The advantages in provenance research of U‐Pb dating different detrital minerals along with simultaneously analyzing trace elements is demonstrated in a study of sand from the mouth of the Merrimack ...River in New England, USA. Zircon ages record episodes of magmatism in the Early Paleozoic, peaking in the Early Devonian, followed by quiescence through the remainder of the Paleozoic and additional magmatic episodes in the Jurassic and Cretaceous. Simultaneous measurement of trace elements in zircons reveals a shift from arc magmatism to crustal melting associated with terrane collision in the Early Devonian, while many Jurassic grains are clearly derived from A‐type granites. Detrital monazites and rutiles have Devonian and Permian ages. Many of the older monazites have trace element characteristics suggestive of igneous origin, while Permian monazites are clearly metamorphic and record orogenesis that is absent from the detrital zircon record. Rutile grains have trace element chemistry indicative of mostly metasedimentary source rocks, and Zr thermometry indicates growth under amphibolite facies conditions. Age offsets between monazite and rutile populations provide information about the region's cooling history. Titanite grains have trace element chemistry mostly consistent with igneous origin and U‐Pb ages lining up with minor zircon age populations in the Ordovician‐Silurian and the Middle Devonian, suggesting that these magmatic episodes produced metaluminous compositions. These results show that combining trace element fingerprinting with dating and analyzing multiple detrital mineral species provide a more complete portrait of the geologic history of the sediment source region than U‐Pb dating of zircon alone.
Key Points
Provenance studies can greatly benefit from the U‐Pb dating of multiple detrital minerals
Combining trace element measurement with dating can help fingerprint source rock types for detrital zircon, monazite, rutile, and titanite
An example of the multiproxy approach utilizing four different minerals is provided here in a study of New England river sand
Molybdenum isotopes are fractionated by Earth‐surface processes and may provide a tracer for the recycling of crustal material into the mantle. Here, we examined the Mo isotope composition of arc ...lavas from Martinique in the Lesser Antilles arc, along with Cretaceous and Cenozoic Deep Sea Drilling Project sediments representing potential sedimentary inputs into the subduction zone. Mo stable isotope composition (defined as δ98Mo in ‰ deviation from the NIST 3134 standard) in lavas older than ∼7 million years (Ma) exhibits a narrow range similar to and slightly higher than MORB, whereas those younger than ∼7 Ma show a much greater range and extend to unusually low δ98Mo values. Sediments from DSDP Leg 78A, Site 543 have uniformly low δ98Mo values whereas Leg 14, Site 144 contains both sediments with isotopically light Mo and Mo‐enriched black shales with isotopically heavy Mo. When coupled with published radiogenic isotope data, Mo isotope systematics of the lavas can be explained through binary mixing between a MORB‐like end‐member and different sedimentary compositions identified in the DSDP cores. The lavas older than ∼7 Ma were influenced by incorporation of isotopically heavy black shales into the mantle wedge. The younger lavas are the product of mixing isotopically light sedimentary material into the mantle wedge. The change in Mo isotope composition of the lavas at ∼7 Ma is interpreted to reflect the removal of the Cretaceous black shale component due to the arrival of younger ocean crust where the age‐equivalent Cretaceous sediments were deposited in shallower oxic waters. Isotopic fractionation of Mo during its removal from the slab is not required to explain the observed systematics in this system.
Key Points
Mo isotopes are useful tracers for subducted sediment contributions to arc magmas
Secular changes in the Mo isotope composition of arc lavas from Martinique reflect changes in the isotope composition of incoming sediment
Mo isotopic fractionation during removal of Mo from the subducting slab is not required to explain isotopic systematics of the arc lavas
Complete sample digestion is a prerequisite for achieving reproducible and accurate analytical results for geological samples. Open-vessel acid digestions successfully dissolve mafic samples, but ...this method cannot achieve complete dissolution of felsic samples, because of the presence of refractory minerals such as zircon. In this study, an efficient and simplified digestion technique using the solid compound NH4HF2 in a screw-top vial has been developed for multielement analysis of different types of rock samples. NH4HF2 has a higher boiling point (239.5 °C) than conventional acids such as HF, HNO3 and HCl, which allows for an elevated digestion temperature in open vessels, enabling the decomposition of refractory phases. Similar to HF, HNO3 and HCl, ultrapure NH4HF2 can be produced using a conventional PFA sub-boiling (heating and cooling) purification system. A digestion time of 2–3 h for 200 mg NH4HF2 in a Savillex Teflon vial at 230 °C is sufficient to digest 50 mg of the felsic rock GSP-2, which is ∼6 times faster than using conventional closed-vessel acid digestion at 190 °C (high-pressure PTFE digestion bomb). The price of a Savillex Teflon vial is far less than the price of a high-pressure PTFE digestion bomb (consisting of a PTFE inner vessel and an outer stainless steel pressure jacket). Moreover, the NH4HF2–open-vessel acid digestion is not hampered by the formation of insoluble fluorides. We have successfully applied the NH4HF2–open-vessel acid digestion to the digestion of a series of international geological reference materials, including mafic to felsic igneous rocks and shales. This method provides an effective, simple, economical, and comparatively safe dissolution method that combines the advantages of both the open- and closed-vessel digestion methods.
