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Site U1586 is the deepest (4692 meters below sea level [mbsl]) and farthest site from shore (170 km) drilled during Expedition 397 (Figures F1, F2, F3). It is located near the toe of the Promontório dos Príncipes de Avis at Common Midpoint (CMP) 1330 on Cruise JC89 Seismic Line 2 near the intersection of Cruise JC089 Line 3 (Figures F4, F5, F6). The continental slope environment is prone to failure and mass transport deposits (MTDs), and large disturbances are recognizable features on seismic profiles. For example, Site U1586 is between two MTDs or disturbed intervals at about 6.3 and 6.5 s two-way traveltime (TWT) on Cruise JC089 Seismic Line 2 near CMPs 1170–1250 and around CMP 1350 (Figure F5). Site U1586 is located where there is good continuity of reflectors to avoid these MTDs, but disturbances may still occur on a shorter length scale than the resolution of the seismic profiles. The target drilling depth of 350 meters below seafloor (mbsf) corresponds to the top of a package of chaotic high-amplitude reflections at 6.6 s TWT that was initially estimated to be late Miocene (~7 Ma) but was later determined biostratigraphically to be middle Miocene (~14 Ma) on the basis of shipboard micropaleontological analyses. The primary scientific objective of Site U1586 was to recover a deep distal record from a water depth of ~4690 mbsl. The sediment thickness thins toward the toe of the Promontório dos Príncipes de Avis owing to lower sedimentation rates with increased distance from shore. Interpretation of the seismic profiles suggests the sequence spans the late Miocene to Quaternary with an average sedimentation rate of 5 cm/ky. Recovery of late Miocene sediment at this site will complement sequences to be drilled during International Ocean Discovery Program (IODP) Expedition 401 to study the exchange between the Mediterranean and Atlantic for the period before, during, and after the Messinian Salinity Crisis (5.96–5.33 Ma) (Flecker et al., 2023). The sediments will also provide a history of surface and deepwater conditions through the Pliocene, including the mid-Pliocene warm period, when atmospheric CO2 was similar to today (400 ppm). Sediments recovered at Site U1586 will also be useful for studying how surface and deep oceanographic conditions responded to the intensification of Northern Hemisphere glaciation in the late Pliocene (~2.9 Ma). Site U1586 is under the influence of Lower Deep Water (LDW), which consists of Antarctic Bottom Water whose properties have been modified significantly from its origin in the high-latitude South Atlantic (Figure F7). This site's great depth may result in carbonate microfossil dissolution, although a 7.45 m piston core (JC089-5-3P) and 4.68 m kasten core (JC089-5-3K) recovered at the same location show continuous preservation of foraminifers during the last glacial stage and Holocene. Results from shipboard analyses during Expedition 397 further show that carbonate preservation and abundance of calcareous microfossils extends back to the Miocene (see Biostratigraphy). Sedimentation rates in the piston core average 11 cm/ky. The Ca/Ti and Zr/Sr measured using core scanning X-ray fluorescence (XRF) show distinct millennial events (Channell et al., 2018), with particularly notable peaks in Zr/Sr marking each of the Heinrich stadials of the last glacial period (Figure F8). Study of Site U1586 cores will permit the reconstruction of changes in ventilation and carbon storage in the deepest Atlantic on glacial–interglacial and millennial timescales with potential implications for atmospheric CO2 changes. Preservation of terrestrial biomarkers and pollen will permit reconstruction of vegetation changes in Europe. Lastly, it should be possible to correlate physical properties at Site U1586 into the Mediterranean cyclostratigraphy, thereby permitting regional climate change to be placed into a global context. 

