ALBERT

Description: To better understand the role of slab melt in the petrogenesis of North Cascades magmas, this study focuses on petrogenesis of high-Mg lavas from the two northernmost active volcanoes in Washington. High-Mg andesites (HMA) and basaltic andesites (HMBA) in the Cascade Arc have high Mg# [molar Mg/(Mg+Fe 2+ )] relative to their SiO 2 contents, elevated Nd/Yb, and are Ni- and Cr-enriched. The rock units examined here include the Tarn Plateau HMBA (51.8–54.0 wt% SiO 2 , Mg# 68–70) and Glacier Creek HMA (58.3–58.7 wt% SiO 2 , Mg# 63–64) from the Mount Baker Volcanic Field, and the Lightning Creek HMBA (54.8–54.6 SiO 2 , Mg# 69–73) from Glacier Peak. This study combines major and trace element compositions of minerals and whole rocks to test several petrogenetic hypotheses and to determine which, if any, are applicable to North Cascades HMA and HMBA. In the Tarn Plateau HMBA, rare earth element (REE) equilibrium liquids calculated from clinopyroxene compositions have high Nd/Yb that positively correlates with Mg#. This correlation suggests an origin similar to that proposed for Aleutian adakites, where intermediate, high Nd/Yb slab-derived melts interact with the overlying mantle to become Mg-rich, and subsequently mix with low Nd/Yb, mantle-derived mafic magmas with lower Mg#. In the Glacier Creek HMA, elevated whole-rock MgO and SiO 2 contents resulted from accumulation of xenocrystic olivine and differentiation processes, respectively, but the cause of high Nd/Yb is less clear. However, high whole-rock Sr/P (fluid mobile/fluid immobile) values indicate a mantle source that was fluxed by an enriched, hydrous slab component, likely producing the observed high Nd/Yb REE signature. The Lightning Creek HMBA is a hybridized rock unit with at least three identifiable magmatic components, but only one of which has HMA characteristics. Cr and Mg contents in Cr-spinel and olivine pairs in this HMA component suggest that its source is a strongly depleted mantle, and high whole-rock Sr/P values indicate mantle melting that was induced through hydration, likely adding the component responsible for the observed high Nd/Yb REE pattern. The elevated SiO 2 contents (54.6 wt%) of the HMA component resulted from differentiation or high degrees of partial melting of ultramafic material through the addition of H 2 O. Therefore the Lightning Creek HMBA is interpreted to have originated from a refractory mantle source that underwent melting through interaction with an enriched slab component. Our results indicate that in addition to slab-derived fluids, slab-derived melts also have an important role in the production of HMA and HMBA in the north Cascade Arc.

Description: Introduction This chapter of the International Ocean Discovery Program (IODP) Expedition 350 Proceedings volume documents the procedures and tools employed in the various shipboard laboratories of the R/V JOIDES Resolution during Expedition 350. This information applies only to shipboard work described in the Expedition Reports section of this volume. Methods for shore-based analyses of Expedition 350 samples and data will be described in the individual scientific contributions to be published in the open literature or in the Expedition Research Results section of this volume. This section describes procedures and equipment used for drilling, coring, and hole completion; core handling; computation of depth for samples and measurements; and sequence of shipboard analyses. Subsequent sections describe specific laboratory procedures and instruments in more details.

