The Bishop Tuff, California: New Insights into Magmatic Timescales and Processes from Micro-Analytical Approaches

<p>The Bishop Tuff is the product of one of the largest eruptions on Earth in the last 1 Myr. This thesis studies the Bishop Tuff in order to better understand the nature of the pre-eruptive magma body, with an emphasis on the processes that occurred within it and the timescales over which they operated. In situ geochemical analyses of crystals and glass from samples collected throughout the Bishop Tuff stratigraphic succession yields insights into the nature of zoning and mixing within this supervolcanic system. Timescales for zircon growth (inferred to represent longevity of the magma chamber) are investigated using U-Pb dating of zircons. Zircon textural and trace element data obtained by SIMS (SHRIMP-RG) are presented from 15 stratigraphically controlled Bishop Tuff samples and two older Glass Mountain (GM) lava samples. The resulting eruption age estimate derived from the weighted mean of 166 rim ages of 766.6±3.1 ka (95% confidence) is identical within uncertainty to published values from ID-TIMS and 40Ar/39Ar techniques. An eruption age is also derived for GM dome YA (the youngest GM dome) of 862±23 ka (95% confidence), significantly older than the widely used 790±20 ka K-Ar age. The oldest zircon cores from late-erupted Bishop material (including those with GM-type textures) have a weighted mean of 838.5±8.8 ka (95% confidence), implying that the Bishop Tuff system was only active for ~80 kyr, and had effectively no temporal overlap with the GM system. Bishop zircon textures are divided into four suites whose proportions change systematically through the eruptive sequence. Trace element variations in Bishop zircons are influenced strongly by sector zoning for many elements, and thus restrict the value of trace element variations in discerning compositional stratification within the magma chamber. In later-erupted units, bright-rim overgrowths are common, and are inferred to have crystallized from the same „bright-rim‟ magma as generated the contrasting rims seen in CL or BSE imaging on quartz, feldspar and orthopyroxene. From zircon zonation patterns, this less-evolved, slightly hotter magma invaded deeper parts of the chamber represented in the late-erupted northern units possibly up to ~10 kyr prior to eruption. In order to better quantify the timescales of interaction with the „bright-rim‟ magma, two-feldspar thermometry data are presented on multiple Bishop Tuff samples to constrain temperature variations within the pre-eruptive magma body and yield values for diffusion modelling. Two-feldspar thermometry agrees well with published Fe–Ti-oxide thermometry and reveals a ~80 °C uniform thermal gradient between the upper and lower regions of the magma chamber. Using this thermometry, diffusion of Ti in quartz, Ba in sanidine, Sr in sanidine and Fe-Mg interdiffusion in orthopyroxene are modelled to estimate timescales for the formation of overgrowth rims on crystals. Ti in quartz and Fe-Mg in orthopyroxene diffusion both yield timescales of <150 years for the formation of overgrowth rims, although differing by about an order of magnitude in their timing. However, Ba and Sr diffusion modelling in sanidine yields disparate timescales 1-2 orders of magnitude longer than for Ti in quartz. The main cause for this discrepancy is inferred to be an incorrect assumption for the initial profile shape for Ba and Sr diffusion modelling (i.e. the profile is influenced by growth zoning). Using the comparison with Sr, constraints are placed on the initial width of the core-rim interface and the initial conditions can be refined, bringing Ba and Sr diffusion timescales into mutual alignment and closer to the values from Ti in quartz. This modelling shows that piecemeal rejuvenation of lower Bishop Tuff magma chamber occurred over a period of ~500 years leading up to eruption. In situ major and trace element analyses of sanidine, plagioclase, biotite, orthopyroxene, clinopyroxene, zircon and matrix glass from the Bishop Tuff and two GM lavas are presented to investigate the pre-eruptive stratification of the Bishop magma chamber and its chemical relationship to the GM system. Analyses of samples from the entire Bishop stratigraphy confirm that the magma chamber was thermally and compositionally zoned prior to growth of crystals and the intrusion of the „bright-rim‟ forming magma. Study of rare mixed swirly and dacitic pumice samples shows enrichments in Ba, Sr and Ti (the elements responsible for bright-rim overgrowths in phenocryst phases) and identifies these pumices as possible representatives of the „bright-rim‟ magma. This integrated study of phenocrysts and glass from the Bishop Tuff leads to development of a revised magma chamber model, in which there is a unitary chamber with a stepped or sloping roof. The chamber has an upper, volumetrically dominant (~2/3) part showing no evidence for convection and with unzoned crystals, and a lower part which had experienced mixing of crystals and interaction with the „bright-rim‟ magma. Intrusion of the „bright-rim‟ magma introduced orthopyroxene and dominantly bright zircon crystals, and caused overgrowth of bright rims enriched in Ti, Sr and Ba on sanidine and quartz phenocrysts. Chemical compositions of GM and Bishop Tuff materials show a shared consanguinity, implying common modes of magma generation, yet the generation of GM and Bishop eruptible magma bodies were physically and temporally separate events.</p>

