Papers by Peter B. Kelemen
Received 28 May 2002; revised 3 October 2002; accepted 25 October 2002; published 24 May 2003. [1... more Received 28 May 2002; revised 3 October 2002; accepted 25 October 2002; published 24 May 2003. [1] Seismic reflection and refraction data from the SE Greenland margin provide a detailed view of a volcanic rifted margin from Archean continental crust to near-to-average ...
Earth and Planetary Science Letters, 2008
Debate over the mechanism of accretion of the lower oceanic crust has centered around the gabbro ... more Debate over the mechanism of accretion of the lower oceanic crust has centered around the gabbro glacier and the sheeted sills models. The thermal profile of the crust, specifically the roles of hydrothermal circulation and cooling rate, is a key component in distinguishing between these two models. Results of this study show no systematic variation of cooling rate with depth
Veins formed by carbonation of peridotite in the large thrust sheet of mantle peridotite in the S... more Veins formed by carbonation of peridotite in the large thrust sheet of mantle peridotite in the Sultanate of Oman are ~ 1000 times younger than previously believed, occurring mainly in a relatively shallow weathering horizon . Accelerating this process via drilling and rock fracture, plus heating, could provide a globally significant sink for atmospheric CO2. Exothermic carbonation of olivine can reach a selfheating condition, in which heating due to reaction balances cooling due to advection and diffusion, maintaining temperature at an optimum value for rapid carbonation, ~ 10 6 faster than at 25°C . Volume changes due to carbonation can cause fracture that in turn exposes unreacted olivine and enhances permeability. We review work on natural peridotite carbonation, including recent observations of carbonate vein mineralogy and crack geometry in peridotites, relevant to evaluating and extending these hypotheses.
Earth and Planetary Science Letters, 2008
Understanding the variation of olivine lattice preferred orientation (LPO) as a function of shear... more Understanding the variation of olivine lattice preferred orientation (LPO) as a function of shear strain is important for models that relate seismic anisotropy to the kinematics of deformation. We present results on the evolution of olivine orientation as a function of shear strain in samples from a shear zone in the Josephine Peridotite (southwest Oregon). We find that the LPO
Earth and Planetary Science Letters, 2007
Trace element abundances in clinopyroxene (cpx) from cumulate xenoliths are used to characterize ... more Trace element abundances in clinopyroxene (cpx) from cumulate xenoliths are used to characterize the nature of primitive magma-forming processes beneath the Aleutian island arc. Clinopyroxenes from deformed mafic and ultramafic xenoliths hosted in a Neogene-age mafic sill from Kanaga Island have widely varying trace element abundances (Sr = 8-32, Y = 2-64, Zr = 2-38, Nd = 0.65-16 ppm) that are generally well correlated with cpx Mg# (Mg/Mg + Fe). Trace element ratios in the Kanaga xenoliths show relatively little variability (Nd/Yb = 1.7-3.5, Sr/Y = 0.20-8.7, Nd/Zr = 0.10-0.43) and trace element patterns are generally parallel to one another, with the most evolved samples showing pronounced negative Eu anomalies (Eu/Eu ⁎ = 0.56-0.66). In contrast, cpx from xenoliths hosted in Holocene-age pyroclastic deposits from Mt. Moffett on Adak Island, have more strongly fractionated trace element patterns, with higher and more variable Sr (8-69 ppm), Nd/Yb (3.1-10.5) and Sr/Y (0.5-47) compared to Kanaga xenolith cpx. Clinopyroxene from the amphibole-bearing and igneous-textured Moffett xenoliths also lack substantial negative Eu anomalies (Eu/Eu ⁎ = 0.84-1.25). With respect to most trace element characteristics, cpx from the Kanaga xenoliths resemble cpx phenocrysts from Aleutians basalts, and are distinct from Moffett xenolith cpx