Geology topics

Brian Jicha

Publications and source records attributed to Brian Jicha.

5 recordsLinked to original sources

USGS2024

Reexamining the Honolulu Volcanics: Hawai‘i's classic case of rejuvenation volcanism

Rejuvenated volcanism is a worldwide phenomenon occurring on many oceanic islands in all of the major ocean basins. This plume-related volcanism follows the main edifice-building stage after a hiatus of variable duration (e.g. 0.6–2 Myrs in Hawai'i). The Honolulu Volcanics (HV), the classic case of rejuvenated volcanism, involved monogenetic eruptions from at least 48 vent areas. Previous studies inferred these vents were aligned along 3 to 11 rifts oriented orthogonal to the propagation direction of the Hawaiian plume. HV basalts are known for having high MgO contents (greater than 10 wt %) and upper mantle xenoliths. Thus, HV magmas are assumed to be relatively primitive and to have ascended rapidly (less than 1 day) through the crust. However, new analyses of olivine cores in basalts from 24 HV vents are mostly too low in forsterite content (74–86 mol %) to be in equilibrium with mantle melts. Olivine and clinopyroxene in HV basalts commonly show reverse zoning indicating magma mixing prior to eruption. These results are inconsistent with the rapid ascent of HV magmas directly from their mantle source. Many of the HV magmas underwent storage (probably in the lower crust or uppermost mantle), crystal fractionation and magma mixing prior to eruption. New 40 Ar/ 39 Ar dates were determined for 11 HV lavas to evaluate their eruptive history. These ages, 80 to 685 ka, combined with our previous and other 40 Ar/ 39 Ar ages for HV lavas reveal long gaps (greater than 50 kyr) between some eruptions. Our comprehensive, whole-rock major and trace element database (63 XRF analyses, 57 ICPMS analyses) of basalts from 37 vents show remarkable compositional diversity with no obvious spatial pattern or temporal trends. The two most recent eruptive sequences have the greatest diversity (basanite and melilitite compositions). HV basanites show systematic trace element trends that may reflect mixing of multiple source components. The nephelinites and melilitites require a complex source history that may have involved residual accessory minerals during mantle melting and a metasomatic component that was not carbonatitic. The new ages and geochemical data show eruptions along most of the previously proposed rift systems were unrelated (except for the Koko Rift). Therefore, geodynamic models that relate HV volcanism to these rift systems are invalid. Lava volumes for two HV eruptions were estimated at 0.11 and 0.23 km 3 using surface mapping and water well data. Similar size, recent monogenetic eruptions in Auckland, New Zealand, were inferred to have lasted several months. Thus, if another HV eruption were to occur, which is possible given the long hiatus between eruptions, it would be extremely disruptive for the nearly 1 million residents of Honolulu. None of the existing geodynamic models fully explain the age duration, volumes and the locations of Hawai'i's rejuvenated volcanism. Thus, the cause of this secondary volcanism remains enigmatic.

Hawaii↗

USGS2024

A Bayesian age from dispersed plagioclase and zircon dates in the Los Chocoyos ash, Central America

The eruption that produced the modern Atitlán caldera in Guatemala has a Volcanic Explosivity Index (VEI) greater than 8, making it the largest of the Quaternary in the Central American Volcanic Arc (CAVA). It generated ∼1220 km 3 (730 km 3 dense rock equivalent) of dispersed rhyolitic ash and unwelded pyroclastic flow deposits known as the Los Chocoyos ash. Despite its scale, the age of the Los Chocoyos ash has been difficult to constrain. Its inferred position in marine sediment off the coast of Ecuador suggests an oxygen isotope-based age of 84 ± 5 thousand years (ka) that has been used for decades. A recent effort to determine the eruption age of the Los Chocoyos ash used U-Th/He and U/Th disequilibrium methods that both yield overdispersed sets of dates from zircon. Using U/Th disequilibrium corrected U-Th/He dates, which include some that are up to 19 ± 8 kyr younger than the youngest U/Th disequilibrium dates, an eruption age of 75 ± 4 ka has been proposed, which overlaps the eruption age of the Youngest Toba Tuff, 40 Ar/ 39 Ar dated at 73.9 ± 0.6 ka. Critically, the above findings have led to the hypothesis that this pair of sulfate-rich supereruptions may be responsible for a global cooling event ∼74 ka. To further test this supereruption doublet hypothesis we present new 40 Ar/ 39 Ar and U-Th disequilibrium data from plagioclase and zircon rim surfaces, respectively, to quantify the timing of the Los Chocoyos eruption. We apply Bayesian estimation to the overdispersed sets of 40 Ar/ 39 Ar plagioclase and U-Th disequilibrium zircon dates. This approach accounts for retention of radiogenic 40 Ar in plagioclase in a subset of crystals and for crystallization of zircon that may precede the time of eruption and yields remarkably consistent ages from two independent chronometers. The Bayesian estimate of 98 ± 6 ka calculated from 308 individual dates identifies the population nearest to the time of the Los Chocoyos eruption. Interpreted as an eruption age, this finding precludes a link between the Los Chocoyos eruption and global cooling ∼74 ka and rules out the supereruption doublet trigger.