The transition from mafic to felsic upper continental crust (UCC) is crucial to habitability of Earth, and may be related to the onset of plate tectonics. Thus, defining when this crustal transition ...occurred has great significance for the evolution of Earth and its inhabitants. We demonstrate that V isotope ratios (reported as δ
V) provide insights into this transition because they correlate positively with SiO
and negatively with MgO contents during igneous differentiation in both subduction zones and intraplate settings. Because δ
V is not affected by chemical weathering and fluid-rock interactions, δ
V of the fine-grained matrix of Archean to Paleozoic (3 to 0.3 Ga) glacial diamictite composites, which sample the UCC at the time of glaciation, reflect the chemical composition of the UCC through time. The δ
V values of glacial diamictites systematically increase with time, indicating a dominantly mafic UCC at ~3 Ga; the UCC was dominated by felsic rocks only after 3 Ga, coinciding with widespread continental emergence and many independent estimates for the onset of plate tectonics.
New in situ laser ablation-inductively coupled plasma-mass spectrometry and sensitive high-resolution ion microprobe U–Pb geochronology of zircons from the Idaho batholith and spatially overlapping ...Challis intrusions reveals a series of discrete magmatic belts of different ages and compositions. Following the accretion of the Blue Mountains province to North America along the Salmon River suture zone, two compositionally diverse belts of metaluminous plutons formed both adjacent to the suture and well inboard of it. These were constructed from ~100 to 85 Ma and were followed by a voluminous pulse of peraluminous magmatism, forming the bulk of the Atlanta lobe and largest fraction of the batholith between ~80 and 67 Ma. Around 70 Ma, a later and more spatially restricted suite of metaluminous plutons formed around the Bitterroot lobe of the batholith. This was followed by another pulse of voluminous peraluminous magmatism in the Bitterroot lobe, lasting from ~66 to 54 Ma. The changes from low volume metaluminous to high volume peraluminous magmatism may reflect a combination of changes in the angle and segmentation of the subducting Farallon plate and over thickening of the continental lithosphere. All of these features were then cut by plutons and dikes associated with the Challis volcanic field, lasting from ~51 to 43 Ma. Inherited components are pervasive in zircons from most phases of the batholith. While Precambrian components are very common, zircons also often contain cores or mantles that are 5–20 million years older than their rims. This suggests that the early phases of the batholith were repeatedly cannibalized by subsequent magmas. This also implies that the older suites may have been originally more aerially extensive than their currently exposed forms.
The use of molybdenum as a quantitative paleo-atmosphere redox sensor is predicated on the assumption that Mo is hosted in sulfides in the upper continental crust (UCC). This assumption is tested ...here by determining the mineralogical hosts of Mo in typical Archean, Proterozoic, and Phanerozoic upper crustal igneous rocks, spanning a compositional range from basalt to granite. Common igneous sulfides such as pyrite and chalcopyrite contain very little Mo (commonly below detection limits of around 10 ng/g) and are not a significant crustal Mo host. By contrast, volcanic glass and Ti-bearing phases such as titanite, ilmenite, magnetite, and rutile contain significantly higher Mo concentrations (e.g., up to 40 µg/g in titanite), and can account for the whole-rock Mo budget in most rocks. However, mass balance between whole-rock and mineral data is not achieved in 4 out of 10 granites analyzed with in-situ methods, where Mo may be hosted in undetected trace molybdenite. Significant Mo depletion (i.e., UCC-normalized Mo/Ce < 1) occurs in nearly every granitic rock analyzed here, but not in oceanic basalts or their differentiates (Greaney et al., 2017; Jenner and O’Neill, 2012). On average, granites are missing ∼60% of their expected Mo contents. There are two possible reasons for this: (1) Mo partitions into an aqueous magmatic vapor/fluid phase that is expelled from cooling plutons, and/or (2) Mo is partitioned into titaniferous phases during partial melting and fractional crystallization of an evolving magma. The first scenario is likely given the high solubility of oxidized Mo. However, correlations between Mo/Ce and Nb/La in several plutonic suites suggest fractionating phases such as rutile or Fe-Ti oxides may sequester Mo in lower crustal rocks or in subducting slabs in arc settings.