which resemble phenocrysts from primitive and geochemically enriched, high-Mg# andesites. The strongly contrasting trace element patterns in the two xenolith suites, which are most clearly evident in the most primitive samples (cpx Mg# N 0.86), are present in cpx with broadly similar major element characteristics (XWo = 0.40-0.50, XCaTs = 0.02-0.25, Mg# = 0.65-0.92), and are interpreted to result from differences in the trace element characteristics of the primitive melts that crystallized to produce the xenoliths. Melts that crystallized to produce the Kanaga xenoliths appear to have been similar to modern Aleutian basalts, whereas those that produced the Moffett samples were more hydrous and perhaps more oxidized and had more strongly fractionated trace element patterns, analogous to those observed in the geochemically enriched primitive andesites. If the trace element-enriched signature in the Moffett xenoliths is produced by melting of the subducting plate in the presence of garnet, then these results support recent thermal modeling which suggests that a 50-60 m.y.-old subducting plate, such as that beneath the central Aleutians, may commonly reach at least those temperatures required to produce eclogite melting under water-saturated conditions (∼850°C). These results indicate that primitive melts arising from the subduction zone are geochemically diverse and may exert primary control over the nature of the distinctive igneous differentiation series (calc-alkaline versus tholeiitic) which are observed in Aleutian volcanoes. (P.B. Kelemen). 1 Tel.: +1 845 365 8728; fax: +1 845 365 8155. 0012-821X/$ -see front matter
Contributions to Mineralogy and Petrology, 2012
ABSTRACT Tectonically exposed mantle peridotite in the Oman Ophiolite is variably serpentinized a... more ABSTRACT Tectonically exposed mantle peridotite in the Oman Ophiolite is variably serpentinized and carbonated. Networks of young carbonate veins are prevalent in highly serpentinized peridotite, particularly near low-temperature alkaline springs emanating from the peridotite. An unusual feature in some samples is the coexistence of serpentine and quartz, which is not commonly observed in serpentinites. This assemblage is unstable with respect to serpentine + talc or talc + quartz under most conditions. Serpentine in the carbonated serpentinites in this study is more iron rich than in most serpentinites reported in previous studies, and samples with co-existing quartz contain the most iron-rich serpentines. Calculations of thermodynamic equilibria in the MgO–SiO2–H2O–CO2 system suggest that serpentine + quartz may be a stable assemblage at low temperatures (e.g., <~15–50 °C) and is stabilized to higher temperatures by preferential cation substitutions in serpentine over talc. Based on these calculations, serpentine + quartz assemblages could result from serpentinization at near-surface temperatures. Clumped isotope thermometry of carbonate veins yields temperatures within error of the observed temperatures in Oman groundwater for all samples analyzed, while the δ18O of water calculated to be in equilibrium with carbonate precipitated at those temperatures is within error of the observed isotopic composition of Oman groundwater for the majority of samples analyzed. As groundwater geochemistry suggests that carbonate precipitation and serpentinization occur concomitantly, this indicates that both hydration and carbonation of peridotite are able to produce extensive alteration at the relatively low temperatures of the near-surface weathering environment.
Earth and Planetary …, 2006
In situ 187Os/188Os ratios are determined on Os-rich platinum-group minerals in podiform chromiti... more In situ 187Os/188Os ratios are determined on Os-rich platinum-group minerals in podiform chromitites both in the Proterozoic ophiolite, Eastern Desert, Egypt, and in the Phanerozoic Oman ophiolite. Because they have very low Re/Os, these primary minerals reflect the ...
