Young Volcanism In The Procellarum KREEP Terrane On The Moon

Volcanism is the most important endogenic process on the Moon,whose products largely shaped the lunar surface.The properties of the magma source,the temperature,composition,pressure,oxygen fugacity,and other key information of the magma,all could be recorded by the lunar volcanic rocks,as well as the flowing and cooling process after eruption.Therefore,studying the lunar volcanic products could help to reconstruct the volcanic eruption process,the internal structure,and the thermal history of the Moon.Procellarum KREEP Terrane(PKT)has the most active volcanism on the Moon that could extend to the Eratosthenian Period;therefore,it is an ideal place to study the lunar volcanology.The Chang’e-5 landing site(43.06°N,41.92°W)is located to the preselected landing region(41-45°N,49-69°W),in northern Oceanus Procellarum within an Eratosthenian-aged mare unit.Isotopic measurements of the returned samples yield an age of ～2.0 Ga.It is the youngest mare basaltic sample that ever returned,at least ～0.8-0.9 Ga younger than the previously collected basaltic samples.This extremely young age makes the Chang’e-5 basalts significant in understanding the late lunar impact and thermal history.The Chang’e-5 sample research has already made lots of important scientific contributions,however just a start of the long task to study the samples.The lack of an overall knowledge of the magma source region,melting mechanism,magma eruption process,and volcanic history of the Chang’e-5 landing region in northern Oceanus Procellarum,largely constrains a comprehensive study of the petrogenesis,volcanology,and thermal evolution process of the Chang’e-5 basalts.The Chang’e-5 basalts represent the intermediate to high-Ti young mare basalts in the PKT,whose laboratory analysis could provide ground truth to those basalts.However,no such research has been done before to link together the orbital remote sensing data,in situ investigation data,and the returned samples;more efforts could be spent on the implications of the Chang’e-5 basalts to the young volcanism in the PKT.In this work,after conducting a comprehensive study of the volcanic features and volcanic history of northern Oceanus Procellarum,we studied the young volcanism in the PKT on the basis of the ground truth provided by the Chang’e-5 in situ data and returned samples,to support the analysis of the volcanic and thermal history of the Moon in the future.Through the study of the volcanic features in northern Oceanus Procellarum,we found Mons Rümker,Mairan domes,sinuous rilles,and mare basalts are four major volcanic landforms in the region.Mons Rümker,～70 km in diameter,develops steep sided domes and shallow domes on the plateau,representing two stages of basaltic eruptions.Mairan domes include South Dome,Middle Dome,Mairan T Dome,and Northwest Dome.They are characterized by their silica-rich composition with high thorium but low iron abundance,that may be products from highly evolved magma.A complex sinuous rille system was found in northern Oceanus Procellarum that is composed of Rima Sharp,Rima Mairan,Rima Harpalus,and Rima Louville.Rima Sharp(～320 km in length)and Rima Harpalus(～76 km in length)are originated from the North Vent;Rima Mairan(～150 km in length)is originated from the South Vent.After leaving the South Vent,the lavas from Rima Mairan were captured by Rima Sharp,entering its channel,and formed internal features within Rima Sharp including inner levee,inner rille,and lava pond.The wrinkle ridges in the Western Maria are mainly NW orientated,close to the dominant trend of wrinkle ridges in Oceanus Procellarum,that may be controlled by the global stress field.The winkle ridges in the Eastern Maria are mainly NE orientated that may be controlled by the Imbrium basin ring system;they are relatively smaller and shorter than the wrinkle ridges in the Western Maria.The mare surface in the study area is generally flat but wrinkle ridges could raise the mare surface to up to ～100-200 m.The elevation decreases from the west to the east;the Western Maria is ～200-300 m higher than the Eastern Maria.Through the study of the volcanic characteristics and history of the northern Oceanus Procellarum,we found the study area could be divided into 14 geological units(3 Rümker plateau unit,3 Imbrian-aged mare basalt unit,4 Eratosthenian-aged mare basalt unit,Rümker dome unit ld,sd,silica-rich dome unit Idm,and highland unit Ith).The Imbrium