ALBERT

Description: Metamorphosed oceanic gabbros provide a record of the cooling history of the lower crust near mid-ocean ridges, but the temperature range, rate, and location of subsolidus events are poorly known. We combine hornblende-plagioclase thermometry, statistical analysis, and thermal models to estimate precisely the temperature, time, distance from axis, and duration of metamorphism in East Pacific Rise gabbros from Hess Deep, ODP Hole 894G. Metamorphic hornblende and plagioclase, which formed during microfracturing and sea water penetration, equilibrated at a mean temperature of 716 +/- 8°C (90% confidence level). Comparison of the properties of the observed temperature distribution with those of model events indicates that metamorphism spanned 〈= 60°C. When combined with thermal models of fast-spreading centers, this implies that metamorphism was rapid (〈= 6000 yr) and occurred 1-4 km off axis. Application of this approach to other gabbros will allow comparison of spatial and temporal characteristics of deformation and fluid flow in the lower oceanic crust as a function of ridge setting.

Description: This database provides measurements on 87Sr/86Sr, d13C from samples of Oman Drilling Project Hole BT1B. The database includes listvenites (n=50), serpentinites (n=14), metamorphic sole rocks (n=11). The sample names and grouping by Units were determined on-board D/V Chikyu from macroscopic observations (Visual Core Description; Kelemen et al. [2020]). Rb and Sr concentrations were determined using a Quadrupole Inductively-Coupled-Plasma-Mass Spectrometer (Q-ICP-MS) at the University of Montpellier (France) and were originally reported by Godard et al. (2021). 87Sr/86Sr were analyzed for interspersed with US National Institute of Standards and Technology (NIST) SRM 987 on a Thermo Scientific Neptune multi-collector ICP-MS at Lamont Doherty Earth Observatory (United States). Total Carbon (TC) was measured from the same bulk rock powder splits as for Strontium isotopes. Total Organic Carbon (TOC, or reduced carbon) was measured from the residual rock powder after the removal of Inorganic Carbon (carbonate carbon) through reaction with dilute (3 N) HCl for at least 3 days, followed by washing with Millipore® water. Concentrations and d13C ratios of Total Carbon (TC) and Total Organic Carbon (TOC), were determined using a Costech element analyzer coupled with a Thermo Scientific Delta V plus mass spectrometer at Lamont Doherty Earth Observatory (United States). References: Reference : Kelemen, P. B., J. M. Matter, D. A. H. Teagle, J. A. Coggon, and the Oman Drilling Project Science Team (2020), Proceedings of the Oman Drilling Project, College Station, TX. and Godard, Marguerite; Carter, Elliot; Decrausaz, Thierry; Lafay, Romain; Bennett, Emma; Kourim, Fatma; de Obeso, Juan-Carlos; Michibayashi, Katsuyoshi; Harris, Michelle; Coggon, Jude; Teagle, Damon A H; Kelemen, Peter B; The Oman Drilling Project Phase 1 Science Party (2021): Lithology, major, volatile and trace element composition of Hole BT1B samples (Semail ophiolite; ICDP Oman drilling project). PANGAEA, https://doi.org/10.1594/PANGAEA.937490

Description: Carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbon-bearing metasediments. Whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find that relatively little carbon may be recycled. If so, input from subduction zones into the overlying plate is larger than output from arc volcanoes plus diffuse venting, and substantial quantities of carbon are stored in the mantle lithosphere and crust. Also, if the subduction zone carbon cycle is nearly closed on time scales of 5–10 Ma, then the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing. Such an increase is consistent with inferences from noble gas data. Carbon in diamonds, which may have been recycled into the convecting mantle, is a small fraction of the global carbon inventory.

