved in the geologic record, particularly in eclogite. Therefore, scarce fresh Lawsonite eclogites in which the composition of Lawsonite can be directly determined are important geochemical archives of fluid-mediated processes during subduction. Of particular interest are Lawsonite-rich veins and layers that may represent former fluid pathways or metasomatic zones and that therefore may record element mobility related to fluid-rock interaction during subduction and/or incipient exhumation. In the Sivrihisar (Turkey) subduction complex, Lawsonite-rich veins and layers comprised of ∼30–50 % Lawsonite occur at pod margins where Lawsonite eclogite has been partially transformed to Lawsonite blueschist. To understand the petrogenesis of these Lawsonite-rich assemblages, we conducted a petrographic and geochemical study of a representative Lawsonite-rich layer at the margin of a meter-scale Lawsonite eclogite pod. Bulk rock and mineral major and trace element analyses were conducted along a transect consisting of the Lawsonite-rich layer (Lws + Grt + Ph), its glaucophane-rich margin (Gln + Grt + Lws + Ph + Rt), and the Lawsonite eclogite host at varying distances from the layer (∼1–2 cm away and >10 cm away). The bulk-rock composition of the Lawsonite eclogite indicates a basaltic protolith that experienced interactions with (meta-)sedimentary rocks before or during the crystallization of high-pressure phases such as garnet and rutile. Integrated major and trace element composition and zoning in high-pressure phases indicate that the Lawsonite-rich layer and its associated glaucophane-rich margin likely formed at or near peak eclogite-facies conditions (2.2–2.4 GPa, 520 °C) as a result of fluid-mediated processes that scavenged Al2O3, MnO, Y, Th, HFSE, and REE from the eclogite immediately adjacent to the vein (∼1–2 cm away). Mass balance calculations also suggest the addition of LILE and transition metals (Ni, Cr, Zn) to the lithologic layers at the pod margin; these elements were likely supplied from an external source, such as serpentinites and/or sediments, both of which occur interspersed with mafic rocks in the Sivrihisar complex. Fluid-rock interaction may have also driven changes in fO2, as omphacite, glaucophane, Lawsonite, and phengite from the pod margin record core-to-rim increases in Fe3+. These changes in bulk composition preferentially stabilized glaucophane-rich (blueschist) and LREE- and LILE-enriched Lawsonite-rich assemblages at the pod margin.

  • Lawsonite geochemistry and stability implication for trace element and water cycles in subduction zones
    Journal of Metamorphic Geology, 2014
    Co-Authors: Laure Martin, Donna L. Whitney, Katherine F. Fornash, Jörg Hermann, Laure Gauthiezputallaz, Vitale A Brovarone, Noreen J. Evans
    Abstract:

    This contribution reviews the existing data on Lawsonite stability and trace element geochemistry, and provides new data for metabasaltic and metasedimentary (quartzite) rocks from New Caledonia, Turkey and California. Lawsonite is a major host of REE, Sr, U, Th and Pb in basaltic compositions. Trace element-rich Lawsonite also occurs in metasedimentary rocks, in which comparatively fewer phases compete for trace elements than in metabasaltic rocks. Trace element patterns in Lawsonite are influenced by the coexistence or breakdown of allanite, titanite, apatite and garnet that compete for these elements in high-P metamorphic rocks. Lawsonite is restricted to cool geotherms and therefore is an indicator mineral for subduction-zone metamorphism. The Lawsonite stability field shows a strong dependence on temperature and composition and it is largest in rocks with a high normative anorthite content and, in basaltic systems, carbon content. Along cold geotherms, Lawsonite can transport water and trace elements to great depths, providing a source for these elements in the deep mantle. Along warmer geotherms, Lawsonite disappears on a continuous reaction, gradually releasing water over a temperature interval of several tens of degrees. During Lawsonite breakdown in complex systems, Th and LREE remain trapped in newly formed accessory allanite. However, owing to extreme LREE content, allanite has lower Pb/Ce and Sr/Nd than Lawsonite, resulting in a relative enrichment of Sr and Pb compared with Ce and Nd in the fluids produced during Lawsonite breakdown. Existing experimental data on the solidus of altered oceanic crust suggest that the Lawsonite-breakdown reaction is within 50 °C of the solidus at sub-arc pressures of 3–4 GPa.