The Bishop Tuff, California: New Insights into Magmatic Timescales and Processes from Micro-Analytical Approaches

<p>The Bishop Tuff is the product of one of the largest eruptions on Earth in the last 1 Myr. This thesis studies the Bishop Tuff in order to better understand the nature of the pre-eruptive magma body, with an emphasis on the processes that occurred within it and the timescales over which they operated. In situ geochemical analyses of crystals and glass from samples collected throughout the Bishop Tuff stratigraphic succession yields insights into the nature of zoning and mixing within this supervolcanic system. Timescales for zircon growth (inferred to represent longevity of the magma chamber) are investigated using U-Pb dating of zircons. Zircon textural and trace element data obtained by SIMS (SHRIMP-RG) are presented from 15 stratigraphically controlled Bishop Tuff samples and two older Glass Mountain (GM) lava samples. The resulting eruption age estimate derived from the weighted mean of 166 rim ages of 766.6±3.1 ka (95% confidence) is identical within uncertainty to published values from ID-TIMS and 40Ar/39Ar techniques. An eruption age is also derived for GM dome YA (the youngest GM dome) of 862±23 ka (95% confidence), significantly older than the widely used 790±20 ka K-Ar age. The oldest zircon cores from late-erupted Bishop material (including those with GM-type textures) have a weighted mean of 838.5±8.8 ka (95% confidence), implying that the Bishop Tuff system was only active for ~80 kyr, and had effectively no temporal overlap with the GM system. Bishop zircon textures are divided into four suites whose proportions change systematically through the eruptive sequence. Trace element variations in Bishop zircons are influenced strongly by sector zoning for many elements, and thus restrict the value of trace element variations in discerning compositional stratification within the magma chamber. In later-erupted units, bright-rim overgrowths are common, and are inferred to have crystallized from the same „bright-rim‟ magma as generated the contrasting rims seen in CL or BSE imaging on quartz, feldspar and orthopyroxene. From zircon zonation patterns, this less-evolved, slightly hotter magma invaded deeper parts of the chamber represented in the late-erupted northern units possibly up to ~10 kyr prior to eruption. In order to better quantify the timescales of interaction with the „bright-rim‟ magma, two-feldspar thermometry data are presented on multiple Bishop Tuff samples to constrain temperature variations within the pre-eruptive magma body and yield values for diffusion modelling. Two-feldspar thermometry agrees well with published Fe–Ti-oxide thermometry and reveals a ~80 °C uniform thermal gradient between the upper and lower regions of the magma chamber. Using this thermometry, diffusion of Ti in quartz, Ba in sanidine, Sr in sanidine and Fe-Mg interdiffusion in orthopyroxene are modelled to estimate timescales for the formation of overgrowth rims on crystals. Ti in quartz and Fe-Mg in orthopyroxene diffusion both yield timescales of <150 years for the formation of overgrowth rims, although differing by about an order of magnitude in their timing. However, Ba and Sr diffusion modelling in sanidine yields disparate timescales 1-2 orders of magnitude longer than for Ti in quartz. The main cause for this discrepancy is inferred to be an incorrect assumption for the initial profile shape for Ba and Sr diffusion modelling (i.e. the profile is influenced by growth zoning). Using the comparison with Sr, constraints are placed on the initial width of the core-rim interface and the initial conditions can be refined, bringing Ba and Sr diffusion timescales into mutual alignment and closer to the values from Ti in quartz. This modelling shows that piecemeal rejuvenation of lower Bishop Tuff magma chamber occurred over a period of ~500 years leading up to eruption. In situ major and trace element analyses of sanidine, plagioclase, biotite, orthopyroxene, clinopyroxene, zircon and matrix glass from the Bishop Tuff and two GM lavas are presented to investigate the pre-eruptive stratification of the Bishop magma chamber and its chemical relationship to the GM system. Analyses of samples from the entire Bishop stratigraphy confirm that the magma chamber was thermally and compositionally zoned prior to growth of crystals and the intrusion of the „bright-rim‟ forming magma. Study of rare mixed swirly and dacitic pumice samples shows enrichments in Ba, Sr and Ti (the elements responsible for bright-rim overgrowths in phenocryst phases) and identifies these pumices as possible representatives of the „bright-rim‟ magma. This integrated study of phenocrysts and glass from the Bishop Tuff leads to development of a revised magma chamber model, in which there is a unitary chamber with a stepped or sloping roof. The chamber has an upper, volumetrically dominant (~2/3) part showing no evidence for convection and with unzoned crystals, and a lower part which had experienced mixing of crystals and interaction with the „bright-rim‟ magma. Intrusion of the „bright-rim‟ magma introduced orthopyroxene and dominantly bright zircon crystals, and caused overgrowth of bright rims enriched in Ti, Sr and Ba on sanidine and quartz phenocrysts. Chemical compositions of GM and Bishop Tuff materials show a shared consanguinity, implying common modes of magma generation, yet the generation of GM and Bishop eruptible magma bodies were physically and temporally separate events.</p>