Site U1587 is the second farthest from shore drilled during Expedition 397 and located at a water depth of 3479 meters below sea level (mbsl) (Figures F1, F2, F3). It is the second deepest site along the bathymetric transect and is bathed today by a mixture of ~75% North Atlantic Deep Water (NADW) and 25% Lower Deep Water (LDW) sourced from the Southern Ocean (Figure F4) (Jenkins et al., 2015). The mixing ratio of these water masses and their vertical position in the water column has changed in the past, which has implications for ventilation and carbon storage in the deep Atlantic Ocean. The location of Site U1587 was motivated by the clear expression of millennial climate variability in proxy records of oxygen isotopes and sea-surface temperature in nearby Piston Core MD95-2042 (Shackleton et al., 2000, 2004; Bard et al., 2000; Pailler and Bard, 2002; Davtian and Bard, 2023). Isotopic, organic biomarker, and pollen results from this core demonstrated the potential of correlating Iberian margin sediments with ice cores from Greenland and Antarctica and with European terrestrial sequences (e.g., Sánchez-Goñi et al., 2000; Margari et al., 2010, 2014, 2020). The sediment record from Site U1587 provides the opportunity to develop sediment proxy records for the Greenland and Antarctic ice cores to the base of the Quaternary and beyond. The piston core (JC089-04-2P) recovered near Site U1587 is 10.7 m long and has a sedimentation rate of 17 cm/ky (Figure F5). Ca/Ti and Zr/Sr show strong evidence of millennial variability during the last glacial cycle. The objective for Site U1587 is to study such variability for older glacial cycles throughout the Quaternary. Site U1587 is located at the intersection of Seismic Lines JC089-6 and JC089-7 (Figure F6). Although mass transport deposits or disturbed intervals are developed nearby, the continuity of reflections is good at Site U1587 (Figures F7, F8). The Upper Miocene to Quaternary sequence at Site U1587 is expanded relative to Site U1586 and is more than 500 m thick. Sedimentation rates are estimated to average ~10 cm/ky at Site U1587, or about twice that of Site U1586. We had permission from the Environmental Protection and Safety Panel (EPSP) to drill to 500 meters below seafloor (mbsf), but we requested and were granted permission to drill an additional 50 m to extend the record well into the late Miocene. Site U1587 provides an expanded sequence of late Miocene to Quaternary sediments with which to address the following objectives: Document how millennial climate variability evolved during the glacial cycles of the Quaternary and Pliocene as boundary conditions changed with the progressive intensification of Northern Hemisphere glaciation (NHG). Reconstruct the history of changing local dominance of northern-sourced versus southern-sourced deep water, as well as ventilation and carbon storage in the deep Atlantic Ocean. Determine interhemispheric phase relationships (leads/lags) by comparing the timing of proxy variables that monitor surface (Greenland) and deepwater (Antarctic) components of the climate system. Investigate climate during past interglacial periods, including the warm Pliocene period prior to the intensification of NHG. Link terrestrial, marine, and ice core records by analyzing pollen and terrestrial biomarkers that are delivered to the deep-sea environment of the Iberian margin. Recover a complete record of the time leading up to, during, and following the Messinian Salinity Crisis, which complements objectives of International Ocean Discovery Program (IODP) Expedition 401 (Flecker et al., 2023) and will permit evaluation of the causes and consequences of this remarkable event in Earth's history. Develop an orbitally-tuned age model for Site U1587 by correlating physical properties to eccentricity-modulated precession and tying them into the record of Mediterranean cyclostratigraphy. 

