Petrological and geochemical studies of an abyssal periodotite from the Atlantis II fracture zone

Lee, Kwok-Lin

doi:10.1575/1912/5686

Cited by 5 publications

(19 citation statements)

References 39 publications

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“…Sample RC27‐9‐6‐2 from Atlantis II Fracture Zone was selected for detailed analysis due to the presence of an unusual clinopyroxenite vein (Figures 2g–2h) and also due to the preexisting data set for other peridotites from the same dredge. While the sample was investigated by Snow et al [1994] and Lee [1997], we present here isotopic analyses for vein and matrix minerals measured separately. The sample is unusually Cpx rich (12%) and clinopyroxenite veins are rarely sampled among abyssal peridotites (one other occurrence has been reported by Constantin et al [1995] in an East Pacific Rise peridotite).…”

Section: Methodsmentioning

confidence: 99%

An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions

et al. 2009

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[1] Abyssal peridotites, the depleted solid residues of ocean ridge melting, are the most direct samples available to assess upper oceanic mantle composition. We present detailed isotope and trace element analyses of pyroxene mineral separates from Southwest Indian Ridge abyssal peridotites and pyroxenites in order to constrain the size and length scale of mantle heterogeneity. Our results demonstrate that the mantle can be highly heterogeneous to <1 km and even <0.1 m length scales. Examination of Nd isotopes in relation to modal, trace, and major element compositions indicate that the length scales and amplitudes of heterogeneities in abyssal peridotites reflect both ancient mantle heterogeneity and recent modification by melting, melt-rock reaction and melt crystallization. The isotopic and trace element compositions of pyroxenite veins in this study indicate that they are not direct remnants of recycled oceanic crust, but instead are formed by recent melt crystallization. Combined with existing data sets, the results show that the average global isotopic composition of peridotites is similar to that of mid-ocean ridge basalts, though peridotites extend to significantly more depleted 143 Nd/ 144 Nd and 87 Sr/ 86 Sr. Standard isotope evolution models of upper mantle composition do not predict the full isotopic range observed among abyssal peridotites, as they do not account adequately for the complexities of ancient and recent melting processes.

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Section: Methodsmentioning

confidence: 99%

An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions

et al. 2009

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“…b Ridge-transform intersection. Lee (1997) data for dredge RC27-9-6 samples. b Abbreviations for lithologies (blank indicates residual peridotite): Dep, Trans and Enr are depleted, transitional and enriched zones in cryptically metasomatized sample; GbVn refers to a gabbroic vein, PyxVn to a pyroxenite vein, CpxVn to a clinopyroxenite vein, and OpxVn to an orthopyroxenite vein; AdjVn is adjacent to a vein, NrMat is <5 cm from a vein and FarMat is >5 cm from a vein.…”

Section: Appendix 5c Armalcolite In Dredge 96mentioning

confidence: 99%

“…Table 5.5: Major element composition of Opx, in wt%. D following a grain number indicates duplicate analysis; L preceding a grain number indicates reprocessed data for sample RC27-9-6-2, from Lee (1997). b Ints refers to interstitial grain, <1 mm diameter; Symp refers to symplectite of Cpx with Sp ± Opx; Numbered analyses and Mdpt (midpoint between core and rim) refer to multiple analyses of grains in cryptically metasomatized samples.…”

Section: Appendix 5c Armalcolite In Dredge 96mentioning

confidence: 99%

“…g Percent error based on average difference among duplicate analyses. L preceding a grain number indicates reprocessed data for sample RC27-9-6-2, from Lee (1997). b Ints refers to interstitial grain, <1 mm diameter; Symp refers to symplectite of Cpx with Sp and Opx; ExLm refers to exsolution lamellae in Cpx.…”

Section: Appendix 5c Armalcolite In Dredge 96mentioning

confidence: 99%

“…c Dep, Trans and Enr are depleted, transitional and enriched zones in cryptically metasomatized samples; GbVn refers to a gabbroic vein, PyxVn to a pyroxenite vein, CpxVn to a clinopyroxenite vein, and OpxVn to an orthopyroxenite vein; AdjVn is adjacent to a vein, NrMat is <5 cm from a vein and FarMat is >5 cm from vein. Lee (1997). b GbVn is a gabbroic vein; NrMat is <5 cm from a vein.…”

Section: Appendix 5c Armalcolite In Dredge 96mentioning

confidence: 99%

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Geochemical and rheological constraints on the dynamics of the oceanic upper mantle

Warren¹

2007

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Abyssal peridotite osmium isotopic compositions from cr‐spinel

Standish

Hart

Blusztajn

et al. 2002

Geochem Geophys Geosyst

102

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[1] Abyssal peridotites, fragments of residual upper oceanic mantle, are believed to have less radiogenic Os compositions and higher Os concentrations than primitive upper mantle (PUM, 187 Os/ 188 Os = 0.129 [Meisel et al., 1996]). We have measured 187 Os/ 188 Os in 10 whole rock abyssal peridotites representing non-plume-influenced mid-ocean ridge segments. The 187 Os/ 188 Os ratios range from 0.1183-0.1582. This large range in Os composition, to both less radiogenic and more radiogenic values than primitive upper mantle, can be attributed to ancient melting and subsequent sequestering of isotopic signatures, melt-rock reaction, or secondary alteration, or a combination of any of the three. Petrographic, electron microprobe, and hand-sample inspection show the peridotites to have experienced varying amounts of serpentinization and weathering, accompanied by heterogeneity in 187 Os/ 188 Os values. In addition, a majority of the peridotites studied here are spatially associated with gabbros having equally, if not more highly, heterogeneous Os isotopic compositions and N-MORBs with homogeneous Sr, Nd, and Pb isotopic signatures. This suggests secondary seawater alteration as the dominant influence on present-day Os signatures. We show that careful separation, leaching, and analysis of Cr-spinel from abyssal peridotite largely removes the radiogenic seawater Os isotopic signature, allowing a more accurate assessment of the Os signature of depleted mid-ocean ridge basalt mantle (DMM). Cr-spinel is a highly refractory mantle mineral and commonly host to tiny sulfide inclusions, which are carrier phases for Os. These sulfides are well protected by the Cr-spinel from high-temperature serpentinization and low-temperature seafloor weathering, thereby preserving nonradiogenic DMM-like Os isotopic signatures. Our analyses of treated Cr-spinel fractions finds 187 Os/ 188 Os ratios that are dominantly less radiogenic (0.1238-0.1482) than corresponding whole rock compositions. Nonmagnetic Cr-spinels show more highly variable Os compositions and concentrations than magnetic Cr-spinels, overlapping with the more radiogenic whole rock values. Magnetic Cr-spinels are dominantly less radiogenic than PUM, with high but variable Os concentrations, possibly resulting from a sulfide ''nugget'' effect. The systematically lower 187 Os/ 188 Os in Cr-spinels compared to whole rocks shows that whole rock abyssal peridotites are largely compromised by radiogenic seawater interaction, often obscuring the mantle signature. Measurements of treated abyssal peridotite Cr-spinels in some cases can circumvent this seawater alteration problem allowing more straightforward interpretation of isotopic values and thus further constraining the Os

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Petrological and geochemical studies of an abyssal periodotite from the Atlantis II fracture zone

Cited by 5 publications

References 39 publications

An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions

An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions

Geochemical and rheological constraints on the dynamics of the oceanic upper mantle

Abyssal peridotite osmium isotopic compositions from cr‐spinel

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