Age of Initial Submarine Volcanism in the Paleo-Tsushima Basin and Implications for Submarine Volcanism in the Opening Stage of the Japan Sea in Northern Kyushu

The Tsushima Lapilli Tuff, the thickest tuff in the Taishu Group on Tsushima Island, underwent a thermal event after deposition, and has not previously yielded a reliable age because various ages have been reported. This study clarifies the eruption age and thermal history of the Tsushima Lapilli Tuff based on fission-track (FT) and U–Pb dating of zircon grains using laser ablation inductively coupled plasma mass spectrometry (ICP-LA-MS) and evaluates submarine volcanism during deposition of the Taishu Group in the southwestern Japan Sea, as well as volcanism change on Tsushima Island. This study revealed that thermal events caused rejuvenation in some single-grain FT ages after deposition in the Tsushima Group, and that the eruption age of the Tsushima Lapilli Tuff was 16.2 ± 0.7 Ma; the age of the largest submarine volcanism event in the Taishu Group in Tsushima Island was thus determined. On the basis of our previous studies, this age and tectonism strongly indicate that felsic submarine volcanism occurred between 18 and 16 Ma, accompanied by rapid subsidence, and the volcanism changed from felsic volcanism originating from melting of old continental crust by asthenospheric upwelling to mafic volcanism originating from small-scale lithospheric mantle upwelling from 13.6 Ma onward.