Site U1588 lies on the broad, gently inclined middle-slope region of the Promontório dos Príncipes de Avis (PPA) on the southwestern Iberian margin (Figures F1, F2, F3, F4). It is the shallowest (1339 meters below sea level [mbsl]) and the closest site to the coast drilled during Expedition 397. Seismic data indicate the location is part of an extensive plastered sediment drift deposit formed under the influence of Mediterranean Outflow Water (MOW) that extends from the Gulf of Cádiz along the Portuguese margin (Hernández-Molina et al., 2003, 2006; Llave et al., 2006). Site U1588 is positioned on Seismic Line TGS-NOPEC PD00-613 (Figures F5, F6) about 8 km northeast of the intersection with Seismic Line TGS-NOPEC PD00-510 (Figure F7), where the distal part of the contourite depositional system occurs as a highly expanded sedimentary record (Hernández-Molina et al., 2014). The combination of Site U1588 (1339 mbsl) and the intermediate, MOW-influenced sites drilled during Expedition 339 (560–1073 mbsl) span the full depth range of the MOW. The record of Site U1588 will be compared with the deeper sites of Expedition 397 (2591–4692 mbsl), which are removed from contourite input and are under the influence of Northeast Atlantic Deep Water (NEADW) and Lower Deep Water (LDW) (Figure F4). Expedition 339 and 397 sites will constitute a bathymetric transect from 560 to 4691 mbsl to be used for paleoconductivity-temperature-depth (paleo-CTD) studies. The record at Site U1588 is expected to be similar to that of Integrated Ocean Drilling Program Site U1391, which was drilled 70 km south–southeast of Site U1588 at a water depth of 1085 mbsl (Figure F2). Three holes were cored and drilled to a total depth of 672 mbsf, spanning the middle Pliocene to Holocene with sedimentation rates of 13–17 cm/ky (Expedition 339 Scientists, 2013). We expected similarly high sedimentation rates at Site U1588. The high accumulation rates associated with contourite deposition provide an expanded sedimentary record to reconstruct millennial climate variability (MCV) with a resolution of a hundred years. In addition, the shallow depth of Site U1588 compared to other Expedition 397 sites is expected to yield excellent preservation of carbonate microfossils. The primary objective of Site U1588 was to drill to 500 mbsf and recover an expanded Pliocene–Pleistocene sedimentary succession formed under the influence of lower MOW (Hernández-Molina et al., 2014). The sediments of Site U1588 and those collected during Expedition 339 provide valuable material for studying how the depth and intensity of the MOW has varied on orbital and millennial timescales. Site U1588 will also provide a marine reference section for studying Quaternary climate variability at high stratigraphic resolution. Surface temperature proxies of Site U1588 will document MCV well beyond the base of the polar ice core. Because Site U1588 is closest to the coast, we expect preservation of terrestrial pollen and biomarkers that will allow linkage of the continental and marine records. Site U1588 should also preserve an MCV record of coastal upwelling along the Portuguese margin. 

This chapter provides an overview of the procedures and methods employed for coring operations and in the shipboard laboratories of the R/V JOIDES Resolution during International Ocean Discovery Program (IODP) Expedition 397. The laboratory information applies only to shipboard work described in the Expedition Report section of the Expedition 397 Proceedings of the International Ocean Discovery Program volume that includes the shipboard sample registry, imaging and analytical instruments, core description tools, and the Laboratory Information Management System (LIMS) database. The shipboard workflow followed standard IODP procedures (as previously described by, e.g., Huber et al., 2019; Winckler et al., 2021; Planke et al., 2023), with revisions and refinements as described in this chapter. Methods used by investigators for shore-based analyses of Expedition 397 data will be documented in separate publications. All shipboard scientists contributed to this volume with the following primary responsibilities (authors are listed in alphabetical order; see Expedition 397 scientists for contact information): Summary chapter: Expedition 397 Scientists Methods and site chapters: Background and objectives: F. Abrantes, D. Hodell Operations: C.A. Alvarez Zarikian, K. Grigar Lithostratigraphy: H.L. Brooks, J.M. Link, J. McManus, C. Pallone, X. Pang, E. Salgueiro, V. dos Santos Rocha, J. Yu Biostratigraphy: C.A. Alvarez Zarikian, W. Clark, J.-A. Flores, M. Peral, K. Verma Paleomagnetism: L. Dauchy-Tric, C. Xuan Geochemistry: S. Hines, B. Mitsunaga, L. Nana Yobo, J. Wu Physical properties and downhole measurements: H.-H.M. Huang, H. Ikeda, J. Kuroda, S. Sanchez Stratigraphic correlation: T. Herbert, H.-H.M. Huang, S. Kaboth-Bahr This introductory section provides an overview of drilling and coring operations, core handling, curatorial conventions, depth scale terminology, and the sequence of shipboard analyses. Subsequent sections of this chapter document specific laboratory instruments and methods in detail. 