  • Lawsonite geochemistry and stability – implication for trace element and water cycles in subduction zones
    Journal of Metamorphic Geology, 2014
    Co-Authors: Laure Martin, Donna L. Whitney, Katherine F. Fornash, Jörg Hermann, Laure Gauthiez-putallaz, A. Vitale Brovarone, Noreen J. Evans
    Abstract:

    This contribution reviews the existing data on Lawsonite stability and trace element geochemistry, and provides new data for metabasaltic and metasedimentary (quartzite) rocks from New Caledonia, Turkey and California. Lawsonite is a major host of REE, Sr, U, Th and Pb in basaltic compositions. Trace element-rich Lawsonite also occurs in metasedimentary rocks, in which comparatively fewer phases compete for trace elements than in metabasaltic rocks. Trace element patterns in Lawsonite are influenced by the coexistence or breakdown of allanite, titanite, apatite and garnet that compete for these elements in high-P metamorphic rocks. Lawsonite is restricted to cool geotherms and therefore is an indicator mineral for subduction-zone metamorphism. The Lawsonite stability field shows a strong dependence on temperature and composition and it is largest in rocks with a high normative anorthite content and, in basaltic systems, carbon content. Along cold geotherms, Lawsonite can transport water and trace elements to great depths, providing a source for these elements in the deep mantle. Along warmer geotherms, Lawsonite disappears on a continuous reaction, gradually releasing water over a temperature interval of several tens of degrees. During Lawsonite breakdown in complex systems, Th and LREE remain trapped in newly formed accessory allanite. However, owing to extreme LREE content, allanite has lower Pb/Ce and Sr/Nd than Lawsonite, resulting in a relative enrichment of Sr and Pb compared with Ce and Nd in the fluids produced during Lawsonite breakdown. Existing experimental data on the solidus of altered oceanic crust suggest that the Lawsonite-breakdown reaction is within 50 °C of the solidus at sub-arc pressures of 3–4 GPa.

  • Petrogenesis of Lawsonite and epidote eclogite and blueschist, Sivrihisar Massif, Turkey
    Journal of Metamorphic Geology, 2006
    Co-Authors: Peter B. Davis, Donna L. Whitney
    Abstract:

    The Sivrihisar Massif, Turkey, is comprised of blueschist and eclogite facies metasedimentary and metabasaltic rocks. Abundant metre- to centimetre-scale eclogite pods occur in blueschist facies metabasalt, marble and quartz-rich rocks. Sivrihisar eclogite contains omphacite + garnet + phengite + rutile ± glaucophane ± quartz + Lawsonite and/or epidote. Blueschists contain sodic amphibole + garnet + phengite + Lawsonite and/or epidote ± omphacite ± quartz. Sivrihisar eclogite and blueschist have similar bulk composition, equivalent to NMORB, but record different P–T conditions: ∼26 kbar, 500 °C (Lawsonite eclogite); 18 kbar, 600 °C (epidote eclogite); 12 kbar, 380 °C (Lawsonite blueschist); and 15–16 kbar, 480–500 °C (Lawsonite-epidote blueschist). Pressures for the Sivrihisar Lawsonite eclogite are among the highest reported for this rock type, which is rarely exposed at the Earth's surface. The distribution and textures of Lawsonite ± epidote define P–T conditions and paths. For example, in some Lawsonite-bearing rocks, epidote inclusions in garnet and partial replacement of matrix epidote by Lawsonite suggest an anticlockwise P–T path. Other rocks contain no epidote as inclusions or as a matrix phase, and were metamorphosed entirely within the Lawsonite stability field. Results of the P–T study and mapping of the distribution of blueschists and eclogites in the massif suggest that rocks recording different maximum P–T conditions were tectonically juxtaposed as kilometre-scale slices and associated high-P pods, although all shared the same exhumation path from ∼9–11 kbar, 300–400 °C. Within the tectonic slices, alternating millimetre–centimetre-scale layers of eclogite and blueschist formed together at the same P–T conditions but represent different extents of prograde reaction controlled by strain partitioning or local variations in fO2 or other chemical factors.