Short-lived alkaline magmatism related to Réunion plume in the Deccan large igneous province: inferences from petrology, 40Ar/39Ar geochronology and paleomagnetism of lamprophyre from the Sarnu-Dandali alkaline igneous complex

Existing geochronological information on Deccan indicates prolonged (started at 68.5 Ma) alkaline magmatism related to the Réunion mantle plume based on the 40Ar/39Ar ages from Sarnu-Dandali and Mundwara alkaline complexes. We studied in detail an alkaline lamprophyre, from the Sarnu-Dandali complex, rich in groundmass (magmatic) as well as xenocrystic phlogopites and clinopyroxenes. 40Ar/39Ar age determinations of the phlogopites from this lamprophyre, reveal two distinct ages of 65.44±1.5 Ma and 68.17±1Ma. However, paleomagnetic results show a VGP at 32.31 N and 298.52 E concordant with that of the Deccan Super Pole at 65.5 Ma and support the younger eruption age at ca. 65.44±1.5Ma. Analyzed phlogopites lack any signs of retention of excess radiogenic Ar and yield similar inverse isochron ages, which suggests that the older age of ca. 68.17±1Ma belongs to the crystallization of xenocrystic phlogopite during mantle metasomatism. Trace element compositions support derivation of lamprophyre magma from an OIB- type enriched (metasomatized) mantle source with an involvement of phlogopite.This finding suggests that the pre-Deccan ages of ca. 68-69 Ma reported previously, may reflect the timing of metasomatism of the subcratonic lithospheric mantle during the separation of Greater-Seychelles from India at ca. ∼68.5 Ma. The absence of pre-Deccan alkaline rocks therefore indicates the short-duration (occurred between 67-65 Ma) of alkaline as well as small-volume, volatile-rich magmatism directly related to the Réunion (Deccan) plume.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5490881

Timing and distribution of the Los Chocoyos supereruption from Atitlán caldera (Guatemala) by zircon 238U-230Th and (U-Th)/He double-dating

&lt;p&gt;The climactic Los Chocoyos (LCY) rhyolitic eruption from Atitl&amp;#225;n caldera (Guatemala) is a key chronostratigraphic marker for the Late Quaternary period that has been widely used for relative dating of paleoenvironmental, paleoclimate, and volcanic events throughout Central America and adjacent marine basins in the Pacific Ocean, the Caribbean Sea, and the Gulf of Mexico. Despite LCY tephra being an important marker horizon, a radioisotopic age for this eruption has remained elusive. LCY tephra has been dated at ca. 84 ka BP based on its occurrence in marine sediments with model &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O ages, but this inferred age has not been independently confirmed through radioisotopic methods. This is due to the inherent limitations of radiocarbon dating (which is practically limited to &amp;#706;50 ka) and a lack of suitable materials for &lt;sup&gt;40&lt;/sup&gt;Ar/&lt;sup&gt;39&lt;/sup&gt;Ar analysis in LCY tephra. To overcome this limitation, we applied &lt;sup&gt;238&lt;/sup&gt;U-&lt;sup&gt;230&lt;/sup&gt;Th and (U-Th)/He zircon double-dating (ZDD). Due to zircon being alteration-resistant this method establishes absolute chronologies for and correlations between silicic tephra deposits, which are unaffected by glass alteration or complex compositional signatures within a single eruption. &lt;sup&gt;238&lt;/sup&gt;U-&lt;sup&gt;230&lt;/sup&gt;Th zircon crystallization rim ages were obtained from LCY proximal tephras (~17 km from Atitl&amp;#225;n caldera) including sub-units that may bear distinct glass compositions (e.g., fallout, ignimbrite, surge) as well as ultra-distal fallout tephra samples (~300 km from source) collected from drill cores at Pet&amp;#233;n Itz&amp;#225; Lake (ICDP) and the Pacific Ocean (IODP). All samples yielded zircon with statistically indistinguishable &lt;sup&gt;238&lt;/sup&gt;U-&lt;sup&gt;230&lt;/sup&gt;Th zircon rim age spectra. These reveal continuous zircon crystallization from ca. 160 ka to ca. 74 ka, with peaks in zircon crystallization between 90-100 ka. ZDD eruption ages from two LCY fallout and one ignimbrite deposit are indistinguishable with error-weighted averages of 75.1 &amp;#177; 3.2 ka (1&amp;#963;; n = 16; MSWD = 4.1), 76.0 &amp;#177; 2.5 ka (n = 16; MSWD = 2.5), and 72.8 &amp;#177; 3.5 ka (n = 16; MSWD = 3.7). Considering all individual zircon results as a single population, a weighted average ZDD age of 74.8 &amp;#177; 1.7 (1&amp;#963;; n = 48; MSWD = 3.3) is obtained and considered as the best estimate for LCY eruption age. GIS-based reassessment of LCY eruptive volume uses thickness information from new 113 outcrops including 6&amp;#8211;10 m thick pyroclastic density currents in Chiapas, Mexico (&gt;130 km from the source) and suggests a minimum estimate volume of ~1200 km&lt;sup&gt;3&lt;/sup&gt;, confirming the LCY eruption as the first&amp;#8208;ever recognized supereruption in Central America. The new ZDD age of 74.8 &amp;#177; 1.7 ka for the LCY eruption is significantly younger than the commonly cited O-isotope stratigraphic age of 84 &amp;#177; 5 ka. This age is close to the voluminous (2,800-5,600 km&lt;sup&gt;3&lt;/sup&gt;) Young Toba Tuff (YTT) supereruption at ca. 73.8 &amp;#177; 0.3 ka from Toba Caldera, Indonesia. Both YTT and LCY eruptions have been previously linked to prominent Quaternary climate excursions. Based on the new LCY eruption age, climate-forcing effects that are usually attributed to YTT may in fact be exacerbated by another supereruption occurring within a short time window of the YTT event.&lt;/p&gt;