During Expedition 339, Site U1385 (37°34.2849′N, 10°7.5616′W) was drilled to a maximum penetration of 151.5 meters below seafloor (mbsf) (Expedition 339 Scientists, 2013) (Figure F1). Site U1385 was a proof of concept to test the continuity and fidelity of the sedimentary record and to support further drilling on the Iberian margin. Results from Site U1385 demonstrated the great promise of the Iberian margin to yield long, continuous records of millennial climate variability (MCV) and detailed land-sea comparisons. Almost exactly 11 y later, we reoccupied Site U1385 (37°34.0128′N, 10°7.6580′W) during Expedition 397 to deepen the sequence. We elected to retain the same site designation, beginning with Hole U1385F, and distinguish the two sites as Site 339-U1385 and Site 397-U1385. Site 397-U1385 is located <1 km southwest of Site 339-U1385 (Figure F2), and seismic data indicate the stratigraphy is continuous between the two locations. On 25–29 November 2011, Site 339-U1385 was drilled in the lower slope of the Portuguese margin to provide a marine reference section of Pleistocene MCV. Five holes were cored (Holes U1385A–U1385E) using the advanced piston corer (APC) system (Figure F3) (Expedition 339 Scientists, 2013). Hole U1385C consisted of a single core in an attempt to retrieve the mudline. With only 4 days of drilling for Site 339-U1385, we were only able to drill to a maximum depth of 156 mbsf. A composite section was constructed using all holes to 166.5 corrected revised meters composite depth (crmcd) by correlating elemental ratios (Ca/Ti) measured by core scanning X-ray fluorescence (XRF) at 1 cm resolution (Hodell et al., 2015). For Site 397-U1385, we eventually intend to correlate the top part of the section to the holes drilled during Expedition 339 and provide an integrated composite reference splice (Hodell et al., 2015). This will permit isotope and other proxy measurements from existing Expedition 339 Holes U1385A–U1385E to be integrated with new data from Expedition 397 Holes U1385F–U1385J. The Site 339-U1385 record extends to 1.45 Ma (Marine Isotope Stage [MIS] 47) with an average sedimentation rate of 11 cm/ky (Figures F3, F4). Sedimentation rates are expected to be similar throughout the Quaternary. The record is mostly complete except for a short hiatus at Termination V that has removed part of late MIS 12 and early MIS 11. It is hoped that drilling at the new location of Site 397-U1385 would avoid this hiatus and provide a continuous sequence, filling the gap at Site 339-U1385. Because the existing working halves of Expedition 339 Holes U1385A–U1385E have been largely depleted to meet intense sampling demand, the uppermost 150 m were duplicated at Site 397-U1385 to provide additional sediment for future studies. Site 339-U1385 has been studied extensively since it was recovered over a decade ago (see the Expedition 339 Expedition-related bibliography [Stow, Hernández-Molina, Alvarez Zarikian, and the Expedition 339 Scientists, 2013]) and has provided a high-resolution benchmark record of MCV for the past 1.45 My (Hodell et al., 2023). Extending this remarkable sediment archive further back in time was the primary goal of reoccupying Site U1385 during Expedition 397. Site 397-U1385 is located ~1 km southwest of Expedition 339 Site U1385 at a water depth of 2591 meters below sea level (mbsl), placing it in the core of Lower Northeast Atlantic Deep Water (LNEADW) today (Figure F5). It is the second shallowest site along the Expedition 397 bathymetric transect (paleo-conductivity-temperature-depth [paleo-CTD]) (Figure F6) and is located along an elevated ridge (Figure F7), thereby decreasing the chances of disturbance by downslope transport. It is located on Seismic Line JC089-9 close to the intersection of Seismic Line JC089-13 (Figure F2) and near the position of Piston Core MD01-2444 (Figure F1). The objective is to recover the deeper part of the section below Site 339-U1385 to the base of the Pliocene (orange reflector) at 400 mbsf (Figures F8, F9), which will more than double the section recovered at Site 339-U1385. The specific objectives of Site 397-U1385 are as follows: Document the nature of MCV for older glacial cycles of the Quaternary beyond the penetration limit of Site 339-U1385 (1.45 Ma). Derive a marine sediment proxy record for Greenland and Antarctic ice cores to examine the amplitude and pacing of MCV during the Quaternary. Determine interhemispheric phase relationships (leads/lags) by comparing the timing of proxy variables that monitor surface (linked to Greenland) and deepwater (linked to Antarctica) components of the climate system. Study how changes in orbital forcing and glacial boundary conditions affect the character of MCV and, in turn, how MCV interacts with orbital geometry to produce the observed glacial-to-interglacial patterns of climate change. Determine how MCV evolved during the Pliocene–Pleistocene as glacial boundary conditions changed with the progressive intensification of Northern Hemisphere glaciation (NHG). Reconstruct the history of changing local dominance of northern-sourced versus southern-sourced deep water by comparing Site U1385 with the other sites along the bathymetric transect (Figure F6). Investigate climate during past interglacial periods, including the warm Pliocene period prior to the intensification of NHG. Link terrestrial, marine, and ice core records by analyzing pollen and terrestrial biomarkers that are delivered to the deep-sea environment of the Iberian margin. Develop an orbitally tuned age model by correlating sediment physical properties at Site U1385 to eccentricity-modulated precession and integrating this record into Mediterranean cyclostratigraphy. 