impact largely shaped the basement of the region where volcanism later happened.After the formation of the Imbrium basin(～3.9 Ga),Imbrium-aged low-Ti mare basalts(Im1 to Im3)erupted between ～3.47-3.40 Ga,covering the entire northern Oceanus Procellarum(exposed in the Western Mairan at present)with a thickness of ～900 m.The low-Ti mare volcanism may produce Mons Rümker at the same time.Concurrently or a little later,silica-rich magma from underplating erupted and formed Mairan domes.Between ～3.4-2.0 Ga,volcanism ceased in northern Oceanus Procellarum;a paleo-regolith and paleoejecta layer formed on the Imbrian-aged basalt with a total thickness of ～10 m.At ～2.0Ga,the volcanism in northern Oceanus Procellarum reactivated and formed the intermediate to high-Ti mare basalts(Em3 and Em4)with a thickness of ～50 m,composing the Eastern Maria.The Mg number is relatively lower for the Eastern Maria than the Western Marian,reflecting a more evolved composition.We propose the majority of Em4 erupted from the North Vent at ～2.0 Ga through Rima Sharp;while the southeast corner of Em4 erupted from the South Vent at ～1.4 Ga through Rima Mairan.In formation of the sinuous rille,sheet flows may be formed in the early stage by high flux eruption followed by Hawaiian and Strombolian eruptions.The lava flow starts to channelization after it cools from the margin to the center and finally eroded down to form sinuous rille.Until the end of the eruption and the solidification of the lava flow,it continues to transport lavas to the terminus of the sinuous rille.After that,there are no major volcanic activities in northern Oceanus Procellarum and a ～6 m thick regolith formed by space weathering on the top of the Eratosthenian-aged mare basalts.Through the study of the young volcanism in the PKT,we found the study area could be divided into 30 geological units(U1 to U30).Chang’e-5 landed on the center of the U2 unit.The Chang’e-5 lunar soil is composed of ～42.8% pyroxene,～33.2% plagioclase,～13.1% olivine,～5.2% iron titanium oxides,and ～3.4% glass according to the Raman spectral measurements.The Chang’e-5 lunar soil has a low abundance of olivine that doesn’t support the young mare basalts rich in olivine that proposed by several previous researchers.The broad and asymmetrical 1 μm absorption and the shallow 2 μm absorption of the young basalts in the reflectance spectra are likely due to the composition of the young basalts are highly evolved with special pyroxene compositions.On the basis of the Moon Mineralogical Mapper hyperspectral data and the lookup table spectral unmixing method that could be applied to variable Mg numbers,we recalculated the mineral abundance of the young mare basalts in the PKT.We found the high-Ca pyroxene content is averaged in ～25±1.0%,low-Ca pyroxene content is averaged in ～19±1.1%,olivine content is averaged in ～6±0.8%,plagioclase content is averaged in ～50±2.7%;olivine is not rich.The less well constrained lunar chronology function in the age range of 1.0-3.0 Ga may be one of the reasons for the large model age difference of the young basalt in PKT.Therefore,we redated the age of the young basalts in the PKT applying the new lunar chronology function calibrated by Chang’e-5 samples and the newly mapped geological boundaries in this study.We found U17 is the youngest unit;and the unit that Chang’e-5 landed(U2)has an age of ～2.0,same as the isotopic age of the returned sample.We found the young basalts have a trend of increasing Ti O2 abundance with decreasing age.The abundance of olivine doesn’t have a clear trend;it is mostly between ～3-9%,far smaller than ～50% previously proposed.We found the lunar mare volcanism peaked at～3.5 Ga,then receded until ～1.2 Ga that large scale lunar mare volcanism finally ceased.Through the current research,we studied the volcanic features and history of the Chang’e-5 landing region in northern Oceanus Procellarum and discussed the young volcanism in the PKT on the basis of the ground truth from the Chang’e-5 samples.We found Chang’e-5 basalts could represent the widespread young basalts in the PKT,which has intermediate to high-Ti composition with an increasing trend with decreasing age.For those young mare units in the PKT,only the U2 unit has returned samples by Chang’e-5.However,they include the youngest mare unit on the Moon(～1.2 Ga)and the unit with the most special composition(U3,highest olivine content),studying them has significant meanings for future lunar sample-return missions.