Trace element partition coefficients and petrogenesis of the 154 ka dacitic Haramul Mic lava dome (Ciomadul, Romania)

&lt;p&gt;Haramul Mic is a ~0.15 km&lt;sup&gt;3 &lt;/sup&gt;volume, crystal-rich, homogeneous, high-K dacite lava dome, which is one of the oldest ones in the Ciomadul Volcanic Complex (Romania, eastern-central Europe). The eruption that formed the lava dome occurred after about 200.000 years of quiescence. Eruption age of the dome determined by (U-Th)/He dating on zircon gave 154 +/- 16 ka that is in agreement with the youngest zircon U-Th outer rim date (142 +18/-16 ka). The apparently continuous crystallization of zircon between the eruption age and the 306 +/- 37 ka oldest zircon core date records a long-living magmatic plumbing system.&lt;/p&gt;&lt;p&gt;The Haramul Mic lava dome rock has 35-40% average crystal content and consists of plagioclase, amphibole, biotite and accessory zircon, apatite, titanite and Fe-Ti oxides. The groundmass is mainly built up by perlitic glass with some microlites and sheared vesicles. The dacite contains sparse mafic enclaves with K-rich, shoshonitic bulk composition, composed of plagioclase and biotite in addition to less amount of amphibole. Felsic crystal clots are more common and they comprise plagioclase, amphibole, biotite and interstitial vesicular glass.&lt;/p&gt;&lt;p&gt;Trace element content of the mineral phases and the groundmass glass was determined by LA-ICP-MS. All of the studied phases show homogeneous trace element compositions and along with the textural characteristics these imply general equilibrium state in the magma storage system before the eruption. Amphibole-plagioclase geothermometer and geobarometer calculations result in 700-800 &lt;sup&gt;o&lt;/sup&gt;C crystallization temperature and 200-300 MPa crystallization pressure.&lt;/p&gt;&lt;p&gt;In order to reveal the magma chamber processes that triggered the eruption and formed the Haramul Mic lava dome after long quiescence time, it is necessary to understand better the behaviour of trace elements as the most sensitive indicators of magma reservoir mechanisms. We determined mineral-liquid trace element partition coefficients and evaluated the result in the context of crystal lattice strain model. They show many similarities with those proposed for the Fish Canyon Tuff dacite except for Li and Sc. The anomalous behaviour of Sc is clearly expressed by the elevated concentration in the glass phase and many times, there are some zonation in Sc from crystal core to rim. This could be explained either by inherently higher Sc content of the melt reflecting the nature of the primary magmas or by partial remelting of biotite just before the eruption. Significant positive anomaly of Li content can be observed in biotite crystals of the mafic enclave compared with the dacitic host rock. Li content of plagioclase varies between 15-30 ppm with slight rimward depletion.&lt;/p&gt;&lt;p&gt;Eruption initiation cannot be explained by physical mixing of mafic recharge magma, but rather by volatile transfer or second boiling. The water-rich nature of the melt is reflected by the abundant vesicles in the glassy groundmass. Furthermore, the amphibole phenocrysts have sharp margin without resorption rim, which suggest hydrous melt phase and relatively fast magma ascent.&lt;/p&gt;&lt;p&gt;This research belongs to the NKFIH-OTKA K135179 project and was supported by the &amp;#218;NKP-19-1 New National Excellence Program of the Ministry for Innovation and Technology.&lt;/p&gt;