During International Ocean Discovery Program Expedition 397, we recovered a total of 6176.7 m of core (104.2% recovery) at four sites (U1586, U1587, U1385, and U1588) from the Promontório dos Príncipes de Avis, a plateau located on the Portuguese continental slope that is elevated above the Tagus Abyssal Plain and isolated from the influence of turbidites. The drill sites are arranged along a bathymetric transect (4692, 3479, 2591, and 1339 meters below sea level [mbsl], respectively) to intersect each of the major subsurface water masses of the eastern North Atlantic. Multiple holes were drilled at each site to ensure complete spliced composite sections, which will be further refined postcruise by a campaign of X-ray fluorescence core scanning. At Site U1586 (4692 mbsl), the deepest and farthest from shore, a 350 m sequence was recovered in four holes that extend as far back as the middle Miocene (14 Ma), which is nearly twice as old as initially predicted from seismic stratigraphy. Sedimentation rates are lower (averaging 5 cm/ky in the Quaternary) at Site U1586 than other Expedition 397 sites, and a few slumped intervals were encountered in the stratigraphic sequence. Despite these limitations, Site U1586 anchors the deep end-member of the bathymetric transect and provides an important reference section to study deepwater circulation, ventilation and carbon storage in the deep eastern North Atlantic. At Site U1587 (3479 mbsl), the second deepest site along the depth transect, we recovered a 567 m sequence of late Miocene to Holocene sediments that accumulated at rates between 6.5 and 11 cm/ky. The high sedimentation rates and long continuous record at this site will permit climate reconstruction at high temporal resolution (e.g., millennial) for the past 7.8 My. The Messinian Stage (7.25–5.33 Ma) was recovered, which provides a valuable opportunity to study the Messinian Salinity Crisis in an open marine setting adjacent to the Mediterranean. Site U1385 (Shackleton site) was a reoccupation of a position previously drilled during Integrated Ocean Drilling Program Expedition 339. Expedition 339 Site U1385 has yielded a remarkable record of millennial-scale climate change for the past 1.45 My (Marine Isotope Stage 47). During Expedition 397, we deepened the site from 156 to 400 meters below seafloor, extending the record to near the base of the Pliocene (5.3 Ma). Sedimentation rates remained high, averaging between 9 and 11 cm/ky throughout the sequence. The newly recovered cores at Expedition 397 Site U1385 will permit the study of millennial climate variability through the entire Quaternary and Pliocene, prior to the intensification of Northern Hemisphere glaciation. Site U1588 is the shallowest, closest to shore, and youngest site drilled during Expedition 397 and is also the one with the highest sedimentation rate (20 cm/ky). The base of the 412.5 m sequence is 2.2 Ma, providing an expanded Pleistocene sequence of sediment deposited under the influence of the lower core of the Mediterranean Outflow Water (MOW). Together with other Expedition 339 sites, Site U1588 will be important for determining how the depth and intensity of the MOW has varied on orbital and millennial timescales. In addition, it also provides a marine reference section for studying Quaternary climate variability at very high temporal resolution (millennial to submillennial). A highlight of the expedition is that sediment at all sites shows very strong cyclicity in bulk sediment properties (color, magnetic susceptibility, and natural gamma radiation). Particularly remarkable are the precession cycles of the Pliocene that can be correlated peak-for-peak among sites. These cyclic variations will be used to derive an orbitally tuned timescale for Expedition 397 sites and correlate them into classic Mediterranean cyclostratigraphy. The cores recovered during Expedition 397 will form the basis of collaborative postcruise research to produce benchmark paleoclimate records for the late Miocene through Quaternary using the widest range of proxy measurements. It will take many years to complete these analyses, but the records will lead to major advances in our understanding of millennial and orbital climate changes and their underlying causes and evolving contextuality. Outreach during Expedition 397 was highly productive, reaching a record number of students and the general public across the world through several diverse platforms, including live ship-to-shore events, webinars, social media, videos, radio pieces, blog posts, and in-person activities. 