1] Abstract: Ophiolites are on-land exposures of igneous crust and residual upper mantle formed b... more 1] Abstract: Ophiolites are on-land exposures of igneous crust and residual upper mantle formed beneath submarine spreading ridges. Upper mantle outcrops in ophiolites provide insight into focusing of melt transport from a $100 km wide region of partial melting into an $5 km wide zone of igneous crustal accretion beneath the ridges. Dunite veins, composed of the minerals olivine and spinel, mark conduits for melt transport through at least the uppermost 30 km of the mantle. New data in this paper, on dunite veins in the Ingalls ophiolite, central Washington Cascades, show a power law relationship between frequency and width, in which frequency/m %0.02 width À3 over a size interval from $0.1 to 2 m. There may be several ways to generate this relationship, but we favor the hypothesis that the dunites represent a coalescing melt transport network. This conclusion is broadly consistent with the related hypothesis that mantle melt extraction occurs in a fractal, branching network, and with recent results on formation of a coalescing network of dissolution channels via flow of a solvent through a partially soluble, compacting porous medium.
ABSTRACT New high-precision U/Pb zircon geochronology from the Oman-United Arab Emirates (U.A.E.)... more ABSTRACT New high-precision U/Pb zircon geochronology from the Oman-United Arab Emirates (U.A.E.) ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust. The new data come from a well-preserved and exposed crustal section in the center of the Wadi Tayin massif. Single grain and grain fragment 206Pb/238U dates from upper-level gabbros, tonalites/trondhjemites and gabbroic pegmatites, corrected for initial Th exclusion, range from 112.55 ± 0.21 to 95.50 ± 0.17 Ma, with most data clustered between 96.40 ± 0.17 to 95.50 ± 0.17 Ma. Zircon dates from upper-level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half-rates of 50-100 km/Ma. Dates from tonalites/trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper-level gabbros, suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates. Three of the dated upper-level gabbros and a single gabbroic pegmatite from the base of the crust have >1 Ma intrasample variability in single grain dates, suggesting assimilation of older crust during the formation or crystallization of the magmas. Whole rock ɛNd(t) of seven samples, including the upper-level gabbros with variable zircon dates, have tightly clustered initial values ranging from ɛNd(96 Ma) = 7.59 ± 0.23 to 8.28 ± 0.31. The ɛNd values are similar to those from other gabbros within the ophiolite, suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas. The new dates suggest that the studied section formed at a fast spreading mid-ocean ridge between ˜96.4-95.5 Ma. The large intrasample variability in zircon dates in some samples is unexpected in this setting, and may be related to propagation of a younger ridge into older oceanic lithosphere.
1] The Talkeetna arc is one of two intraoceanic arcs where much of the section from the upper man... more 1] The Talkeetna arc is one of two intraoceanic arcs where much of the section from the upper mantle through the volcanic carapace is well exposed. We reconstruct the vertical section of the Talkeetna arc by determining the (re)crystallization pressures at various structural levels. The thermobarometry shows that the tonalites and quartz diorites intruded at $5-9 km into a volcanic section estimated from stratigraphy to be 7 km thick. The shallowest, Tazlina and Barnette, gabbros crystallized at $17-24 km; the Klanelneechena Klippe crystallized at $24-26 km; and the base of the arc crystallized at $35 km depth. The arc had a volcanic:plutonic ratio of $1:3-1:4. However, many or most of the felsic plutonic rocks may represent crystallized liquids rather than cumulates so that the liquid:cumulate ratio might be 1:2 or larger. The current 5-to 7-km structural thickness of the plutonic section of the arc is $15-30% of the original 23-to 28-km thickness. The bulk composition of the original Talkeetna arc section was $51-58 wt % SiO 2 .
1] Drilling during ODP Leg 209, dredging, and submersible dives have delineated an anomalous stre... more 1] Drilling during ODP Leg 209, dredging, and submersible dives have delineated an anomalous stretch of the Mid-Atlantic Ridge north and south of the 15°20 0 N Fracture Zone. The seafloor here consists dominantly of mantle peridotite with gabbroic intrusions that in places is covered by a thin, discontinuous extrusive volcanic layer. Thick lithosphere (10-20 km) in this region inhibits magma from reaching shallow levels beneath the ridge axis, thereby causing plate accretion to be accommodated by extensional faulting rather than magmatism. The bathymetry and complex fault relations in the drill-core suggest that mantle denudation and spreading are accommodated by a combination of high-displacement, rolling-hinge normal faults and secondary lower-displacement normal faults. These extensional faults must also accommodate corner flow rotation (up to 90°) of the upwelling mantle within the shallow lithosphere, consistent with remnant magnetic inclinations in denuded peridotite and gabbro from Leg 209 core that indicate up to 90°of sub-Curie-temperature rotation.