Earth and Planetary Science Letters↗

USGS2020

The eruptive history, magmatic evolution, and influence of glacial ice at long-lived Akutan volcano, eastern Aleutian Islands, Alaska, USA

New 40 Ar/ 39 Ar and whole-rock geochemical data are used to develop a detailed eruptive chronology for Akutan volcano, Akutan Island, Alaska, USA, in the eastern Aleutian island arc. Akutan Island (166°W, 54.1°N) is the site of long-lived volcanism and the entire island comprises volcanic rocks as old as 3.3 Ma. Our current study is on the 225 km 2 western half of the island, where our results show that the focus of volcanism has shifted over the last ∼700 k.y., and that on occasion, multiple volcanic centers have been active over the same period, including within the Holocene. Incremental heating experiments resulted in 56 40 Ar/ 39 Ar plateau ages and span 2.3 Ma to 9.2 ka. Eruptive products of all units are primarily tholeiitic and medium-K, and range from basalt to dacite. Rare calc-alkaline lavas show evidence suggesting their formation via mixing of mafic and evolved magmas, not via crystallization-derived differentiation through the calc-alkaline trend. Earliest lavas are broadly dispersed and are almost exclusively mafic with high and variable La/Yb ratios that are likely the result of low degrees of partial mantle melting. Holocene lavas all fall along a single tholeiitic, basalt-to-dacite evolutionary trend and have among the lowest La/Yb ratios, which favors higher degrees of mantle melting and is consistent with the increased magma flux during this time. A suite of xenoliths, spanning a wide range of compositions, are found in the deposits of the 1.6 ka caldera-forming eruption. They are interpreted to represent completely crystallized liquids or the crystal residuum from tholeiitic fractional crystallization of the active Akutan magma system. The new geochronologic and geochemical data are used along with existing geodetic and seismic interpretations from the island to develop a conceptual model of the active Akutan magma system. Collectively, these data are consistent with hot, dry magmas that are likely stored at 5−10 km depth prior to eruption. The prolonged eruptive activity at Akutan has also allowed us to evaluate patterns in lava-ice interactions through time as our new data and observations suggest that the influence of glaciation on eruptive activity, and possible magma composition, is more pronounced at Akutan than has been observed for other well-studied Aleutian volcanoes to the west.

Alaska↗

USGS2019

Ar-Ar age constraints on the timing of Havre Trough opening and magmatism

The age and style of opening of the Havre Trough back-arc system is uncertain due to a lack of geochronologic constraints for the region. 40 Ar/ 39 Ar dating of 19 volcanic rocks from across the southern Havre Trough and Kermadec Arc was conducted in three laboratories to provide age constraints on the system. The results are integrated and interpreted as suggesting that this subduction system is young (<2 Ma) and coeval with opening of the continental Taupo Volcanic Zone of New Zealand. Arc magmatism was broadly concurrent across the breadth of the Havre Trough.

Kermadec Arc–Havre Trough↗

USGS2019

Isotopic and petrologic investigation, and a thermomechanical model of genesis of large-volume rhyolites in arc environments: Karymshina Volcanic Complex, Kamchatka, Russia

The Kamchatka Peninsula of eastern Russia is currently one of the most volcanically active areas on Earth where a combination of >8 cm/yr subduction convergence rate and thick continental crust generates large silicic magma chambers, reflected by abundant large calderas and caldera complexes. This study examines the largest center of silicic 4-0.5 Ma Karymshina Volcanic Complex, which includes the 25 × 15 km Karymshina caldera, the largest in Kamchatka. A series of rhyolitic tuff eruptions at 4 Ma were followed by the main eruption at 1.78 Ma and produced an estimated 800 km 3 of rhyolitic ignimbrites followed by high-silica rhyolitic post-caldera extrusions. The postcaldera domes trace the 1.78 Ma right fracture and form a continuous compositional series with ignimbrites. We here present results of a geologic, petrologic, and isotopic study of the Karymshina eruptive complex, and present new Ar-Ar ages, and isotopic values of rocks for the oldest pre- 1.78 Ma caldera ignimbrites and intrusions, which include a diversity of compositions from basalts to rhyolites. Temporal trends in δ 18 O, 87 Sr/ 86 Sr, and 144 Nd/ 143 Nd indicate values comparable to neighboring volcanoes, increase in homogeneity, and temporal increase in mantle-derived Sr and Nd with increasing differentiation over the last 4 million years. Data are consistent with a batholithic scale magma chamber formed by primarily fractional crystallization of mantle derived composition and assimilation of Cretaceous and younger crust, driven by basaltic volcanism and mantle delaminations. All rocks have 35–45% quartz, plagioclase, biotite, and amphibole phenocrysts. Rhyolite-MELTS crystallization models favor shallow (2 kbar) differentiation conditions and varying quantities of assimilated amphibolite partial melt and hydrothermally-altered silicic rock. Thermomechanical modeling with a typical 0.001 km 3 /yr eruption rate of hydrous basalt into a 38 km Kamchatkan arc crust produces two magma bodies, one near the Moho and the other engulfing the entire section of upper crust. Rising basalts are trapped in the lower portion of an upper crustal magma body, which exists in a partially molten to solid state. Differentiation products of basalt periodically mix with the resident magma diluting its crustal isotopic signatures. At the end of the magmatism crust is thickened by 8 km. Thermomechanical modeling show that the most likely way to generate large spikes of rhyolitic magmatism is through delamination of cumulates and mantle lithosphere after many millions of years of crustal thickening. The paper also presents a chemical dataset for Pacific ashes from ODDP 882 and 883 and compares them to Karymshina ignimbrites and two other Pleistocene calderas studied by us in earlier works.

Kamchatka↗