The Idaho batholith and spatially overlapping Challis intrusive province in the North American Cordillera have a history of magmatism spanning some 55 Myr. New isotopic data from the ∼98 Ma to 54 Ma ...Idaho batholith and ∼51 Ma to 43 Ma Challis intrusions, coupled with recent geochronological work, provide insights into the evolution of magmatism in the Idaho segment of the Cordillera. Nd and Hf isotopes show clear shifts towards more evolved compositions through the batholith's history and Pb isotopes define distinct fields correlative with the different age and compositionally defined suites of the batholith, whereas the Sr isotopic compositions of the various suites largely overlap. The subsequent Challis magmatism shows the full range of isotopic compositions seen in the batholith. These data suggest that the early suites of metaluminous magmatism (98-87 Ma) represent crust-mantle hybrids. Subsequent voluminous Atlanta peraluminous suite magmatism (83-67 Ma) results primarily from melting of different crustal components. This can be attributed to crustal thickening, resulting from either subduction processes or an outboard terrane collision. A later, smaller crustal melting episode, in the northern Idaho batholith, resulted in the Bitterroot peraluminous suite (66-54 Ma) and tapped different crustal sources. Subsequent Challis magmatism was derived from both crust and mantle sources and corresponds to extensional collapse of the over-thickened crust.
The composition of the fine-grained matrix of glacial diamictites from the Mesoarchean, Paleoproterozoic, Neoproterozoic, and Paleozoic, collected from four modern continents, reflects the secular ...evolution of the average composition of the upper continental crust (UCC). The effects of localized provenance are present in some cases, but distinctive geochemical signatures exist in diamictites of the same age from different localities, suggesting that these are global signatures. Archean UCC, dominated by greenstone basalts and to a lesser extent komatiites, was more mafic, based on major elements and transition metal trace elements. Temporal changes in oxygen isotope ratios, rare earth elements, and high field strength elements indicate that the UCC became more differentiated and that tonalite–trondhjemite–granodiorite suites became less important with time, findings consistent with previous studies. We also document the concentrations of siderophile and chalcophile elements (Ga, Ge, Cd, In, Sn, Sb, W, Tl, Bi) and lithophile Be in the UCC through time, and use the data for the younger diamictites to construct a new estimate of average UCC along with associated uncertainties.
The upper continental crust (UCC) is an important reservoir of Ba within the Earth. We report high precision (≤±0.05‰, 2SD) Ba isotopic data for 71 samples (including granites, granodiorites, loess, ...glacial diamictites, and river sediments) to constrain the Ba isotopic composition of the UCC. I-type granites from the Fogang batholith, Southeastern (SE) China, exhibit variable δ137/134Ba (−0.16‰ to 0.01‰), which may be due to isotopic fractionation during the latest stages of magmatic differentiation. The δ137/134Ba of S-type granites from Darongshan-Shiwandashan, SE China (−0.03‰ to 0.11‰) correlate with ɛNd (t), likely reflecting mixing of heterogeneous crustal source materials. Five A-type granites with high SiO2 contents (∼76 wt%) from Nankunshan, SE China have remarkably low δ137/134Ba (−0.47‰ to −0.33‰), which possibly arose from magmatic differentiation or assimilation of crustal materials with light Ba isotopic compositions. Loess from northwestern China has homogeneous δ137/134Ba (−0.02‰ to 0.03‰) that shows no correlation with bulk compositions, sample locations, or degree of chemical weathering, suggesting that loess is representative of the average Ba isotopic composition of the UCC. Three river sediments from northern China have δ137/134Ba similar to that of loess. The δ137/134Ba of glacial diamictites vary with CIA values: those with high CIA (≥60) have heterogeneous δ137/134Ba (−0.19‰ to 0.35‰), while those with low CIA (<60) have δ137/134Ba around 0‰, identical to the values of loess and river sediments, suggesting that Ba isotopes can be fractionated during chemical weathering. In all, samples from the UCC have highly heterogeneous δ137/134Ba, ranging from −0.47‰ to 0.35‰. Using the weighted average of samples in this study, the δ137/134Ba of the UCC is estimated to be 0.00 ± 0.03‰ (2SD/√n, n = 71), which is similar to the average Ba isotopic composition of the upper mantle, but significantly lower than δ137/134Ba of seawater.
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