Journal of Petrology, Jun 1, 2006
The Early to Middle Jurassic Talkeetna Arc section exposed in the Chugach Mountains of south–cent... more The Early to Middle Jurassic Talkeetna Arc section exposed in the Chugach Mountains of south–central Alaska is 5–18 km wide and extends for over 150 km. This accreted island arc includes exposures of upper mantle to volcanic upper crust. The section comprises six lithological units, in order of decreasing depth:(1) residual upper mantle harzburgite (with lesser proportions of dunite);(2) pyroxenite;(3) basal gabbronorite;(4) lower crustal gabbronorite;(5) mid-crustal plutonic rocks;(6) volcanic rocks. The pyroxenites ...
J. geophys. Res
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. B7, PAGES 10,077-10,094, JUNE 10, 1995 ... Origin ... more JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. B7, PAGES 10,077-10,094, JUNE 10, 1995 ... Origin of thick, high-velocity igneous crust ... Peter B. Kelemen and W. Steven Holbrook Department of Geology and Geophysics, Woods Hole ...
Geochemistry, Geophysics, Geosystems, 2003
1] Abstract: We investigate the use of loess as a proxy for the concentration and isotopic compos... more 1] Abstract: We investigate the use of loess as a proxy for the concentration and isotopic composition of highly siderophile elements, specifically Os, in the upper continental crust. The 187 Os/ 188 Os, platinum group element, and Re concentrations of 16 loess samples from China, Europe, and South America, previously analyzed for major, trace element, and Sr and Nd isotope composition, reveal subtle differences between loess provinces. Despite those differences, the 187 Os/ 188 Os of 1.05 ± 0.23 is surprisingly homogenous. Average 187 Os/ 188 Os as well as average Os (31 pg/g) and Ir (22 pg/g) concentrations are similar to the lower limit of previous estimates for average upper continental crust, whereas Ru, Pt, and Pd concentrations are intermediate between previous estimates. We argue that hydrogenous enrichment of Os in riverine sediments led Esser and Turekian [1993] to overestimate the Os concentration of upper continental crust (50 pg/g). On the basis of this argument and correlations with major and trace elements we propose that average platinum group element concentrations of loess (i.e., 31 pg Os/g, 22 pg Ir/g, 210 pg Ru/g, 510 pg Pt/g, 520 pg Pd/g) are a proxy for the upper continental crust. We further suggest that the nonchondritic average Os/Ir of 1.4 reflects the combined effects of radiogenic ingrowth of Os from Re decay over the mean lifetime of the upper continental crust and preferential return of Os to the crust during subduction. Rhenium concentrations scatter significantly, with highest values in loess derived from organic-rich sedimentary rocks. Low median Re concentrations most likely reflect depletion of loess in organic matter, an important sink for Re in the upper continental crust. An average 187 Re/ 188 Os of 34.5 was calculated on the basis of the measured 187 Os/ 188 Os and Nd model ages. This value corresponds to a Re concentration of 198 pg/g. Correcting measured 187 Os/ 188 Os = 1.05 and inferred 186 Os/ 188 Os = 0.119871 (from 190 Pt/ 188 Os = 0.0176) for the older mean age (2.2 Gyr) of upper continental crust compared to loess (1.6 Gyr) yields average upper crustal 187 Os/ 188 Os of 1.40 and 186 Os/ 188 Os of 0.119885.
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Papers by Peter B. Kelemen