Description: International Ocean Discovery Program (IODP) Hole U1436A (proposed Site IBM-4GT) lies in the western part of the Izu fore-arc basin, ~60 km east of the arc-front volcano Aogashima, ~170 km west of the axis of the Izu-Bonin Trench, and 1.5 km west of Ocean Drilling Program (ODP) Site 792, at 1776 meters below sea level (mbsl). It was drilled as a 150 m deep geotechnical test hole for potential future deep drilling (5500 meters below seafloor [mbsf]) at proposed Site IBM-4 using the D/V Chikyu. Core from Site U1436 yielded a rich record of Late Pleistocene explosive volcanism, including a distinctive black glassy mafic ash layer that may record a large-volume subaqueous eruption on the Izu arc front. Because of the importance of this discovery, Site U1436 was drilled in three additional holes (U1436B, U1436C, and U1436D), as part of a contingency operation, in an attempt to get better recovery on the black glassy mafic ash layer and its enclosing sediments and to better constrain its thickness. IODP Site U1437 is located in the Izu rear arc, ~330 km west of the axis of the Izu-Bonin Trench and ~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll, at 2117 mbsl. The primary scientific objective for Site U1437 was to characterize “the missing half of the subduction factory” because numerous ODP/Integrated Ocean Drilling Program sites had been drilled in the arc-front to fore-arc region (i.e., ODP Site 782A Leg 126), but this was the first site to be drilled in the rear-arc region of the Izu arc. A complete view of the arc system is needed to understand the formation of oceanic arc crust and its evolution into continental crust. Site U1437 on the rear arc had excellent core recovery in Holes U1437B and U1437D, and we succeeded in hanging the longest casing ever in the history of R/V JOIDES Resolution scientific drilling (1085.6 m) in Hole U1437E and cored to 1806.5 mbsf. The stratigraphy at Site U1437 was divided into seven lithostratigraphic units (I–VII) that were distinguished from each other based on the proportions and characteristics of tuffaceous mud/mudstone and interbedded tuff, lapilli-tuff, and tuff-breccia. The section is much more mud rich than expected, with ~60% tuffaceous mud for the section as a whole (89% in the uppermost 433 m) and high sedimentation rates of 100–260 m/My for the upper 1320 m (Units I–V). The proportion (40%) and grain size of volcaniclastics are much smaller than expected for an intra-arc basin, composed half of ash/tuff and half of lapilli-tuff of fine grain size (clasts 〈3 cm). These volcaniclastics were deposited by suspension settling through water and from density currents, in relatively distal settings. Volcanic blocks are only sparsely scattered through the lowermost 25% of the section (Units VI and VII, 1320–1806.5 mbsf), which includes hyaloclastite, in situ quench-fragmented blocks, and a rhyolite peperite intrusion (i.e., proximal deposits). The transition from unconsolidated to lithified rocks occurred progressively; however, sediments were considered lithified from 427 mbsf (top of Hole U1437D) downward. Alteration resulted in destruction of fresh glass from ~750 mbsf downward, but minerals are less altered. Because of the alteration, the deepest biostratigraphic datum was at ~850 mbsf and the deepest paleomagnetic datum was at ~1300 mbsf. Additional age control deeper than ~1300 mbsf is provided by an age range of 10.97–11.85 Ma inferred from a nannofossil assemblage at ~1403 mbsf and a preliminary U-Pb zircon concordia intercept age of 13.6 +1.6/−1.7 Ma, measured postcruise on a rhyolite peperite in Unit VI at ~1390 mbsf. Based on the seismic profiles, the Miocene–Oligocene hiatus (~17–23 Ma) was predicted to lie at ~1250 mbsf, but strata at that depth (Unit V, 1120–1312 mbsf) are much younger (~9 Ma), indicating that we recovered a thicker Neogene section of volcaniclastics and associated igneous rocks than anticipated. Our preliminary interpretation of shipboard geochemistry of solids is that arc-front versus rear-arc sources can be distinguished for individual intervals in the upper, relatively distal 1320 m of the section (Units I–V), whereas data for the lower, proximal 25% of the section (Units VI–VII) overlap and exceed the compositional fields for Neogene rear-arc seamounts and Quaternary arc-front volcanoes. This suggests that the compositional divergence between arc-front and rear-arc magmas only fully developed after ~13 Ma.