Throughout the course of an organism’s life, the chemical signatures of environment, food consumption, and weather are recorded into their carbonate structures; these signatures can be directly linked to a time-resolved lifespan. Here we present trace element data from benthic foraminifera and tropical molluscs determined using an ESI NWR193UC excimer laser coupled with an Agilent 8900 triple quadrupole mass spectrometer in the MicroAnalytical Geochemistry and Isotope Characterization (MAGIC) Laboratory at the University of Maine. Benthic foraminifera are protists that live on the sea floor and produce calcite shells, progressively adding chambers. Changes in Mg/Ca in foraminifera are used as a proxy for ocean temperature. Laser ablation ICP-MS data for 18 trace elements were collected in individual growth chambers in foraminifera of the genus Uvigerina from the Bay of Plenty. Line scans were performed within thin (~10 µm) chamber walls using a spot size of 8 µm, beam energy density of 3 J/cm2, repetition rate of 12 Hz, and scan speeds of 2-3 µm/s. Concentrations were determined relative to the NIST610 glass. Ratios of Mg/Ca and other trace elements record the same range of values as those determined via bulk wet chemistry analysis of ~10 foraminifera for a given population, which suggests that LA-ICP-MS may be a viable alternative to wet chemistry. Trace element data were collected across shells of the warm-tropical mollusc species Chione subrugosa from the Ostra Base Camp area, Peru (78°37’22”W, 8°54’46”S). Previous studies of the area have suggested that a large climate transition occurred, transforming a warm water tropical bay into a desert surrounded by a coastal stand with cool waters. This area was occupied by humans at 6250-5450 radiocarbon years BP. This study examines Chione subrugosa, which were found in the living position at the fossilized Ostra Beach and are thought to have been the final living warm-tropical molluscs in the bay. Studies of modern molluscs have revealed that molluscs record massive climatic changes, such as El Niño, in their chemistry. Laser ablation provides a unique opportunity to examine chemical changes directly related to the changing coastal environment. Line scans transverse growth bands along the length of the shell, providing a high resolution record of daily variation in trace element chemistry over the lifespan of the mollusc. Eleven elements were analysed with a beam energy density of 2.4 J/cm2, repetition rate of 15 Hz, spot size of 5 x 25 µm, and a scan speed of 5 µm/s. Preliminary data suggest the preservation of yearly oscillations in trace elements, with high concentrations of La, Ce, U, and Pb during early shell growth. Continued study will examine catastrophic mollusc life events in an effort to link these with environmental climate changes over daily timescales. 

