Geochemical and Petrologic Systematics in Lavas from Haleakala Volcano, East Maui: Implications for the Evolution of Hawaiian Mantle

Haleakala volcano on East Maui, Hawaii, consists of a tholeiitic basalt shield which grades into a younger alkalic series that was followed by a post-erosional alkalic series. Fresh, stratigraphically controlled samples from the three volcanic series were analyzed for major element abundances, Sr and Nd isotopic ratios and trace element (K, Rb, Sr, Ba, Th, Ta, Nb, REE, Hf, Zr, Sc, V, Cr, Co, Ni and Zn) abundances. These samples include tholeiites, transitional basalts, alkalic basalts, ankaramites, hawaiites and mugearites which range widely in abundances of major elements, trace elements and isotopic ratios. Most of the variations among tholeiites can be explained by olivine±clinopyroxene fractionation and different degrees of partial melting. Olivine composition and Ni abundance in one tholeiite which contains 16.6% MgO suggest that this tholeiite could be close to a primary melt in equilibrium with mantle olivine with composition of Fo9 0. Isotopic ratios, incompatible trace element abundances, compatible element abundances and major element abundances indicate that hawaiites and mugearites can be derived from their stratigraphically associated alkalic basalts by fractionation of abundant clinopyroxene and plagioclase and minor amounts of olivine and magnetite. Olivine and clinopyroxene compositions suggest a cumulative origin for ankaramites. Isotopic ratios and incompatible element abundance ratios (e.g., Ba/La, Nb/La, La/Ce, La/Sm) vary systematically with age. The youngest series (post-erosional alkalics) has the highest Rb/Sr, Ba/La, Nb/La, La/Ce and 1 Nd/ 14 4 Nd ratios but the lowest 8 7 Sr/8 6 Sr ratios whereas the oldest series (dominantly tholeiitic basalts) has the lowest Rb/Sr, Ba/La, Nb/La, La/Ce and 14 3 Nd/ 14 4 Nd ratios but the highest 8 7 Sr/ 8 6 Sr ratios. The post-erosional basalts have La/Ce and La/Sm ratios much higher than chondrites whereas these ratios in the tholeiites are similar to those in chondrites. All the Haleakala samples fall in the 1 Nd/1 4 Nd8 7 Sr/ 8 Sr mantle array with ratios of the post-erosional basalts overlapping the MORB field and ratios in the tholeiites lying closer to estimated bulk earth ratios. The most striking feature of the trace element and isotopic data is the inverse correlations between isotopic ratios and parent/daughter abundance ratios in the Sr and Nd systems. These inverse correlation trends are opposite to the well-known "mantle isochron" established by 1 -4 1 -, , t -i ;, -h l~U*h ~ a~ . 4; 4A lsbl~~~Y oceanic basalts. Decoupling of isotopic ratios with parent/daughter abundance ratios is common in Hawaiian basalts. For example, basalts from other Hawaiian volcanoes such as Loihi seamount, East Molokai volcano and Koolau volcano also show this phenomenon. A mixing model is proposed to explain the contrasting isotopic and trace element characteristics in Hawaiian basalts. The mixing end-members are proposed to be primitive mantle plumes and incipient melts (IM) derived by small degrees (0.1%-1%) of melting of a MORB-source. Peridotites with compositions similar to a MORB source are presumed to be the wall-rocks for the ascending mantle plume which induces melting in its wall rocks. The mixing process may be IM mixed with mantle plumes to form the sources of Hawaiian basalts, or, mixing of IM with melts derived by large degrees (12%-16%) melting of mantle plumes. The contribution of the plume material to the basalts decreases as Hawaiian volcanoes evolve and the basalts become more enriched in incompatible trace elements but less radiogenic in 8 Sr/ 8 6 Sr ratios. Quantitative calculations show that this model accurately predicts the observed isotopic ratios and incompatible trace element abundances in representative samples from the three Haleakala volcanic series. An important implication of this model is that the inferred degrees of melting for the mixing components require that the lower lithosphere and much of the asthenosphere beneath Hawaiian volcanoes is involved in creating these volcanoes. Both the MORB reservoir and primitive mantle components required in the proposed mixing model may be abundant in the Earth's mantle, thus this model may be applicable for other oceanic and continental areas where alkalic basalts have lower 8 7 Sr/ 8 Sr and higher 1 Nd/ 14 4 Nd than geographically associated tholeiites. Thesis Supervisor: F. A. Frey, Professor of Geochemistry ,.~. ' r'~;li~i#c' 'i..'"~~i ;xisl~r* ~', ~k-*t;L~nS~L~LJhb~k~;r ~i+L~ ?C~fii*jejl r ~ Acknowledgements First of all, I would like to express my deep gratitude to my advisor, Fred Frey, for his continuous advice and support. His enthusiasm towards petrology and geochemistry has been a great source of inspiration throughout my graduate years at M.I.T. I would also like to thank him and Mike Garcia (University of Hawaii) for initiating my interest in studying the Haleakala volcano. I benefited from numerous discussions with Stan Hart, Tim Grove, Mike Garcia, Nobu Shimizu, David Kohlstedt, and many of my graduate student colleagues. Their constructive comments on my thesis are appreciated. G. Thompson (Woods Hole Oceanographic Institution) made the shatter box, XRF equipment and CHN analyzer in his laboratory available to me. So did M. Rhodes (University of Massachusetts) with his XRF equipment. In addition, this thesis would not have reached its present stage without the mass spectrometer in the laboratories of Stan Hart. Special thanks to M.I.T. Campus police for their escort services during all those late night sample changes. Dorothy Frank patiently spent weekends and nights typing my thesis over many times. Cathy and Tom Sando and Peter Molnar provided shelter during the last three weeks of this thesis writing. Last but not least, I would like to thank my family in Taiwan and my husband, Wang-Ping, for consistent support and encouragement throughout all