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In situ apatite U-Pb petrochronology and Sr-Nd isotope geochemistry requires well-characterized and matrix-matched references materials (RMs), yet only a few suitable apatite RMs are currently available. To ameliorate this issue, we determined the U-Pb, Sm-Nd, and Sr isotopic and elemental compositions of a suite of prospective apatite RMs using isotope dilution (ID) TIMS and laser ablation (LA) ICP-MS. The two RMs, from Morocco (MRC-1) and Brazil (BRZ-1), are cm-sized and available in significant quantities. The U-Pb ID-TIMS data yield an isochron age of 153.3 ± 0.2 Ma for MRC-1. This age is consistent with laser ablation split stream ICP-MS (LASS) analyses that produce an isochron age of 152.7 ± 0.6 Ma. The weighted mean of ID-TIMS analyses for 143Nd/144Nd analyses is 0.512677 ± 3, for 147Sm/144Nd is 0.10923 ± 9, and for 87Sr/86Sr is 0.707691 ± 2. The range and mean of TIMS Sm-Nd isotopic data are reproducible by LA-ICP-MS, but laser ablation Sr data are consistently offset towards more radiogenic values. For BRZ-1 apatite, ID-TIMS U-Pb analyses are dispersed, but a subset of the data yields a coherent age intercept of 2078 ± 13 Ma. The vast majority of LASS spot transects across the apatite produce an isochron that define a younger age of 2038 ± 14 Ma. We interpret this as incorporation of cryptic, younger altered domains within BRZ-1. Discordant U-Pb spot analyses are associated with chemically distinct cracks, likely a result of fluid infiltration. The weighted means of ID-TIMS analyses of BRZ-1 yield 143Nd/144Nd = 0.510989 ± 5, 147Sm/144Nd = 0.10152 ± 8, and 87Sr/86Sr = 0.709188 ± 3. The distribution of Nd isotopic compositions of this RM measured by LA-MC-ICP-MS analyses are comparable to TIMS analyses. By contrast, 87Sr/86Sr measurements by LA-ICP-MS are inaccurate and exhibit large uncertainties, but this RM can be useful for empirically correcting in situ 87Sr/86Sr measurements. The data indicate that MRC-1 apatite may serve well as a U-Pb, Sm-Nd, and Sr RM, whereas BRZ-1 apatite has the most potential as a Sm-Nd RM. These potential RMs provide new benchmarks for in situ apatite chemical analyses and inter-laboratory calibrations. 

Abstract This study addresses the question of how and where arc magmas obtain their chemical and isotopic characteristics. The Wooley Creek batholith and Slinkard pluton are a tilted, mid- to upper-crustal part of a vertically extensive, late-Jurassic, arc-related magmatic system in the Klamath Mountains, northern California. The main stage of the system is divided into an older lower zone (c. 159 Ma) emplaced as multiple sheet-like bodies, a younger upper zone (c. 158–156 Ma), which is gradationally zoned upward from mafic tonalite to granite, and a complex central zone, which represents the transition between the lower and upper zones. Xenoliths are common and locally abundant in the lower and central zones and preserve a ghost stratigraphy of the three host terranes. Bulk-rock Nd isotope data along with ages and Hf and oxygen isotope data on zircons were used to assess the location and timing of differentiation and assimilation. Xenoliths display a wide range of εNd (whole-rock) and εHf (zircon), ranges that correlate with rocks in the host terranes. Among individual pluton samples, zircon Hf and oxygen isotope data display ranges too large to represent uniform magma compositions, and very few data are consistent with uncontaminated mantle-derived magma. In addition, zoning of Zr and Hf in augite and hornblende indicates that zircon crystallized at temperatures near or below 800 °C; these temperatures are lower than emplacement temperatures. Therefore, the diversity of zircon isotope compositions reflects in situ crystallization from heterogeneous magmas. On the basis of these and published data, the system is interpreted to reflect initial MASH-zone differentiation, which resulted in elevated δ18O and lowered εHf in the magmas prior to zircon crystallization. Further differentiation, and particularly assimilation–fractional crystallization, occurred at the level of emplacement on a piecemeal (local) basis as individual magma batches interacted with partial melts from host-rock xenoliths. This piecemeal assimilation was accompanied by zircon crystallization, resulting in the heterogeneous isotopic signatures. Magmatism ended with late-stage emplacement of isotopically evolved granitic magmas (c. 156 Ma) whose compositions primarily reflect reworking of the deep-crustal MASH environment. 

Rhyolitic melt that fuels explosive eruptions often originates in the upper crust via extraction from crystal-rich sources, implying an evolutionary link between volcanism and residual plutonism. However, the time scales over which these systems evolve are mainly understood through erupted deposits, limiting confirmation of this connection. Exhumed plutons that preserve a record of high-silica melt segregation provide a critical subvolcanic perspective on rhyolite generation, permitting comparison between time scales of long-term assembly and transient melt extraction events. Here, U-Pb zircon petrochronology and 40 Ar/ 39 Ar thermochronology constrain silicic melt segregation and residual cumulate formation in a ~7 to 6 Ma, shallow (3 to 7 km depth) Andean pluton. Thermo-petrological simulations linked to a zircon saturation model map spatiotemporal melt flux distributions. Our findings suggest that ~50 km 3 of rhyolitic melt was extracted in ~130 ka, transient pluton assembly that indicates the thermal viability of advanced magma differentiation in the upper crust. 