Titanite has the ability to incorporate significant amounts of common Pb, which leads to uncertainty when applying the U-Pb decay series for geochronology. The isobaric interference of 204Hg on 204Pb poses an additional complexity in applying common Pb corrections. Here we investigate the removal of 204Hg interferences during titanite U-Pb dating using reaction cell gas chemistry via triple quadrupole mass spectrometry. U-Pb dates were determined for the natural titanite reference materials MKED-1 and BLR1 using an ESI NWR193UC excimer laser coupled to an Agilent 8900 ‘triple quad’ mass spectrometer. The 8900 is equipped with an octopole collision/reaction cell, which enables online interference removal. Two experiments were run, one in which we collected data in NoGas mode, and one in which NH3 was used as a reaction cell gas in MS/MS mode, in order to assess the feasibility of determining U/Pb ratios with mass shifted isotopes. In all experiments, a signal smoothing device was placed inline just before the ICP-MS interface, downstream from the addition of the Ar nebulizer gas to the He carrier gas stream. For the NoGas experiment, titanite was ablated using a 25 µm spot, with a beam energy density of 3 J/cm2, and a pulse rate of 4 Hz. In NoGas mode, signal intensities for the isotopes 201Hg, 202Hg, 204Pb, 206Pb, 207Pb, 232Th, 235U, and 238U were counted. In MS/MS mode, titanite was ablated using a 40 µm spot, with a beam energy density of 5 J/cm2, and a pulse rate of 4 Hz. A larger spot size in this experiment was used to counteract the decrease in signal intensity due to use of the reaction cell. In MS/MS mode, NH3 was flowed through the reaction cell in order to enable a charge transfer reaction between NH3 and Hg+, effectively neutralizing Hg. The isotopes 201Hg, 202Hg, 204Pb, 206Pb, and 207Pb were measured on-mass, as the isotopes of Pb are not affected by the NH3 gas. Uranium and Th both exhibit partial reaction with NH3 gas; therefore, the isotopes 232Th, 235U, and 238U were measured mass-shifted up 15 mass units, at masses 247, 250, and 253 respectively. Ratios of 207Pb/235U, 206Pb/238U, and 207Pb/206Pb were determined using the UPbGeochron4 DRS in Iolite (v.3.71) with MKED-1 as the primary reference material. Dates were calculated using IsoplotR by applying the Stacey-Kramers correction for common Pb. All isotopes of Hg were effectively neutralized by the NH3 charge transfer reaction in MS/MS mode; zero counts were detected for Hg isotopes. Dates for the BLR-1 titanite were 1050.55 ± 2.72 (2σ, n=12) Ma in NoGas mode, and 1048 ± 1.88 (2σ, n=15) Ma in MS/MS mode. These dates are in excellent agreement with the TIMS 206Pb/238U date for the BLR-1 titanite of 1047.1 ± 0.4 Ma. This method has the potential to enable measurement of 204Pb without needing to correct for Hg interferences. 

Titanite and apatite can incorporate significant amounts of common Pb (204Pb) into their mineral structures, which leads to uncertainty when applying the U-Pb decay series for geochronology. The isobaric interference between 204Pb and 204Hg creates an additional complexity when calculating common lead corrections. Here we investigate the removal of 204Hg interferences during titanite U-Pb dating using reaction cell gas chemistry via triple quadrupole mass spectrometry compared with traditional methods that calculate U-Pb ages using a common lead correction. U-Pb dates for titanite natural reference materials MKED-1 and BLR-1 were determined using an ESI NWR193UC excimer laser coupled with an Agilent 8900 ‘triple quadrupole’ mass spectrometer. The 8900 is equipped with an octopole collision/reaction cell, which enables online interference removal. In order to compare traditional methods for U-Pb dating with interference removal methods, two experiments were run, one in which data was collected in NoGas mode, and one in which the 8900 was run in MS/MS mode, in order to assess the feasibility of determining U/Pb ratios with mass shifted isotopes. In MS/MS mode, NH3 was flowed through the reaction cell in order to enable a charge transfer reaction between NH3 and Hg+, effectively neutralizing Hg. During spot analyses in NoGas mode, masses 202Hg, 204Hg, 204Pb, 206Pb, 207Pb, 208Pb, 232Th, 235U, and 238U were monitored. For spot analyses in MS/MS mode, Th and U isotopes were measured on-mass at 232Th, 235U, 238U and mass-shifted to 247Th, 250U, and 253U. Pb isotopes were measured on-mass since Pb does not react with NH3. Ratios for 207Pb/235U, 206Pb/238U, and 207Pb/206Pb were calculated in Iolite (v.3.7.1) using the Geochron4 DRS using MKED-1 as the primary reference material and BLR-1 as a secondary reference material. Dates were calculated using IsoplotR. Weighted mean ages for titanite BLR-1 in MS/MS mode are 1043.8 ± 10.5 Ma (2σ, MSWD=1.08) for U isotopes measured on mass, and 1039.7 ± 8.3 Ma (2σ, MSWD=1.08) for mass-shifted U isotopes. These dates are both in agreement with the TIMS 206Pb/238U date for the BLR-1 titanite of 1047.1 ± 0.4 Ma. The use of NH3 for reaction cell chemistry has the potential to enable measurement of 204Pb without needing to correct for Hg interferences.