Description: A central debate for the evolution of the Himalayan orogen is how the Greater Himalayan Crystalline complex in its core was emplaced during the Cenozoic Indo-Asian collision. Addressing this problem requires knowledge of the structural relationship between the South Tibet detachment fault (STD) and the Main Central thrust (MCT) that bound these rocks from above and below. The fault relationship is exposed in the Himachal Himalaya of northwestern India, where they merge in their updip direction and form a frontal branch line that has been warped by subsequent top-to-the-southwest shear deformation. To elucidate how the two major crustal-scale faults evolved in the western Himalaya, we conducted integrated geologic research employing field mapping, pressure-temperature (P-T) analyses, U-Pb zircon geochronology, trace and rare earth element (REE) geochemistry, and thermochronology. Our field study reveals complex geometric relationships among major thrusts with large-magnitude shortening within each thrust sheet. Three successive stages of top-to-the-southwest thrust development are recognized: (1) imbricate stack development, (2) translation of large thrust sheets along low-angle detachments and backthrusting along the STD, and (3) development of duplex systems via underplating. This kinematic process can be quantified by our new analytical data: (1) P-T determinations show 7-9 kbar and 450-630 {degrees}C conditions across the STD. The lack of a metamorphic discontinuity across the fault is consistent with a backthrust interpretation. (2) U-Pb zircon geochronology yields ca. 830 Ma and ca. 500 Ma ages of granitoids in the MCT hanging wall, ca. 1.85 Ga ages of granitic gneisses in both the MCT hanging wall and footwall, and 8-6 Ma ages of granitic pegmatites in the MCT footwall. These ages help define regional chronostratigraphy, and the youngest ages reveal a previously unknown intrusion phase. (3) Trace element and REE geochemistry of 1.85 Ga, 830 Ma, and 500 Ma granitoids are characteristic of remelted continental crust, constraining the protolith tectonic setting. (4) U-Pb geochronology of detrital zircon reveals that siliciclastic sedimentary sequences above the STD, below the MCT, and between these two faults have similar age spectra with Neoproterozoic youngest age peaks. This result implies that the STD and MCT each duplicated the same stratigraphic section. (5) Th-Pb geochronology of monazite included in MCT hanging-wall garnet yields Paleozoic and early Tertiary ages, indicating Paleozoic and early Tertiary metamorphism in these rocks. (6) The 40Ar/39Ar thermochronology of the K-feldspar from southern MCT hanging-wall rocks evinces cooling below 220-230 {degrees}C ca. 13-19 Ma or later, constraining the thrust development history. We use these results to derive a tectonic model of crustal shortening across the Himachal Himalaya involving early thickening, tectonic wedging emplacement of the Greater Himalayan Crystalline complex between the MCT and STD, and continued growth of the Himalayan thrust wedge by accretion of thrust horses from the Indian footwall.

Description: The solubility of OH in pure synthetic rutile was experimentally constrained at 0.5-2.0 GPa and 500-900 {degrees}C, in equilibrium with four oxygen fugacity (fO2) buffering mineral assemblages: hematite-magnetite (HM), nickel-nickel oxide (NNO), cobalt-cobalt oxide (CCO), and iron-wustite (IW). The hydroxyl concentration ([OH], in parts per million H2O by weight) of equilibrated rutile crystals was characterized by FTIR spectroscopy. Measurements at 1 GPa at individual fO2 buffers demonstrate that [OH] in rutile depends strongly on temperature: at HM, [OH] increases from 48 to 267 ppm as temperature rises from 500 to 900 {degrees}C, whereas at NNO, [OH] increases from 108 to 956 ppm over the same temperature range. The [OH] in rutile also increases strongly with decreasing fO2 at any pressure and temperature, and exhibits a slight, linear, positive dependence on pressure at a given temperature and fO2. The observed systematic dependences on pressure, temperature, and fO2 indicate that hydrogen substitutes into rutile as hydroxyl, (OH), via forward progress of the reaction Ti4+O2 + [1/2]H2O = Ti3+O(OH) + [1/4]O2. Our measured [OH] values are significantly greater than those determined in previous studies on finer-grained, polycrystalline rutile, which likely suffered diffusive loss of H during quenching. This is supported by our observation of narrow, OH-depleted rims on otherwise high-OH run products, pointing to minor but important diffusive H loss from crystal rims during quenching. Fitting of isothermal variations in composition with fO2 at 1 GPa and temperature indicates nearly ideal, multi-site mixing of the TiO2-TiOOH solid solution. A fit to the entire data set suggests standard volume, enthalpy, and entropy of the hydration reaction of, respectively, 1.90 {+/-} 0.48 cm3/mol, 219.3 {+/-} 1.3 kJ/mol, and 19.9 {+/-} 1.4 J/(mol{middle dot}K) (1{sigma} uncertainty). These constraints form the basis for use of [OH] in rutile as a thermobarometer and oxybarometer in experimental and natural systems. The moderate to high [OH] in nominally anhydrous rutile at all investigated temperatures, pressures, and fO2 values imply that Ti3+ may be higher than previously suspected in some terrestrial geologic settings.