Description: International Ocean Discovery Program (IODP) Hole U1436A (proposed Site IBM-4GT) lies in the western part of the Izu fore-arc basin, ~60 km east of the arc-front volcano Aogashima, ~170 km west of the axis of the Izu-Bonin Trench, 1.5 km west of Ocean Drilling Program (ODP) Site 792, and at 1776 meters below sea level (mbsl). It was drilled as a 150 m deep geotechnical test hole for potential future deep drilling (5500 meters below seafloor [mbsf]) at proposed Site IBM-4 using the D/V Chikyu. Core from Site U1436 yielded a rich record of Late Pleistocene explosive volcanism, including distinctive black glassy mafic ash layers that may record large-volume eruptions on the Izu arc front. Because of the importance of this discovery, Site U1436 was drilled in three additional holes (U1436B, U1436C, and U1436D), as part of a contingency operation, in an attempt to get better recovery on the black glassy mafic ash layers and enclosing sediments and to better constrain the thickness of the mafic ash layers. IODP Site U1437 is located in the Izu rear arc, ~330 km west of the axis of the Izu-Bonin Trench and ~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll, at 2117 mbsl. The primary scientific objective for Site U1437 was to characterize “the missing half of the subduction factory”; this was because numerous ODP/Integrated Ocean Drilling Program sites had been drilled in the arc to fore-arc region (i.e., ODP Site 782A Leg 126), but this was the first site to be drilled in the rear part of the Izu arc. A complete view of the arc system is needed to understand the formation of oceanic arc crust and its evolution into continental crust. Site U1437 on the rear arc had excellent core recovery in Holes U1437B and U1437D, and we succeeded in hanging the longest casing ever in the history of R/V JOIDES Resolution scientific drilling (1085.6 m) in Hole U1437E and cored to 1806.5 mbsf. The stratigraphy at Site U1437 was divided into seven lithostratigraphic units (I–VII) that were distinguished from each other based on the proportions and characteristics of tuffaceous mud/mudstone and interbedded tuff, lapilli tuff, and tuff breccia. The section is much more mud rich than expected, with ~60% tuffaceous mud for the section as a whole (89% in the uppermost 433 m) and high sedimentation rates of 100–260 m/My for the upper 1320 m (Units I–V). The proportion (40%) and grain size of tephra are much smaller than expected for an intra-arc basin, composed half of ash/tuff and half of lapilli tuff of fine grain size (clasts 〈 3 cm). These were deposited by suspension settling through water and from density currents, in relatively distal settings. Volcanic blocks are only sparsely scattered through the lowermost 25% of the section (Units VI and VII, 1320–1806.5 mbsf), which includes hyaloclastite, in situ quench-fragmented blocks, and a rhyolite peperite intrusion (i.e., proximal deposits). The transition from unconsolidated to lithified rocks occurred progressively; however, sediments were considered lithified from 427 mbsf (top of Hole U1437D) downward. Alteration resulted in destruction of fresh glass from ~750 mbsf downward, but minerals are less altered. Because of the alteration, the deepest biostratigraphic datum was at ~850 mbsf and the deepest paleomagnetic datum was at ~1300 mbsf. Additional age control deeper than this depth is provided by an age range of 10.97–11.85 Ma inferred from a nannofossil assemblage at ~1403 mbsf and a preliminary U-Pb zircon concordia intercept age of 13.6 +1.6/–1.7 Ma, measured postcruise on a rhyolite peperite in Unit VI at ~1390 mbsf. Based on the seismic profiles, the Miocene–Oligocene hiatus (~17–23 Ma) was predicted to lie at ~1250 mbsf, but strata at that depth (Unit V, 1120–1312 mbsf) are much younger (~9 Ma), indicating that we recovered a thicker Neogene section of volcaniclastics and associated igneous rocks than anticipated. Our preliminary interpretation of shipboard geochemistry is that arc-front versus rear-arc sources can be distinguished in the upper, relatively distal 1320 m of section (Units I–V), whereas the lower, proximal 25% of the section (Units VI–VII) may be geochemically heterogeneous, suggesting that the rear-arc magmas only fully compositionally diverged after ~13 Ma.