Abstract Lavas erupted at hotspot volcanoes provide evidence of mantle heterogeneity. Samoan Island lavas with high⁸⁷Sr/⁸⁶Sr (>0.706) typify a mantle source incorporating ancient subducted sediments. To further characterize this source, we target a single high⁸⁷Sr/⁸⁶Sr lava from Savai’i Island, Samoa for detailed analyses of⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd isotopes and major and trace elements on individual magmatic clinopyroxenes. We show the clinopyroxenes exhibit a remarkable range of⁸⁷Sr/⁸⁶Sr—including the highest observed in an oceanic hotspot lava—encompassing ~30% of the oceanic mantle’s total variability. These new isotopic data, data from other Samoan lavas, and magma mixing calculations are consistent with clinopyroxene⁸⁷Sr/⁸⁶Sr variability resulting from magma mixing between a high silica, high⁸⁷Sr/⁸⁶Sr (up to 0.7316) magma, and a low silica, low⁸⁷Sr/⁸⁶Sr magma. Results provide insight into the composition of magmas derived from a sediment-infiltrated mantle source and document the fate of sediment recycled into Earth’s mantle. 

Abstract Coupled U‐Pb and trace‐element analyses of accessory phases in crustal xenoliths from the Late Devonian Udachnaya kimberlite (Siberian craton, Russia) are used to constrain Moho temperature and crustal heat production at the time of kimberlite eruption. Rutile and apatite in lower‐crustal garnet granulites record U‐Pb dates that extend from 1.8 Ga to 360 Ma (timing of kimberlite eruption). This contrasts with upper‐crustal tonalites and amphibolites that contain solely Paleoproterozoic apatite. Depth profiling of rutile from the lower‐crustal xenoliths show that U‐Pb dates increase gradually from rim to core over μm‐scale distances, with slower‐diffusing elements (e.g., Al) increasing in concentration across similar length‐scales. The U‐Pb and trace element gradients in rutile are incompatible with partial Pb loss during slow cooling, but are consistent with neocrystallization and re‐heating of the lower crust for <1 Myr prior to eruption. Because Paleoproterozoic rutile and apatite dates are preserved, we infer that long‐term ambient lower‐crustal temperatures before this thermal perturbation were cooler than the Pb closure temperature of rutile and probably apatite (<400°C). The lower‐crustal temperature bounds from these data are consistent with pressure‐temperature arrays of Udachnaya peridotite xenoliths that suggest relatively cool geothermal gradients, signifying that the mantle xenoliths accurately capture the thermal state of the lithosphere prior to eruption. Combined, the xenolith data imply low crustal heat production for the Siberian craton (∼0.3 μW/m³). Nevertheless, such values produce surface heat flow values of 20–40 mW/m², higher than measured around Udachnaya (average 19 mW/m²), suggesting that the surface heat flow measurements are inaccurate. 

Rare earth element (REE) ore-bearing carbonatite dikes and a stock at Mountain Pass, California, are spatially associated with a suite of ultrapotassic plutonic rocks, and it has been proposed that the two are genetically related. This hypothesis is problematic, given that existing geochronological constraints indicate that the carbonatite is ∼15–25 Myr younger than the ultrapotassic rocks, requiring alternative models for the formation of the REE ore-bearing carbonatite during a separate event and/or via a different mechanism. New laser ablation split-stream inductively coupled plasma mass spectrometry (LASS-ICP-MS) petrochronological data from ultrapotassic intrusive rocks from Mountain Pass yield titanite and zircon U–Pb dates from 1429 ± 10 to 1385 ± 18 Ma, expanding the age range of the ultrapotassic rocks in the complex by ∼20 Myr. The ages of the youngest ultrapotassic rocks overlap monazite Th–Pb ages from a carbonatite dike and the main carbonatite ore body (1396 ± 16 and 1371 ± 10 Ma, respectively). The Hf isotope compositions of zircon in the ultrapotassic rocks are uniform, both within and between samples, with a weighted mean εHf i of 1·9 ± 0·2 (MSWD = 0·9), indicating derivation from a common, isotopically homogeneous source. In contrast, in situ Nd isotopic data for titanite in the ultrapotassic rocks are variable (εNd i = –3·5 to –12), suggesting variable contamination by an isotopically enriched source. The most primitive εNd i isotopic signatures, however, do overlap εNd i from monazite (εNd i = –2·8 ± 0·2) and bastnäsite (εNd i = –3·2 ± 0·3) in the ore-bearing carbonatite, suggesting derivation from a common source. The data presented here indicate that ultrapotassic magmatism occurred in up to three phases at Mountain Pass (∼1425, ∼1405, and ∼1380 Ma). The latter two stages were coeval with carbonatite magmatism, revealing previously unrecognized synchronicity in ultrapotassic and carbonatite magmatism at Mountain Pass. Despite this temporal overlap, major and trace element geochemical data are inconsistent with derivation of the carbonatite and ultrapotassic rocks by liquid immiscibility or fractional crystallization from common parental magma. Instead, we propose that the carbonatite was generated as a primary melt from the same source as the ultrapotassic rocks, and that although it is unique, the Mountain Pass ultrapotassic and carbonatite suite is broadly similar to other alkaline silicate–carbonatite occurrences in which the two rock types were generated as separate mantle melts.