Magmatic history of the Oldest Toba Tuff inferred from zircon U–Pb geochronology

The magmatic history of the Oldest Toba Tuff (OTT), the second largest in volume after the Youngest Toba Tuff (YTT), northern Sumatra, Indonesia, was investigated using U–Pb zircon dating by LA-ICP-MS. Zircon dates obtained from surface and interior sections yielded ages of 0.84 ± 0.03 Ma and 0.97 ± 0.03 Ma, respectively. The youngest OTT zircon ages were in accordance with the 40Ar/39Ar eruption age of ~ 0.8 Ma, whereas the oldest zircon dates were ~ 1.20 Ma. Therefore, the distribution of zircon U–Pb ages is interpreted to reflect protracted zircon crystallization, suggesting that the estimated 800–2,300 km3 of OTT magma accumulated and evolved for at least 400,000 years prior to eruption. This result is comparable to the volume and timescales of YTT magmatism. The similarities of both magmatic duration and geochemistry between OTT and YTT may indicate that they are similar in size and that the caldera collapse that generated OTT might be much larger previously interpreted.

Simultaneous Middle Pleistocene eruption of three widespread tholeiitic basalts in northern California (USA): Insights into crustal magma transport in an actively extending back arc

Mapping and chronology are central to understanding spatiotemporal volcanic trends in diverse tectonic settings. The Cascades back arc in northern California (USA) hosts abundant lava flows and normal faults, but tholeiitic basalts older than 200 ka are difficult to discriminate by classic mapping methods. Paleomagnetism and chemistry offer independent means of correlating basalts, including the Tennant, Dry Lake, and Hammond Crossing basalt fields. Paleomagnetic analysis of these chemically similar basalts yield notable overlap, with statistical analysis yielding 7 chances in 1,000,000 that their similar mean remanent directions are random. These basalts also have overlapping 40Ar/39Ar ages of 272.5 ± 30.6 ka (Tennant), 305.8 ± 23.9 ka (Dry Lake), and 300.4 ± 15.2 and 322.6 ± 17.4 ka (Hammond Crossing). Chemical and paleomagnetic analyses indicate that these spatially distributed basalts represent simultaneous (&lt;100 yr uncertainty) eruptions, and thus we use 305.5 ± 9.8 ka (weighted mean) as the eruption age. Their vents align on a N25°W trend over a distance of 39 km. Tennant erupted the largest volume (3.55 ± 0.75 km3) at the highest elevation; both factors decay to the south-southeast at Dry Lake (0.75 ± 0.15 km3) and Hammond Crossing (0.15 ± 0.05 km3). We propose vertical magma ascent beneath the Tennant vent area, where the most evolved, high-SiO2 magma erupted, with lateral dike propagation in the brittle crust. Propagation was near orthogonal to east-west extension (0.3–0.6 mm/yr) along north-northwest–trending normal faults.