de Obeso Juan Carlos

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de Obeso
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Juan Carlos

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  • Article
    A Mg isotopic perspective on the mobility of magnesium during serpentinization and carbonation of the Oman Ophiolite
    (American Geophysical Union, 2020-12-04) de Obeso, Juan Carlos ; Santiago Ramos, Danielle ; Higgins, John A. ; Kelemen, Peter B.
    Alteration of mantle peridotite in the Samail ophiolite forms secondary minerals, mainly serpentine and Mg‐rich carbonates. Magnesium accounts for ∼25 – 30% of peridotite mass and its mobility can be used to trace this alteration. We report the first set of Mg isotope measurements from peridotites and their alteration products in Oman. Partially serpentinized peridotites have Mg isotope ratios that are indistinguishable from estimates for the average mantle and bulk silicate earth (δ26Mg = −0.25 ± 0.04‰). However, more extensively altered peridotite samples show large shifts in Mg isotopic composition. The range of δ26Mg values for our suite of alteration products from the mantle section is ∼4.5‰ (from −3.39‰ to 1.19‰), or >60% of the total range of terrestrial variability in δ26Mg values. Serpentine veins are typically enriched in 26Mg (max δ26Mg value = 0.96‰) whereas Mg‐carbonate veins are associated with low 26Mg/24Mg ratios (magnesite δ26Mg = −3.3‰, dolomite δ26Mg = −1.91‰). Our preferred explanation for the range in δ26Mg values involves coprecipitation of serpentine and carbonates at water‐to‐rock ratios >103. The coincidence of alteration products characterized by δ26Mg values that are both lower and higher than bulk silicate Earth and the finite 14C ages of the carbonates suggest that both serpentinization and carbonation are ongoing in Oman. Rates of calcite precipitation in travertines inferred from Δ26Mgcal‐fl suggest that travertine formation in Oman sequesters a total of 106–107 kg CO2/yr, consistent with previous estimates.
  • Article
    Timing of magnetite growth associated with peridotite-hosted carbonate veins in the SE Samail ophiolite, Wadi Fins, Oman
    (American Geophysical Union, 2020-04-06) Cooperdock, Emily H. G. ; Stockli, Daniel F. ; Kelemen, Peter B. ; de Obeso, Juan Carlos
    Carbonate‐altered peridotite are common in continental and oceanic settings and it has been suggested that peridotite‐hosted carbonate represent a significant component of the carbon‐cycle and provide an important link in the CO2 dynamics between the atmosphere, hydrosphere, and lithosphere. The ability to constrain the timing of carbonate and accessory phase growth is key to interpreting the mechanisms that contribute to carbonate alteration, veining, and mineralization in ultramafic rocks. Here we examine a mantle section of the Samail ophiolite exposed in Wadi Fins in southeastern Oman where the peridotite is unconformably overlain by Late Cretaceous‐Paleogene limestone and crosscut by an extensive network of carbonate veins and fracture‐controlled alteration. Three previous 87Sr/86Sr measurements on carbonate vein material in the peridotite produce results consistent with vein formation involving Cretaceous to Eocene seawater (de Obeso & Kelemen, 2018, We employ (U‐Th)/He chronometry to constrain the timing of hydrothermal magnetite in the calcite veins in the peridotite. Magnetite (U‐Th)/He ages of crystal sizes ranging from 1 cm to 200 μm record Miocene growth at 15 ± 4 Ma, which may indicate (1) fluid–rock interaction and carbonate precipitation in the Miocene, or (2) magnetite (re)crystallization within pre‐existing veins. Taken together with published Sr‐isotope values, these results suggest that carbonate veining at Wadi Fins started as early as the Cretaceous, and continued in the Miocene associated with magnetite growth. The timing of hydrothermal magnetite growth is coeval with Neogene shortening and faulting in southern Oman, which points to a tectonic driver for vein (re)opening and fluid‐rock alteration.